This is one of the webpages of Libarid A. Maljian at the Department of Physics at CSLA at NJIT.

New Jersey Institute of Technology

College of Science and Liberal Arts

Department of Physics

The Earth in Space

Phys 203

Spring 2023

Second Examination lecture notes

Mineralogy

Mineralogy is the study of minerals, and a mineralogist is someone who studies minerals. A mineral is defined as a naturally occurring solid inorganic object with a definite chemical structure. As we will discuss shortly, a rock does not have a definite chemical structure. This is the most important difference between a mineral and a rock; whereas a mineral has a definite chemical structure, a rock does not have a definite chemical structure. Every mineral has at least two names: a mineral name and a chemical name. For example, the mineral thenardite has the chemical name sodium sulfate Na₂SO₄. As another example, the mineral magnesite has the chemical name magnesium carbonate MgCO₃. Some minerals have a third name, its common designation. For example, the mineral halite has the chemical name sodium chloride NaCl and the common designation table salt. As another example, the mineral hematite has the chemical name ferric oxide Fe₂O₃ and the common designation rust.

There are various properties of minerals that enable us to uniquely identify a mineral. For example, the color of a mineral may help us to identify a mineral. However, the color of a mineral is the least reliable way to identify a mineral, since there can be some variations in color of even a specific mineral. The streak of a mineral is its color in its powdered form, which more reliably identifies a mineral than its color in its solid form. The luster of a mineral is how the mineral reflects light. For example, minerals that reflect light very well have a metallic luster, since most metals reflect light very well. A dull luster is no luster, meaning the mineral does not reflect light well at all. Yet another property of minerals is how the mineral is able to transmit light through it. Some minerals are more transparent, meaning they easily transmit light through them. Some minerals are more opaque, meaning that light is not transmitted through them. In between these two extremes are translucent minerals, which permit light to be transmitted through them but not particularly well.

The hardness of a mineral is its resistance to scratching or rubbing. We can test the hardness of two minerals by rubbing or scratching them against each other. If one mineral is able to scratch a second mineral, then the first mineral is more hard than the second mineral. If instead the second mineral is able to scratch the first mineral, then the second mineral is more hard than the first mineral. The hardness of minerals is quantified with the Mohs scale, named for the German mineralogist Friedrich Mohs who defined this scale in the year 1812. The Mohs scale is numbered from one through ten. Lower numbers on the Mohs scale are less hard minerals, while higher numbers on the Mohs scale are more hard minerals. The least hard mineral on the Mohs scale is talc, which can easily be crumbled into talcum powder, more commonly known as baby powder. The most hard mineral on the Mohs scale is diamond, which is one of the mineral forms of carbon. Since diamond is the most hard mineral on the Mohs scale, no mineral can scratch a diamond except another diamond.

The tenacity of a mineral is its resistance to breaking or deforming. We must break or at least deform a mineral to test its tenacity. There is no tenacity scale analogous to the Mohs scale for hardness. Instead, we describe the tenacity of minerals with various terms, such as brittle if the mineral is easily breakable or malleable if the mineral is easily deformable.

The cleavage of a mineral is its crystal geometry when broken. This means that we must break a mineral to test its cleavage. Some examples of cleavage include cubic cleavage if the mineral breaks into right angles, diagonal cleavage if the mineral breaks into slanted angles, and lateral cleavage if the mineral breaks into thin sheets. The habit of a mineral is its crystal geometry if it happens to form without confinement. Some examples of habit include equant habit if the mineral forms with equal angles, bladed habit if the mineral forms into rods, and fibrous habit if the mineral forms with long threads. Most minerals form with some amount of confinement; therefore, the habit of a mineral usually does not manifest itself.

The density of anything in the universe is the ratio of its mass to its volume. We can measure the density of a solid by immersing it in a liquid. If the object is more dense than the liquid, then the solid will sink until it lands at the bottom of the container. If the object is less dense than the liquid, then the solid will rise until it floats on the surface of the liquid. We almost always measure the density of a mineral by immersing it in liquid water. The density of a mineral as compared with the density of liquid water is called the specific gravity of the mineral. For example, if a mineral has a specific gravity of two, this means that its density is twice the density of liquid water; such a mineral would sink in liquid water. As another example, if the specific gravity a mineral is one-half, this means that its density is half the density of liquid water; such a mineral would float in liquid water.

There are many other properties of minerals, such as the taste of a mineral or the smell of a mineral. Some minerals have magnetic properties, which can be tested using an ordinary bar magnet such as a refrigerator magnet. Some minerals are highly chemically reactive. By using all of these properties of minerals, we can uniquely identify minerals. This enables us to classify minerals based on their chemical composition.

There are many classes of minerals, but by far the most abundant group of minerals are the silicate minerals, which are based on the silicon-oxygen tetrahedral group SiO₄^4–. The carbon atom usually forms covalent bonds with four other atoms with a tetrahedral geometry. The silicon atom also usually forms covalent bonds with four other atoms with a tetrahedral geometry. Therefore, we can build an almost infinite diversity of molecules based on either the carbon atom or the silicon atom. Molecules based on the carbon atom are called organic molecules, an enormous category of molecules that is the basis of all life on planet Earth. For example, carbohydrates (including simple sugars and complex carbohydrates), lipids (fats), proteins, and nucleic acids (such as deoxyribonucleic acid DNA and ribonucleic acid RNA) are all organic molecules. Just as organic molecules form an enormous category of molecules, silicate minerals form an enormous category of minerals. More than ninety percent of all minerals are silicate minerals.

Although the carbon-carbon covalent bond is strong enough to build organic molecules, the silicon-silicon covalent bond is not strong enough to build silicate minerals. However, the silicon-oxygen covalent bond is strong enough to build silicate minerals. Therefore, there is always an oxygen atom between any two silicon atoms in a silicate mineral. Other atoms also bond with the silicon atoms within silicate minerals, often metals such as iron and magnesium. This is also analogous to organic molecules, where other atoms such as hydrogen, oxygen, and nitrogen bond with the carbon atoms within organic molecules.

All life on planet Earth has evolved based on organic molecules because of the nearly infinite diversity of molecules based on the carbon atom. It is theoretically possible that life could evolve on another planet based on silicate minerals, which themselves compose rocks, as we will discuss shortly. There is a nearly infinite diversity of silicate minerals, just as there is a nearly infinite diversity of organic molecules. The tetrahedral geometry of silicate minerals and organic molecules is also the same. On a hypothetical planet where life evolved based on silicate minerals, all life would be composed of rocks, which would themselves be composed of silicate minerals, just as all life on planet Earth is composed of organic molecules. Imagine a planet with rock people, rock dogs, rock cats, and rock trees. This would be a planet where all life evolved based on silicate minerals instead of organic molecules.

The simplest organic molecule would be a single carbon atom covalently bonded to other atoms. These are the methanes. We may chemically bond two methanes to each other, resulting in two carbon atoms covalently bonded to each other and to other atoms. These are the ethanes, which are essentially double methanes. We may chemically bond another methane to an ethane (a double methane), resulting in three carbon atoms covalently bonded to one another and to other atoms. These are the propanes, which are essentially triple methanes. We may continue this process until we have constructed long chains of carbon atoms covalently bonded to one another and to other atoms. These are one-dimensional, single-chain organic molecules. We may chemically bond two single-chain organic molecules to each other, resulting in a double-chain organic molecule. We may chemically bond another single-chain organic molecule to a double-chain organic molecule, resulting in a triple-chain organic molecule. We may continue this process until we have constructed a two-dimensional, single-sheet organic molecule. We may chemically bond two single-sheet organic molecules to each other, resulting in a double-sheet organic molecule. We may chemically bond another single-sheet organic molecule to a double-sheet organic molecule, resulting in a triple-sheet organic molecule. We may continue this process until we have constructed three-dimensional organic molecules. Remarkably, our discussion of inorganic silicate minerals will be completely analogous to our discussion of organic molecules. This further persuades us that life could evolve on a hypothetical planet based on inorganic silicate minerals instead of organic molecules.

The simplest silicate mineral would be a single silicon atom covalently bonded to other atoms. These are the nesosilicates, such as the olivines and the garnets. We may chemically bond two nesosilicates to each other, resulting in two silicon atoms covalently bonded to each other (with an oxygen atom between them of course) as well as to other atoms. These are the sorosilicates, such as the epidotes. The sorosilicates are essentially double nesosilicates. We may chemically bond another nesosilicate to a sorosilicate (a double nesosilicate), resulting in three silicon atoms covalently bonded to one another (with oxygen atoms between them of course) as well as to other atoms. These are triple nesosilicates. We may continue this process until we have constructed long chains of silicon atoms covalently bonded to one another (with oxygen atoms between them of course) as well as to other atoms. These are one-dimensional, single-chain inosilicates, such as the pyroxenes. We may chemically bond two single-chain inosilicates to each other, resulting in a double-chain inosilicate, such as the amphiboles. We may chemically bond another single-chain inosilicate to a double-chain inosilicate, resulting in a triple-chain inosilicate. We may continue this process until we have constructed two-dimensional, single-sheet phyllosilicates, such as the serpentines and the micas. We may chemically bond two single-sheet phyllosilicates to each other, resulting in a double-sheet phyllosilicate. We may chemically bond another single-sheet phyllosilicate to a double-sheet phyllosilicate, resulting in a triple-sheet phyllosilicate. We may continue this process until we have constructed three-dimensional tectosilicates, such as the zeolites, the feldspars, and quartz.

The nesosilicates have highest oxygen-to-silicon ratios as well as the most amount of metals, such as iron and magnesium, chemically bonded with the silicon atoms. This is because a single silicon atom is free to bond with four other atoms simultaneously. However, as we progress from nesosilicates to sorosilicates (double nesosilicates) to triple nesosilicates to single-chain inosilicates, the oxygen-to-silicon ratios become smaller and smaller and there are fewer and fewer metals, such as iron and magnesium, chemically bonded with the silicon atoms. This is because more and more bonding sites of the silicon atoms are occupied by silicon-oxygen covalent bonds that hold the entire molecular structure of the mineral together, leaving fewer and fewer sites where metals may bond with the silicon atoms. This progression continues from single-chain inosilicates to double-chain inosilicates to triple-chain inosilicates to single-sheet phyllosilicates. As more and more bonding sites of the silicon atoms are occupied by silicon-oxygen covalent bonds that hold the entire molecular structure of the mineral together, there are fewer and fewer sites where metals may bond with the silicon atoms. This progression continues from single-sheet phyllosilicates to double-sheet phyllosilicates to triple-sheet phyllosilicates and ultimately to tectosilicates, which have the lowest oxygen-to-silicon ratios and the least amount of metals, such as iron and magnesium, chemically bonded with the silicon atoms. In particular, the tectosilicate mineral quartz is the only silicate mineral that is pure silicon and oxygen, with no other atoms such as metals within the chemical structure of this mineral.

The abundance of metals in nesosilicates and sorosilicates gives these minerals a darker color. Consequently, these silicate minerals are called the dark silicates. The scarcity of metals in tectosilicates gives these minerals a lighter color. Consequently, these silicate minerals are called the light silicates. In between these two extremes are the inosilicates and the phyllosilicates, which have intermediate amounts of metals giving them an intermediate color. Consequently, these silicate minerals are called the intermediate silicates. The abundance of metals within the dark silicates also makes these minerals the most dense among all silicates. The scarcity of metals within the light silicates also makes these minerals the least dense among all the silicates. The intermediate silicates have intermediate densities between these two extremes.

Suppose we begin with tectosilicates and warm the temperature by adding thermal energy (heat). The added thermal energy (heat) breaks chemical bonds. As we continue to make the minerals warmer and warmer by adding more and more thermal energy (heat), we break more and more chemical bonds. Eventually, we have broken so many chemical bonds that we have destroyed the three-dimensional structures of the tectosilicates, leaving two-dimensional molecular sheets that are no longer bonded with one another. By warming the temperature, we have changed the tectosilicate into a collection of phyllosilicates. If we continue to add thermal energy (heat), we continue to break chemical bonds. Eventually, we have broken so many chemical bonds that we have destroyed the two-dimensional structure of the phyllosilicates, leaving one-dimensional molecular chains that are no longer bonded with one another. By warming the temperature, we have changed the phyllosilicates into a collection of inosilicates. If we continue to add thermal energy (heat), we continue to break chemical bonds. Ultimately, we have broken so many chemical bonds that we have destroyed the one-dimensional structure of the inosilicates, leaving individual silicon atoms that are no longer bonded with one another. By warming the temperature, we have changed the inosilicates into a collection of nesosilicates. We deduce that the dark silicates (nesosilicates and sorosilicates) have the hottest melting temperatures, the light silicates (tectosilicates) have the lowest melting temperatures, and the intermediate silicates (inosilicates and phyllosilicates) have intermediate melting temperatures. We may confirm this progression of melting temperatures by beginning with nesosilicates and cooling the temperature by extracting thermal energy (heat). The extraction of thermal energy (heat) forms chemical bonds. As we continue to make the minerals cooler and cooler by extracting more and more thermal energy (heat), we form more and more chemical bonds. Eventually, we have formed so many chemical bonds that we no longer have individual silicon atoms that are separate from one another; we have one-dimensional molecular chains of silicon-oxygen bonds. By cooling the temperature, we have changed the nesosilicates into inosilicates. If we continue to extract thermal energy (heat), we continue to form chemical bonds. Eventually, we have formed so many chemical bonds that we no longer have inosilicates that are separate from one another; we have two-dimensional molecular sheets of silicon-oxygen bonds. By cooling the temperature, we have changed the inosilicates into phyllosilicates. If we continue to extract thermal energy (heat), we continue to form chemical bonds. Ultimately, we have formed so many chemical bonds that we no longer have phyllosilicates that are separate from one another; we have three-dimensional molecular structures of silicon-oxygen bonds. By cooling the temperature, we have changed the phyllosilicates into tectosilicates. Again, we deduce that the dark silicates (nesosilicates and sorosilicates) have the hottest melting temperatures, light silicates (tectosilicates) have the lowest melting temperatures, and intermediate silicates (inosilicates and phyllosilicates) have intermediate melting temperatures.

In summary, most silicate minerals can be subdivided into three subcategories: the dark silicates, the intermediate silicates, and the light silicates. At one extreme, the dark silicates have the most amount of metals such as iron and magnesium chemically bonded with the silicon atoms, which are themselves bonded with oxygen atoms between them. This abundance of metals give these dark silicates a darker color, hence their name. This abundance of metals also makes the dark silicates the most dense silicates. Dark silicates also have the warmest melting temperatures and the highest oxygen-to-silicon ratios. Caution: dark silicates have the least complex chemical structure. The dark silicates include nesosilicates, such as olivines and garnets, and sorosilicates, such as epidotes. At the opposite extreme, the light silicates have the least amount of metals chemically bonded with the silicon atoms, which are themselves bonded with oxygen atoms between them. This scarcity of metals gives these light silicates a lighter color, hence their name. This scarcity of metals also makes the light silicates the least dense silicates. Light silicates also have the coolest melting temperatures and the lowest oxygen-to-silicon ratios. Caution: light silicates have the most complex chemical structure. The light silicates include tectosilicates, such as zeolites, feldspars, and quartz. The intermediate silicates are in between these two extremes, with an intermediate amount of metals, an intermediate color, intermediate densities, intermediate melting temperatures, intermediate oxygen-to-silicon ratios, and intermediate chemical complexity. The intermediate silicates include inosilicates, such as pyroxenes and amphiboles, and phyllosilicates, such as serpentines and micas. In brief, most silicate minerals can be categorized into a spectrum. At one extreme, the dark silicates have the greatest abundance of metals, the darkest color, the highest densities, the warmest melting temperatures, the highest oxygen-to-silicon ratios, and the least complex chemical structure. At the other extreme, the light silicates have the least abundance of metals, the lightest color, the lowest densities, the coolest melting temperatures, the lowest oxygen-to-silicon ratios, and the most complex chemical structure. The intermediate silicates are in between these two extremes.

Consider an organic molecule where a carbon atom covalently bonds to another carbon atom which covalently bonds to another carbon atom which covalently bonds to another carbon atom and so on and so forth, but ultimately the last carbon atom covalently bonds back to the first carbon atom. These rings of carbon-carbon covalent bonds (also bonded with other atoms) are the cycloalkane organic molecules. Once again, there are silicate minerals analogous to these organic molecules, providing further evidence that life could hypothetically evolve based on inorganic silicate minerals instead of organic molecules. Consider a silicate mineral where a silicon atom covalently bonds to another silicon atom (with an oxygen atom between them of course) which covalently bonds to another silicon atom (with an oxygen atom between them of course) which covalently bonds to another silicon atom (with an oxygen atom between them of course) and so on and so forth, but ultimately the last silicon atom covalently bonds back to the first silicon atom (with an oxygen atom between them of course). These rings of silicon-oxygen covalent bonds (also bonded with other atoms) are the cyclosilicates. Examples of cyclosilicates include three-silicon rings such as the benitoites, four-silicon rings such as the axinites, and six-silicon rings such as the beryls and the tourmalines.

Although the vast majority of all minerals are silicate minerals, there are other mineral groups. The sulfates are a mineral group based on the sulfur-oxygen tetrahedral group SO₄^2–. Examples of sulfate minerals include gypsum, anglesite, epsomite, and thenardite. The carbonates are a mineral group based on the carbon-oxygen trigonal planar group CO₃^2–. Examples of carbonate minerals include calcite, magnesite, dolomite, and smithsonite. The oxides are the mineral group where metals bond with oxygen. Examples of oxide minerals include iron oxides such as hematite, magnetite, and wüstite, aluminum oxides such as corundum, and copper oxides such as cuprite and tenorite. The halides are the mineral group where halogens such as fluorine, chlorine, and bromine bond with metals. Examples of halide minerals include halite, villiaumite, fluorite, and sylvite. The native elements are minerals composed of a single type of atom, usually metals such as pure aluminum, pure iron, pure nickel, pure copper, pure zinc, pure silver, pure platinum, pure gold, or pure lead. Caution: if a metal bonds with oxygen, the mineral is classified as an oxide, not as a native element. Also, if a metal bonds with a halogen, the mineral is classified as a halide, not as a native element.

