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

Fall 2024

Fifth (Final) Examination lecture notes

 

 

 

Introduction to Landscapes/Environments

 

We divide all landscapes/environments on planet Earth into three categories: terrestrial landscapes/environments, marine landscapes/environments, and transitional landscapes/environments.  Terrestrial landscapes/environments are on the continents.  Marine landscapes/environments are at the ocean floor.  Transitional landscapes/environments are where oceans and continents meet each other, commonly known as shores.  Most of the landscapes/environments on planet Earth are marine, since most of planet Earth is covered with oceans.  Unfortunately, the fact that marine landscapes/environments are at the bottom of the ocean is precisely the reason that much is not known about marine landscapes/environments.  We presented an overview of marine landscapes/environments earlier in the course, and we also discussed transitional landscapes/environments (shores) earlier in the course.  Therefore, we will devote the rest of the course to discussing terrestrial landscapes/environments.

 

Temperatures are hot at and near the equator, while temperatures are cold at and near the poles.  Humidity is high (aridity is low) at and near the equator due to the equatorial low.  Humidity is low (aridity is high) at and near roughly thirty degrees latitude in both hemispheres due to the subtropical highs.  Humidity is high (aridity is low) at and near roughly sixty degrees latitude in both hemispheres due to the subpolar lows.  Finally, humidity is low (aridity is high) at and near the poles in both hemispheres due to the polar highs.  We conclude that it is hot and humid at and near the equatorial low at the equator, it is hot and arid at and near the subtropical highs at roughly thirty degrees latitude in both hemispheres, it is cold and humid at and near the subpolar lows at roughly sixty degrees latitude in both hemispheres, and it is cold and arid at and near the polar highs at the poles in both hemispheres.

 

Since it is hot and humid at and near the equatorial low, most terrestrial landscapes/environments at and near the equator are tropical forests, such as the Amazon rainforest in northern South America, the Congo rainforest in central Africa, and the Indonesian rainforests.  Since it is hot and arid at and near the subtropical highs, most terrestrial landscapes/environments at and near roughly thirty degrees latitude in both hemispheres are hot deserts, such as the Basin and Range in southwestern United States and northwestern Mexico (including the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert), the Sahara in northern Africa, the Arabian Desert in the Arabian peninsula, the Gobi in China and Mongolia, the Patagonian Desert in Argentina, the Kalahari in southern Africa, and the Great Australian Desert in Australia (including the Great Victoria Desert, the Great Sandy Desert, the Tanami Desert, the Simpson Desert, and the Gibson Desert).  Since it is cold and humid at and near the subpolar lows, most terrestrial landscapes/environments at and near roughly sixty degrees latitude in the northern hemisphere are boreal forests (cold forests or taigas), including the Canadian boreal forests, the Scandinavian boreal forests, and the Russian boreal forests.  There are no terrestrial landscapes/environments at and near roughly sixty degrees latitude in the southern hemisphere, but theoretically any such terrestrial landscapes/environments would also be boreal forests (cold forests or taigas).  Finally, since it is cold and arid at and near the polar highs, most terrestrial landscapes/environments at and near the poles in both hemispheres are permafrost covered with continental ice sheets, such as the continent Antarctica at the south pole and the microcontinent Greenland near the north pole.

 

There is a gradual progression of terrestrial landscapes/environments from forests (most humid) to deserts (most arid).  After forests (most humid) would be less humid (or more arid) terrestrial landscapes/environments where there are fewer trees as compared with forests; these are woodlands.  After woodlands would be less humid (or more arid) terrestrial landscapes/environments where bushes and shrubs could grow but not trees; these are shrublands.  After shrublands would be less humid (or more arid) terrestrial landscapes/environments where grass could grow but not bushes or shrubs; these are grasslands.  After grasslands would be less humid (or more arid) terrestrial landscapes/environments just outside of deserts; these are steppes, also known as semi-arid landscapes/environments.  The Latin root semi- means half.  For example, a semicircle is half of a full circle, and a semiformal event is halfway between a formal event and an informal event.  Therefore, the term semi-arid literally means half-desert, meaning that steppes are just outside of deserts.  After steppes are deserts, the most arid (least humid) terrestrial landscapes/environments.  We may begin this discussion with deserts, the most arid (least humid) terrestrial landscapes/environments.  After deserts (most arid or least humid) would be less arid (or more humid) terrestrial landscapes/environments; these are steppes, also known as semi-arid landscapes/environments.  After steppes would be less arid (or more humid) terrestrial landscapes/environments where grass could grow; these are grasslands.  After grasslands would be less arid (or more humid) terrestrial landscapes/environments where bushes or shrubs could grow; these are shrublands.  After shrublands would be less arid (or more humid) terrestrial landscapes/environments just outside of forests; these are woodlands.  After woodlands are forests, the most humid (least arid) terrestrial landscapes/environments.  Just as steppes are half-deserts since steppes are just outside of deserts and hence steppes are not as arid as deserts, woodlands are half-forests since woodlands are just outside of forests and hence woodlands are not as humid as forests with fewer trees as compared with forests.

 

Beginning at the tropical forests at the equator and traveling toward the hot deserts at the subtropical highs, we would encounter tropical forests followed by tropical woodlands followed by tropical shrublands followed by savannas (hot grasslands) followed by hot steppes followed by hot deserts.  If we continue this journey toward the boreal forests (cold forests or taigas) at the subpolar lows, we would encounter hot deserts followed by midlatitude steppes followed by prairies (midlatitude grasslands) followed by midlatitude shrublands followed by midlatitude woodlands followed by boreal forests (cold forests or taigas).  This journey would only be theoretical in the southern hemisphere, since there are no terrestrial landscapes/environments at these latitudes in the southern hemisphere.  If we continue this journey toward the permafrost at the polar highs, we would encounter boreal forests followed by subpolar woodlands followed by subpolar shrublands followed by subpolar grasslands followed by tundras followed by permafrost covered with continental ice sheets.  This journey would be mostly theoretical in the southern hemisphere, since there are few terrestrial landscapes/environments at these latitudes in the southern hemisphere.

 

Although the midlatitude seasons (summertime and wintertime) are actually caused by the alternation between more direct sunlight and less direct sunlight due to the obliquity of the Earth as the Earth orbits the Sun, we may interpret the midlatitude seasons as being caused by the apparent motion of the Sun relative to the Earth.  Although this is not actually the case, it appears to be the case from the frame of reference of the Earth itself.  From this frame of reference, the Sun appears to be on top of the Tropic of Cancer (roughly 23˝ degrees north latitude) on roughly June 21st every year.  Over the next six months, the Sun appears to move south, arriving on top of the equator (zero degrees latitude) three months later on roughly September 21st every year and arriving on top of the Tropic of Capricorn (roughly 23˝ degrees south latitude) three months after that on roughly December 21st every year.  Over the next six months, the Sun appears to move north, arriving on top of the equator three months later on roughly March 21st every year and finally returning to the Tropic of Cancer three months after that on roughly June 21st every year.  From the frame of reference of the Earth, the midlatitude seasons (summertime and wintertime) are caused by this apparent motion of the Sun.  Note that summertime is during June, July, and August in the midlatitudes of the northern hemisphere, while summertime is during December, January, and February in the midlatitudes of the southern hemisphere.  Conversely, note that wintertime is during December, January, and February in the midlatitudes of the northern hemisphere, while wintertime is during June, July, and August in the midlatitudes of the southern hemisphere.

 

At and near the equator, temperatures are hot throughout the year.  Therefore, we do not discuss the seasons as being summertime and wintertime at the equatorial latitudes.  The corresponding seasons at the equatorial latitudes are the high-sun season and the low-sun season.  The high-sun season is during June, July, and August in the tropical latitudes north of the equator, while the high-sun season is during December, January, and February in the tropical latitudes south of the equator.  Conversely, the low-sun season is during December, January, and February in the tropical latitudes north of the equator, while the low-sun season is during June, July, and August in the tropical latitudes south of the equator.  Actually, the altitude of the Sun is high throughout the entire year at these latitudes.  During the low-sun season, the altitude of the Sun is simply not as high as the altitude of the Sun during the high-sun season.  We may regard the high-sun season at the equatorial latitudes as analogous to midlatitude summertime, and we may regard the low-sun season at the equatorial latitudes as analogous to midlatitude wintertime.

