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
Fall 2018
Fifth (Final) Examination lecture notes
All landscapes/environments
on planet Earth can be divided 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 bottom of the oceans. Transitional landscapes/environments are
shores (where oceans meet continents).
Most of the landscapes/environments on planet Earth are marine, since
most of planet Earth is covered with oceans.
Unfortunately, it is precisely for this reason that much is not known
about marine landscapes/environments. We
discussed transitional landscapes/environments (shores) earlier in the
course. Therefore, we will spend the
rest of the course 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 due to the polar highs. Putting all of this together, 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 thirty degrees latitude in both hemispheres
are hot deserts, such as the Basin and Range in southwestern United States and
northwestern Mexico, 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. Since it is cold and
humid at and near the subpolar lows, most terrestrial landscapes/environments
at and near sixty degrees latitude in the northern hemisphere are boreal
forests (taigas), such as the Canadian boreal forests and the Russian boreal
forests. There are no terrestrial
landscapes/environments at and near sixty degrees latitude in the southern
hemisphere, but theoretically any such terrestrial landscapes/environments
would also be boreal forests (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 Antarctica at the
South Pole and 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 than 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. After steppes are deserts, the most arid
(least humid) terrestrial landscapes/environments. Suppose we 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. 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.
Beginning at the tropical
forests at the Equator and travelling 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 (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 (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 (summer and winter) 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, the midlatitude
seasons may be interpreted 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 21 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 21 every year and arriving on top of the Tropic of
Capricorn (roughly 23 ˝ degrees south latitude) three months later on roughly
December 21 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 21 every year and finally returning to the Tropic
of Cancer three months later on roughly June 21 every year. This apparent motion of the Sun may be
interpreted as causing the midlatitude seasons
(summer and winter). Note that summer is
during June, July, and August in the midlatitudes of
the northern hemisphere while summer is during December, January, and February
in the midlatitudes of the southern hemisphere. Conversely, note that winter is during
December, January, and February in the midlatitudes
of the northern hemisphere while winter 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 summer and winter at
the tropical latitudes. The corresponding
seasons at the tropical 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.
At and near the poles in both
hemispheres, temperatures are cold throughout the year. Therefore, we do not discuss the seasons as
being summer and winter at the polar latitudes.
The corresponding seasons at the polar latitudes are the daytime season
and the nighttime season, since entire months can pass with nearly-continuous
daytime or nearly-continuous nighttime 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.
As the Sun appears to move
relative to the Earth throughout the year, the humidity of forests or the
aridity of deserts (as the case may be) moves with the Sun. In other words, 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 situation seems to be reversed in the two hemispheres, 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 tropical
latitudes in both hemispheres.
As another example, 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 terrestrial landscapes/environments at the midlatitudes
in the southern hemisphere. During June,
July, and August, the humidity of the 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 situation seems to
be reversed in the two hemispheres, 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 southern hemisphere, since there are no terrestrial
landscapes/environments at these latitudes in the southern hemisphere.
As yet another example,
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
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 boreal
forests at the subpolar low (at roughly sixty degrees south latitude) intrudes
to the south, causing more humid daytime seasons. Although the situation seems to be reversed
in the two hemispheres, 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 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. As a result, there are many exceptions to the
preceding discussion. This having been
said, the preceding discussion is mostly correct. This reveals that latitude is the primary
variable determining the characteristics of terrestrial
landscapes/environments, while ocean-current effects, marine-versus-continental
effects, and orographic effects are secondary (but still important) variables
determining the characteristics of terrestrial landscapes/environments.
High mountaintops are
terrestrial landscapes/environments that belong in their own 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. If the mountain is tall enough,
we would encounter subpolar woodlands at higher elevations and subpolar shrublands at even higher elevations. If the mountain is tall enough, we reach an
elevation where bushes and shrubs can no longer grow. This is the timberline, more commonly known
as the tree line. At higher elevations
we encounter subpolar grasslands, and at even higher elevations we 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! 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 claim 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 latitude and elevation up a mountain is 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.
Geomorphology is the study of
how and why landscapes/environments change, and a geomorphologist is someone
who studies the evolution of landscapes/environments. A geomorphic agent is anything that changes
landscapes/environments. For example,
wind can change landscapes/environments.