Petrology

Petrology is the study of rocks, and a petrologist is someone who studies rocks. These words are derived from the Greek root petro- for rock. For example, the name Peter means rock. A rock is defined as a naturally occurring solid inorganic object that is an aggregate (a mixture) of minerals. Since a rock is an aggregate (a mixture) of many different minerals, a rock does not have a definite chemical structure. This is the most important difference between a rock and a mineral. Both minerals and rocks are naturally occurring solid inorganic objects. However, a mineral has a definite chemical structure, while a rock does not have a definite chemical structure, since it is an aggregate (a mixture) of minerals. Since a rock does not have a definite chemical structure, there are significant variations in density, composition, and other properties even for a particular type of rock. In other words, whereas a particular mineral is unique, a particular rock is not unique. Petrography is the classification of rocks, the study of different types of rocks. Petrogenesis is the study of how rocks form. Broadly speaking, there are three different ways that rocks can form. Hence, there are three broad categories of rocks: igneous rocks, sedimentary rocks, and metamorphic rocks.

If we add sufficient heat to a rock, it melts to become molten rock (liquid rock). If molten rock cools, it may crystallize (solidify) back into solid rock. Any rock that forms from the crystallization of molten rock is called igneous rock, one of the three main types of rock. We can classify igneous rocks based on where they form. Molten rock deep within the Earth is called magma, while molten rock that has extruded out of the Earth is called lava. We will discuss shortly a significant difference between molten rock deep within the Earth and molten rock that has extruded out of the Earth that justifies using these two different terms, magma and lava. For now, we simply mention that different names are often used for the same thing in colloquial languages. While they are living animals, chickens, turkeys, and ducks are called fowl, but after they have been slaughtered and are prepared as food they are called poultry. While they are living animals, bulls and cows are called cattle, but after they have been slaughtered and are prepared as food they are called beef. While they are living animals, pigs are called swine, but after they have been slaughtered and are prepared as food they are called pork. While they are living animals, deer, elk, moose, and reindeer are called cervids, but after they have been slaughtered and are prepared as food they are called venison. The Spanish word for living fish is pez, but the Spanish word for slaughtered fish that are prepared as food is pescado. There are countless other examples in colloquial languages. Magma that crystallizes into solid rock is called intrusive igneous rock, since it forms deep within the Earth. Lava that crystallizes into solid rock is called extrusive igneous rock, since it formed from lava that extruded out of the Earth. Another term for intrusive igneous rock is plutonic igneous rock, and another term for extrusive igneous rock is volcanic igneous rock.

Since they form deep within the Earth where it is hot, intrusive/plutonic igneous rocks typically take a long time to cool and crystallize. This slower, more gradual cooling builds large crystals throughout the rock. The result is a rock with a coarse-grained texture, meaning that it feels more rough to the touch. Since they form from lava that has extruded out of the Earth, extrusive/volcanic igneous rocks typically take a short time to cool and crystallize. This faster, more rapid cooling only permits small crystals to be built throughout the rock. The result is a rock with a fine-grained texture, meaning that it feels more smooth to the touch. The term for the coarse-grained texture of igneous rocks is phaneritic, since the Greek root phanero- means visible, meaning that the crystals in a phaneritic igneous rock are large enough to be visible with the naked (unaided) eye. The term for the fine-grained texture of igneous rocks is aphanitic, since again the Greek root phanero- means visible and the Greek root a- means not, meaning that the crystals in an aphanitic igneous rock are too small to be visible with the naked (unaided) eye. We require at least a magnifying glass, sometimes even a microscope, to see the crystals in an igneous rock with an aphanitic texture. An igneous rock that takes an extremely long time to cool and crystalize would have an extremely coarse-grained texture. This extreme form of the phaneritic texture is called pegmatitic. An igneous rock can take an extremely short time to cool and crystalize. This virtually instantaneous crystallization is called quenching. In this case, the igneous rock has an extremely fine-grained texture. This extreme form of the aphanitic texture is called glassy, since the rock feels as smooth as glass. The extrusive/volcanic igneous rock obsidian has a glassy texture, since it forms by quenching. Obsidian is often black in color, which together with its glassy texture makes obsidian a gorgeous rock. The extrusive/volcanic igneous rock pumice also has a glassy texture, since it forms by quenching. However, the abundance of vesicles (cavities) within pumice causes this rock to feel rough to the touch, even though it has a glassy texture. The abundance of vesicles (cavities) within pumice also gives this rock a density that is usually less than liquid water, permitting this rock to float in liquid water. In summary, igneous rocks that take an extremely long time to cool and crystallize have a pegmatitic (extremely coarse-grained) texture, igneous rocks that take a moderately long time to cool and crystallize have a phaneritic (moderately coarse-grained) texture, igneous rocks that take a moderately short time to cool and crystallize have an aphanitic (moderately fine-grained) texture, and igneous rocks that take an extremely short time to cool and crystallize (quenching) have a glassy (extremely fine-grained) texture. Typically, extrusive/volcanic igneous rocks have an aphanitic texture, perhaps even glassy, since they take a short amount of time to cool and crystallize. Typically, intrusive/plutonic igneous rocks have a phaneritic texture, perhaps even pegmatitic, since they take a long amount of time to cool and crystallize. In brief, we can calculate the cooling history of an igneous rock (how long it took the rock to form) by simply feeling its texture, whether it feels more rough to the touch (coarse-grained texture) or more smooth to the touch (fine-grained texture). Caution: some igneous rocks have both large crystals and small crystals in the same rock. This occurs when there is an interruption in the cooling history of the rock. This unusual texture is called porphyritic. The large crystals in a porphyritic igneous rock are called the phenocrysts, while the small crystals in a porphyritic igneous rock are called the groundmass.

We can also classify igneous rocks based on their mineral composition. Since the vast majority of all minerals are silicates, we classify igneous rocks based on their silicate mineral composition. Igneous rocks that are composed predominantly of dark silicates are called mafic igneous rocks. The word mafic is a combination of the words magnesium and ferrum, the Latin word for iron. As we discussed, dark silicates have the most amount of metals such as iron and magnesium. Igneous rocks that are composed predominantly of light silicates are called felsic igneous rocks. The word felsic is a combination of the words feldspar and silica, the word for molten quartz. As we discussed, feldspar and quartz are two examples of light silicates. Igneous rocks that are composed predominantly of intermediate silicates are simply called intermediate igneous rocks. Everything we have discussed about silicates therefore also applies to the igneous rocks that they compose. In particular, mafic igneous rocks have the most amount of metals, are darkest in color, are most dense, and have the hottest melting temperatures, while felsic igneous rocks have the least amount of metals, are lightest in color, are least dense, and have the lowest melting temperatures. Intermediate igneous rocks are between these two extremes. The Bowen reaction series quantifies the melting-temperature spectrum for igneous rocks, named for the Canadian petrologist Norman L. Bowen who discovered this progression of melting temperatures for igneous rocks in the early twentieth century. According to the Bowen reaction series, as we add more and more heat to igneous rocks, felsic rocks melt first, intermediate rocks then melt at somewhat hotter temperatures, and finally mafic rocks melt at the hottest temperatures. Conversely, if we extract more and more heat from molten rock, mafic rocks crystallize first, intermediate rocks then crystallize at somewhat lower temperatures, and finally felsic igneous rocks crystallize at the lowest temperatures.

The two most important mafic igneous rocks are basalt and gabbro. Both basalt and gabbro have large quantities of metals, are dark in color, have high densities, and have hot melting temperatures. The only difference between basalt and gabbro is where they form and thus their cooling histories and their textures. Gabbro forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time. Thus, gabbro forms with large crystals resulting in a coarse-grained (rough) texture. Basalt forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time. Thus, basalt forms small crystals resulting in a fine-grained (smooth) texture. In other words, gabbro is the intrusive/plutonic form of basalt, or basalt is the extrusive/volcanic form of gabbro. The two most important felsic igneous rocks are rhyolite and granite. Both rhyolite and granite have small quantities of metals, are light in color, have low densities, and have low melting temperatures. The only difference between rhyolite and granite is where they form and thus their cooling histories and their textures. Granite forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time. Thus, granite forms with large crystals resulting in a coarse-grained (rough) texture. Rhyolite forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time. Thus, rhyolite forms small crystals resulting in a fine-grained (smooth) texture. In other words, granite is the intrusive/plutonic form of rhyolite, or rhyolite is the extrusive/volcanic form of granite. The two most important intermediate igneous rocks are andesite and diorite. Both andesite and diorite have intermediate quantities of metals, are intermediate in color, have intermediate densities, and have intermediate melting temperatures. The only difference between andesite and diorite is where they form and thus their cooling histories and their textures. Diorite forms intrusively/plutonically deep within the Earth and therefore cools and crystallizes over a long period of time. Thus, diorite forms with large crystals resulting in a coarse-grained (rough) texture. Andesite forms extrusively/volcanically on the surface of the Earth and therefore cools and crystallizes over a short period of time. Thus, andesite forms small crystals resulting in a fine-grained (smooth) texture. In other words, diorite is the intrusive/plutonic form of andesite, or andesite is the extrusive/volcanic form of diorite.

Rocks on the surface of the Earth are subjected to wind, rain, and other natural forces that degrade (weaken and destroy) the rocks, ultimately breaking them into small pieces called sediments. These natural forces also move these sediments from one location to another. Layer upon layer of sediments may accumulate at a particular location until the weight of the sediments begins to compress the sediments. Chemical reactions may cement the sediments together. Eventually, the sediments become lithified, meaning that they have become a rock. Any rock that forms from the lithification of sediment is called sedimentary rock, one of the three main types of rock. Sedimentary rocks are classified into three subcategories: clastic sedimentary rocks, chemical sedimentary rocks, and biogenic sedimentary rocks.

A clastic sedimentary rock lithifies through the action of physical forces. If large sediments are lithified to form a clastic sedimentary rock, then the rock will have a coarse-grained texture, meaning that it will feel rough to the touch. If small sediments are lithified to form a clastic sedimentary rock, then the rock will have a fine-grained texture, meaning that it will feel smooth to the touch. The Wentworth scale is a sediment size scale, named for the American geologist Chester K. Wentworth who defined this scale in the year 1922. According to the Wentworth scale, sediments are classified as gravels, sands, silts, or clay/mud based on their size. The largest sediments are gravels, which lithify into the extremely coarse-grained clastic sedimentary rocks conglomerate (if the sediments are rounded) or breccia (if the sediments are angular). Sands are somewhat smaller sediments, which lithify into the moderately coarse-grained clastic sedimentary rock sandstone. Silts are even smaller sediments, which lithify into the moderately fine-grained clastic sedimentary rock siltstone. Finally, the smallest sediments are clay/mud, which lithify into the extremely fine-grained clastic sedimentary rock shale.

Whenever natural forces degrade (weaken and destroy) rocks, the resulting sediments always form with irregular, jagged shapes. The technical term for this irregular, jagged shape is angular. If the sediments are eroded (moved) over a far distance, the sediments collide with each other. These collisions tend to smooth out the shapes of sediments. Liquid water and even water vapor in the air will also contribute to the smoothing of the shapes of the sediments if they are eroded (moved) over a far distance. The technical term for this smoothed shape is rounded. If the sediments are eroded (moved) over a short distance, the sediments do not have the opportunity to collide with each other significantly, which would have smoothed out their shapes. Also, liquid water as well as water vapor in the air will also not have much opportunity to smooth out the shapes of the sediments if they are eroded (moved) over a short distance. Thus, the sediments retain their original angular shape. In summary, we can calculate the distance (from how far away) sediments were eroded (moved) from the shape of the sediments, whether the shape is angular (more irregular or jagged) or rounded (more smooth). For example, the clastic sedimentary rock conglomerate is lithified from gravels with more rounded shapes, while the clastic sedimentary rock breccia is lithified from gravels with more angular shapes. Therefore, we conclude that the gravels that lithified to form conglomerate were eroded (moved) over a far distance, while the gravels that lithified to form breccia were eroded (moved) over a short distance.

Some clastic sedimentary rocks are lithified from poorly sorted sediments, meaning that the sediments are all different sizes (some large and some small). Some clastic sedimentary rocks are lithified from well sorted sediments, meaning that the sediments are all roughly the same size (all small). This sorting of sediments within a clastic sedimentary rock reveals the energy of the natural force that eroded (moved) the sediment. A major river has a large quantity of energy, meaning that it is able to erode (move) large sediments and certainly small sediments. Hence, the resulting clastic sedimentary rocks will be poorly sorted. Conversely, a small stream has a small quantity of energy, meaning that it is only able to erode (move) small sediments. Hence, the resulting clastic sedimentary rocks will be well sorted. As we will discuss later in the course, a glacier is a giant mass of ice with tremendous energy. Hence, a glacier is able to move giant boulders in addition to small sediments. Hence, the resulting clastic sedimentary rocks will be poorly sorted if the sediments were eroded (moved) by a glacier. As we will discuss shortly, we can determine the history of the Earth from rocks, in particular from sedimentary rocks. Our current discussion already reveals how this is possible. By studying the sorting of sediments within a clastic sedimentary rock, we can actually determine the history of the surrounding landscape. For example, the sorting of sediments within a clastic sedimentary rock may reveal that perhaps there may have been a small stream in a particular landscape followed by perhaps an ice age with glaciers followed by perhaps a major river, and so on and so forth.

A chemical sedimentary rock lithifies through the process of chemical reactions. Common examples of chemical sedimentary rocks include limestone which is the lithification of the mineral calcite, dolostone which is the lithification of the mineral dolomite, and chert which is the lithification of the mineral quartz.

A biogenic sedimentary rock is lithified from organic materials (lifeforms) together with inorganic sediments. Chalk, coquina, and bituminous coal are three common examples of biogenic sedimentary rocks. The biogenic sedimentary rock chalk forms from the lithification of microscopic ocean plankton. Caution: the chalk that is used to write on chalkboards is artificially manufactured from the minerals gypsum and calcite. The chalkboards themselves are manufactured from slate, which is a rock that we will discuss shortly. Coquina is a gorgeous biogenic sedimentary rock that is lithified from many different types of shells from various invertebrate animals. The formation of the biogenic sedimentary rock bituminous coal is as follows. We begin with many layers of dead plants that are compacted with clay/mud. In addition to compacting the organic matter, the clay/mud also serves to prevent the decomposition of the organic matter. Over thousands of years, the accumulation of sediments over the organic matter compresses it to high densities until it is classified as peat. As sedimentary rock continues to lithify over the peat, it is further compressed until it is classified as lignite (or brown coal). If the lignite (or brown coal) is compressed to even higher densities over millions of years, it is eventually lithified to the biogenic sedimentary rock bituminous coal. Note that if bituminous coal continues to be subjected to high pressures, it may become anthracite, which is another type of rock that we will discuss shortly. If anthracite is subjected to further pressures, it may become graphite, one of the mineral forms of carbon. Graphite is used in writing utensils such as pencils, but note that the graphite in these writing utensils is often incorrectly referred to as lead because humans used to write with lead, which no one should do since lead is poisonous! If the mineral graphite is subjected to enormous pressures over millions of years, it is compressed to diamond, another mineral form of carbon that is the most hard mineral on the Mohs scale, as we discussed. Lignite (or brown coal), bituminous coal, and anthracite are all particular examples of fossil fuels. All fossil fuels can be divided into three broad categories: petroleum (crude oil), natural gas, and coal. Natural gas forms when particular microorganisms that generate methane are compacted under modest pressures over millions of years. As we discussed, coal forms when plants are compacted under high pressures over millions of years. Geologists continue to debate how petroleum (crude oil) forms. Older theories claimed that petroleum (crude oil) forms over millions of years like coal and natural gas, but some modern theories claim that petroleum (crude oil) can form in only a few decades. One form of petroleum is bitumen, which is also called asphalt. However, the word asphalt is colloquially used for an artificially manufactured substance that is a mixture of bitumen and various minerals. This manufactured substance should be called asphalt concrete (or asphalt cement) to distinguish it from naturally occurring asphalt, which should itself be called bitumen to avoid confusion with asphalt concrete (or asphalt cement).

Heat, pressure, and chemical reactions can gradually change one rock into a completely new rock. Any rock that forms by changing a pre-existing rock is called a metamorphic rock, one of the three main types of rock. The original rock is called the parent rock, while the new metamorphic rock that formed from the parent rock is called the daughter rock. Contact metamorphism is the process by which metamorphic rocks form primarily from heat, with pressure and chemical reactions being less important processes. Regional metamorphism is the process by which metamorphic rocks form primarily from pressure, with heat and chemical reactions being less important processes. Hydrothermal metamorphism is the process by which metamorphic rocks form primarily from chemical reactions, with heat and pressure being less important processes. The term hydrothermal is derived from the Greek root hydro- meaning water, since the chemicals are almost always dissolved within water.