 

At and near the poles in both hemispheres, temperatures are cold throughout the year.  Therefore, we do not discuss the seasons as being summertime and wintertime at the polar latitudes.  The corresponding seasons at the polar latitudes are the daytime season and the nighttime season, since entire weeks or even entire months of nearly continuous daytime or nearly continuous nighttime occurs at these latitudes.  The daytime season is during June, July, and August in the polar latitudes near the north pole, while the daytime season is during December, January, and February in the polar latitudes near the south pole.  Conversely, the nighttime season is during December, January, and February in the polar latitudes near the north pole, while the nighttime season is during June, July, and August in the polar latitudes near the south pole.  We may regard the daytime season at the polar latitudes as analogous to midlatitude summertime, and we may regard the nighttime season at the polar latitudes as analogous to midlatitude wintertime.

 

As the Sun appears to move north and south relative to the Earth over the course of the year, the humidity of forests and the aridity of deserts both shift with latitude corresponding to the apparent motion of the Sun.  Consequently, the humidity of forests or the aridity of deserts (as the case may be) intrudes upon neighboring terrestrial landscapes/environments.  For example, consider terrestrial landscapes/environments at the tropical latitudes north of the equator.  During June, July, and August, the humidity of the tropical forests at the equatorial low intrudes to the north, causing more humid high-sun seasons.  During December, January, and February, the aridity of the hot deserts at the subtropical high (at roughly thirty degrees north latitude) intrudes to the south, causing more arid low-sun seasons.  The situation is somewhat reversed for terrestrial landscapes/environments at the tropical latitudes south of the equator.  During June, July, and August, the aridity of the hot deserts at the subtropical high (at roughly thirty degrees south latitude) intrudes to the north, causing more arid low-sun seasons.  During December, January, and February, the humidity of the tropical forests at the equatorial low intrudes to the south, causing more humid high-sun seasons.  Although the seasons seem to be reversed in the two hemispheres, in actuality only the time of year is reversed.  The seasons are actually the same.  High-sun seasons are more humid and low-sun seasons are more arid in the equatorial latitudes in both hemispheres.

 

As another example of shifting climates corresponding to the apparent motion of the Sun, consider terrestrial landscapes/environments at the midlatitudes in the northern hemisphere.  During June, July, and August, the aridity of the hot deserts at the subtropical high (at roughly thirty degrees north latitude) intrudes to the north, causing more arid summers.  During December, January, and February, the humidity of the boreal forests at the subpolar low (at roughly sixty degrees north latitude) intrudes to the south, causing more humid winters.  The situation is somewhat reversed for (theoretical) terrestrial landscapes/environments at the midlatitudes in the southern hemisphere.  During June, July, and August, the humidity of the (theoretical) boreal forests at the subpolar low (at roughly sixty degrees south latitude) intrudes to the north, causing more humid winters.  During December, January, and February, the aridity of the hot deserts at the subtropical high (at roughly thirty degrees south latitude) intrudes to the south, causing more arid summers.  Although the seasons seem to be reversed in the two hemispheres, in actuality only the time of year is reversed.  The seasons are actually the same.  Summers are more arid and winters are more humid in the midlatitudes in both hemispheres.  Admittedly, this discussion is only theoretical for the midlatitudes of the southern hemisphere, since there are no terrestrial landscapes/environments at these latitudes in the southern hemisphere.

 

As yet another example of shifting climates corresponding to the apparent motion of the Sun, consider terrestrial landscapes/environments at the polar latitudes near the north pole.  During June, July, and August, the humidity of the boreal forests at the subpolar low (at roughly sixty degrees north latitude) intrudes to the north, causing more humid daytime seasons.  During December, January, and February, the aridity of the permafrost at the polar high (at the north pole) intrudes to the south, causing more arid nighttime seasons.  The situation is somewhat reversed for (mostly theoretical) terrestrial landscapes/environments at the polar latitudes near the south pole.  During June, July, and August, the aridity of the permafrost at the polar high (at the south pole) intrudes to the north, causing more arid nighttime seasons.  During December, January, and February, the humidity of the (theoretical) boreal forests at the subpolar low (at roughly sixty degrees south latitude) intrudes to the south, causing more humid daytime seasons.  Although the seasons seem to be reversed in the two hemispheres, in actuality only the time of year is reversed.  The seasons are actually the same.  Daytime seasons are more humid and nighttime seasons are more arid in the polar latitudes in both hemispheres.  Admittedly, this discussion is mostly theoretical for the polar latitudes of the southern hemisphere, since there are few terrestrial landscapes/environments at these latitudes in the southern hemisphere.

 

There are many other effects that we have neglected in the preceding discussion, such as ocean-current effects, marine-versus-continental effects, and orographic effects.  Therefore, there are many exceptions to our overview of the climates of various terrestrial landscapes/environments.  Nevertheless, our generalizations are more often correct and less often incorrect.  This reveals that latitude is the primary variable determining the climate of terrestrial landscapes/environments, while ocean-current effects, marine-versus-continental effects, and orographic effects are secondary (but still important) variables determining the climate of terrestrial landscapes/environments.

 

High mountaintops are terrestrial landscapes/environments that belong in their own unique category.  Consider a tall mountain at the equator.  At the base of the mountain, we have a tropical forest, since we are at the equatorial low.  However, the temperature gradually becomes colder as we climb the mountain.  Eventually, the temperature may be so cold that we have a boreal forest at higher elevations up the mountain.  If the mountain is even taller, we would encounter subpolar woodlands at higher elevations and subpolar shrublands at even higher elevations.  If the mountain is taller still, we would reach an elevation where bushes and shrubs can no longer grow.  This is the timberline, more commonly known as the tree line, of the mountain.  At higher elevations we would encounter subpolar grasslands, and at even higher elevations we would encounter tundras.  If the mountain is tall enough, the summit of the mountain will be permafrost covered with a glacier, even though the mountain is at the equator where the sea-level temperature is quite hot!  There are many glaciers at the equator, even though sea-level temperatures are hot throughout the entire year at the equator!  This discussion reveals that elevation up a mountain is analogous to latitude.  We can turn this logic completely around and assert that latitude is analogous to elevation up a mountain.  We may even go so far as to assert that the entire planet Earth has a timberline, an extreme latitude in each hemisphere beyond which bushes and shrubs can no longer grow, just as there is an extreme elevation up a mountain beyond which bushes and shrubs can no longer grow.  A splendid example of this correspondence between elevation up a mountain and latitude is the dual meaning of the term icecap.  Summits of tall mountains are called icecaps if they are permafrost covered with glaciers, and poles of planets are also called icecaps if they are permafrost covered with ice sheets.

 

 

Introduction to Geomorphology

 

Geomorphology is the study of the evolution of landscapes/environments, and a geomorphologist is someone who studies the evolution of landscapes/environments.  A geomorphic agent is anything that changes landscapes/environments.  For example, wind changes landscapes/environments.  The technical term for the action of wind as a geomorphic agent is aeolian processes.  The adjective aeolian is derived from Aeolus, the ancient mythological Greek god of wind.  Gravity changes landscapes/environments by pulling rocks and sediments downward.  The technical term for the action of gravity as a geomorphic agent is mass wasting.  Running water in rivers and streams changes landscapes/environments.  The technical term for the action of running water as a geomorphic agent is fluvial processes.  The adjective fluvial is derived from a Latin word meaning to flow.  Underground water changes landscapes/environments.  The technical term for the action of underground water as a geomorphic agent is groundwater processes.  Glaciers (giant masses of ice) change landscapes/environments.  The technical term for the action of glaciers as a geomorphic agent is glacial processes.  Although there are many more geomorphic agents, these five (fluvial processes, groundwater processes, glacial processes, aeolian processes, and mass wasting) are the most important geomorphic agents.  By far, the single most important geomorphic agent is fluvial processes.  Nothing changes landscapes/environments more dramatically in a shorter amount of time than running water in rivers and streams.

 

When geomorphic agents change a landscape/environment, the geomorphic agents first destroy the existing landscape/environment.  This is called degradation.  More technically, degradation is the action of natural forces weakening rocks and breaking them down into sediments.  Next, the geomorphic agents transport or move these sediments from one location to another.  This is called erosion.  Finally, the geomorphic agents build a new landscape/environment from these transported sediments.  This is called aggradation.  To summarize, geomorphic agents first degrade a landscape/environment into sediments, then erode these sediments to another location, then finally aggrade a new landscape/environment from these eroded sediments.