The technical term for the action of wind as a geomorphic agent is aeolian processes, named after Aeolus the Greek god of
wind. Gravity can change
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 (rivers and streams) can change landscapes/environments. The technical term for the action of running
water as a geomorphic agent is fluvial processes. Underground water can change
landscapes/environments. The technical
term for the action of underground water as a geomorphic agent is groundwater
processes. Glaciers (giant masses of
ice) can 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 rivers and streams.
When any geomorphic agent
changes a landscape/environment, it first destroys the existing
landscape/environment. This is called
degradation. More technically,
degradation is the action of breaking down rocks into sediments. Next, the geomorphic agent transports (moves)
these sediments from one location to another.
This is called erosion. Finally,
the geomorphic agent uses these sediments to build a new
landscape/environment. This is called
aggradation. To summarize, any
geomorphic agent first degrades a landscape/environment into sediments, then
erodes these sediments to another location, then finally aggrades a new
landscape/environment from these 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 frost
wedging, salt wedging, and exfoliation.
There are many examples of chemical weathering, including carbonation, oxidation,
and hydrolysis.
Chemical weathering requires
some understanding of acids and bases.
According to the Arrhenius theory of acids and bases, an acid is a
solution with an excess of hydronium cations (H3O+),
while a base is a solution with an excess of hydroxide anions (OH–). Also according to this Arrhenius theory, the
reaction of an acid and a base yields a salt plus water. 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 acid’s conjugate base plus the base’s conjugate acid. 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 we 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. In other words, even in the complete absence
of human pollution, naturally-occurring unpolluted
rain water is actually carbonic acid.
There are still 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).
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
the 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 “fresh” water to
provide a steady source of food, and agriculture is impossible without
soil. For this reason, a discussion of
soil sciences is now in order.
Soil is a particular type of
regolith that is a mixture of inorganic minerals, air, water, and organic
materials (both living and decomposing).
Recall that the geosphere is the solid inorganic part of the Earth, the
atmosphere is the sum total of all air of the Earth, the hydrosphere is the sum
total of all water of the Earth, and the biosphere is the sum total of all life
of the Earth. Since soil is a mixture of
inorganic minerals, air, water, and organic materials (both living and
decomposing), this means that soil is the intersection of the geosphere, the
atmosphere, the hydrosphere, and the biosphere.
The sum total of all soil of the Earth is the pedosphere. In other words, the pedosphere is where all
the other Earth spheres meet one another.
Recall that according to the
Wentworth Scale, the largest sediments are gravels, smaller sediments are sand,
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 (sand). Soil will feel more fine-grained if its
mineral components are small (clay/mud).
Soil will feel intermediate between these extremes if its mineral
components are between large and small (silt).
Soil with roughly equal mixtures of sand, silt, and clay/mud is called
loam.
The classification of the
water part of soil is as follows. Water
falling into the soil from rain for example is called free water. This free water may then begin to attract
itself. 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 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 material is called illuviation. To summarize, just as geomorphological
processes in general begin with degradation then follow with erosion then end
with aggradation, geomorphological processes in soil begin with eluviation then
follow with leaching then end with 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. This O horizon decomposes and
ultimately creates the A horizon. To
summarize, first we have the O horizon below which 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.
Although the quality of the
parent material does affect whether or not soil will be agriculturally
productive, the most important factor affecting soil quality 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 make clear
which climates are good for agriculture and which climates are bad for
agriculture in a moment.) Another factor
affecting soil quality is the topography of the land. Yet another factor affecting soil quality is
the abundance of living plants and animals within the soil. Such lifeforms 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, extract water and minerals for their survival from the
soil, and excrete water and minerals harmful to their survival back into the
soil. As a result, earthworms can
completely rejuvenate dead soil in a relatively short amount of time. It is not an exaggeration to say that the
health of soil can be determined by digging up some of
the soil and simply counting the number of living earthworms within it. Rapid bioturbation of soil is essential to
rejuvenating dead soil quickly so that farmers are able to
continually produce large amounts 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.