Metamorphic rocks can be subclassified based on their shape. Metamorphic rocks that have a folded shape resulting from asymmetrical regional metamorphism (asymmetrical pressures) are called foliated metamorphic rocks, while metamorphic rocks that do not have a folded shape are called non-foliated metamorphic rocks. Non-foliated metamorphic rocks may form from symmetrical regional metamorphism (symmetrical pressures), but non-foliated metamorphic rocks may also form from contact metamorphism (heat) or from hydrothermal metamorphism (chemical reactions). If we begin with siltstone or shale as the parent rock and apply asymmetrical pressures, the result is the daughter rock slate, a foliated metamorphic rock. We can begin with slate itself as the parent rock and apply further asymmetrical pressures to yield the daughter rock phyllite, a more severely foliated metamorphic rock. We can begin with phyllite as the parent rock and apply even further asymmetrical pressures to yield the daughter rock schist, an even more severely foliated metamorphic rock. We can begin with schist as the parent rock and continue to apply asymmetrical pressures to yield the daughter rock gneiss, a severely foliated metamorphic rock. If we begin with the biogenic sedimentary rock bituminous coal as the parent rock and apply symmetrical pressures, the result is the daughter rock anthracite, a non-foliated metamorphic rock. Another non-foliated metamorphic rock is marble, the daughter rock to the chemical sedimentary rocks limestone and dolostone. Yet another non-foliated metamorphic rock is quartzite, the daughter rock to the clastic sedimentary rock sandstone. Another non-foliated metamorphic rock is hornfels, the daughter rock to the clastic sedimentary rock shale.

Metamorphic rocks that form deep within the Earth may be subjected to sufficient heat to melt them. That molten rock may later cool and crystallize into an igneous rock. Metamorphic rocks may also be thrust to the surface of the Earth by a violet event, such as an earthquake. This metamorphic rock would now be subjected to wind, rain, and other natural forces that will degrade (weaken and destroy) the rock, ultimately breaking it into sediments, which may later lithify into a sedimentary rock. An intrusive/plutonic igneous rock that formed deep within the Earth is subjected to heat, pressure, and chemical reactions that may gradually change the igneous rock into a metamorphic rock. In summary, rocks are continuously changing from one type to another, and any rock can become any other type of rock. This is called the rock cycle. It is difficult for us to believe that the rock cycle actually occurs, since in most cases we do not witness rocks change before our eyes. Most rocks change very slowly over long periods of time, although there are rare cases when we can witness before our very eyes rocks forming in a short period of time, such as quenching resulting in igneous rocks with a glassy texture. Throughout this course, we will discuss innumerable manifestations of the dynamic planet Earth. The word dynamic means continuously changing. The opposite of the word dynamic is the word static, meaning not changing. Our planet Earth is not static. Our planet Earth is dynamic, since it is continuously changing, and the rock cycle is one of the many processes we will discuss in this course that reveals that our planet Earth is continuously changing.

If heat changes a parent rock into a metamorphic daughter rock, that heat must not melt the rock. If the rock were to melt and recrystallize into a solid rock, then we must classify the rock as an igneous rock, not as a metamorphic rock. The rock migmatite forms from heat that melts some parts of the rock which then recrystallize into solid rock, but the heat also changes other parts of the rock without melting those parts of the rock. Some petrologists classify migmatite as igneous, while other petrologists classify migmatite as metamorphic. Still other petrologists classify migmatite as both igneous and metamorphic, and yet other petrologists actually place migmatite in a unique category of rock that is intermediate between igneous and metamorphic. There will never be a consensus among petrologists on the classification of migmatite. Therefore, migmatite is a rock that cannot be classified. We now realize that not all rocks can be classified as either igneous, sedimentary, or metamorphic. As another example, the rock tuff partly forms from the crystallization of molten rock and partly forms from the lithification of sediment. Some petrologists classify tuff as igneous, while other petrologists classify tuff as sedimentary. Still other petrologists classify tuff as both igneous and sedimentary, and yet other petrologists actually place tuff in a unique category of rock that is intermediate between igneous and sedimentary. Again, there will never be a consensus among petrologists on the classification of tuff. Therefore, tuff is another rock that cannot be classified. A more subtle example is the sedimentary rock marlstone, which is intermediate between the sedimentary rocks limestone and shale. Although both limestone and shale are sedimentary rocks and therefore marlstone is also a sedimentary rock, limestone is a chemical sedimentary rock while shale is a clastic sedimentary rock. Therefore, marlstone is a sedimentary rock that is intermediate between chemical sedimentary and clastic sedimentary. There will never be a consensus among petrologists on the precise classification of marlstone. Therefore, marlstone is yet another rock that cannot be classified.

The Structure and the Composition of the Geosphere

The Earth is mostly covered with oceans, while a small fraction of the surface of the Earth is continents. The continents are composed of mostly felsic igneous rock, for reasons we will make clear shortly. On the surface of the continents is a thin layer of sedimentary rock, having formed from the lithification of sediments that themselves formed from natural forces degrading rocks into sediments. Therefore, if we were to drill into the continent, we would first drill through a layer of sedimentary rock followed by rhyolite (extrusive/volcanic felsic rock, having crystallized near the surface of the Earth) followed by granite (intrusive/plutonic felsic rock, having crystallized deep within the Earth). The ocean basins (at the bottom of the ocean) are composed of mostly mafic igneous rock, for reasons we will make clear shortly. On the surface of the ocean basins (at the bottom of the ocean) is a thin layer of sedimentary rock, having formed from the lithification of sediments that themselves formed from natural forces degrading rocks into sediments. Therefore, if we were to drill into the ocean basins (at the bottom of the ocean), we would first drill through a layer of sedimentary rock followed by basalt (extrusive/volcanic mafic rock, having crystallized near the surface of the Earth) followed by gabbro (intrusive/plutonic mafic rock, having crystallized deep within the Earth).

The geosphere (the solid part of the Earth) is layered. The most dense layer of the geosphere is the core at its center. The core is composed primarily of metals, such as iron and nickel. The next layer of the geosphere surrounding the core is the mantle, which is less dense than the core. The mantle is composed primarily of rocks, which are themselves composed primarily of silicate minerals. There are also fair amounts of metals in the mantle, such as iron. In summary, the mantle is composed of iron-rich silicate rock. The outermost layer of the geosphere surrounding the mantle is the crust. The crust is the least dense layer of the geosphere and the thinnest layer of the geosphere. The crust is also composed of silicate rock, but there are fewer metals such as iron in the crust as compared with the mantle. In summary, the crust is composed of iron-poor silicate rock.

The Earth’s core is itself layered. At the very center of the geosphere is the inner core, composed primarily of metals such as iron and nickel. The temperature of the Earth’s core is very hot, for reasons we will discuss shortly. These hot temperatures should be sufficient to melt metals into the molten state. However, the pressure of the inner core is so enormous that the metals are compressed into the solid state even though they are at temperatures where they should be in the molten state. Since the inner core is composed of solid metal, the inner core is also called the solid core. The layer around the solid (inner) core is the outer core, which is also composed primarily of metals such as iron and nickel. Again, the temperature of the outer core is sufficiently hot to melt metals into the molten state. Although the pressure of the outer core is enormous by human standards, the pressure is nevertheless not as high as the pressure of the inner core. Therefore, the pressure of the outer core is not sufficient to compress the metals into the solid state. The outer core is therefore molten, as metals should be at these hot temperatures. Since the outer core is composed of molten metal, the outer core is also called the molten core. Surrounding the molten (outer) core is the first layer of the mantle: the mesosphere. The Greek root meso- means middle. For example, Central America is sometimes called Mesoamerica, as in Middle America. Therefore, the word mesosphere simply means middle sphere or middle layer. The next layer of the mantle that surrounds the mesosphere is the asthenosphere. This layer of the mantle is composed of weak rock. Indeed, the Greek root astheno- means weak. The asthenosphere is composed of weak rock that is still mostly solid, although parts of the asthenosphere are composed of rock that is partially molten. Finally, the rest of the mantle together with the entire crust is called the lithosphere. The lithosphere is of varying thickness. Some parts of the lithosphere are so thick that they protrude out of the oceans. These thick parts of the lithosphere are called continents. The parts of the lithosphere that are thin are the ocean basins at the bottom of the ocean.

Since density is defined as mass divided by volume, density is inversely proportional to volume. Therefore, more dense rock must occupy a smaller volume, while less dense rock must occupy a larger volume. Some parts of the lithosphere are so thick that they protrude out of the oceans; these are the continents. Since the continental parts of the lithosphere are thick occupying a large volume, they must be composed of less dense rock. This reveals why the continents are composed primarily of felsic rock, since felsic rock is the least dense igneous rock. Other parts of the lithosphere are so thin that they do not protrude out of the ocean; these are the ocean basins at the bottom of the ocean. Since the oceanic parts of the lithosphere are thin occupying a small volume, they must be composed of more dense rock. This reveals why the ocean basins are composed primarily of mafic rock, since mafic rock is the most dense igneous rock.

How have geophysicists determined the layers of the geosphere, their thicknesses, their compositions, and their physical states (solid or molten)? Have geologists drilled to the center of the Earth and directly studied the interior of the Earth? No, we have come nowhere near drilling to the center of the Earth. We have barely drilled through the Earth’s crust, which is by far the thinnest layer of the geosphere. We will never have technology advanced enough to drill far beyond the crust, and reaching the core is out of the question. Geophysicists have determined the layers of the geosphere, their thicknesses, their compositions, and their physical states (solid or molten) using seismic waves. We will discuss earthquakes in more detail shortly. For now, earthquakes cause waves that propagate (travel) throughout the entire geosphere. These waves are called seismic waves, and a seismometer is a device that detects seismic waves. There are millions of earthquakes on planet Earth every day, as we will discuss shortly. Most earthquakes are so weak that humans cannot feel them, but seismometers sensitive (accurate) enough can detect incredibly weak seismic waves. Geophysicists have placed thousands of seismometers all over planet Earth to detect these seismic waves. Surface seismic waves propagate (travel) along the surface of the geosphere, while body seismic waves propagate (travel) throughout the interior of the geosphere. There are two different types of body seismic waves: pressure waves and shear-stress waves. A pressure is a force exerted directly onto (perpendicularly onto) an area. Consequently, a pressure wave may propagate (travel) through solids, liquids, and even gases, since all that is necessary for a pressure wave to propagate is for atoms or molecules to collide with other atoms or molecules in the direction of propagation. A shear-stress is a force exerted along (parallel across) an area. Consequently, a shear-stress wave may propagate (travel) through solids but not through liquids or gases, since there must be strong chemical bonding for atoms or molecules to pull other atoms or molecules along (parallel across) an area. Also, pressure waves propagate faster than shear-stress waves. Therefore, a seismometer will always detect pressure waves first. For this reason, pressure waves are also called primary waves. A seismometer will always detect shear-stress waves second, since they propagate slower than pressure (primary) waves. For this reason, shear-stress waves are also called secondary waves. By an amazing coincidence, the words pressure and primary both begin with the same letter. Therefore, these waves are also called P-waves. By another amazing coincidence, the words shear, stress, and secondary also all begin with the same letter! Therefore, these waves are also called S-waves. We can calculate how distant an earthquake occurred from a seismometer from the arrival times of the P-waves and the S-waves. If a seismometer detects the S-waves a long duration of time after the P-waves, the earthquake must have occurred far from the seismometer, since the P-waves had sufficient distance to propagate (travel) far ahead of the S-waves, resulting in a long delay between them. If the seismometer detects the S-waves immediately after the P-waves, the earthquake must have occurred near the seismometer, since the P-waves did not have sufficient distance to propagate (travel) that far ahead of the S-waves, resulting in a short delay between them. In summary, we calculate the distance the seismic waves propagated from the arrival times of the P-waves and the S-waves. We now have the distance of propagation, and we certainly know the time of propagation, since we know when the earthquake occurred and when the seismometer detected the seismic waves. We can then calculate the speed of the seismic waves, since the speed of anything equals its distance traveled divided by its time of travel. Once we have the speed of the seismic waves, we can determine the properties of the materials that would cause that speed of propagation, whether the material was solid metal, molten metal, solid rock, or molten rock. We can program a computer with all of these data, and the computer can calculate the layers of the geosphere, their thicknesses, their compositions, and their physical states (solid or molten) from all of these data. Also, we are certain that the interior of the Earth is at least partially molten, since seismometers on the opposite side of planet Earth from an earthquake do not detect S-waves; these seismometers only detect P-waves. As we discussed, S-waves cannot propagate through liquids; they can only propagate through solids. However, P-waves can propagate through either solids or liquids, as we discussed. In fact, the opposite side of planet Earth from any earthquake is called the shadow zones of that particular earthquake. This term comes from the idea that the molten (outer) core casts a shadow, preventing those seismometers from detecting S-waves. In actuality, the S-waves cannot propagate through the molten (outer) core, while the P-waves can propagate through the molten (outer) core. Hence, seismometers in the shadow zones only detect P-waves from earthquakes on the opposite side of planet Earth. In summary, by using thousands of seismometers all over planet Earth that detect seismic waves from millions of earthquakes each day and by running computer simulations, geophysicists have determined the layers of the geosphere, their thicknesses, their compositions, and their physical states. One of the earliest examples of the successful use of these techniques was when the Croatian geophysicist Andrija Mohorovičić discovered the boundary between the crust and the mantle in the year 1909 using seismic waves. Consequently, the boundary between the crust and the mantle is called the Mohorovičić discontinuity in his honor.

It cannot be accidental or coincidental that the geosphere (the solid part of the Earth) is layered according to density. Why are the inner layers more dense while the outer layers are less dense? To explain this layering, we must discuss the formation of the Earth. The Earth, its Moon, the other planets and their moons, the Sun, and the entire Solar System formed roughly 4.5 billion years ago. The four planets closer to the Sun (Mercury, Venus, Earth, and Mars) were born as small, dense planetesimals (baby planets) that grew larger through accretion, which is the gaining of mass through sticky collisions. During a sticky collision, a significant fraction of the kinetic energy (moving energy) of the colliding objects is converted into thermal energy (heat energy). Thus, objects that suffer from sticky collisions become significantly warmer. Therefore, as the Earth was forming and growing larger through accretion, it became warmer. Eventually, the Earth became so hot that it became almost entirely molten. While the Earth was almost entirely molten, more dense materials were able to sink toward the center of the planet while less dense materials were able to rise toward the surface of the planet. Most metals are more dense than most rocks. That is, most rocks are less dense than most metals. Therefore, most of the metals sank toward the center of the planet, forming the core. Most of the rocks rose toward the surface of the planet, forming the mantle and the crust. The process by which any planet separates materials according to density is called differentiation, and the planet is said to be differentiated. A planet larger than the Earth would be more severely differentiated than the Earth, since it would have more mass and therefore stronger gravity that would pull the metals towards the center of the planet more strongly. A planet smaller than the Earth would be less severely differentiated than the Earth, since it would have less mass and therefore weaker gravity that would pull the metals toward the center of the planet less strongly. For example, planet Mars is smaller than the Earth. Therefore, Mars has less mass and thus weaker gravity as compared with the Earth. Hence, Mars is less severely differentiated as compared with the Earth. Of course, planet Mars is still differentiated. It has a dense core composed primarily of metals such as iron, and it has less dense outer layers composed primarily of rock. Nevertheless, Mars is less severely differentiated as compared with the Earth, meaning that not all of the metals sank toward the center of Mars to form its core. This is also the case with the Earth. Although most of the metals sank toward the center of the Earth to form its core, there are still fair amounts of metals in the mantle and small amounts of metals in the crust. Since Mars is less severely differentiated as compared with the Earth, we find more iron on the surface of Mars as compared with the amount of iron on the surface of the Earth. The abundance of iron on the surface of Mars has oxidized. As we discussed, iron oxide is commonly known as rust, which has a reddish color. This is why Mars is red. In fact, the nickname of planet Mars is the Red Planet. Mars even appears red to the naked eye (without the aid of a telescope or even binoculars) in our sky.

The Earth has a magnetic field, but we do not completely understand how this magnetic field is generated. According to older theories, the Earth’s magnetic field is generated by its solid (inner) core. This theory may seem reasonable, since the solid (inner) core is composed of ferromagnetic metals such as iron and nickel. However, we now realize that this old model is too simplistic. According to more modern theories, the Earth’s magnetic field is created by its molten (outer) core. This more modern theory also seems reasonable, since the molten (outer) core is also composed of ferromagnetic metals such as iron and nickel. These modern theories claim that the rotation of the Earth causes circulating currents of molten metal in the outer core. These circulating currents of molten metal in turn generate the Earth’s magnetic field. These more modern theories seem reasonable, but nevertheless these theories are not fully developed. If these models are correct, then the two equally important variables that create a metallic-rocky planet’s magnetic field is a metallic core that is at least partially molten and reasonably rapid rotation. Indeed, among the four planets closer to the Sun (Mercury, Venus, Earth, and Mars), the Earth has the strongest magnetic field, since it is the only one of these four planets that has both a partially molten metallic core and reasonably rapid rotation. For example, Venus probably has a partially molten metallic core, but Venus has very slow rotation. Hence, Venus has a very weak magnetic field. As another example, Mars has reasonably rapid rotation, but Mars has a metallic core that is no longer partially molten. Hence, Mars also has a very weak magnetic field. Caution: we are only comparing the magnetic fields of the four inner planets closer to the Sun. Although the Earth has the strongest magnetic field among the inner planets, its magnetic field is still weak compared with all four outer planets further from the Sun (Jupiter, Saturn, Uranus, and Neptune).

The overall structure of the Earth’s magnetic field is very much similar to the magnetic field created by a bar magnet, such as a refrigerator magnet. In fact, the only difference between the Earth’s magnetic field and a bar magnet’s magnetic field is strength. The Earth’s magnetic field is thousands of times weaker than a bar magnet’s magnetic field. That is, a bar magnet’s magnetic field is thousands of times stronger than the Earth’s magnetic field. Students often cannot believe that a small refrigerator magnet could create a stronger magnetic field than an entire planet, but this stands to reason actually. A refrigerator magnet’s magnetic field is strong enough to lift paper clips for example, but the Earth’s magnetic field is not this strong. The Earth’s magnetic field is everywhere around us, yet we do not see paper clips floating around us! A refrigerator magnet’s magnetic field is strong enough to lift paper clips against planet Earth’s gravitational field, but planet Earth’s magnetic field is not strong enough to lift paper clips against its own gravitational field. Hence, the Earth’s magnetic field is indeed thousands of times weaker than a bar magnet’s magnetic field.