 

Another term for degradation is weathering.  The two main types of weathering are mechanical weathering and chemical weathering.  Chemical weathering is the degradation of landscapes/environments with compositional changes in the rocks and sediments, while mechanical weathering is the degradation of landscapes/environments without compositional changes in the rocks and sediments.  Mechanical weathering is always executed through the action of physical forces.  There are many examples of mechanical weathering, including exfoliation, frost wedging, and salt wedging.  There are many examples of chemical weathering, including carbonation, oxidation, and hydrolysis.

 

Chemical weathering requires some understanding of acids and bases.  The first modern theory of acids and bases was formulated by the Swedish physical chemist Svante Arrhenius.  According to the Arrhenius theory of acids and bases, an acid is a solution with an excess of hydronium cations (H3O1+), while a base is a solution with an excess of hydroxide anions (OH1–).  Also according to this Arrhenius theory, the reaction of an acid and a base yields a salt plus water.  The Danish physical chemist Johannes Nicolaus Brřnsted and the British physical chemist Martin Lowry formulated a second theory of acids and bases that is regarded as more general than the Arrhenius theory of acids and bases.  According to the Brřnsted-Lowry theory of acids and bases, an acid is still a solution with an excess of hydronium cations, but a base is a solution that is deficient of hydronium cations.  Also according to this Brřnsted-Lowry theory, the reaction of an acid and a base yields the conjugate base of the acid plus the conjugate acid of the base.  The American physical chemist Gilbert N. Lewis formulated a third theory of acids and bases that is regarded as even more general than the Brřnsted-Lowry theory of acids and bases.  According to the Lewis theory of acids and bases, an acid is a solution that is deficient of electron pairs, while a base is a solution with an excess of electron pairs.  Regardless of which theory of acids and bases we decide to use, the pH is a measure of whether a solution is an acid or a base.  An acid has a pH that is less than seven, while a base has a pH that is greater than seven.  Only pure water has a pH exactly equal to seven.  For example, carbonation is the dissolving of carbon dioxide into water yielding carbonic acid, a solution with a pH less than seven.  Whenever it rains, the naturally occurring carbon dioxide in the atmosphere has dissolved within the raindrops, making the rain acidic.  Note that this is naturally occurring.  Even in the complete absence of human pollution, naturally occurring unpolluted rainwater is actually carbonic acid.

 

Although mechanical weathering and chemical weathering are the two main types of weathering, there are other types of weathering.  Biological weathering is the degradation of landscapes/environments by lifeforms.  Differential weathering is the degradation of different parts of a landscape/environment at varying rates.  Common landforms resulting from differential weathering are inselbergs (such as Zuma Rock in Niger State, Nigeria) and volcanic necks (such as Shiprock in New Mexico, United States).

 

 

Soil Science

 

As a result of weathering of all kinds, nearly the entire surface of planet Earth is covered with a thin layer of sediments commonly called dirt or ground or earth.  The technical term for this thin layer of sediments is regolith, derived from the Greek words for blanket and rock.  As far as the survival of the human species is concerned, the most important type of regolith is soil.  Thousands of years ago when the population of our species was small, humanity was able to survive nomadically, randomly gathering plants and randomly killing animals for food.  However, roughly six thousand years ago the population of the human species became large enough that agriculture became necessary for its survival.  Agriculture is the domestication of plants and animals near a supply of limnological water (such as a river or a lake) to provide a steady source of food, and agriculture is impossible without soil.  Since soil is essential for the survival of the human species, we must devote a significant amount of time in this course to soil science.

 

Soil is a particular type of regolith that is a mixture of inorganic minerals, water, air, and organic materials (both living and decomposing).  The sum total of all soil of the Earth is called the pedosphere.  As we discussed earlier in the course, the geosphere is the solid inorganic part of the Earth, the hydrosphere is the sum total of all water of the Earth, the atmosphere is the sum total of all air of the Earth, and the biosphere is the sum total of all life of the Earth.  Since soil is a mixture of inorganic minerals, water, air, and organic materials (both living and decomposing), we conclude that the pedosphere, the sum total of all soil of the Earth, is the intersection of the geosphere, the hydrosphere, the atmosphere, and the biosphere.  In other words, the pedosphere is where all the other Earth spheres meet one another.

 

As we discussed earlier in the course, the Wentworth scale quantifies the size of sediments.  According to the Wentworth scale, the largest sediments are gravels, smaller sediments are sand, even smaller sediments are silt, and the smallest sediments are clay/mud.  The texture of soil is determined by its inorganic mineral component.  Soil will feel more coarse-grained if its mineral components are large (sandy).  Soil will feel more fine-grained if its mineral components are small (muddy).  Soil will feel intermediate between these extremes if its mineral components are between large and small (silty).  Soil with roughly equal mixtures of sand, silt, and clay/mud is called loam, which is regarded as the soil with the ideal mixture of inorganic materials for productive agriculture.  However, other factors are more important in determining whether or not soil will be agriculturally productive, as we will discuss.

 

The classification of the water component of soil is as follows.  Water falling into the soil (usually from rain) is called the free water.  The water molecules within this free water may then begin to attract each other.  This is called the water of cohesion.  Water that sticks to inorganic or organic parts of the soil is called the water of adhesion.  Finally, water that has completely merged with the inorganic or organic part of the soil is called the combined water.

 

The degradation of one part of the soil is called eluviation.  The erosion of this degraded material from one part of the soil to another part of the soil is called leaching.  The aggradation of a new part of the soil from this leached material is called illuviation.  To summarize, just as geomorphological processes in general begin with degradation then follow with erosion followed by aggradation, geomorphological processes in soil begin with eluviation then follow with leaching followed by illuviation.

 

Eluviation, leaching, and illuviation develop layers of soil, which are called soil horizons.  The uppermost layer of soil is the A horizon, commonly known as the topsoil.  Below the A horizon is the B horizon, commonly known as the subsoil.  Below the B horizon is the C horizon, and below the C horizon is what is commonly known as the bedrock.  Although this bedrock is not part of the soil, many soil scientists call it the R horizon, meaning the rock horizon.  On top of the A horizon is a layer of dead plants and animals that have not yet decomposed.  This is commonly known as the litter.  Although this litter is not yet part of the soil, many soil scientists call it the O horizon, meaning the organic horizon.  As this O horizon decomposes, it mixes with air, water, and inorganic minerals to ultimately create the A horizon.  To summarize, below the O horizon is the A horizon below which is the B horizon below which is the C horizon below which is the R horizon.  Eluviation usually but not always occurs in the A horizon, leaching usually but not always occurs from the A horizon to the B horizon, and illuviation usually but not always occurs in the B horizon.

 

The quality of the parent material is one factor that determines whether or not soil will be agriculturally productive.  Ideally, there should be an optimum amount of inorganic sediments, an optimum quantity of water, an optimum amount of organic materials, and an optimum quantity of air within soil to maximize its agricultural productivity.  Either large amounts of inorganic sediments within soil or only small amounts of inorganic sediments within soil may result in that soil having low agricultural productivity.  Similarly, either large quantities of water within soil or only small quantities of water within soil may result in that soil having low agricultural productivity, and so on and so forth.  Again, there should be an optimum amount of inorganic sediments, an optimum quantity of water, an optimum amount of organic materials, and an optimum quantity of air within soil to maximize its agricultural productivity.  Nevertheless, the most important factor affecting the agricultural productivity of soil is climate.  Soil derived from poor parent material will nevertheless yield a productive crop if it is in a good climate, while soil derived from good parent material will nevertheless yield a poor crop if it is in a bad climate.  We will discuss shortly which climates result in high agricultural productivity and which climates result in poor agricultural productivity.  Another factor affecting the agricultural productivity of soil is the topography of the land.  Yet another factor affecting the agricultural productivity of soil is the abundance of living plants and animals within the soil.  Lifeforms within soil mix the soil, thus rejuvenating formerly dead soil.  This process is called bioturbation.  Although prairie dogs are efficient at bioturbation, earthworms are much more important.  As earthworms dig through soil, they eat the soil, they extract water and minerals from the soil, and they excrete water and minerals back into the soil.  As a result, earthworms can completely rejuvenate dead soil in a relatively short amount of time.  We can actually determine the agricultural productivity of soil by digging up some of the soil and simply counting the number of living earthworms within the soil.  Rapid bioturbation of soil is essential to rejuvenating dead soil quickly so that farmers are able to continually produce large quantities of crop to feed the billions of members of the human species.  Therefore, it is not an exaggeration to conclude that the single most important animal necessary for the survival of the human species is the earthworm.