Taxonomy is the study of
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, by the time we get to biological species there have been so many
subdivisions that there are millions of biological species that have been
classified. Soil taxonomy is very much
similar. 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, by the time we get to soil series there have been so
many subdivisions that there are more than nineteen thousand soil series that
have been classified. Whereas it is
impossible to discuss all of the soil series, discussing all twelve of the 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 words recent and soil. 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, aridsols,
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 aridsols are soils found in deserts. The word aridsol is derived from the word arid, which means dry. The gelisols are
the soils above permafrost. Eluviation,
leaching, and illuviation are not primarily due to bioturbation in gelisols but due to cryoturbation, also known as frost
churning. The histosols
are organically-rich soils found in wetlands (swamps, marshes, and
quagmires). Now suppose these young
soils are subjected to even more eluviation, leaching, and illuviation. The soil thus develops mature horizons, and
these soils are the best soils for agriculture.
There are two soil orders in this category: the alfisols
and the mollisols.
These two soil orders are found abundantly in the midwestern
United States, which is precisely why the midwestern
United States produces enough food to feed the entire world. These soils are so rich that they have a deep
black color. You are strongly urged to
take a road trip across the United States so that you can see these soils with
your own eyes. You will also see with
your 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 words ultimate and soil.
Unfortunately, this is not exactly correct since this soil is not quite
a dead soil. 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. Since woodlands are just outside of forests,
the spodosols and the 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 the andisols, aridsols, gelisols, histosols, inceptisols, and vertisols, the mature soil orders that are best for agriculture
are the alfisols and mollisols,
the dying yet still alive soil orders are the spodosols
and ultisols, and the oldest soil order is the oxisols. The best
soils for agriculture are the alfisols and the mollisols, while the worst soils for agriculture are the entisols (since these are the newly born soils) and the oxisols (since these are the dead soils).
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. The regolith will move downward.
The fastest type of
mass-wasting event is called a fall. Somewhat
slower than a fall is a slide or flow.
Finally, 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. 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,
and 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.
The action of rivers and
streams 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 rivers
and streams. This is remarkable
considering that water in rivers and streams makes up 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 three percent is in giant glaciers called continental ice
sheets. 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, the 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.
We begin our discussion of
fluvial processes with a basic discussion of running water. Precipitation on or near a mountaintop or
glacial meltwater near a mountaintop begins the river. This is the head of the river. Water flows down the mountain along rills
that merge to form brooks or creeks.
These brooks or creeks eventually merge to form streams which eventually
merge to form rivers. 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, the head of the river is rills
that merge to form brooks or creeks that merge to form streams that merge to
form rivers that merge to form large 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 word dendritic means treelike. If you were to look at a dendritic drainage
pattern from an airplane flying overhead, the river would look like the trunk
of a tree, the streams would look like large branches attached to the trunk,
and the brooks or creeks would look like small branches attached to the large
branches. There are other drainage
patterns as well. 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. The opposite of a perennial river is an
ephemeral river, more commonly known as a temporary river. The drainage pattern associated with
ephemeral rivers is the interior drainage pattern.
The total area of land that
feeds water into a watercourse is the watershed of the watercourse. A watershed is also known as a drainage
basin. 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, it is not an exaggeration to say
that the Mississippi River watershed is 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 feeds one ocean while the other enormous watershed
often feeds a different ocean.
The river gradient is
commonly considered to be the steepness of a river. More correctly, 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). 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. The longitudinal profile is a graph of the
river gradient from the head to the mouth.
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
correctly, the river discharge is the volume of water flowing per unit
time. 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 it downhill.
Through the action of both
mechanical weathering and chemical weathering, running water carves out a river
channel for itself. The sides of the
river channel are called the river banks, and the bottom of the river channel
is called the river bed.
The rocks and sediments that
a river erodes 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
river bed). 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
completely dissolved within the 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.
These two concepts sound like the same thing, but they are completely
different. 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.
The first river has such a large competence that it is able to erode bed
load. The river may even look brown and
dirty from the large load being eroded.