The Earth’s magnetic field reverses itself once every few hundred thousand years. We do not understand how or why this occurs. We do know that it does happen from the magnetization of iron within rocks. The uppermost layer of rock has its iron magnetized in the same direction of the Earth’s magnetic field; this is called normal polarity. However, a deeper layer of rock has its iron magnetized in the opposite direction of the Earth’s magnetic field; this is called reverse polarity. An even deeper layer of rock has normal polarity again, and an even more deep layer of rock has reverse polarity again. In other words, the magnetization of iron within rock (the polarity) alternates from normal to reverse and back again. The reason for this is clear. When new rock forms, the iron within that rock will become magnetized in whichever direction the Earth’s magnetic field happens to point at the time of that rock’s formation. The Earth’s magnetic field must reverse itself periodically to cause the alternating polarity of rock layers. Therefore, we are certain that the Earth’s magnetic field reverses itself once every few hundred thousand years, but again we do not understand how or why this occurs.

It is a common misconception that the Earth’s magnetic field begins at the north pole and ends at the south pole. This is false for a couple of reasons. Firstly, it is a basic law of physics that magnetic field lines are not permitted to begin or end anywhere; magnetic field lines must form closed loops. The magnetic field lines of a bar magnet, such as a refrigerator magnet for example, do not begin at the north pole of the magnet, nor do they end at the south pole of the magnet. The magnetic field lines of a bar magnet go straight through the magnet, coming out of its north pole, circulating around to go into its south pole, going straight through the magnet, and coming out its north pole again. Similarly, the magnetic field lines of planet Earth go straight through the planet, coming out one end, circulating around to go into the opposite end, going straight through the planet, and coming out again. Moreover, the Earth’s magnetic field lines do not come out from nor do they go into the geographical poles. The Earth’s magnetic field lines come out from and go into the magnetic poles, which are different from the geographical poles. Indeed, a magnetic compass does not point toward geographical north as is commonly believed. A magnetic compass points toward magnetic north, which again is different from geographical north. Admittedly, the Earth’s magnetic poles are somewhat close to the planet’s geographical poles, but there is no reason to expect that the magnetic poles should be the same or even close to the geographical poles. The solid (inner) core is literally floating within the molten (outer) core. Therefore, the solid (inner) core is actually detached from the rest of the planet. Consequently, there is no reason to expect that the solid (inner) core is rotating in precisely the same direction as the rest of the planet. In addition, there are circulating currents of molten metal within the molten (outer) core. In brief, the rotation of the Earth is complicated; the entire planet does not rotate together as one solid unit. Therefore, there is no reason to expect that the magnetic poles should be the same or even close to the geographical poles. To ask why the Earth’s magnetic poles are different from the geographical poles is a wrong question to ask, since there is no reason to expect that the magnetic poles should be the same as the geographical poles. The correct question to ask is why are the Earth’s magnetic poles even close to the Earth’s geographical poles. We do not understand why this is the case. Indeed, other planets have magnetic poles that are completely different from their geographical poles.

The solar wind is a stream of charged particles from the Sun composed primarily of protons and electrons. This solar wind is capable of substantially ionizing the Earth’s atmosphere in a fairly short amount of time. Fortunately, the Earth’s magnetic field, although weak compared with the magnetic field of bar magnets, is sufficiently strong to deflect most of the Sun’s solar wind. Some of the charged particles in the solar wind do however become trapped within the Earth’s magnetic field. These charged particles execute helical trajectories around the Earth’s magnetic field lines. These regions of the Earth’s magnetic field are called the Van Allen belts, named for the American physicist James Van Allen who discovered these belts in the year 1958. The charged particles within the Van Allen belts execute helical trajectories while drifting along the Earth’s magnetic field lines toward the magnetic poles. The charged particles eventually collide with the molecules of the Earth’s atmosphere, surrendering their kinetic energy by emitting light. The result is gorgeous curtains of light or sheets of light across the sky near the Earth’s magnetic poles. This is called an aurora. Near the Earth’s north magnetic pole it is called aurora borealis (or more commonly the northern lights), and near the Earth’s south magnetic pole it is called aurora australis (or more commonly the southern lights). If the Sun happens to be less active, its solar wind would be weaker, the resulting aurorae would appear less spectacular, and we would only be able to enjoy them near the Earth’s magnetic poles. If the Sun happens to be more active, its solar wind would be stronger, the resulting aurorae would appear more spectacular, and we would be able to enjoy them further from the Earth’s magnetic poles. For example, the Battle of Fredericksburg in December 1862 during the American Civil War was interrupted by the appearance of the aurora borealis in the sky, even though Fredericksburg, Virginia is quite far from the Earth’s north magnetic pole.

The interior of the geosphere is hot due to geothermal energy. This geothermal energy drives geologic activity, as we will discuss shortly. The source of this geothermal energy is primarily radioactive decay. There are certain atoms that have an unstable nucleus, since the nucleus has too much energy. To stabilize itself, the nucleus will emit particles to decrease its own energy. These atoms are called radioactive atoms, and the emission of these particles is called radioactive decay. A certain naturally occurring fraction (percentage) of all the atoms in the universe are radioactive. A certain fraction (percentage) of the atoms that compose the geosphere are radioactive. When these atoms suffer from radioactive decay, the emitted particles are themselves a source of energy. This is the source of the Earth’s geothermal energy. A planet larger than the Earth would have a longer geologic lifetime than the Earth, since it would have more mass and therefore more radioactive atoms and hence a greater supply of geothermal energy. A planet smaller than the Earth would have a shorter geologic lifetime than the Earth, since it would have less mass and therefore less radioactive atoms and hence a smaller supply of geothermal energy. Billions of years from now, most of the radioactive atoms of planet Earth will have decayed. With very little radioactive atoms remaining to provide geothermal energy, geologic activity will end, and the Earth will become a geologically dead planet. This geological death has already occurred for other planets smaller than the Earth. The planets Mercury and Mars for example are geologically dead planets, since they are significantly smaller than the Earth. However, planet Venus has almost the same size as the Earth. Presumably, the geologic lifetime of Venus is roughly the same as the geologic lifetime of the Earth. Indeed, both Venus and the Earth are currently geologically alive.

The three mechanisms by which heat (energy) is transported from one location to another are conduction, convection, and radiation. Conduction is the transfer of heat (energy) from one object to another because they are in direct contact with one another. For example, our hand becomes hot when we grab a hot object because our hand is in direct contact with the hot object when we grab it. The heat (energy) is transferred from the hot object to our hand by conduction, since our hand is in direct contact with the hot object. Convection is the transfer of heat (energy) from one object to another by moving materials. It is commonly known that hot air rises and cold air sinks. This is not just the case for air; this is true for any gas and for any liquid as well. In physics, gases and liquids are called fluids. Caution: in colloquial English, the word fluid refers to liquids only, but in physics the word fluid refers to liquids or gases. Convection is the transfer of heat (energy) by moving fluids, since hot fluids rise and cool fluids sink. For example, a heater on the first floor of a house also warms the second floor by convection. The heater on the first floor warms the air on the first floor; since hot fluids rise, the hot air rises to the second floor and warms the second floor. When the air that has risen to the second floor loses its heat and cools, it sinks back to the first floor, where it is warmed again by the heater. Radiation is the transfer of heat (energy) without direct contact and without moving fluids. Radiation typically transports heat (energy) using electromagnetic waves. For example, heat (energy) is not transported from the Sun to the Earth by conduction, since the Earth is not in direct contact with the Sun, thank God! Heat (energy) is not transported from the Sun to the Earth by convection, since there is no fluid in outer space that takes the Sun’s heat (energy) and moves it to the Earth. The Sun’s heat (energy) is transported to the Earth across outer space by radiation, through electromagnetic waves.

The Theory of Plate Tectonics

Christopher Columbus discovered America in the year 1492, more than five hundred years ago. Shortly thereafter, the Italian navigator Amerigo Vespucci further explored the American continents and was the first to recognize that America was in fact a New World, and hence this New World was named America in his honor. Over the following decades, other navigators continued to explore the American continents. As a result of all of their achievements, the first maps of the entire world were eventually drawn for the first time in human history. In these world maps, cartographers noticed that the east coast of South America has a similar shape to the west coast of Africa. Most cartographers believed that this was only a coincidence, since the coastlines appeared to have similar shapes but not precisely the same shape. The first person to seriously investigate why these two coastlines share similar shapes was the German meteorologist Alfred Wegener during the early twentieth century. Although Wegener was not a geologist, he discovered that the rocks on the east coast of South America and the rocks on the west coast of Africa are similar to one another. He began to imagine that South America and Africa were connected to each other in the distant past and have moved apart from one another since that ancient time. He also imagined that North America and Europe were connected to each other in the distant past and have also moved apart from one another since that ancient time. When Wegener connected maps of North America and Europe to each other like a jigsaw puzzle, he noticed that the Appalachian Mountains in North America connect with the Scandinavian Mountains in Europe. Although Wegener was not a geologist, he then discovered that the rocks that compose the Appalachian Mountains are similar to the rocks that compose the Scandinavian Mountains! Wegener then discovered that South America and Africa were in the same cold climate in the distant past, leading him to imagine that these continents were not only connected to each other in the distant past but also further south where it is cold. If South America and Africa were connected to each other and further south, Wegener imagined that North America and Europe were not only connected to each other in the distant past but also further south, perhaps near the equator where it is warm. Wegener then discovered that during the same ancient cold climate in South America and Africa, North America and Europe were indeed in the same warm climate! Finally, Wegener discovered fossils of the same animals in South America and Africa; these were fossils of land animals that could not swim across an ocean. Putting all of these observations together, Alfred Wegener proposed the hypothesis of continental drift in his textbook The Origin of Continents and Oceans, published in the year 1915. According to his hypothesis, the continents were connected to one another in the distant past and have drifted apart from one another since that ancient time. Wegener’s hypothesis of continental drift was rejected and ridiculed by the geological community at the time. Today, we realize that some of Alfred Wegener’s ideas were indeed incorrect. For example, Wegener proposed that gravitational forces from the Moon causes the continents to drift. Although gravitational forces from the Moon (and the Sun) cause tides in the oceans as we will discuss later in the course, these gravitational forces are not strong enough to push continents. Wegener also proposed that the continents cut through the rock at the ocean floor as they drift. Today, we realize that this is incorrect as well. Nevertheless, today we do realize that Wegener’s basic idea that the continents are moving is in fact correct. Alfred Wegener died in the year 1930, shortly after his fiftieth birthday while on an expedition surveying the enormous ice sheet that covers the microcontinent Greenland. If he had lived a few decades longer, he would have lived to see that his hypothesis of continental drift was essentially correct, although not entirely correct. In the late 1920s, the British geologist Arthur Holmes proposed that convection within the mantle may transfer thermal energy (heat) from the lower mantle deep within the geosphere to the upper mantle near the surface of the geosphere. He also proposed that this thermal energy could be transferred to the continents, pushing them as Alfred Wegener envisioned. Unfortunately, it would take a few decades before the ideas of Alfred Wegener and Arthur Holmes would be combined to formulate the correct theory of the geology of planet Earth.

The Second World War occurred from the year 1939 to the year 1945. Before the Second World War, the United States maintained a small standing army. Whenever the United States needed to wage war, the federal government would assemble a massive army that would fight and win the war. After the war ended, the army was almost entirely disbanded. This occurred after the end of the American Civil War and after the end of the First World War for example. However, the Axis Powers (Germany, Italy, and Japan) almost succeeded in literal global domination during the Second World War. Although the Axis Powers were defeated, that threat was immediately replaced by another threat; the Union of Soviet Socialist Republics (the U.S.S.R. or the Soviet Union) had used the Second World War as an opportunity to conquer half of the entire world. After the United States defeated three nations that almost succeeded in literal global domination and upon realizing the new global threat that it faced, the political leadership of the United States decided against disbanding its army, and the United States continues to maintain a large standing army to the present day. Enormous amounts of funds have been invested in the armed forces of the United States since the end of the Second World War. As a result, the armed forces of the United States have pioneered many branches of scientific research. In the context of this course, the United States Navy founded the Office of Naval Research (ONR) in the year 1946. The Office of Naval Research (ONR) discovered the continental shelves, which are the parts of the continents that are submerged under the ocean. The true shape of a continent is only obtained when we include the continental shelves together with the rest of the continent. When we include the continental shelves, the east coast of South America in fact fits perfectly into the west coast of Africa. This is no longer a coincidence that we can simply ignore. The Office of Naval Research (ONR) also discovered oceanic ridges, which are mountain ranges at the bottom of the ocean. In particular, the Mid-Atlantic Ridge runs along the middle of the Atlantic Ocean and itself has the same shape as the east coast of South America and the west coast of Africa. This is also not a coincidence that we can simply ignore. The Office of Naval Research (ONR) later discovered a continuum of rock ages at the bottom of the ocean. The rocks that compose oceanic ridges are youngest, and the rocks that compose the ocean basin are progressively older and older further and further from the oceanic ridges, closer and closer to continents. Indeed, the rocks that compose the ocean basins nearest to continents are oldest. The Office of Naval Research (ONR) also discovered a continuum of sediment thickness at the bottom of the ocean. The sediments that cover the ocean floor are thinnest near the oceanic ridges, and the sediments that cover the ocean floor are progressively thicker and thicker further and further from the oceanic ridges, closer and closer to continents. Indeed, the sediments that cover the ocean floor nearest to continents are thickest. This continuum of sediment thickness is consistent with the continuum of rock age. If rock is older, then there has been more time for sediment to accumulate, and so the sediment must be thicker; if rock is younger, then there has been less time for sediment to accumulate, and so the sediment must be thinner. The Office of Naval Research (ONR) also mapped the global distribution of seismic activity such as earthquakes and the global distribution of igneous activity such as volcanic eruptions. These two global distributions closely match each other. Some locations on planet Earth have an abundance of both seismic and igneous activity, while other locations on planet Earth have very little seismic or igneous activity. Finally, the Office of Naval Research (ONR) discovered alternating magnetic polarities within rocks on the ocean floor. This alternating magnetic polarity is not just vertically downward to deeper rock layers but also laterally (horizontally) across neighboring rocks, providing the strongest evidence that the ocean floor must be moving. All of these observations led geologists to conclude that continents and ocean basins slowly move over millions of years. During the 1960s, American geophysicists Harry Hammond Hess, Robert S. Dietz, and Robert R. Coats, British geophysicists Drummond Matthews and Frederick Vine, Canadian geophysicists Lawrence Morley and John Tuzo Wilson, American geophysicist W. Jason Morgan, British geophysicists Dan P. McKenzie and Robert L. Parker, French geophysicist Xavier Le Pichon, and American geophysicists Jack Oliver, Bryan Isacks, and Lynn R. Sykes, together formulated the fundamental theory of the geology of planet Earth: the Theory of Plate Tectonics. Whereas Alfred Wegener and Arthur Holmes were the grandfathers of this theory, these fourteen geophysicists were the true fathers of the Theory of Plate Tectonics.

According to the Theory of Plate Tectonics, the lithosphere is fractured (or broken) into roughly twenty pieces called tectonic plates. The adjective tectonic is derived from a Greek word meaning construction or building. In other words, the Theory of Plate Tectonics states that the lithosphere is composed of (built from or constructed from) pieces. These tectonic plates are floating on the asthenosphere and are being pushed very slowly by convection cells in the asthenosphere. Mostly solid but nevertheless weak rock in the lower layers of the asthenosphere are heated by the Earth’s geothermal energy. Since hot fluids rise, this hot rock rises until it reaches a tectonic plate of the lithosphere. The risen rock then pushes the tectonic plate in a certain direction, since the thermal energy (heat energy) of the rock is transformed into the kinetic energy (moving energy) of the tectonic plate. Since the tectonic plate gains kinetic energy at the expense of the thermal energy of the rock, the rock must become cooler as it pushes the tectonic plate. Since cool fluids sink, the now cooler rock sinks back into the asthenosphere, where it is warmed again by the Earth’s geothermal energy. In brief, there are circulating currents of weak rock within the asthenosphere. These are called convection cells. These convection cells push the tectonic plates very slowly, a few centimeters per year on average. According to the Theory of Plate Tectonics, much geologic activity occurs at the boundary between two tectonic plates. There are three different types of tectonic plate boundaries: divergent plate boundaries, convergent plate boundaries, and transform fault boundaries.

At a divergent plate boundary, two tectonic plates are being pushed away from each other. The verb to diverge means to spread out. As the tectonic plates are pushed away from each other, hot rock within the asthenosphere can rise and deposit itself onto the tectonic plates. As the rock cools, it may crystallize into solid igneous rock. Hence, this is the youngest part of a tectonic plate, since newborn rock forms at divergent plate boundaries. Entire mountain ranges can be built at divergent plate boundaries. For example, the Mid-Atlantic Ridge runs along the middle of the Atlantic Ocean and assumes the same shape as the east coast of South America and the west coast of Africa. The South American continent is actually a part of a much larger tectonic plate called the South American plate, and the African continent is actually a part of a much larger tectonic plate called the African plate. The South American plate and the African plate are being pushed away from each other, permitting hot rock to rise out of the asthenosphere. The hot rock has deposited onto both plates, has cooled, and has crystallized to form the Mid-Atlantic Ridge. This is why the shape of the east coast of South America, the west coast of Africa, and the Mid-Atlantic Ridge between them are all similar to each other. South America and Africa were connected to each other roughly two hundred million years ago, but they have been pushed apart from one another, thus forming the Mid-Atlantic Ridge at the boundary between these two tectonic plates. North America and Europe were also connected to each other roughly two hundred million years ago, and they are also being pushed apart from one another. The Atlantic Ocean is continuously becoming wider very slowly, only a few centimeters per year. The Red Sea between the Arabian peninsula and northeastern Africa is the result of another divergent plate boundary. The Arabian peninsula was ripped off of the continent of Africa millions of years ago, opening up the Red Sea. The African continent is still being ripped apart in southeastern Africa, forming the African Rift Valleys. If we wait millions of years, the African Rift Valleys will rip open to become a narrow sea like the Red Sea. If we wait millions of more years, that narrow sea will open even wider to become a growing ocean like the Atlantic Ocean.