 

Crops extract from the soil the minerals and water they need to grow.  Therefore, an essentially dead soil remains immediately after crops are harvested.  Earthworms need time to rejuvenate this dead soil.  In the meantime, abundant crops cannot be grown from this dead soil.  In order to continually produce crops, a farmer must not grow crops from his entire area of land.  Farmers must leave some of the land fallow, meaning no crop must be grown from some of the land so that it has time to be rejuvenated for future agricultural use.  During a future growing season, crops may be grown from the rejuvenated soil that was left fallow during a previous growing season, while the land that was just harvested is now left fallow so that it has time to be rejuvenated for future agricultural use.  This practice of alternating the lands with actively growing crops with the lands left fallow season by season so that crops may always be produced every season is called crop rotation.  When humans developed agriculture roughly six thousand years ago, they used a two-field crop rotation system.  In this technique, crops are grown on half of the land while the other half of the land is left fallow.  The following season, crops are grown on the half of the land that was left fallow the previous season while the other half of the land that was harvested the previous season is now left fallow.  This entire two-season procedure is then repeated every two years.  During the Middle Ages from roughly 1500 years ago to roughly 500 years ago, Europeans used a three-field crop rotation system.  In this technique, some crops are grown on one-third of the land, other crops are grown on a second-third of the land, and the final-third of the land is left fallow.  The following season, the first-third of the land is left fallow, the crops that were grown the previous season on the first-third of the land are now grown on the second-third of the land, and the crops that were grown the previous season on the second-third of the land are now grown on the final-third of the land.  During the third season, crops that were grown the previous season on the final-third of the land are now grown on the first-third of the land, the second-third of the land is left fallow, and the crops that were grown the previous season on the second-third of the land are now grown on the final-third of the land.  This entire three-season procedure is then repeated every three years.  During the Modern Ages beginning roughly 500 years ago, farmers began using a four-field crop rotation system.  In this technique, various crops are grown on three-quarters of the land while the final-quarter of the land is left fallow.  The various crops as well as the fallow land are all rotated over four seasons, and this entire four-season procedure is then repeated every four years.  As agricultural knowledge continues to advance and as agricultural efficiency continues to increase, farmers are able to leave less and less of the land fallow while using more and more of the land to continually produce the large quantities of crops that are necessary to feed the billions of members of the human species.

 

Taxonomy is the study of classification, and a taxonomist is someone who studies classification.  In biological taxonomy, all lifeforms are categorized into five biological kingdoms.  Each biological kingdom is subdivided into several biological phyla.  Each biological phylum is subdivided into several biological classes.  Each biological class is subdivided into several biological orders.  Each biological order is subdivided into several biological families.  Each biological family is subdivided into several biological genera.  Finally, each biological genus is subdivided into several biological species.  Whereas there are only five biological kingdoms, there are so many subdivisions from kingdoms to phyla to classes to orders to families to genera to species that there are millions of biological species that have been classified.  Soil taxonomy is very much similar to biological taxonomy.  All soils are categorized into twelve soil orders.  Each soil order is subdivided into several soil suborders.  Each soil suborder is subdivided into several soil great groups.  Each soil great group is subdivided into several soil subgroups.  Each soil subgroup is subdivided into several soil families.  Finally, each soil family is subdivided into several soil series.  Whereas there are only twelve soil orders, there are so many subdivisions from soil orders to soil suborders to soil great groups to soil subgroups to soil families to soil series that there are more than nineteen thousand soil series that have been classified.  Although it is impossible to discuss the thousands of different soil series, discussing all twelve soil orders provides tremendous insight into soil morphology.

 

Consider soils that have just been born so that no eluviation, leaching, or illuviation has taken place.  These are the entisols, the youngest soil order.  The word entisol is derived from the word recent.  In other words, entisols are recently born soils.  Now suppose some eluviation, leaching, and illuviation has taken place so that the soil has horizons, although they are still immature horizons.  These are young soils, and six soil orders are in this category: andisols, aridisols, gelisols, histosols, inceptisols, and vertisols.  The andisols are the volcanic ashes.  The word andisol is derived from the Andes Mountains where there is active vulcanism.  The aridisols are soils found in deserts.  The word aridisol is derived from the word arid meaning dry.  The gelisols are the soils above permafrost.  The word gelisol is derived from the Latin root gel- meaning cold.  For example, a gelatin dessert is a cold dessert.  The rejuvenation of dead gelisols is not primarily due to bioturbation but due primarily to cryoturbation instead, also known as frost churning.  During the nighttime season, the gelisols freeze.  During the next daytime season, the gelisols thaw.  During the next nighttime season, the gelisols freeze again, and during the next daytime season, the gelisols thaw again.  This is cryoturbation or frost churning, the mixing and hence rejuvenating of dead gelisols by the seasonal freezing and thawing of the soil.  The histosols are organically rich soils found in wetlands (swamps, marshes, and quagmires).  The word histosol is derived from the Greek root histo- meaning living tissue.  For example, histology is the study of living tissue, and a histologist is someone who studies living tissue.  The word inceptisol is derived from the word inception, although note that inceptisols are not quite newly born soils.  These soils are called inceptisols not because the soil is newly born but because the horizons within the soil are newly born.  Now suppose these young soils are subjected to further eluviation, leaching, and illuviation.  The soil thus develops mature horizons, and hence these soils are the most agriculturally productive soils.  There are two soil orders in this category: the alfisols and the mollisols.  The word alfisol is derived from a combination of the words aluminum and ferrum, the Latin word for iron, since these alfisols have optimum amounts of inorganic minerals in addition to optimum amounts of air, water, and organic materials.  The word mollisol is derived from the Latin root moll- meaning soft.  For example, to mollify someone is to appease (to soften) their strong emotions, an emollient is a moisturizer that softens skin, and mollusks are invertebrate animals with soft bodies such as squid, octopi, snails, and slugs.  The optimum amounts of inorganic minerals, air, water, and organic materials give mollisols a soft texture.  These alfisols and mollisols are found abundantly in the midwestern United States, which is one of several reasons why American farmers are able to produce enough food in just a handful of midwestern states to feed the entire world.  These soils are so rich that they have a deep black color.  We are all strongly urged to take a road trip across the midwestern United States so that we can see these alfisols and mollisols with our own eyes.  We will also see with our own eyes that the most abundant crop produced in the United States (and the entire world) is corn, with no other crop being produced anywhere nearly as abundantly.  Now suppose these soils are subjected to even more eluviation, leaching, and illuviation.  The result is old, dying soils.  There are two soil orders in this category: the spodosols and the ultisols.  The word ultisol is derived from the word ultimate, although note that ultisols are not quite dead soils.  When soil is subjected to a maximum amount of eluviation, leaching, and illuviation, the soil is dead.  These are the oxisols, the oldest soil order.  These oxisols are found in tropical forests at the equatorial low where there is the most amount of rain in the entire world.  The word oxisol is derived from the word oxidation, since the oxygen within the overabundant water has reacted (oxidized) with the soil to result in large amounts of metal oxides within the soil.  Since woodlands are just outside of forests, spodosols and ultisols are found in woodlands where the precipitation is high but not as high as in forests.  To summarize, the youngest soil order is the entisols, the slightly older but still immature soil orders are andisols, aridisols, gelisols, histosols, inceptisols, and vertisols, the mature soil orders that are best for agriculture are the alfisols and the mollisols, the dying yet still alive soil orders are spodosols and ultisols, and the oldest soil order is the oxisols.  The most agriculturally productive soils are the alfisols and the mollisols, while the least agriculturally productive soils are the entisols (since these are newly born soils) and the oxisols (since these are dead soils).