Now suppose the second river has such a low competence that it is only
able to erode solution load, which cannot be seen since it is completely
dissolved. The river may actually look
clean and clear. Although the first
river definitely has a larger competence and the second river definitely has a
smaller competence, no conclusion can be drawn about the capacities. It is entirely possible that the total amount
of solution load of the second river may add up to a larger amount than the bed
load in the first river. In other words,
a river that looks clean and clear may in fact be eroding a large amount of
load. To go further, 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 thorough
discussion of fluvial geomorphology.
Throughout this discussion, we will appreciate how dramatically running
water changes landscapes/environments.
First, the running water will try to carve out a longitudinal profile so
that it will take 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 will 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. The running water carves out the arc of an
inverted cycloid until it encounters more weathering-susceptible rock, at which
point the water will dig 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
happens again. We now see that the
longitudinal profile is actually a collection of inverted cycloids, and two
neighboring cycloids will 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 is called a waterfall, since the water falls
downward. 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 smooth out 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.
Once the longitudinal profile
is fairly developed, the energy of the running water begins to develop the
lateral profile. In other words, the
running water begins to change 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, and the narrow river valley
eventually becomes a wide river valley.
Now suppose at the bottom of this river valley the river channel is
straight. The water in this straight
channel will not flow straight however, since it must maneuver around sand
deposits within the river channel. The
flow therefore follows a curved path, even though the river channel is
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 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 leaving the 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 word
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 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 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 it 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
but the load that it erodes also overflows.
Some of this load may be deposited on the river banks. With each flooding, more and more of these
rocks and sediments accumulate on the river banks until walls of rock have been
built 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 are built by humans. There
is, however, a disadvantage of natural levees.
If there happens to be so much flooding one year 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 major river since it is
being prevented from being a tributary to the major river. The word 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 are carved out by humans. A natural cutoff enables the 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 most destructive of all natural disasters.
Humans attempt to control flooding through artificial levees and
artificial cutoffs. The digging 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.
Most deaths due to flooding in the United States are
automobile-related. For your own safety,
never ever drive through a road that is flooded. Wait for the flood to completely subside first. Again, most deaths in the United States due
to flooding, which is itself the most common and most destructive of all
natural disasters, is automobile-related.
Groundwater processes is
another 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 a rock is able to
transmit water through it is called the permeability of the rock. These two concepts sound like the same thing,
but they are different. 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, and the vast majority of all groundwater is of this type.
If you were to drill into the
ground, you 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 filled with water (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).
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 certain
landscapes/environments where groundwater processes are more important as
compared with other geomorphic agents. 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. Speleology is
the study of caverns (caves), and a speleologist is someone who studies caverns
(caves). A speleothem is any rock formation whatsoever found within
caverns. Long and skinny speleothems
that grow from cavern ceilings are called stalactites, while short and fat
speleothems that grow from cavern grounds are called stalagmites. A stalactite and a stalagmite may eventually
merge becoming one speleothem connecting the cavern ceiling to the cavern
ground. These speleothems are called
columns.
Aeolian processes is another
important geomorphic agent. This is the
action of wind as a geomorphic agent.
Aeolian processes are more important in deserts as compared with other
geomorphic agents, although it should again be emphasized that fluvial
processes is the single most important geomorphic agent, even in deserts. For example, an ephemeral river that forms
after rain may last only a few minutes, but it nevertheless
sculpts the desert more than the action of the wind over the rest of the entire
year. Nevertheless, whereas aeolian processes may not be the most important geomorphic
agent in deserts, aeolian processes are more
important 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 about ten percent of the Sahara is covered with sand;
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 one-third
covered with sand; two-thirds of the Arabian Desert is not covered with
sand. The parts of a desert covered with
solid rocks are called hamadas, the parts of a desert
covered with gravels are called regs, and the parts
of a desert covered with sands are called ergs.
Just as the sediments eroded
by running water is called fluvial load, the sediments eroded by wind is called
aeolian load.
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 as wind blows the sands roll, slide,
or bounce 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 small rock that has been
smoothed by the action of the wind is called a ventifact,
and 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 the plateau, it becomes smaller and smaller over time. The plateau is worn down to a mesa. As winds continue to degrade the mesa, it
becomes smaller and smaller over time.