At a convergent plate boundary, two tectonic plates are being pushed toward each other. The verb to converge means to come together. As the tectonic plates are pushed toward each other, they eventually collide. The more dense tectonic plate will sink into the asthenosphere, while the less dense tectonic plate will rise out of the asthenosphere. Hence, the more dense plate will sink underneath the less dense plate. As the more dense tectonic plate sinks into the asthenosphere, it is heated by the Earth’s geothermal energy. Eventually, the plate becomes sufficiently hot that it melts. Hence, this is the oldest part of a tectonic plate, since the tectonic plate is being destroyed as it melts into molten rock. Liquids are usually less dense than solids. Hence, this less dense molten rock may rise and collide underneath the other tectonic plate; the molten rock may even push through the other tectonic plate, forming active volcanoes. For example, the Nazca plate at the bottom of the Pacific Ocean and the South American plate are being pushed toward each other. The Nazca plate is more dense, since it is an oceanic plate composed primarily of mafic igneous rock. The South American plate is less dense, since it is a continental plate composed primarily of felsic igneous rock. Thus, the Nazca plate is sinking underneath the South American plate. As the Nazca plate sinks, it is heated by the Earth’s geothermal energy. The Nazca plate melts, and the less dense molten rock rises. The molten rock collides underneath and even pushes through the South American plate, forming the Andes Mountains on the west coast of South America. Indeed, many of the mountains in the Andes are volcanically active. The Nazca plate is an example of an oceanic plate, since it is at the bottom of the Pacific Ocean. The South American plate is an example of a continental plate, since it is mostly the South American continent. Hence, the Andes Mountains have formed at an oceanic-continental convergent plate boundary. It is always the oceanic plate that sinks underneath the continental plate at an oceanic-continental convergent plate boundary, since oceanic plates are composed primarily of more dense mafic igneous rock and continental plates are composed primarily of less dense felsic igneous rock. These oceanic-continental convergent plate boundaries form continental volcanic arcs, such as the Andes Mountains. At an oceanic-oceanic convergent plate boundary, two oceanic plates collide. Again, the more dense plate will sink underneath the less dense plate. The more dense plate is heated and melts. The molten rock rises and collides underneath and may even push through the other tectonic plate, again forming active volcanoes. These are volcanic island arcs, such as the Aleutian Islands in Alaska or the Japanese Islands or the Philippine Islands. A subduction zone is wherever one tectonic plate sinks underneath another tectonic plate. There are two different types of subduction zones: oceanic-continental convergence forming continental volcanic arcs such as the Andes Mountains and oceanic-oceanic convergence forming volcanic island arcs such as the Aleutian Islands, the Japanese Islands, or the Philippine Islands. However, continental-continental convergence does not result in a subduction zone, since continental plates are too thick to permit one plate to sink underneath the other plate. If two continental plates collide and if the convection cells in the asthenosphere continue to push them toward each other, the rock of both plates becomes folded. If we apply a large force to rock in a short duration of time, we will fault (break) the rock. However, if we apply a small force to rock over a long duration of time, we will bend or deform or warp the rock. The technical term for this bending or deforming or warping is folding. If we apply a small force to rock over a long duration of time, we will fold the rock. Hence, two colliding continental plates become folded. The rocks are folded upward forming non-volcanic mountains, since there is no subduction. The technical term for these non-volcanic mountains is fold mountains. For example, millions of years ago the Indian plate was near the South Pole. The Indian plate was pushed northward and closed up an ancient ocean that no longer exists. The Indian plate eventually collided with the Eurasian plate. As the plates continue to be pushed toward each other, they have folded upward forming the Himalayas, the tallest mountains in the world. The Himalayas continue to grow taller, although very slowly, as the Indian plate and the Eurasian plate continue to be pushed toward each other. Mount Everest is in the Himalayas, is the tallest mountain in the world, and is still growing taller, although only a few centimeters per year. The Appalachian Mountains are another example of fold mountains. Two hundred million years ago, the North American plate and the Eurasian plate collided and folded upward, forming the Appalachian Mountains. The Appalachian Mountains were the tallest mountains in the world roughly two hundred million years ago. However, over the past two hundred million years, the North American plate and the Eurasian plate have diverged from one another, and thus the Appalachian Mountains are no longer growing taller. In fact, the Appalachian Mountains are continuously becoming shorter as natural forces such as wind and rain degrade (weaken and destroy) their rocks, ultimately breaking them down into sediment and moving the sediment to other landscapes. The Ural Mountains are also fold mountains. Hundreds of millions of years ago, Europe and Asia were two separate continents. They collided, forming the Ural Mountains. For hundreds of years, humans believed that the Ural Mountains were an arbitrary geographical boundary between Europe and Asia. The Theory of Plate Tectonics has revealed that the Ural Mountains are not an arbitrary geographical boundary between Europe and Asia. The Theory of Plate Tectonics has revealed that the Ural Mountains are the actual geological boundary between Europe and Asia! In summary, there are three different types of convergent plate boundaries: oceanic-continental convergence, oceanic-oceanic convergence, and continental-continental convergence. At oceanic-continental convergence, we have continental volcanic arcs such as the Andes Mountains. At oceanic-oceanic convergence, we have volcanic island arcs such as the Aleutian Islands, the Japanese Islands, and the Philippine Islands. These two cases are subduction zones. However, at continental-continental convergence, we do not have subduction; we have fold mountains such as the Himalayas, the Appalachians, or the Urals.

The Wilson cycle describes the evolution (the birth, growth, decline, and death) of ocean basins, named for the Canadian geophysicist John Tuzo Wilson, one of the fathers of the Theory of Plate Tectonics. According to the Wilson cycle, we begin with no ocean basin at a continental rift valley, such as the African Rift Valleys. After millions of years, the continental rift valley will rip open to form a narrow widening sea, such as the Red Sea. After millions of more years, the narrow sea will gradually widen to form a growing ocean, such as the Atlantic Ocean. After millions of more years, the margins of that ocean basin may begin to subduct beneath other tectonic plates. The result is a dying ocean, such as the Pacific Ocean. Although the Pacific Ocean is much larger than the Atlantic Ocean, the Pacific Ocean is a dying ocean that is gradually closing up. After millions of more years, the ocean basin has closed up so much that it is now a narrow closing sea, such as the Mediterranean Sea. Finally, that narrow sea will close entirely forming fold mountains, such as the Himalayas, the Appalachians, or the Urals. In millions of years, fold mountains will form when Africa collides with Europe thus closing up the Mediterranean Sea. Geologists have given these future mountains a name even though they have not yet been born; they are called the Mediterranean Mountains. The rock around the Mediterranean Sea has already started folding, forming the Atlas Mountains in Africa and the Alps in Europe.

At a transform fault boundary, two tectonic plates slide across each other. Tectonic plates are not smooth, since they are composed of rock. Hence, the sliding of the tectonic plates across each other causes seismic activity, such as weak vibrations. On occasion, the two tectonic plates become stuck, but the convection cells in the asthenosphere continue to try to push the plates across each other. It may take years or even decades, but eventually the plates become unstuck and slide across each other by a tremendous amount, perhaps several meters. An enormous amount of energy is liberated from years or even decades of accumulated stored energy within the rock, thus causing intense vibrations. This powerful type of seismic activity is called an earthquake. For example, the San Andreas Fault is the boundary between the North American plate and the Pacific plate. The North American plate is being pushed south, while the Pacific plate is being pushed north. The sliding of these two tectonic plates across each other causes seismic activity, including earthquakes. This explains why there is an abundance of seismic activity, including earthquakes, in California, since it is on the west coast of North America at the boundary between the North American plate and the Pacific plate. This also explains why the shape of Baja California fits into the shape of Mexico. Millions of years ago, Baja California was connected to Mexico. As these two tectonic plates were pushed in two opposite directions, Baja California was ripped off of the Mexican mainland, opening up the Sea of Cortez. If we wait millions of years, all of Baja California together with upper California (the State of California) will be ripped off of the North American mainland forming an enormous island off the west coast of North America. Geologists have given this future island a name even though it has not yet been born; it is called the Island of California. The Island of California will then continue to move north as the Pacific plate continues to be pushed north.

Most geologic activity occurs at ocean basins (at the bottom of the ocean), simply because most of the world is covered with ocean. For example, most seismic activity such as earthquakes occurs at the bottom of the ocean. Most igneous activity such as volcanic eruptions occurs at the bottom of the ocean. Most mountains are at the bottom of the ocean. Even most landslides occur at the bottom of the ocean. The San Andreas Fault is actually an example of a continental-continental transform fault boundary. The Queen Charlotte Fault off the west coast of Canada is an example of an oceanic-oceanic transform fault boundary that causes earthquakes at the bottom of the Pacific Ocean and near the west coast of Canada. Just as it is popularly known among many Americans that the San Andreas Fault continually causes earthquakes near their west coast, it is popularly known among many Canadians that the Queen Charlotte Fault continually causes earthquakes near their west coast.

According to the Theory of Plate Tectonics, geologic activity does not just occur at plate boundaries. Geologic activity also occurs at hotspots, which are caused by mantle plumes. A mantle plume is a rising mass of hot rock within the asthenosphere. The rising mantle plume eventually collides underneath a tectonic plate. This collision can occur anywhere underneath the tectonic plate depending on the location of the mantle plume. The collision need not be located at a plate boundary; the collision could even occur in the middle of a tectonic plate. After the collision, the hot rock may push through the tectonic plate, causing geologic activity at this location on the tectonic plate. This is called a hotspot. For example, there is volcanic activity in the Hawaiian Islands, but the Hawaiian Islands are nowhere near a plate boundary; the Hawaiian Islands are in the middle of the Pacific plate. The volcanic activity in the Hawaiian Islands is caused by a hotspot, which is itself caused by a mantle plume. Molten rock pushes out of the Pacific plate at the bottom of the Pacific Ocean. The extruding molten rock builds a volcano at the bottom of the Pacific Ocean. More extruding molten rock grows the volcano taller and taller until it protrudes out of the Pacific Ocean. It is now a volcanic island. We all learn in primary (elementary) school that Mount Everest in the Himalayas is the tallest mountain in the world. Actually, if we measure the height of the Hawaiian Islands above their true base, which is at the bottom of the Pacific Ocean, then the Hawaiian Islands are in fact the tallest mountains in the world. As measured from their true base at the bottom of the Pacific Ocean, the Hawaiian Islands are much taller than Mount Everest. It is however strictly correct to state that Mount Everest has the highest elevation above sea level.

The rocks that compose the westernmost Hawaiian island are oldest, while the rocks that compose the Hawaiian Islands further and further east are younger and younger. The rocks that compose the easternmost Hawaiian island (the main Hawaiian island) are youngest, and that is the only Hawaiian island with active volcanism. All the other volcanoes on all the other Hawaiian Islands are extinct. This suggests that the hotspot is moving east, but in actuality the Pacific plate is moving west over the hotspot. As the Pacific plate moves west, the hotspot pushes up a Hawaiian island with active volcanism. As the Pacific plate continues to move west, that Hawaiian island moves west and off of the hotspot causing its volcanoes to become extinct, while the hotspot pushes up a new Hawaiian island with active volcanism. As the Pacific plate continues to move west, that Hawaiian island moves west and off of the hotspot causing its volcanoes to become extinct, while the hotspot pushes up a new Hawaiian island, and so on and so forth. If we wait millions of years, the main Hawaiian island will move off of the hotspot, its volcanoes will become extinct, and the hotspot will push up a new Hawaiian island to the east of the easternmost (main) Hawaiian island. This is already beginning to occur. To the east of the easternmost (main) Hawaiian island, there is a small active volcano at the bottom of the Pacific Ocean. If we wait millions of years, that active volcano will grow to become the new Hawaiian island.

Although the Hawaiian Islands are volcanic islands, we cannot classify the Hawaiian Islands as a volcanic island arc, since we must reserve that term for volcanic islands that are caused by subduction, such as the Aleutian Islands, the Japanese Islands, or the Philippine Islands. The Hawaiian Islands are not caused by subduction; the Hawaiian Islands are caused by a hotspot, which is itself caused by a mantle plume. Geologists use the term nematath to describe volcanic islands caused by a hotspot caused by a mantle plume. The word nematath literally means string/trail/chain of dung/excrement/defecation/feces/turds. The vulgar word for dung/excrement/defecation/feces/turds is sh!#$&*t. Just as a string/trail/chain of dung/excrement/defecation/feces/turds, vulgarly known as sh!#$&*t, left by geese reveals in which direction the geese have walked, a nematath such as the Hawaiian Islands reveals in which direction a tectonic plate is moving. We can even calculate the speed of the tectonic plate from the spacing of the islands of the nematath and the ages of the rocks that compose the islands of the nematath. This is why geologists use the term nematath for volcanic islands caused by a hotspot caused by a mantle plume, since such volcanic islands is rather like a string/trail/chain of dung/excrement/defecation/feces/turds, vulgarly known as sh!#$&*t.

Imagine a globe that depicts the shapes of the continents and the oceans. The Theory of Plate Tectonics has revealed that the Earth has never looked this way before and that the Earth will never look this way again. If the tectonic plates are slowly but continuously being pushed by convection cells in the asthenosphere, then the shapes of the continents and the oceans are slowly but continuously changing. We do not notice this occurring, but we would notice significant changes if we wait millions of years. During some eras of Earth’s history, the continents may have been more uniformly (evenly) distributed around the planet. During other eras in Earth’s history, the continents may have all been together as one giant landmass called a supercontinent. A supercontinent is surrounded by a superocean, the unity of all the oceans around the supercontinent. According to the supercontinent cycle, once every roughly five hundred million years, all the continents are together as a supercontinent surrounded by a superocean. If planet Earth is roughly 4.5 billion years old and if there is a supercontinent every roughly five hundred million years, this means that there have been roughly nine or ten supercontinents thus far in Earth’s history. In roughly five billion years, the Sun will swell to become a red giant, thus destroying all four of the inner planets, including the Earth. If the Earth will be destroyed in roughly five billion years and if there is a supercontinent every roughly five hundred million years, then there will be roughly ten supercontinents in the Earth’s future. Therefore, over the entire history of planet Earth, there have been and there will be very roughly twenty supercontinents. Many students believe that we happen to be alive when the continents are uniformly (evenly) distributed around the planet, but this is not the case. The continents are presently crowded together on the opposite side of planet Earth from the Pacific Ocean, which is by far the largest ocean in the entire world. The continents are also presently more crowded toward the northern hemisphere as compared with the southern hemisphere. Therefore, we happen to be alive not when the continents are more uniformly (evenly) distributed; we happen to be alive when the continents are rather crowded together. This is because rather recently, roughly two hundred million years ago, all the continents were together in one giant supercontinent called Pangaea surrounded by a superocean called Panthalassa. It is a common misconception that when the Earth formed, the continents were all together as Pangaea and have been spreading apart ever since. This is false. Again, there have been roughly ten supercontinents in Earth’s history thus far, and there will be another roughly ten supercontinents in the Earth’s future. Not only was Pangaea not the first supercontinent in Earth’s history, Pangaea was in fact the most recent supercontinent in Earth’s history. There were several other supercontinents before Pangea ever formed, and there will be several other supercontinents in the Earth’s future.

The Theory of Plate Tectonics has explained seismic activity, such as earthquakes in California. The Theory of Plate Tectonics has explained subductive igneous activity, such as the volcanic activity in the Andes Mountains, the Aleutian Islands, the Japanese Islands, and the Philippine Islands. The Theory of Plate Tectonics has explained the formation of non-volcanic mountains, such as the Himalayas, the Appalachians, and the Urals. The Theory of Plate Tectonics has explained why the continents fit together like a jigsaw puzzle, such as the east coast of South America fitting into the west coast of Africa and the Arabian peninsula fitting into northeastern Africa and Baja California fitting into Mexico. The Theory of Plate Tectonics has even explained the igneous activity of the Hawaiian Islands. The Theory of Plate Tectonics is truly the fundamental theory of the geology of planet Earth. Caution: the Theory of Plate Tectonics has not explained all geologic phenomena on planet Earth; there are still many examples of geologic activity that geologists do not completely understand. Nevertheless, whenever a geologist tries to understand any geologic activity on planet Earth, the geologist does so within the context of the Theory of Plate Tectonics.

During eras of Earth’s history when there was a supercontinent at the equator, the climate of the entire planet would be warm, since the equator is warm throughout the entire year. During other eras of Earth’s history when there were continents and/or microcontinents that were relatively isolated at one or both poles, the climate of the entire planet would be cold, since the poles are cold throughout the entire year. An ice age is a long period of time, lasting millions of years, when the climate of the entire planet is cold. There have been a few ice ages throughout Earth’s history thus far. It is truly remarkable how we have just applied the Theory of Plate Tectonics to climatology. The Theory of Plate Tectonics is the fundamental theory of the Earth’s geology, the study of the geosphere (the solid part of the Earth). We might suspect that this theory therefore has nothing to do with meteorology and climatology, which are the study of the Earth’s atmosphere. However, we have just applied the Theory of Plate Tectonics to explain ice ages, a climatological phenomenon. Therefore, the Theory of Plate Tectonics is not just the fundamental theory of geology of planet Earth; the Theory of Plate Tectonics is one of the fundamental theories of all the Earth Sciences.