 

Modern farmers require tractors, irrigation sprinklers, and other industrialized machinery to continually produce the large quantities of crop necessary to feed the billions of members of the human species.  Nevertheless, modern farmers also continue to use the same simple tools that farmers have used for thousands of years, including shovels, chopping axes, pickaxes, pitchforks, sickles, scythes, rakes, and hoes.  A shovel is used to dig and move soil.  A hoe is used to shape soil for planting, while a rake is used to level soil and gather together materials other than soil.  Consequently, rakes and hoes often complement each other.  A sickle is a curved blade used to reap (harvest).  Since it has a short handle, the sickle is a handheld tool that is used to separate crops from weeds while reaping (harvesting).  A scythe is also a curved blade used to reap (harvest), but the scythe has long handle.  Therefore, the scythe is a two-handed tool that can reap (harvest) larger quantities than the sickle.  However, there is a drawback to the scythe.  Since it is a two-handed tool, the scythe is not used to separate crops from weeds while reaping (harvesting).  Therefore, crops and weeds must be separated later after scything.

 

 

Mass Wasting

 

Mass wasting is the action of gravity as a geomorphic agent.  Mass-wasting events are commonly called landslides, but this is not a word used in technical geomorphology.  There are many different ways a mass-wasting event can be triggered, such as heavy rains, snowmelts, seismic activity (such as earthquakes), and forest fires.  However, no such violent event is even necessary to trigger a mass-wasting event.  A mass-wasting event may be caused by an over-steepened slope.  The maximum steepness that a hillside or mountainside can endure before gravity can finally move regolith downward is called the angle of repose.  In other words, the moment the aggrading of sediments upon a hillside or mountainside results in a steepness greater than the angle of repose, gravity will degrade the hillside or mountainside, and the regolith will move downward.  The regolith will continue to move downward until the steepness of the hillside or mountainside is more shallow than the angle of repose.  Of course, geomorphic agents will continue to aggrade sediments upon the hillside or mountainside after the mass-wasting event.  Again, the moment the aggrading of sediments results in a steepness greater than the angle of repose, gravity will degrade the hillside or mountainside, and the regolith will again move downward.

 

The fastest type of mass-wasting event is called a fall.  Somewhat slower than a fall is a slide or flow.  The slowest type of mass-wasting event is called creep (or solifluction when glacier-related).  The regolith moving during a mass-wasting event may be primarily rock, debris, earth, or mud.  An avalanche is a mass-wasting event that is not necessarily glacier-related, since there are rock avalanches and mud avalanches for example.  An avalanche that is glacier-related should properly be called a snow avalanche.  A lahar is a mudflow composed of pyroclastic materials down the side of a volcano.

 

After many mass-wasting events, there may be an accumulation of sediments at the base of a hill or mountain.  These are called talus slopes, since the sediment itself is called talus.  A mass-wasting event may leave behind a steep cliff.  These steep cliffs are often called scarps, but they are more properly called escarpments.

 

 

Fluvial Processes

 

The action of running water as a geomorphic agent is called fluvial processes.  This is the single most important geomorphic agent, in general.  Nothing changes a landscape/environment more dramatically in a shorter period of time than running water.  This is remarkable considering that all the running water in all the rivers and streams in the entire world adds up to only a tiny fraction of all the water in the hydrosphere.  Roughly ninety-seven percent of all the water in the hydrosphere is in the oceans, and the remaining roughly three percent of all the water in the hydrosphere is in giant masses of ice called continental ice sheets, as we will discuss shortly.  Much much less than one percent of all the water in the hydrosphere is in the air as water vapor, underground as groundwater, and on the continents in lakes, rivers, and streams.  Nevertheless, running water in rivers and streams is the single most important geomorphic agent, in general.  The water cycle, more properly called the hydrologic cycle, is the continuous motion of water from one part of the Earth to another.  Evaporation is the transfer of water from the hydrosphere to the atmosphere, precipitation is the transfer of water from the atmosphere onto the geosphere, and runoff together with infiltration is the transfer of water from the geosphere back to the hydrosphere.  In addition, water is transferred from the biosphere to the atmosphere through transpiration.

 

Precipitation on or near a mountaintop or glacial meltwater near a mountaintop begins a river.  This is the head of the river.  Water flows down the mountain along very small bodies of running water called rills.  These rills merge to form larger bodies of running water called brooks or creeks.  These brooks or creeks eventually merge to form even larger bodies of running water called streams, and these streams eventually merge to form rivers, the largest bodies of running water.  Rivers may merge to form even larger rivers.  Eventually, the river ends at the ocean or some other large body of water.  This is called the mouth of the river.  To summarize, water flows from the head of the river down rills that merge to form brooks or creeks that merge to form streams that merge to form rivers that merge to form larger rivers ending at the mouth of the river.  The generic word for a rill, a brook or a creek, a stream, or a river is a watercourse.  A smaller watercourse that feeds water into a larger watercourse is called a tributary.  In other words, rills are tributaries of brooks or creeks, brooks or creeks are tributaries of streams, streams are tributaries of rivers, and rivers can be tributaries of larger rivers.  The opposite of a tributary is a distributary, which is a smaller watercourse that takes water out of a larger watercourse.

 

The pattern with which smaller watercourses merge to become larger watercourses is called a drainage pattern.  The most common type of drainage pattern is the dendritic drainage pattern.  The adjective dendritic means treelike.  If we were to view a dendritic drainage pattern from an airplane flying overhead, the river would resemble the trunk of a tree, the streams would resemble large branches attached to the trunk, and the brooks or creeks would resemble small branches attached to the large branches.  There are several other less common drainage patterns.  The radial drainage pattern is found around steep mountains and volcanoes, the rectangular drainage pattern is found in weathering-resistant rocks that have been faulted, and the trellis drainage pattern is found in landscapes/environments with both weathering-susceptible rock and weathering-resistant rock.  All of these drainage patterns apply to perennial rivers, more commonly known as permanent rivers, which carry running water throughout the entire year.  By contrast, an ephemeral river, more commonly known as a temporary river, does not carry running water throughout the entire year.  Some ephemeral rivers only carry water for a few hours per year, and some ephemeral rivers only carry water for a few minutes per year!  The drainage pattern associated with ephemeral rivers is the interior drainage pattern.  We will concentrate on perennial rivers during this discussion of fluvial processes.  We will briefly discuss ephemeral rivers in the context of aeolian processes, which are important in deserts as compared with other landscapes/environments.

 

The total area of land that feeds water into a watercourse is the watershed of the watercourse, also known as the drainage basin of the watercourse.  The watershed or drainage basin of a rill may be smaller than a room.  However, the watershed of a brook or creek will be larger, the watershed of a stream will be even larger, and the watershed of a river will be larger still.  The watersheds of the largest rivers in the world take up enormous areas of continents.  For example, the Mississippi River watershed is nearly the entire area of the United States between the Appalachian Mountains and the Rocky Mountains.  The boundary between two watersheds is called a drainage divide.  A drainage divide between two enormous watersheds is called a continental divide, since one of the enormous watersheds delivers all of its water to one ocean while the other enormous watershed often delivers all of its water to a different ocean.

 

The river gradient is commonly considered to be the steepness of a river.  More precisely, the river gradient is the vertical drop of a river divided by the horizontal distance of the river.  For example, if a river drops by one hundred feet over a horizontal distance of twenty miles, then the river gradient is five feet per mile, since one hundred divided by twenty is five.  In other words, the river drops by a vertical distance of five feet for every mile of horizontal distance.  The river gradient is typically steepest at the head but becomes more and more shallow downstream, ultimately becoming most shallow at the mouth.  In other words, the river gradient typically decreases from the head to the mouth.  A graph of the river gradient from the head to the mouth is called the longitudinal profile of the river.

 

The river discharge is commonly considered to be the amount of water flowing in the river combined with how fast the water is flowing.  More precisely, the river discharge is the volume of water flowing per unit time within the river.  The river discharge is typically smallest at the head and becomes greater and greater downstream, ultimately becoming greatest at the mouth.  In other words, the river discharge typically increases from the head to the mouth.  This is for two reasons.  Firstly, there is more and more water downstream as smaller watercourses merge to form larger watercourses.  Secondly, water moves faster downstream, since gravity has pulled the water downhill.

 

Through the action of both mechanical weathering and chemical weathering, running water degrades landscapes/environments into rocks and sediments.  Thus, the running water carves out a river channel for itself.  The sides of the river channel are called the riverbanks, and the bottom of the river channel is called the riverbed.  Moreover, the running water erodes (transports or moves) the rocks and sediments downstream.  The rocks and sediments that a river erodes (transports or moves) is called the river load.  The largest river load is called the bed load, since this load is so heavy that it rolls, slides, or bounces at the bottom of the river channel (the riverbed).  Smaller than the bed load is the suspended load.  These are sediments that are light enough to be carried by the running water.  The smallest river load is the solution load.  This is river load so small that it is dissolved within the river water.