The mesa is worn down to a butte.
To summarize, aeolian processes degrade
plateaus into mesas and degrade mesas into buttes.
The final geomorphic agent to
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 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 this course,
planet Earth entered into an ice age roughly thirty million years ago that
continues to this day. 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 this planet.
One covers the continent Antarctica at the South Pole. This continental ice sheet is the single
largest mass of ice in the entire world; ninety percent of all the ice in the
cryosphere makes up that continental ice sheet.
The remaining 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 add up to a small fraction of the ice in the
cryosphere. Continental ice sheets are
so enormous that entire mountain ranges are buried beneath them. These buried mountain ranges are called nunataks.
Continental ice sheets expand over the ocean waters that surround the
continent or microcontinent. These
intrusions are called ice shelves. The
Ross Ice Shelf is the largest ice shelf in the world. Pieces of an ice shelf may break off
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 interesting, there are only two continental ice
sheets on this planet presently, and there would be none if planet Earth were
not presently within an ice age. There
are thousands of alpine glaciers all over planet Earth on every continent
(except Australia), and these alpine glaciers would exist whether or not planet
Earth were within an ice age. Therefore,
we will henceforth concentrate our attention on alpine glaciers.
We begin with the formation
of a glacier. Consider precipitation
upon a high mountaintop. The mountaintop
is so high that the precipitation is solid.
This precipitation is called normal ice.
As normal ice accumulates, the weight of the abundance of this normal
ice begins to squeeze out the air within itself. The ice eventually becomes firn, a high-density form of ice with the consistency of
course 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 made of glacial ice, the uppermost layer of a glacier is made of
normal ice since it 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 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 processes 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, the terminus is said to be advancing. When a glacial terminus moves up a mountain,
the terminus is said to be retreating.
When a glacial terminus does not move, the terminus is said to be
stationary. How can ice flow
uphill? In fact, the glacial ice within
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; while the terminus retreats, the
markers still move downhill. 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
glacial ice 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
glacial ice became stuck and then finally became unstuck and flowed
downhill. The glacial ice continued to
flow downhill all the years the terminus was stationary. What actually happened is the overall rate of
accumulation was equal to the overall rate of ablation for several years. Finally, 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 as the glacier flows over them. An alpine glacier may smooth the shapes of
entire mountain ranges through this process, 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 surrounding landscapes/environments. The rocks are then moved by the glacier.
As glaciers degrade valleys,
they may leave behind a sharp and narrow ridge of mountains. This is called an aręte, which is a French
word meaning not only ridge but sharp knife as well. Glaciers may also leave behind a single steep
mountain after it has degraded the landscapes/environments around it. This 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.
This term derives from 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
finally understood that this is not correct; the boulders were moved by
glaciers, not a great flood.
Nevertheless, the word drift became stuck in the vocabulary. Adding the word glacial before the word drift
helps to make this term at least half correct.
To summarize, the rocks and sediments that glaciers erode is called
glacial drift. Large boulders that do
not match surrounding landscapes/environments are called glacial erratics.
We now discuss glacial
aggradation. Glacial till is rocks and
sediments that are directly deposited by a glacier. Since glaciers are strong enough to erode
large rocks as well as small sediments, glacial till is more heterogeneous
(poorly sorted). When the glacier melts,
the meltwater itself carries sediments and deposits them elsewhere. Stratified drift is sediments that are
deposited by this glacial meltwater. We
may argue that glacial meltwater is simply running water and therefore should be considered a fluvial process. This is precisely why stratified drift is
also called glaciofluvial sediment. Since glacial meltwater is too weak to erode
large rocks, only small sediments are eroded by glacial meltwater. For this reason, stratified drift is more
homogenous (well 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 edges of glaciers.
Medial moraines are down the middle of glaciers and result 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.
This is a tarn. A string of tarns
is called paternoster lakes, since they appeared to be a string of rosary beads
to monks who saw them. 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.
New Jersey Institute of Technology
College of Science and Liberal Arts at NJIT
Department of Physics at CSLA at NJIT
Libarid A. Maljian at web.njit.edu
Libarid A. Maljian at the Department of Physics at CSLA at NJIT
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