Evidence for the Theory of Plate Tectonics includes the matching shapes of different continents, similar rocks on different continents, matching mountain ranges on different continents, similar ancient climates on different continents, fossils of the same land animal on different continents, the continuum of rocks ages of ocean-basin rocks, the continuum of sediment thickness on the ocean basins, the magnetic reversals within ocean-basin rocks, seismic activity, and igneous activity. There is also astronomical evidence for the Theory of Plate Tectonics. Quasars are the most distant galaxies in the universe from us. These quasars are so distant that they should not appear to move. However, astronomical observations of quasars using ground-based telescopes reveal that they do appear to move slowly over periods of several years. We must conclude that it is not the quasars that are moving; in fact, it is the telescope that is itself moving in the opposite direction. Since the ground-based telescope is upon a continent which is itself a part of a tectonic plate, the apparent motion of quasars reveals that the lithosphere is indeed fractured (broken) into tectonic plates that are all moving in various directions. We can even use the apparent motion of quasars to measure the actual motion of tectonic plates. Furthermore, the global positioning system (GPS) is further evidence of the Theory of Plate Tectonics. The global positioning system (GPS) is a collection of many artificial satellites that humans have placed in orbit around the Earth that accurately measure the motion of ground-based receivers, such as mobile telephones for example. However, the global positioning system (GPS) has measured the motion of apparently stationary ground-based receivers. Once again, these apparently stationary ground-based receivers are upon continents which are themselves part of tectonic plates. Hence, the global positioning system (GPS) has further revealed that the lithosphere of the Earth is fractured (broken) into tectonic plates that are all moving in various directions. Before the global positioning system (GPS), we were only able to measure the relative motions of tectonic plates, the motions of tectonic plates relative to one another. Using the global positioning system (GPS), we are now able to measure the absolute motions of tectonic plates, the motions of tectonic plates relative to the center of the Earth.

Although the Theory of Plate Tectonics is the fundamental theory of the geology of planet Earth, the Theory of Plate Tectonics is not a universal law of geology. In other words, the Theory of Plate Tectonics does not apply to every metallic-rocky planet in the universe. The Theory of Plate Tectonics does not even apply to every metallic-rocky planet in our Solar System. In particular, the Theory of Plate Tectonics is not the correct theory to explain the geologic activity on planet Venus. Geophysicists continue to search for the correct theory of the geology of planet Venus.

Orology

Orology is the study of mountains, and an orologist is someone who studies mountains. These words are derived from the Greek root oro- for mountain. A collection of a few mountains is called a mountain massif. A collection of a few mountain massifs, which is therefore a large number of mountains, is called a mountain range. A collection of a few mountain ranges, which is therefore a large number of mountain massifs and thus an enormous number of mountains, is called a cordillera. Orography is the classification of mountains, the study of different types of mountains. Orogenesis is the study of how mountains form. Again, these words are derived from the Greek root oro- for mountain. In order to discuss orogenesis (the formation of mountains), we must make a distinction between ductile deformation and brittle deformation. If we apply a large force to a rock in a short duration of time, we will break the rock. This is brittle deformation, and the technical term for a break within rock resulting from brittle deformation is a fault. If we apply a small force to a rock over a long duration of time, we will bend or deform or warp the rock. This is ductile deformation, and the technical term for the bending or deforming or warping of rock due to ductile deformation is a fold. In brief, ductile deformation causes folds, while brittle deformation causes faults.

Small forces over long periods of time result in folded rock through ductile deformation. Some parts of rock will be folded upward, while other parts of the rock will be folded downward. Rocks that are folded upward are called anticlines, which grow larger to become domes, which eventually over millions of years grow to become fold mountains. The rocks that are folded downward are called synclines, which grow larger to become basins, which eventually over millions of years grow to become valleys between fold mountains. Since they do not form from subduction, fold mountains are non-volcanic cordilleras. Examples of fold mountains include the Himalayas, the Appalachians, and the Urals. The Atlas Mountains, the Alps, and the Caucasus Mountains are also fold mountains in the process of formation.

Large forces over short periods of time result in faulted rock through brittle deformation. Vertical faults are called dip-slip faults, while horizontal faults are called strike-slip faults. Note that dip-slip faults need not be perfectly vertical. A fault that is slanted in elevation would still be classified as a dip-slip fault. Caution: most faults are small breaks within a tectonic plate. Some faults are between different tectonic plates, but these are the largest faults in the entire world. The largest dip-slip faults in the world are called thrust faults, which is simply another term for subduction zones. The largest strike-slip faults in the world are called transform faults. We need vertical motion to create mountains and valleys. Therefore, strike-slip faults do not form mountains and valleys; only dip-slip faults form mountains and valleys. With a dip-slip fault, some parts of the rock will be faulted upward, while other parts of the rock will be faulted downward. Rocks that are faulted upward are called horsts, which eventually grow to become fault mountains. Rocks that are faulted downward are called grabens, which eventually grow to become rift valleys between fault mountains. The steep cliff between a fault mountain and a rift valley is called a fault scarp. The African Rift Valleys in southeastern Africa is an example of fault mountains and rift valleys. Thrust faults (subduction zones) create volcanic arcs, either continental volcanic arcs such as the Andes Mountains or volcanic island arcs such as the Aleutian Islands, the Japanese Islands, and the Philippine Islands.

Most mountains are at the bottom of the ocean, simply because most of the world is covered with ocean. Some oceanic mountains are volcanic island arcs that form from oceanic-oceanic subduction, such as the Aleutian Islands, the Japanese Islands, and the Philippine Islands. Other oceanic mountains are nemataths that form from hotspots caused by mantle plumes, such as the Hawaiian Islands. Still other oceanic mountains are oceanic ridges that form from oceanic-oceanic divergent plate boundaries, such as the Mid-Atlantic Ridge.

We have discussed the formation of the Himalayas, the Appalachians, the Urals, the Atlas Mountains, the Alps, the Caucasus Mountains, the African Rift Valleys, the Andes Mountains, the Aleutian Islands, the Japanese Islands, the Philippine Islands, the Hawaiian Islands, and the Mid-Atlantic Ridge. However, there is one cordillera we have yet to discuss: the Rocky Mountains. We have avoided a discussion of these mountains because orologists continue to debate how they formed. The most popular theory to explain the formation of the Rocky Mountains is the subduction of an oceanic plate underneath the western margin of the North American plate. This theoretical oceanic plate is called the Juan de Fuca plate. One argument in favor of this theory is the three parallel mountain ranges near the west coast of North America: the Rocky Mountains, the Cascade Range further west, and the Pacific Coastal Range furthest west directly along the west coast of the North American continent. As an oceanic plate subducts underneath a continental plate, some of the rock that composes the subducting plate is heated and melts into molten rock. This molten rock rises and pushes its way through the margin of the overlying continental plate, forming a mountain range. As the oceanic plate continues to subduct deeper within the Earth, the hotter temperatures melt more of its rock. This molten rock rises and pushes its way through the overlying continental plate, forming another mountain range parallel to the first mountain range but toward the interior of the continent. As this process continues, the result is mountain ranges on the continent that are parallel to one another and parallel to the coastline where the oceanic plate is subducting. The same is true with the formation of the Andes Mountains near the west coast of South America. As we discussed, the Nazca plate is currently subducting underneath the western margin of the South American plate, forming the Andes Mountains. In actuality, the Andes Mountains are a collection of three parallel mountain ranges: Cordillera Oriental furthest east, Cordillera Occidental to the west, and the Chilean Coastal Range furthest west directly along the west coast of the South American continent.

Further evidence that the Rocky Mountains, the Cascade Range, and the Pacific Coastal Range formed from the subduction of the Juan de Fuca plate underneath the North American plate is the accretionary wedge along the west coast of North America. When an oceanic plate subducts underneath a continental plate, the continental plate rips islands, microcontinents, and entire island arcs off of the subducting oceanic plate. These pieces stick to the continental plate, causing the continent to grow larger and larger as the oceanic plate subducts beneath it. This edge of the continent, which is an amalgamation of islands, microcontinents, and island arcs that were ripped off of a subducting oceanic plate, is called an accretionary wedge, since accretion is the gaining of mass through sticky collisions. Each piece of an accretionary wedge has its own unique geology, different from the geology of neighboring parts of the accretionary wedge. Each of these parts of an accretionary wedge with its own unique geology is called a terrane. Caution: the common English word terrain is spelled differently. The common English word terrain simply means a description of the topography of a landscape, commonly known as the lay of the land. The western margin of North America is indeed an accretionary wedge, since it is a collection of terranes, each with its own unique geology, different from the geology of even neighboring terranes.

When the Earth first formed, there were probably small accumulations of felsic igneous rock distributed somewhat uniformly (evenly) throughout the lithosphere. These were the first microcontinents. As these microcontinents were pushed by convection cells in the asthenosphere, some of them collided with each other to form larger masses of felsic igneous rock: small-sized continents. These small-sized continents grew even larger as oceanic plates subducted underneath them, since the continents ripped pieces off of these oceanic plates as they subducted. Hence, the small-sized continents grew larger by gaining accretionary wedges, eventually becoming medium-sized and even large-sized continents. Therefore, the deep center of a modern-day continent was originally an ancient microcontinent from when the Earth was much younger. Indeed, the oldest rocks in the entire world are found within the deep centers of modern-day continents. The ancient deep center of a modern-day continent is called the craton of the continent, originally an ancient microcontinent that first formed billions of years ago when the Earth was young. As we will discuss shortly, these cratons are composed of batholiths, enormous masses of intrusive/plutonic felsic igneous rock.

If a tectonic plate becomes heavier, it will sink into the asthenosphere. If a tectonic plate becomes lighter, it will rise out of the asthenosphere. This vertical motion of a tectonic plate is called isostasy. For example, during the glacial period of an ice age, water that evaporates from the oceans will form enormous ice sheets upon continents. This extra weight will push the continental plates downward into the asthenosphere. As another example of isostasy, at the end of a glacial period of an ice age, the enormous ice sheets upon continents will melt into liquid water that returns to the oceans. This removed weight permits the continental plates to rise out of the asthenosphere. As yet another example of isostasy, natural forces degrade (weaken and destroy) mountains, breaking their rocks into sediment. These same natural forces move this sediment to other locations. Thus, weight is removed from the mountains, causing that particular part of the continental plate to become lighter. Hence, it will rise out of the asthenosphere. In other words, as mountains become shorter, they are buoyed upward to become taller! The isostatic upward motion of a continental plate is comparable to the shortening of mountains by degradation. Hence, the actual elevation of mountaintops above sea level only changes by small amounts as they are degraded by natural forces, since the shortened mountain is compensated by the isostatic rising of the continental plate. A similar compensation occurs with climatological effects upon sea level. As we will discuss later in the course, we may naïvely predict sea level to drop during the glacial period of an ice age, since water that has evaporated from the oceans will form enormous ice sheets upon continents instead of returning to the ocean through rivers. However, these enormous ice sheets also push the continental plates into the asthenosphere, and hence we would predict that sea level should rise relative to the sinking continents during the glacial period of an ice age! The dropping sea level from subtracted water is compensated by the rising sea level relative to the isostatic sinking of the continental plates. Hence, the actual change in sea level is small. As we will also discuss later in the course, we may naïvely predict sea level to rise at the end of a glacial period of an ice age, since enormous ice sheets will now melt into liquid water that will return to the oceans through rivers. However, this removed weight permits the continental plates to rise out of the asthenosphere, and hence we would predict that sea level should drop relative to the rising continents at the end of a glacial period of an ice age! The rising sea level from added water is compensated by the dropping sea level relative to the isostatic rising of the continental plates. Again, the actual change in sea level is small.

Seismology

Seismology is the study of vibrations within the geosphere, and a seismologist is someone who studies vibrations within the geosphere. These words are derived from the Greek root seismo- for shake. Any vibration in the geosphere is called seismic activity, and earthquakes are the most powerful form of seismic activity. Although seismic activity can occur anywhere in the world, seismic activity is most clearly understood at transform fault boundaries. At a transform fault boundary, tectonic plates are pushed across each other. This slow, continuous motion of two tectonic plates across each other at a transform fault boundary is called fault creep. Since tectonic plates are composed of rock, they are not smooth. Hence, fault creep results in frequent weak vibrations. On occasion, the tectonic plates may become stuck, but the convection cells within the asthenosphere continue to try to push the tectonic plates across each other. When there is active fault creep, the thermal energy (heat energy) of the rock within the asthenosphere is being transformed into the kinetic energy (moving energy) of the tectonic plates, as we discussed. If there is not active creep, the tectonic plates do not have kinetic energy (moving energy). Thus, the thermal energy (heat energy) of the rock in the asthenosphere must be transformed instead into elastic potential energy. Potential energy is any form of stored energy, and elastic potential energy is compressional stored energy. In other words, if there is not active creep, the rock must be suffering from compression over many years, perhaps many decades. Eventually the rock rebounds to its original shape, releasing the stored energy accumulated over years or even decades. The result is intense vibrations, an earthquake. In other words, the mechanism by which an earthquake occurs is the elastic rebound of compressed rock to its original shape, liberating the potential (stored) energy. Note that all solids have elastic properties. Just as stretching an elastic band stores energy in the elastic band which can be liberated by relaxing the elastic band, a rock or a table or a chair or a mobile telephone can store energy by being compressed, and that stored energy can be liberated by alleviating the applied pressure, thus causing the solid object to elastically rebound to its original shape. If there is no active fault creep at a transform fault boundary, all the transferred thermal energy of the rock within the asthenosphere is being transformed into elastic potential energy, thus compressing the rock. This stored energy is eventually liberated, and hence a powerful earthquake more likely to occur where there is no active fault creep. If there is active fault creep at a transform fault boundary, a significant fraction of the thermal energy of the rock within the asthenosphere is being transformed into kinetic energy, thus moving the tectonic plates. Hence, less energy is stored as elastic potential energy, making a powerful earthquake less likely to occur where there is active fault creep. In summary, a powerful earthquake is less likely to occur at a transform fault boundary where there is active creep, but a powerful earthquake is more likely to occur at a transform fault boundary where there is no active creep!

As we discussed, seismic waves propagate (travel) throughout the entire geosphere from any seismic event. As we also discussed, there are surface seismic waves that propagate (travel) along the surface of the geosphere, and there are body seismic waves that propagate (travel) throughout the interior of the geosphere. The two types of body seismic waves are P-waves (pressure waves or primary waves) and S-waves (shear-stress waves or secondary waves). A seismometer is a device that detects seismic waves. The operation of a seismometer is based on the principle of inertia, which states that it is more difficult to change the motion of a more massive (heavier) object and it is less difficult (easier) to change the motion of a less massive (lighter) object. If an object is at rest, it has a tendency to remain at rest, and a more massive (heavier) object has a greater tendency to remain at rest. To build a primitive seismometer, we suspend a very massive (very heavy) object from the ceiling. This object is so massive (heavy) that it will not move, even during intense vibrations. We attach writing utensils such as pencils to the heavy object, and these writing utensils will make marks upon paper during vibrations. The pencils make marks not because the pencils move; the pencils are attached to the massive (heavy) object, which does not move. The writing utensils make marks upon the paper because the paper moves! We can attach three mutually perpendicular writing utensils to the massive (heavy) object to record vibrations in all three dimensions. The piece of paper with the pencil marks is called a seismograph, the seismometer’s recording of the detected seismic waves.

From the arrival times of the P-waves and the S-waves, we can calculate the distance between the seismic event and the seismometer, as we discussed. However, this does not reveal precisely where the seismic event occurred. For example, suppose we calculate that a seismic event occurred one hundred kilometers from a seismometer. Was the seismic event one hundred kilometers to the north? Was the seismic event one hundred kilometers to the west? Was the seismic event one hundred kilometers to the southeast? We can draw a circle with a radius of one hundred kilometers centered on the seismometer, and all we can deduce is that the seismic event occurred somewhere on the circumference of that circle. Suppose a second seismometer at a different location, again using the arrival times of the P-waves and the S-waves, reveals that the same seismic event occurred fifty kilometers from that seismometer. We can draw a circle with a radius of fifty kilometers centered on that seismometer, and we can deduce that the seismic event occurred somewhere on the circumference of that second circle. However, if the seismic event occurred somewhere on the circumference of the first circle and somewhere on the circumference of the second circle, then the seismic event must have occurred somewhere on the intersection of these two circles. In general, two circles intersect at two different points. If we had a third seismometer at a third location, we could construct a third circle centered on that seismometer, and therefore the seismic event must have occurred at the intersection of all three circles. In general, three circles intersect at a single point. Therefore, to pinpoint the precise location of a seismic event, we must have at least three seismometers. The geometrical procedure of finding the intersection of three circles is called trilateration. Therefore, we determine the precise location of a seismic event using trilateration.