 

The competence of a river is the maximum size load that the river erodes, while the capacity of a river is the total amount of load that the river erodes.  At first glance, competence and capacity seem to be the same concept, but in fact competence and capacity are two completely different characteristics of a river.  For example, it is entirely possible for a river to have a large competence but small capacity.  It is also possible for a river to have a small competence but large capacity.  As a concrete example, consider two rivers.  Suppose the first river has such a large competence that it is able to erode bed load.  This first river may even appear brown and dirty from the large load that it erodes.  Now suppose the second river has such a low competence that it is only able to erode solution load, which cannot be seen with the naked (unaided) eye since it is dissolved within the river water.  This second river may actually appear clean and clear.  Although the first river definitely has a larger competence and the second river definitely has a smaller competence, we cannot draw any conclusions about the capacities of these two rivers from their appearances.  It is entirely possible that the total amount of solution load of the second river may add up to a larger amount of load than the bed load in the first river.  In other words, a river that appears clean and clear may in fact be eroding a large capacity of load.  Also note that we can never draw the conclusion that a river is not eroding any load.  Every river, without exception, is eroding some capacity of load.

 

We now begin a detailed discussion of fluvial morphology.  Throughout this discussion, we will appreciate how dramatically running water changes landscapes/environments.  Running water tries to carve out a longitudinal profile that will give it the least amount of time to travel from the head to the mouth.  Most people believe that the path of least time between two points is a straight line, but in the presence of gravity this is not correct.  The geometrical question of finding the path of least time between two points in the presence of a constant force such as gravity is such a difficult geometrical question that it was solved somewhat recently, only about three hundred years ago.  This mathematical problem is called the brachistochrone problem, and the solution to the brachistochrone problem is a cycloid, or more correctly an arc of an inverted cycloid.  In other words, the path of least time between two points in the presence of a constant force such as gravity is an arc of an inverted cycloid.  Therefore, running water tries to carve out a longitudinal profile that looks like the arc of an inverted cycloid.  This explains why the river gradient is steepest at the head and becomes less steep (more shallow) downstream, becoming most shallow at the mouth.  An arc of an inverted cycloid starts out steep and ends up shallow.  The bottom of such a longitudinal profile is called the base level of the river.

 

If all of the rock from the head to the mouth had uniform (constant) resistance to weathering, this would summarize the entire development of the longitudinal profile of a river.  However, this is obviously not the case.  As the river water flows from the head to the mouth, the water encounters different landscapes/environments composed of a variety of rocks that all have different susceptibilities and resistances to weathering.  The running water tries to carve out an arc of an inverted cycloid until it encounters more weathering-susceptible rock, at which point the water will mechanically and chemically degrade downward to carve out another inverted cycloid.  This process is called downcutting.  The running water may eventually encounter even more weathering-susceptible rock, at which point downcutting occurs again.  As a result, the longitudinal profile of a river develops into a collection of inverted cycloids, and two neighboring cycloids join where the river gradient changes abruptly from shallow to steep.  Here, running water will approach with laminar flow (orderly flow) but will then fall down and crash with turbulent flow (disorderly flow).  This part of the river where the water falls downward is called a waterfall.  In other words, a waterfall is located at a base level of a particular inverted cycloid where the river gradient abruptly changes.  This is called a local base level, since the base level of the very last inverted cycloid at the mouth of the river is called the ultimate base level.  Through mechanical weathering and chemical weathering, a waterfall will gradually smoothen the abrupt change in the river gradient at its local base level, causing the water to eventually fall down less abruptly.  In other words, waterfalls eventually become more mild waterfalls, and these mild waterfalls are called rapids.  To summarize, waterfalls are located at local base levels with abrupt changes in the river gradient, and waterfalls eventually become rapids, which are located at local base levels with less abrupt changes in the river gradient.

 

The energy of the running water not only develops the longitudinal profile of the river but the lateral profile of the river as well.  In other words, the running water also changes the landscapes/environments on both sides of the river.  Consider waterfalls or rapids that flow over a narrow river valley.  Mass wasting will cause rocks to fall down, thus gradually widening the narrow river valley until it eventually becomes a wide river valley.  Now suppose that at the bottom of this river valley the river channel is straight.  The water in this straight channel will not flow straight however, since the water must maneuver around sand deposits within the river channel.  The flowing water therefore follows a curved path, even though the river channel is itself straight.  The deepest part of a river channel is the thalweg, which is almost never straight even if the river channel is straight.  (By international agreement, the border between two countries separated by a river is not just the river itself.  The precise border is the thalweg of the river.  This international agreement is called the thalweg doctrine or the thalweg principle.)  Since the flowing water follows a curved path to maneuver around the sand deposits, the actual shape of the river channel changes as the water cuts into the outer banks of the river channel and leaves sand deposits on the inner banks of the river channel.  The straight river channel has become a sinuous river channel.  The curves in the river channel become even more severe as the water cuts into the outer bank of every curve.  The sinuous river channel has become a meandering river channel.  The verb to meander means to walk aimlessly, without a logical direction.  In other words, the curves in meandering river channels are so severe that it seems as if the flowing water curved randomly without any logical reason.  To summarize, straight river channels become sinuous river channels which eventually become meandering river channels.  These meandering river channels cut into the surrounding wide river valley, making it wider and wider until the river has carved out a vast flat area of land around itself.  This is called a floodplain, since this entire area of land will be flooded if the river overflows by even a small amount.  To summarize the development of the lateral profile around a river, we begin with narrow river valleys which then become wide river valleys which eventually become floodplains.

 

A river may overflow rather periodically, causing the floodplain around the river to flood regularly.  Remarkably, the river aggrades landforms that protect its own floodplain.  For example, each time there is flooding, it is not just water that overflows from the river.  The load that the river erodes also overflows.  Some of this load may be deposited on the riverbanks.  With each flooding, more and more of these rocks and sediments accumulate on the riverbanks until walls of rock have been aggraded that prevent the water from further flooding the floodplain.  These are sometimes called levees, but they are more properly called natural levees to distinguish them from artificial levees that humans build, as we will discuss shortly.  There is however a disadvantage of natural levees.  If there happens to be so much flooding during a particular storm that the water actually floods over the levees, then the levees will prevent water from draining back into the river.  The result is a backswamp on either side of the river.  The water in a backswamp may try to take its own path to the ocean or major body of water.  The result is a yazoo tributary, a smaller river that runs parallel to a larger river since the natural levees of the larger river prevent the smaller river from becoming a tributary of the larger river.  That is, the natural levees of the larger river prevent the water flowing in the smaller river from draining back into the larger river.  The term yazoo is derived from the Yazoo River, which is itself a yazoo tributary of the Mississippi River.  Another means by which rivers protect their own floodplains is through connections between meanders.  These are sometimes called cutoffs, but they are more properly called natural cutoffs to distinguish them from artificial cutoffs that humans excavate, as we will discuss shortly.  A natural cutoff enables the flowing water to take a shorter path to the ocean or major body of water, thus decreasing the time that the floodplain is flooded.  Once a natural cutoff forms, a meander will become an oxbow lake, the oxbow lake will then become an oxbow swamp, and the oxbow swamp will eventually become a meander scar.

 

Floods are the most common and the most destructive of all natural disasters.  Humans attempt to control flooding through the building of artificial levees and the excavation of artificial cutoffs.  Artificial levees are commonly called dikes, although we must be careful when using this word.  For example, we used this same word dike earlier in the course for a pluton (an intrusive igneous landform) that formed within a fault.  The excavation of artificial cutoffs is called channelization.  For example, the United States Army Corps of Engineers has used channelization to shorten the length of the Mississippi River by more than two hundred and forty kilometers (one hundred and fifty miles) in order to ease flooding in the midwestern United States.  Historically, the longest river in North America was the Mississippi River.  However, the United States Army Corps of Engineers has shortened the Mississippi River through channelization to such a degree over the past two centuries that the Mississippi River is now the second longest river in North America.  The longest river in North America is currently the Missouri River, which is itself a tributary of the Mississippi River.  Most deaths due to flooding in the United States are automobile related.  We must never ever drive through a road that is flooded.  We must first wait for the flood to completely subside.  Again, most deaths in the United States due to flooding, which is itself the most common and the most destructive of all natural disasters, are automobile related.

 

 

Groundwater Processes

 

Groundwater processes is an important geomorphic agent.  While there are actual underground rivers as depicted in movies and while such underground rivers are indeed valid examples of groundwater, the vast majority of all groundwater is not of this type.  The vast majority of all groundwater flows through the solid rock beneath us.  Any rock whatsoever is not entirely solid.  A certain amount of the space of any rock is empty.  The fraction of the volume of a rock that consists of empty space is called the porosity of the rock.  The degree to which water is able to be transmitted through a rock is called the permeability of the rock.  At first glance, porosity and permeability seem to be the same concept, but in fact porosity and permeability are two completely different characteristics of a rock.  For example, it is entirely possible for a rock to have a large porosity but small permeability.  It is also possible for a rock to have a small porosity but large permeability.  All rocks have a certain amount of porosity, and all rocks have a certain amount of permeability.  Therefore, water is able to flow through rock.  Although underground rivers are valid examples of groundwater, the vast majority of all groundwater is not underground rivers.  The vast majority of all groundwater flows through the rock beneath us, due to the porosity and the permeability of rock.

 

If we were to drill into the ground, we would eventually reach a layer of rock that is completely filled with water flowing through it.  This is the phreatic zone, commonly known as the zone of saturation.  The word saturated means filled.  In other words, the zone of saturation is rock that is filled with flowing water.  Above this layer is the vadose zone, commonly known as the zone of aeration.  The word aerated means exposed to air.  In other words, the zone of aeration is rock that is not completely filled with water, rock that still has some air within its pores.  Between these two zones is the water table.  To summarize, if we were to drill into the ground, first we would encounter the vadose zone (the zone of aeration), then we would encounter the water table, then we would encounter the phreatic zone (the zone of saturation).  The term water table is misleading, since the water table is not horizontal like a table.  In actuality, the depth of the water table beneath the ground varies tremendously depending on the porosity and the permeability of the underground rock.

 

Layers of rock that are highly porous and permeable to groundwater are called aquifers, commonly composed of conglomerate, sandstone, faulted igneous rock, or faulted metamorphic rock.  Layers of rock that are highly nonporous and impermeable to groundwater are called aquitards, commonly composed of siltstone, shale, unfaulted igneous rock, or unfaulted metamorphic rock.  An aquifer between two aquitards is called a confined aquifer.  The buildup of pressure within a confined aquifer may eventually shoot groundwater out of the ground.  These are called artesian wells.

 

Although fluvial processes is the single most important geomorphic agent in general, there are some landscapes/environments where groundwater processes are nevertheless important geomorphological processes as compared with other landscapes/environments.  One such landscape/environment is the karst topography, where we find caverns (caves), natural bridges, sinkholes (natural wells), springs, artesian wells, hot springs, and geysers.

 

A cavern (or cave) is a large empty cavity within rock.  The study of caverns (caves) is called speleology, and a speleologist is someone who studies caverns (caves).  These words are derived from the Greek root speleo- for cavern (cave).  Speleogenesis is the study of how caverns (caves) form.  Although caverns (caves) can form from igneous activity, we concentrate our discussion on caverns (caves) that form from groundwater processes.  As groundwater flows through underground rock, the groundwater uses mechanical weathering and chemical weathering to degrade the underground rock into sediment.  The groundwater then erodes (transports or moves) that sediment to another location, ultimately carving out a large empty cavity within the underground rock, ultimately carving out a cavern (cave).  A speleothem is any rock formation whatsoever found within caverns.  Long and thin speleothems that grow from cavern ceilings are called stalactites, while short and wide speleothems that grow from cavern grounds are called stalagmites.  A stalactite and a stalagmite may eventually merge becoming a single speleothem connecting the cavern ceiling to the cavern ground.  These speleothems are called columns.

 

 

Aeolian Processes

 

Aeolian processes is the action of wind as a geomorphic agent.  Aeolian processes are important geomorphological processes in deserts as compared with other landscapes/environments, although we emphasize again that fluvial processes is the single most important geomorphic agent, even in deserts.  For example, an ephemeral river may carry running water for just a few minutes after it rains, but nevertheless the running water within this ephemeral river sculpts the desert more in those few minutes than the action of the wind over the rest of the entire year!  Hence, even in deserts, fluvial processes is the single most important geomorphic agent.  Nevertheless, aeolian processes are important geomorphological processes in deserts as compared with other landscapes/environments.

 

It is a common misconception that deserts are entirely sand.  Not only is this false, deserts are not even mostly sand.  For example, the Sahara in northern Africa is the largest hot desert in the world.  Nevertheless, only roughly ten percent of the Sahara is covered with sand; roughly ninety percent of the Sahara is not covered with sand.  As another example, the sandiest desert in the world is the Arabian Desert.  Nevertheless, this sandiest desert in the world is only roughly one-third covered with sand; roughly two-thirds of the Arabian Desert is not covered with sand.  The regions of a desert covered with solid rocks are called hamadas, the regions of a desert covered with gravels are called regs, and the regions of a desert covered with sands are called ergs.

 

A small rock that has been smoothed by the action of the wind is called a ventifact, while a large rock that has been smoothed by the action of the wind is called a yardang.  Entire landscapes/environments are degraded by aeolian processes.  A plateau is a large area of land with high elevation but shallow relief.  As winds degrade a plateau, the plateau becomes smaller and smaller over time.  The plateau is eventually worn down to a mesa.  As winds continue to degrade the mesa, the mesa becomes smaller and smaller over time.  The mesa is eventually worn down to a butte.  To summarize, aeolian processes degrade plateaus into mesas and degrade mesas into buttes.  The Basin and Range is a collection of deserts in southwestern United States and northwestern Mexico, including the Mojave Desert, the Sonoran Desert, and the Chihuahuan Desert.  The Basin and Range has been and continues to be degraded by aeolian processes.  In particular, the Colorado Plateau has gradually decreased in size as a result of aeolian degradation, causing the southern terminus of the Colorado Plateau to slowly migrate northward, leaving behind small mesas and even smaller buttes that were once part of the Colorado Plateau.  Aeolian processes also degrade landscapes within the Colorado Plateau, carving out small mesas and even smaller buttes within the plateau itself.  A beautiful landscape of buttes within the Colorado Plateau is Monument Valley at the border of Utah and Arizona.

 

As aeolian processes degrade landscapes/environments into sediments, the wind also erodes (transports or moves) the sediments to other landscapes/environments.  Just as the sediments eroded by running water is called fluvial load, the sediments eroded by wind is called aeolian load.  As we discussed earlier in the course, the Wentworth scale quantifies the size of sediments.  According to the Wentworth scale, the largest sediments are gravels, smaller sediments are sand, and even smaller sediments are silt.  Gravels are too heavy to be eroded by wind; consequently, gravels are not part of aeolian load.  Although sands are smaller and lighter than gravels, they are still heavy enough that wind is only able to roll, slide, or bounce sand upon the ergs of the desert.  For this reason, sands are considered aeolian bed load, in perfect analogy with fluvial bed load.  Finally, silts are small enough and light enough that they can actually be carried by the wind.  For this reason, silts are considered aeolian suspended load, in perfect analogy with fluvial suspended load.

 

A number of landforms are aggraded by aeolian processes.  Since silts constitute aeolian suspended load, winds can carry silts and deposit them upon landscapes, forming a layer of silt covering a large area of land.  This silt deposit is called loess.  Since sands constitute aeolian bed load, winds can push sands and sculpt them into mounds of sand called dunes.