The source of the seismic energy from a seismic event is a point deep within the geosphere called the focus. The seismic waves propagate outward from the focus in all directions. The point on the surface of the geosphere directly above the focus is called the epicenter, but the epicenter is not the source of the seismic energy. Again, the focus deep within the geosphere is the source of the seismic energy. However, while standing on the surface of the geosphere, it appears as if the epicenter is the source of the seismic energy, when in fact the source of the seismic energy is the focus deep within the geosphere underneath the epicenter. As seismic waves propagate outward, the total energy of the seismic event is spread over a larger and larger sphere. Hence, the seismic energy becomes diluted, and seismometers further and further from the seismic event will detect weaker and weaker vibrations. Seismometers closer to the seismic event will detect stronger vibrations from the same seismic event. The total energy liberated from a seismic event is called the magnitude of the seismic event, while the strength of vibrations detected by a seismometer is called the local intensity of the vibrations. The local intensity measured by a seismometer depends upon how near or how distant the seismometer is from the seismic event. In other words, the local intensity measured by a seismometer depends upon the distance of the seismometer from the seismic event. However, the magnitude of the seismic event is a fixed number, since it is the total energy liberated by the seismic event. The Mercalli scale is a local intensity scale, named for the Italian geologist and Catholic priest Giuseppe Mercalli who formulated this scale in the year 1902. Weaker vibrations measured by a seismometer are lower numbers on the Mercalli scale, while stronger vibrations measured by a seismometer are higher numbers on the Mercalli scale. We can calculate the magnitude (the total energy) liberated by the seismic event from the local intensity of the vibrations measured by a seismometer together with the distance between the seismic event and the seismometer, as determined from the arrival times of the P-waves and the S-waves. The Richter scale is a magnitude scale that measures the total energy liberated by seismic events, named for the American seismologist Charles Francis Richter who formulated this scale in the year 1935. Less energetic seismic events have lower Richter magnitudes, while more energetic seismic events have higher Richter magnitudes. The Richter scale is also a logarithmic scale, meaning that higher Richter magnitudes are unimaginably more powerful than lower Richter magnitudes, which are unimaginably weaker. In particular, every unit on the Richter scale is roughly thirty times more powerful than the previous unit on the Richter scale. For example, a five on the Richter scale is roughly thirty times more powerful than a four. A six on the Richter scale is roughly thirty times more powerful than a five, which makes a six roughly nine hundred (thirty times thirty) times more powerful than a four. A seven on the Richter scale is roughly thirty times more powerful than a six, which makes a seven roughly nine hundred (thirty times thirty) times more powerful than a five, which makes a seven roughly twenty-seven thousand (thirty times thirty times thirty) times more powerful than a four. This reveals that higher Richter magnitudes are unimaginably more energetic than lower Richter magnitudes, since we must multiply by a string of thirties to calculate their relative energies. This also reveals that lower Richter magnitudes are unimaginably less energetic than higher Richter magnitudes, since we must divide by a string of thirties to calculate their relative energies. Higher Richter magnitude seismic events are much more rare, while lower Richter magnitude seismic events are much more common. For example, there are roughly ten million first-magnitude seismic events on planet Earth every year. This is roughly once every three seconds! However, eighth-magnitude earthquakes only occur roughly once a year on planet Earth. Ninth-magnitude earthquakes only occur roughly once a decade on planet Earth, and tenth magnitude earthquakes only occur roughly once a century on planet Earth. Note that there is no upper limit to the Richter scale. Theoretically, an eleventh-magnitude earthquake would only occur roughly once a millennium! There is also no lower limit to the Richter scale. For example, there are roughly one hundred million zeroth-magnitude seismic events on planet Earth every year. There are roughly one billion negative-first-magnitude seismic events on planet Earth every year, and there are roughly ten billion negative-second-magnitude seismic events on planet Earth every year.

Seismology was an inexact science before the 1960s. In particular, all Richter magnitudes before the 1960s were estimates, not exact calculations. Seismology became an exact science in the 1960s thanks to the underground detonation of thermonuclear fusion bombs. Throughout the 1950s, thermonuclear fusion bombs were detonated above ground; entire islands in the Pacific Ocean were obliterated during these above-ground tests. In the year 1963, the United States and the Union of Soviet Socialist Republics (the U.S.S.R. or the Soviet Union) signed the Limited Test Ban Treaty, banning the test-detonation of any nuclear weapon underwater or above ground, including outer space. However, this treaty did not ban underground detonations. Hence, when the United States and the Soviet Union began test-detonating thermonuclear fusion bombs underground, seismologists began detecting the resulting seismic waves. From knowing the precise location of the detonated bomb and its precise energy yield, seismologists calibrated the Richter scale into a precise energy scale. For example, a Richter magnitude of 3.21 has an energy yield of one ton of trinitrotoluene (one ton of TNT), a Richter magnitude of 5.21 has an energy yield of one thousand tons of trinitrotoluene (one kiloton of TNT), and a Richter magnitude of 7.21 has an energy yield of one million tons of trinitrotoluene (one megaton of TNT).

Often, not all of the elastic potential (stored) energy is liberated during an earthquake. Several hours after the earthquake, weaker vibrations may occur called aftershocks. Small amounts of energy may also be liberated before the earthquake occurs. These are called foreshocks. The word foreshock is derived from the word before. Our discussion of seismology may have given the impression that earthquakes can only occur at transform fault boundaries without active creep always following foreshocks. In actuality, we cannot predict when or where an earthquake will occur. Firstly, an earthquake may not be preceded by any foreshocks. Secondly, earthquakes not only occur at transform fault boundaries without active creep; earthquakes may occur at transform fault boundaries where there is active creep. Furthermore, earthquakes need not even occur at transform fault boundaries. Earthquakes also occur at convergent plate boundaries and at divergent plate boundaries for example. Finally, earthquakes need not even occur near plate boundaries. For example, it is a common misconception that earthquakes in the United States can only occur near its west coast due to the San Andreas Fault. In actuality, earthquakes occur throughout the United States. The east coast of the United States is far from any plate boundary, yet there are earthquakes near its east coast. These earthquakes need not be weak either. In fact, some of the most powerful earthquakes that have ever occurred in the United States have occurred near its east coast. Geologists do not yet understand how powerful earthquakes occur far from plate boundaries.

Vulcanology

Vulcanology is the study of igneous activity, and a vulcanologist is someone who studies igneous activity. These words are derived from Vulcan, the ancient mythological Roman god of fire. The study of intrusive/plutonic igneous activity deep within the Earth is simply called intrusive/plutonic vulcanology, while the study of extrusive/volcanic igneous activity near the surface of the Earth is simply called extrusive/volcanic vulcanology. Not much is known about intrusive/plutonic vulcanology, since we cannot directly observe the deep interior of the geosphere. Intrusive molten rock deep within the geosphere is called magma, while extrusive molten rock near the surface of the geosphere is called lava. Magma is not just molten rock; magma is actually a mixture of the three states of matter. The molten (liquid) component of magma is called the melt. There are solid rocks mixed with the melt, called the crystallized solids. There are gases trapped within magma, called the volatiles. The most abundant volatile within magma is water vapor.

We can classify molten rock based on its mineral composition. Since the vast majority of all minerals are silicate minerals, we therefore classify molten rock based on its silicate composition. In this course, we will refer to molten rock (either magma or lava) composed predominantly of dark silicates as mafic molten rock, since such molten rock when cooled will crystallize to form mafic igneous rock. In particular, mafic magma crystallizes to form gabbro since gabbro is an intrusive/plutonic mafic igneous rock, while mafic lava crystallizes to form basalt since basalt is an extrusive/volcanic mafic igneous rock. In this course, we will refer to molten rock (either magma or lava) composed predominantly of light silicates as felsic molten rock, since such molten rock when cooled will crystallize to form felsic igneous rock. In particular, felsic magma crystallizes to form granite since granite is an intrusive/plutonic felsic igneous rock, while felsic lava crystallizes to form rhyolite since rhyolite is an extrusive/volcanic felsic igneous rock. In this course, we will refer to molten rock (either magma or lava) composed predominantly of intermediate silicates as intermediate molten rock, since such molten rock when cooled will crystallize to form intermediate igneous rock. In particular, intermediate magma crystallizes to form diorite since diorite is an intrusive/plutonic intermediate igneous rock, while intermediate lava crystallizes to form andesite since andesite is an extrusive/volcanic intermediate igneous rock. Since continental plates are composed predominantly of felsic rock, we find felsic magma underneath these continental plates. If this felsic magma extrudes out of the continent, it becomes felsic lava. Since oceanic plates are composed predominantly of mafic rock, we find mafic magma underneath oceanic plates, which are themselves at the bottom of the ocean. If this mafic magma extrudes out of the ocean basins at the ocean floor, it becomes mafic lava.

As we discussed, quartz is a light silicate, and molten quartz is called silica. Hence, we redefine felsic molten rock as either magma or lava composed of the greatest quantity of silica. We also redefine mafic molten rock as either magma or lava composed of the least quantity of silica. Finally, we redefine intermediate molten rock as either magma or lava composed of an intermediate quantity of silica. The quantity of silica within molten rock determines the viscosity of the molten rock. The viscosity of any fluid is a measure of the internal friction within the fluid. A high viscosity fluid has much internal friction, thus making it difficult for the high viscosity fluid to flow. A low viscosity fluid has little internal friction, thus making it easier for the low viscosity fluid to flow. For example, honey and molasses have high viscosity, and indeed honey and molasses do not flow easily. Liquid water by comparison has a low viscosity, and indeed liquid water does flow much more easily than honey or molasses. Since felsic molten rock is composed of the most silica, felsic molten rock is most viscous. If high viscosity felsic lava extrudes out of a continent, it will build a tall, steep volcano, since the high viscosity felsic lava cannot flow far before it cools and crystallizes into rhyolite. These tall, steep volcanoes on continents are called stratovolcanoes. Since mafic molten rock is composed of the least silica, mafic molten rock is least viscous. If low viscosity mafic lava extrudes out of an ocean basin, it will build a wide, shallow volcano, since the low viscosity mafic lava can flow far before it cools and crystallizes into basalt. These wide, shallow volcanoes in the ocean are called shield volcanoes, such as the Hawaiian Islands for example.

When an oceanic plate subducts underneath a continental plate, the oceanic plate melts into mafic magma with the least quantities of silica. Underneath the continental plate, there is felsic magma with the greatest quantities of silica. Hence, the mafic magma with the least silica mixes with the felsic magma with the most silica to form intermediate magma with intermediate quantities of silica at these subduction zones. This intermediate magma may extrude out of the overlying continent, becoming intermediate lava. For example, the Andes Mountains near the west coast of South America formed from the subduction of the Nazca plate underneath the west coast of the South American plate, as we discussed. The mixture of the mafic magma from the melting Nazca plate and the felsic magma underneath the South American plate forms intermediate magma, which extrudes out of the South American plate as intermediate lava. Therefore, the Andes Mountains are built from intermediate igneous activity. Andesite is an extrusive/volcanic intermediate igneous rock, and indeed andesite is named for the Andes Mountains, much of which is composed of andesite.

When magma deep within the geosphere extrudes out of the geosphere to become lava, the trapped volatiles are liberated to the Earth’s atmosphere. This is an important distinction between magma and lava. Magma is not only intrusive/plutonic molten rock deep within the geosphere; magma also has large quantities of volatiles trapped within it. Lava is not only extrusive/volcanic molten rock near the surface of the geosphere; lava has also liberated most of its volatiles. This significant difference warrants these two different terms for molten rock, magma for intrusive molten rock deep within the geosphere and lava for extrusive molten rock near the surface of the geosphere. The large quantities of silica within felsic magma are able to trap an abundance of volatiles within it. Hence, when felsic lava erupts from a stratovolcano, all of these trapped volatiles are liberated. Thus, stratovolcanic eruptions are the most violent eruptions. These eruptions are often explosive; a stratovolcano can obliterate itself when it erupts! The small quantities of silica within mafic magma can only trap a small amount of volatiles within it. Hence, when mafic lava erupts from a shield volcano, only small quantities of trapped volatiles are liberated. Thus, shield-volcanic eruptions are the most gentle eruptions. For example, the active volcanoes on the main Hawaiian island suffer from continuous but very gentle eruptions. These eruptions are so quiet that there are actually vulcanologists living on the side of these active volcanoes directly studying these gentle but continuous eruptions!

It is not just lava that extrudes from igneous activity. Gases are also liberated, predominantly water vapor. These gases were formerly the trapped volatiles within the magma deep within the geosphere. The liberation of gases from igneous activity is called outgassing. Solids are also ejected from igneous activity; these solids are called pyroclastic materials. The smallest pyroclastic material is called ash, which is small enough and therefore light enough to drift in the Earth’s atmosphere. Other pyroclastic materials are heavy enough that after ejecting from the eruption, the Earth’s gravity pulls them back downward onto either the side of the volcano or onto the surrounding landscapes. With every eruption from a volcano, pyroclastic materials are ejected, many of which land on the side of the volcano. Eventually, many layers of pyroclastic materials on the side of the volcano will slide down the volcano. This landslide composed primarily of pyroclastic materials is called a lahar. With each lahar, pyroclastic materials slide down the volcano and land at the base of the volcano. After many lahars, an accumulation of pyroclastic materials builds a small volcano at the base of the major volcano. This small volcano is called a cinder cone. These cinder cones are the most common type of volcano. Caution: the definition of a volcano is not a mountain with lava extruding out from it, as is commonly believed. The strict definition of a volcano is any landform built from extrusive igneous activity. Lava is not required to extrude out of a landform for it to satisfy this definition and be classified as a volcano. Since cinder cones are indeed landforms built from extrusive igneous activity, they are classified as volcanoes. Lava could still extrude out of a cinder cone, since lava is not required to extrude out of a major volcano or any volcano for that matter. Lava may extrude out from any location on the surface of the geosphere. In fact, on most occasions lava does not extrude out of volcanoes. Most extrusive igneous activity is from fissures, cracks on the surface of the geosphere from which lava may erupt.

Any landform built from extrusive igneous activity is called a volcano. Therefore, any landform built from intrusive igneous activity should properly be called a plutano. However, all vulcanologists call any landform built from intrusive igneous activity a pluton. If magma deep within the geosphere flows within a fault and cools, it may crystalize into an intrusive/plutonic landform. These plutons are called dikes. Two other types of plutons are sills and laccoliths. Mafic molten rock flows easily, since it has low viscosity. Hence, when mafic magma deep within the geosphere cools, it may crystallize to form wide and thin intrusive/plutonic landforms. These plutons are called sills, since they are similar in shape to a windowsill. Felsic molten rock does not flow easily, since it has high viscosity. Hence, when felsic magma deep within the geosphere cools, it may crystallize to form bulged intrusive/plutonic landforms. These plutons are called laccoliths. The largest laccoliths in the world are called batholiths. These batholiths compose cratons, the deep and ancient interiors of continents. Not much more is known about plutons or intrusive igneous activity in general, since it occurs deep within the geosphere.

The entire margin of the Pacific Ocean is subduction zones, resulting in igneous activity nearly everywhere along the margin of the Pacific Ocean. This is called the Pacific Ring of Fire. For example, the Andes Mountains on the west coast of South America are volcanically active. The Aleutian Islands in Alaska are volcanically active. There is igneous activity in the Japanese Islands and the Philippine Islands. There is also igneous activity near the west coast of North America. For example, Mount Saint Helens in the Cascade Range suffered from a violent eruption in the year 1980.

Every igneous eruption ejects pyroclastic materials, and the smallest type of pyroclastic material is called ash. This ejected ash is small enough and therefore light enough to drift in the Earth’s atmosphere. A single igneous eruption powerful enough could eject enough ash into the atmosphere to surround the entire planet, significantly reducing the amount of incoming sunlight to the Earth. The result is global cooling, significantly cold temperatures across the entire planet. For example, the eruption of Laki Fissure in Iceland in June 1783 caused the winter of late 1783 to early 1784 to be bitterly cold. The eruption of Mount Tambora in Indonesia in April 1815 caused the year 1816 to be so cold that that year is commonly known as the Year Without a Summer; there was heavy snow in June and frost in July and August of that year! The eruption of Mount Krakatoa/Krakatau in Indonesia in August 1883 caused five years of global cooling. The eruption of Mount Pinatubo in the Philippines in June 1991 caused a measurable cooling of planet Earth.

The Newhall-Self scale is a volcanic explosivity scale, named for American geologists Christopher Newhall and Stephen Self who together formulated the scale in the year 1982. Lower numbers on the Newhall-Self scale are more gentle igneous eruptions that occur more frequently, while higher numbers on the Newhall-Self scale are more violent igneous eruptions that are more rare. The continuous but gentle eruptions of the volcanoes on the main Hawaiian island ranks zero, the lowest rank, on the Newhall-Self scale. Eruptions of rank one on the Newhall-Self scale occur roughly once per day, eruptions of rank two on the Newhall-Self scale occur roughly twice per week, and eruptions of rank three on the Newhall-Self scale occur roughly once per month. The eruption of Laki Fissure in the year 1783 ranks four on the Newhall-Self scale. Such eruptions occur roughly once per year. The eruption of Mount Saint Helens in the year 1980 and the eruption of Mount Vesuvius anno Domini 79, which destroyed several ancient Roman cities such as Pompeii, both rank five on the Newhall-Self scale. Such eruptions occur roughly once per decade. The eruption of Mount Krakatoa/Krakatau in the year 1883 and the eruption of Mount Pinatubo in the year 1991 both rank six on the Newhall-Self scale. Such eruptions occur roughly once per century. The eruption of Mount Tambora in the year 1815 ranks seven on the Newhall-Self scale. Such eruptions occur roughly once per millennium. The last eruption of the underground supervolcano Yellowstone roughly 630,000 years ago ranks eight, the highest rank, on the Newhall-Self scale. Such eruptions occur roughly once every ten thousand years. If the underground supervolcano Yellowstone were to erupt again, it would cause a catastrophic loss of life across most of the North American continent.