 

 

Glaciology

 

The final geomorphic agent we discuss in this course is glacial processes.  The sum total of all ice of the Earth is called the cryosphere.  Giant masses of ice that sculpt entire mountains and valleys are called alpine glaciers, but even these alpine glaciers are small compared to continental ice sheets, enormous masses of ice that are the sizes of entire continents or microcontinents.  Continental ice sheets only exist on planet Earth during ice ages.  As we discussed earlier in the course, planet Earth entered into an ice age roughly thirty million years ago that continues to the present day.  This is called the Current Ice Age.  During glacial periods of the Current Ice Age, continental ice sheets expand beyond the poles, while during interglacial periods of the Current Ice Age, continental ice sheets retreat back to the poles.  We are currently within an interglacial period of the Current Ice Age.  Consequently, there are presently only two continental ice sheets on planet Earth.  One continental ice sheet covers the continent Antarctica at the south pole.  This continental ice sheet is the single largest mass of ice in the entire world; roughly ninety percent of all the ice in the cryosphere composes that continental ice sheet.  The remaining roughly ten percent of all the ice in the cryosphere is in the other continental ice sheet that covers the microcontinent Greenland near the north pole.  All of the alpine glaciers in the world combined together add up to only a tiny fraction of the ice in the cryosphere.  The average thickness of the Greenland continental ice sheet is roughly 1.5 kilometers, and the average thickness of the Antarctic continental ice sheet is more than two kilometers.  In other words, whenever we walk upon Antarctica or Greenland, we are not actually walking upon Antarctica or Greenland; we are actually walking a couple kilometers in elevation above Antarctica or Greenland!  Entire mountain ranges are buried beneath these continental ice sheets.  These buried mountain ranges are called nunataks.  Continental ice sheets also expand over the ocean waters surrounding the continent or microcontinent.  These intrusions are called ice shelves.  The Ross Ice Shelf is the largest ice shelf in the world.  Giant pieces of ice may break off from an ice shelf, resulting in giant slabs of ice floating in the ocean.  These are called ice floes.  Whereas the study of continental ice sheets, ice shelves, and ice floes is fascinating, there are currently only two continental ice sheets on planet Earth, and there would be none at all if planet Earth were not presently within an ice age.  There are thousands of alpine glaciers all over planet Earth on nearly every continent (Australia being the only exception), and these alpine glaciers would exist whether or not planet Earth were within an ice age.  Therefore, we will henceforth concentrate our discussion of glacial processes on alpine glaciers.

 

We begin our discussion of alpine glaciers with the formation of a glacier.  Consider precipitation upon a high mountaintop.  The mountaintop is so high that the precipitation is solid.  This solid precipitation is called normal ice.  As normal ice accumulates, the weight of the abundance of this normal ice begins to squeeze air out of the ice itself.  The normal ice eventually becomes firn, a high-density form of ice with the consistency of coarse sand.  As even more ice accumulates, the firn is still further compressed until it becomes glacial ice, an extremely high-density form of ice that flows somewhat like a liquid.  Although most of a glacier is composed of glacial ice, the uppermost layer of a glacier is composed of normal ice, since the uppermost layer of a glacier has no overlying weight to compress it to exotic densities.  This uppermost layer of a glacier is sometimes called the glacial crust, but it is more properly called the zone of fracture, since normal ice can break.  Cracks within this zone of fracture are called glacial crevasses.  Since the zone of fracture is roughly fifty meters thick (roughly one hundred and sixty feet thick), many glaciologists surveying glaciers have been killed while walking upon the zone of fracture and then falling into a glacial crevasse.

 

The beginning of a glacier upon a high mountaintop is called the glacial head, in perfect analogy with the beginning of a river being called the river head.  However, while the end of a river is called the river mouth, the end of a glacier is called the glacial terminus.  At and near the glacial terminus, the glacier loses mass through melting, subliming, and calving.  Calving is the process by which pieces of a glacier break and fall into the ocean, becoming mountains of floating ice called icebergs.  Any process by which a glacier loses mass is called glacial ablation.  In other words, melting, subliming, and calving are the three different forms of glacial ablation.  The zone of accumulation of a glacier is the region at and near its head where the glacier gains more mass than it loses.  The zone of wastage of a glacier is the region at and near its terminus where the glacier loses more mass than it gains.  The boundary between the zone of accumulation and the zone of wastage is called the equilibrium line.

 

When a glacial terminus moves down a mountain, we say that the terminus is advancing.  When a glacial terminus moves up a mountain, we say that the terminus is retreating.  When a glacial terminus does not move, we say that the terminus is stationary.  How can ice flow uphill?  In fact, the ice that composes a glacier always flows downhill whether the terminus is advancing, retreating, or stationary.  A retreating terminus simply reveals that the glacier is shrinking in size overall because the overall rate of ablation is greater than the overall rate of accumulation, but the glacial ice still flows downhill in this case.  This can be proven by driving markers into the zone of fracture; the markers still move downhill even while the terminus retreats uphill.  An advancing terminus simply reveals that the glacier is growing in size overall because the overall rate of accumulation is greater than the overall rate of ablation.  Finally, a stationary terminus reveals that the glacier is maintaining the same size overall because the overall rate of accumulation is equal to the overall rate of ablation, but the ice that composes the glacier still flows downhill in this case as well.  If a glacial terminus was stationary for several years then suddenly advances by a large amount, it is not because the glacier became stuck and then finally became unstuck and moved downhill.  The ice that composes the glacier continued to flow downhill all the years the terminus was stationary.  In actuality, the overall rate of accumulation was equal to the overall rate of ablation for several years.  Eventually, the precipitation happened to be greater one year, causing the overall accumulation to become greater than the overall ablation.  The overall size of the glacier grew, and the terminus advanced.  This is called a glacial surge.

 

There are two forms of glacial weathering (glacial degradation): glacial abrasion and glacial plucking.  Glacial abrasion is the smoothing of rocks by the glacier as it flows downhill.  An alpine glacier may smooth the shapes of entire mountain ranges through glacial abrasion, turning V-shaped valleys into U-shaped valleys.  A U-shaped valley is more properly called a glacial trough.  A glacial trough that has been drowned by rising sea levels is called a fjord.  Glacial plucking is the ripping of rocks out of landscapes/environments.  The glacier then erodes (transports or moves) these rocks downhill.  As glaciers degrade valleys, they may leave behind a narrow and sharp ridge of mountains called an aręte, which is a French word used to describe anything shaped narrow and sharp, such as a fishbone or a knife.  Glaciers may also leave behind a single steep mountain after the glacier has degraded the surrounding landscapes/environments.  Such a steep mountain is called a glacial horn.

 

The rocks and sediments eroded by rivers is called fluvial load, and the sediments eroded by wind is called aeolian load.  Unfortunately, the rocks and sediments eroded by glaciers is not called glacial load; it is called glacial drift.  The use of the word drift is rooted in the Biblical story of Noah’s flood.  Whenever geomorphologists encountered large boulders that did not match surrounding landscapes/environments, they believed that these boulders drifted in the great flood that the Bible records covered the entire world.  Geomorphologists eventually understood that these boulders were moved by glaciers, not a great flood.  Nevertheless, the word drift became engrained in the vocabulary.  Adding the adjective glacial before the noun drift helps to make this term glacial drift at least half-correct.  In brief, the rocks and sediments that glaciers erode is called glacial drift.  A large boulder that does not match the surrounding landscapes/environments is called a glacial erratic.

 

We now discuss glacial aggradation.  Rocks and sediments that are directly deposited by a glacier is called glacial till.  Since glaciers have sufficient energy to erode large rocks as well as small sediments, glacial till is heterogeneous (poorly sorted).  When the glacier melts, the meltwater itself carries sediments and deposits them at other landscapes/environments.  Sediments that are deposited by this glacial meltwater is called stratified drift.  We may argue that glacial meltwater is simply running water, and therefore stratified drift is actually caused by a fluvial process.  This is precisely why stratified drift is also called glaciofluvial sediment.  Glacial meltwater has insufficient energy to erode large rocks; only small sediments are eroded by glacial meltwater.  For this reason, stratified drift is more homogenous (well sorted) as compared with glacial till, which is more heterogeneous (poorly sorted).

 

A moraine is a layer or ridge of sediment that is either drift being eroded by a glacier or till that has been deposited by a glacier.  There are several different types of moraines.  Lateral moraines are along the side edges of glaciers.  Medial moraines are down the middle of glaciers and form from the merging of two lateral moraines.  End moraines are deposited by stationary termini of glaciers, while ground moraines are deposited by retreating termini of glaciers.

 

After a glacier melts, a lake may be all that is left of the glacier.  A lake composed of glacial meltwater is called a tarn.  A string of tarns is called paternoster lakes, since monks believed that they resembled a string of rosary beads.  The word paternoster is Latin for Our Father, which is one of the prayers recited when praying with rosary beads.  After a glacier melts, all that is left of the head of the glacier is an amphitheater-shaped excavation at the top of the mountain.  This is called a cirque.

 

 

 

copyeditor: Michael Brzostek (Spring2023)

 

 

 

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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