Paleogeology

Paleogeology is the study of the ancient geosphere, and a paleogeologist is someone who studies the ancient geosphere. These words are derived from the Greek root paleo- for ancient. The ultimate goal of paleogeology is to determine the history of planet Earth. Nearly everything paleogeologists have deduced about the history of the Earth is from rocks, and all of the information about Earth history recorded in rocks is called the rock record. In determining the history of planet Earth, we must make a distinction between relative dating and absolute dating. With relative dating, we can determine the events of Earth history and even the correct ordering of these events, but we cannot determine how long ago these events occurred. In other words, relative dating reveals that event A was followed by event B which was itself followed by event C for example, but relative dating does not reveal how long ago these events occurred. Did these events occur thousands of years ago? Did they occur millions of years ago? Did they occur billions of years ago? Relative dating cannot answer these questions. Absolute dating does reveal how long ago events in Earth history occurred. Paleogeologists have actually succeeded in determining the events in Earth history and their proper order using relative dating. Therefore, most of our discussion on paleogeology will be in the context of relative dating. We will however conclude our discussion of paleogeology with a brief overview of absolute dating.

Sedimentary rock lithifies from layers of sediments, as we discussed. A layer of lithified sedimentary rock is called a stratum. This word stratum is derived from a Latin word meaning layer. The lower strata of sedimentary rock are older since they were deposited first, and higher strata of sedimentary rock are younger since they were deposited later on top of the lower strata. This obvious truth is the most fundamental technique of relative dating and is called the Law of Superposition. Another technique of relative dating is the Principle of Original Horizontality, which states that a geologic event that folded rock for example must have occurred later (more recently) than the formation of the sedimentary rock in the first place. More plainly, sedimentary rock forms originally horizontally, hence the name of this technique of relative dating. Another technique of relative dating is the Principle of Cross-Cutting Relationships, which states that anything that cuts across strata of sedimentary rock such as a fault for example must have occurred later (more recently) than the formation of the strata themselves. A fossil is the remains of any ancient organism that has been lithified into rock. Fossils are always found within sedimentary rock in a certain particular order, the order of biological evolution (from less evolved to more evolved). This is called the Principle of Fossil Succession, and it is another technique of relative dating. There are some species of organisms that only existed on this planet for a brief period of time, perhaps only a few million years. Whenever we find such a fossil in one stratum in one rock formation, we can be certain that that stratum corresponds with another stratum in another rock formation with the same fossil. These valuable fossils are called index fossils, and they provide an essential technique of relative dating.

Roughly one century ago, the oldest rocks paleogeologists discovered were roughly five hundred and fifty million years old. Therefore, paleogeologists formerly believed that planet Earth is roughly that age. It was only later that much older rocks were discovered, and today we now know that planet Earth is nearly ten times older, roughly 4.5 billion years old. Nevertheless, paleogeologists from roughly a century ago took what they believed was the entire history of planet Earth and divided it into three eras: the Paleozoic Era, the Mesozoic Era, and the Cenozoic Era. The Greek root paleo- means ancient, as we discussed, and the Greek root zoo- means animal. For example, zoology is the study of animals, a zoologist is someone who studies animals, and humans visit animals in a zoo. Paleogeologists use the Greek root zoo- for life in general, not just animal life. Therefore, the term Paleozoic Era literally means the era of ancient life. The Greek root meso- means middle, as we discussed, and again paleogeologists use the Greek root zoo- for life in general. Therefore, the term Mesozoic Era literally means the era of middle life. The Greek root ceno- means recent, and therefore the term Cenozoic Era literally means the era of recent life. Today, we understand that these three eras together only constitute the most recent five hundred and fifty million years of Earth history. The first four billion years of Earth history is placed into an extremely long period of time called the Precambrian. In summary, the first four billion years of the history of planet Earth is the Precambrian followed by the Paleozoic Era (the era of ancient life) followed by the Mesozoic Era (the era of middle life) followed by the Cenozoic Era (the era of recent life).

Because of the rock cycle, rocks are continuously changing from one type to another. Hence, it is more likely for an old rock to have already changed into another younger rock; that is, it is less likely for an old rock to remain the same type of rock to the present day. Therefore, young rocks are more common, while old rocks are more rare. As a result, the rock record is more and more scarce as paleogeologists attempt to push our understanding of the history of the Earth further and further backward in time. Paleogeologists have determined recent Earth history, over tens of millions of years, in detail. Paleogeologists are less certain about what occurred in Earth history hundreds of millions of years ago, and paleogeologists are most uncertain about what occurred in Earth history billions of years ago. As we discussed, the Earth formed almost entirely molten. Hence, there were virtually no rocks when the Earth first formed, and therefore there is no rock record left from the actual formation of the Earth. Without a rock record, paleogeologists will never know with certainty what occurred during the earliest periods of Earth history. We have deduced that the Earth formed almost entirely molten and differentiated itself into a core, mantle, and crust from the laws of physics, not from the rock record.

The Precambrian began when the Earth formed roughly 4.5 billion years ago and ended roughly five hundred and fifty million years ago. At the beginning of the Precambrian, the Earth formed almost entirely molten and differentiated itself into a most dense metallic core, a less dense mantle composed of iron-rich silicate rock, and a least dense crust composed of iron-poor silicate rock. The felsic parts of the lithosphere were the first microcontinents, which collided with one another to form small-sized continents. The small-sized continents eventually became medium-sized and even large-sized continents as oceanic plates subducted beneath them, as we discussed. We will discuss the formation of the Earth’s atmosphere and the Earth’s oceans later in the course. Roughly one billion years after the Earth formed (roughly 3.5 billion years ago), the most primitive lifeforms evolved in the Earth’s oceans. These lifeforms were unicellular microorganisms, perhaps bacteria and blue-green algae. Over a couple of billion years, these primitive lifeforms transformed the composition of the Earth’s atmosphere, as we will discuss later in the course. Eventually, these unicellular microorganisms evolved into multicellular microorganisms. By the end of the Precambrian, the most primitive macroscopic organisms had evolved, such as invertebrate animals. All life that ever existed throughout the entire Precambrian was entirely in the Earth’s oceans.

The Paleozoic Era began roughly five hundred and fifty million years ago and ended roughly two hundred and fifty million years ago. The beginning of the Paleozoic Era is commonly known as the age of invertebrates, since the most evolved organisms on this planet were invertebrate animals, such as modern-day jellyfish. Over many millions of years, some invertebrate animals evolved into vertebrate animals. The most primitive vertebrate animals are fishes, which can only breathe underwater with gills. The first fish that evolved were the jawless fish, such as certain modern-day species of eel. Over many millions of years, some of the jawless fish evolved into cartilaginous fish, such as modern-day sharks and rays. Over many more millions of years, some of the cartilaginous fish evolved into bony fish, such as modern-day tuna and salmon. Hence, the middle of the Paleozoic Era is commonly known as the age of fishes. By the end of the Paleozoic Era, some of the fishes evolved to amphibians. The strict definition of an amphibian is a vertebrate animal that spends its infancy breathing underwater with gills, loses its gills during adolescence and grows lungs, and spends its adulthood breathing air with lungs. Examples of modern-day amphibians include frogs, toads, and salamanders. Hence, the end of the Paleozoic Era is commonly known as the age of amphibians. As some fish evolved into amphibians, there must have been vertebrate animals that were intermediate between fish (which can only breathe underwater with gills) and amphibians (which as adults can only breathe air with lungs). These intermediate vertebrate animals may have required both gills and lungs to breathe. A modern-day example of such an intermediate vertebrate is the lungfish, which has both gills and lungs. Some species of lungfish must inhale air with their lungs and must exhale underwater with their gills.

At the end of the Paleozoic Era, some amphibians evolved further to become reptiles. The strict definition of a reptile is a vertebrate animal that spends its entire life breathing air with lungs after hatching from a hard-shelled egg. Examples of modern-day reptiles include alligators, crocodiles, lizards, turtles, tortoises, and snakes. The entire Mesozoic Era is commonly known as the age of reptiles. Since the dinosaurs were particularly dominant during this era, the entire Mesozoic Era is also commonly known as the age of dinosaurs. The Mesozoic Era began roughly two hundred and fifty million years ago and ended roughly sixty-six million years ago.

At the end of the Mesozoic Era, most of the organisms on planet Earth suddenly became extinct, thus ending the Mesozoic Era and beginning the Cenozoic Era. During the Cenozoic Era, the mammals are the most evolved organism on this planet. Hence, the entire Cenozoic Era is commonly known as the age of mammals. A mammal is a vertebrate animal that spends its entire life breathing air with lungs after being born live. The strict definition of a mammal is a vertebrate animal where the females feed their young with mammary glands, hence the term mammal. As mammals evolved, there must have been vertebrate animals that were intermediate between reptiles (which lay and hatch from eggs) and mammals (which are born live). A modern-day example of such an intermediate vertebrate is the platypus, which although it is a mammal nevertheless lays and hatches from eggs. Mammals are also endothermic, commonly known as warm blooded, while all the other vertebrate animals we have discussed are ectothermic, commonly known as cold blooded. Ectothermic animals are not particularly adaptable, since they can only regulate their body temperatures within a narrow range of the surrounding environmental temperature. Mammals are endothermic; they are able to regulate their body temperatures over a fairly wide range of the surrounding environmental temperature. As a result, mammals are extraordinarily adaptable, and indeed mammals have adapted to live in every environment on planet Earth. This makes the diversity of mammals enormous. Examples of modern-day mammals include rodents (mice, rats, hamsters, gerbils, lemmings, muskrats, voles, guinea pigs, porcupines, gophers, beavers, squirrels, chipmunks, groundhogs/woodchucks, and prairie dogs), lagomorphs (rabbits and hares), moles, shrews, hedgehogs, bats, anteaters, sloths, hyenas, mongooses, felids (cats, lions, tigers, leopards, jaguars, cheetahs, cougars/pumas, and lynxes), canids (dogs, wolves, coyotes, jackals, and foxes), ursids (bears), raccoons, mustelids (weasels, badgers, ferrets, minks, wolverines, and otters), skunks, aardvarks, pinnipeds (seals and walruses), elephants, equids (horses, donkeys, and zebras), rhinoceroses, tapirs, bovids (cattle, bison, buffaloes, antelopes, yaks, sheep, and goats), suids (swine), giraffes, camelids (camels and llamas), cervids (deer, elk, moose, and reindeer), hippopotami, and cetaceans (whales, dolphins, and porpoises). Late in the Cenozoic Era, the most intelligent group of mammals evolved: the primates (lemurs, monkeys, baboons, gibbons, orangutans, gorillas, and chimpanzees). Even later in the Cenozoic Era, the most intelligent group of primates evolved, making them the most evolved mammals and therefore the most evolved organisms on planet Earth: humans. The first species of humans evolved roughly four million years ago, and the particular species Homo sapiens evolved much more recently, very roughly two hundred thousand years ago. Homo sapiens are the most evolved organisms on Earth because they are sentient, meaning that they are consciously aware of their own existence, consciously aware of the existence of the world around them, and can express abstract ideas with spoken language. The Cenozoic Era began roughly sixty-six million years ago and continues to the present day.

Note that there is a clear evolutionary progression of vertebrate animals, from fish to amphibians to reptiles to mammals. We see this clear evolutionary progression when we study not just the anatomy of vertebrate animals but also the physiology of vertebrate animals. Consider the respiratory systems of vertebrate animals for example. Fish are only able to breathe underwater with gills. Amphibians spend their infancy breathing underwater with gills and spend their adulthood breathing air with lungs. Reptiles spend their entire lives breathing air with lungs after hatching from hard-shelled eggs. Mammals spend their entire lives breathing air with lungs after being born live. Consider the cardiovascular/circulatory systems of vertebrate animals as another example. Fish have a two-chambered heart. Amphibians have a three-chambered heart. Reptiles have a three-and-a-half chambered heart. Mammals have a four-chambered heart. Consider the nervous systems of vertebrate animals as yet another example. The fish brain has an enormous medulla oblongata, a medium-sized cerebellum, and a tiny cerebrum. The amphibian brain has a smaller medulla oblongata, and the reptile brain has an even smaller medulla oblongata. Finally, the mammal brain has a tiny medulla oblongata, a medium-sized cerebellum, and an enormous cerebrum. Again, note the clear evolutionary progression of vertebrate animals from fish to amphibians to reptiles to mammals. Birds are regarded as intermediate in evolution between reptiles and mammals. For example, birds hatch from hard-shelled eggs like reptiles, but birds are endothermic like mammals. In the correct order from less evolved to more evolved, the vertebrate animals include jawless fish, cartilaginous fish, bony fish, amphibians, reptiles, birds, and mammals. There are also vertebrates that are intermediate in evolution between these classes, such as the lungfish (intermediate between bony fish and amphibians) and the platypus (intermediate between birds and mammals).

Once every roughly one hundred million years, almost all the organisms on planet Earth suddenly become extinct. This is called an extinction level event. The most recent extinction level event ended the Mesozoic Era and began the Cenozoic Era roughly sixty-six million years ago. One theory to explain extinction level events is the asteroid-impact theory. An asteroid is a large chunk of metal and rock in outer space. According to the asteroid-impact theory, once every roughly one hundred million years, an asteroid falls from outer space toward the Earth and collides with the surface of the Earth. It is not difficult to calculate that such a collision would unleash roughly one thousand times the combined nuclear arsenal of the entire world! This much liberated energy would completely obliterate all life at and near the point of impact. In addition, this much energy would shatter the asteroid into innumerable extremely hot fragments that would shoot outward from the point of impact and then rain back downward, igniting global forest fires and heating most of the Earth’s atmosphere to inhospitable temperatures, eradicating life over an even larger area. Most significantly, this much energy would pulverize rock at the point of impact into dust and ash, and the force of the collision would eject that dust and ash into the atmosphere, surrounding the entire planet and blocking most sunlight for several months, perhaps even a couple of years. Without sunlight, most plants would die, since plants require the energy of sunlight to synthesize their own food (photosynthesis). Most herbivorous animals would then die, since herbivores eat plants. Most carnivorous animals would then die, since carnivores eat herbivores. The result is an extinction level event, when most of the organisms across the entire planet suddenly become extinct. In the late 1970s, a crater was discovered in Yucatán in Mexico, named the Chicxulub crater. This crater is roughly the correct size that would be caused by an asteroid impact. The Chicxulub crater is also the correct age, having formed at the stratum of sedimentary rock between the end of the Mesozoic Era and the beginning of the Cenozoic Era. Finally, this stratum of rock is rich in dense elemental metals such as iridium and osmium that are less abundant on the surface of the Earth but more abundant in asteroids. Indeed, this stratum of rock between the end of the Mesozoic Era and the beginning of the Cenozoic Era is rich in iridium and osmium throughout the entire planet. Hence, this asteroid-impact theory for extinction level events has been proven, at least for the most recent extinction level event that ended the Mesozoic Era and began the Cenozoic Era. We are certain an extinction level event occurs once every roughly one hundred million years from the sudden disappearance of fossils within the rock record. From the observations of the orbits of asteroids around the Sun relative to the Earth’s orbit around the Sun and calculating probabilities, we estimate that an asteroid should collide with the Earth once every roughly one hundred million years. This further supports the theory that all extinction level events are caused by asteroid impacts. There are other theories to explain extinction level events however. Another theory states that roughly once every one hundred million years, there is an accumulation of poisonous gases within the oceans from a sudden increase in submarine volcanic activity. These poisonous gases would kill most life in the oceans. In addition, these poisonous gases would bubble out of the oceans, also killing most life on the continents. Perhaps all of the poisonous gases in the oceans accumulate at a certain location within the ocean and bubble out of the ocean as a single burst, killing most life on the continents. To summarize this alternative theory, once every roughly one hundred million years, the ocean flatulates (farts), and the stink kills most life on Earth!

The most reliable method of absolute dating is radioactive dating. As we discussed, certain atoms in the universe are radioactive, meaning that their nucleus is unstable because it has too much energy. This radioactive nucleus emits particles to lower its own energy to achieve greater stability. The original radioactive atom is called the parent atom, and the new atom transmuted from the original atom is called the daughter atom. The radioactive half-life is the amount of time it takes for one-half of the radioactive parent to transmute into a more stable daughter. This means that after two half-lives, one-fourth of the radioactive parent remains, since one-half of one-half is one-fourth. After three half-lives, one-eighth of the radioactive parent remains, since one-half of one-half of one-half is one-eighth. After four half-lives, one-sixteenth of the radioactive parent remains, since one-half of one-half of one-half of one-half is one-sixteenth. Hence, by measuring the quantity of radioactive parent within a rock, we can determine how many half-lives have passed since the formation of the rock. The half-life of the radioactive carbon-fourteen isotope is roughly six thousand years. Hence, this radioactive isotope is used to date comparatively young rocks. The half-life of the radioactive uranium-235 isotope is roughly seven hundred million years. Hence, this radioactive isotope is used to date comparatively middle-aged rocks. The half-life of the radioactive uranium-238 isotope is roughly 4.5 billion years. Hence, this radioactive isotope is used to date comparatively old rocks. Using radioactive dating, paleogeologists have determined that the Cenozoic Era began roughly sixty-six million years ago, the Mesozoic Era began roughly two hundred and fifty million years ago, and the Paleozoic Era began roughly five hundred and fifty million years ago. The oldest rocks that paleogeologists have ever dated are roughly 4.5 billion years old. We assume that planet Earth is slightly older than these oldest rocks, since the Earth was born almost entirely molten when there were no rocks, leaving no rock record available to us from the Earth’s initial formation. Hence, a somewhat more accurate estimate for the age of the Earth is roughly 4.6 billion years.

Links

Libarid A. Maljian homepage at the Department of Physics at CSLA at NJIT

Libarid A. Maljian profile at the Department of Physics at CSLA at NJIT

Department of Physics at CSLA at NJIT

College of Science and Liberal Arts at NJIT

New Jersey Institute of Technology

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