What is geomorphology?
· Definition:
The study of the landscape: Geomorphology entails the systematic description of
landforms, the analysis of the processes that form them, as well as
understanding the function landforms and their response to changes in energy
(Geo, G. the Earth; Morph, G. Form, ology G. the science of)
· Landscape:
Mountainous Terrain--the combined effect of numerous landforms
· Landform:
An individual feature--a slope, valley or mountain
· Geomorphology
draws upon all fields of geology, such as: structural geology, geophysics,
mineralogy, petrology, sedimentation and stratigraphy, hydrology, glacial
geology, paleoclimatology, etc.
What produces a landscape?
· A landscape is
the product of the interaction between the follow factors through time:
· Energy:
Driving forces behind geomorphic change; drives the hydrologic cycle, chemical
reactions on the earth's surface, uplift, subsidence, etc.
· Solar energy:
2 cal/cm2-min reaches the outer atmosphere; 30 to 14 % is absorbed
into the system--depending on latitude
· Geothermal
energy : derived from decay of radioactive elements and residual heat :
gradient ± 20-30°C/km (crust)
· Gravity
:g= acceleration due to gravity=GM/r2 =9.80 m/s2
·
Resisting framework: lithology and structure
· Structure:
Defines the grain of the topography (joints, fold patterns, layering,
arrangement of rocks of varying resistance)
· Process: The
manner in which the ambient forces are applied to cause change. Processes
(endogenic and exogenic) are understood by applying the Law of
Uniformitarianism
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Important considerations
· Scale of the
system: Relationship between size and duration
· Geologic history
(relaxation time of past events)
· Magnitude and frequency
of events affecting the system
· Geologic
inhomogeneities (differences in structure, lithology, climate, etc.)
Some general classification
landforms based on processes
·
Constructional vs Destructional
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The landforms that are found on the surface of the Earth can be grouped into
4 categories:
(1) Structural Landforms - landforms that are
created by massive earth movements due to plate tectonics. This includes
landforms with some of the following geomorphic features: fold mountains, rift
valleys, and volcanoes.
(2) Weathering Landforms - landforms that
are created by the physical or chemical decomposition of rock through weathering.
Weathering produces landforms where rocks and sediments are decomposed and
disintegrated. This includes landforms with some of the following geomorphic
features: karst, patterned ground, and soil profiles.
(3) Erosional Landforms - landforms formed
from the removal of weathered and eroded surface materials by
wind, water, glaciers, and gravity. This includes landforms with some of the
following geomorphic features: river valleys, glacial valleys, and coastal
cliffs.
(4) Depositional Landforms - landforms formed
from the deposition of weathered and eroded surface
materials. On occasion, these deposits can be compressed, altered by pressure,
heat and chemical processes to become sedimentary rocks. This includes
landforms with some of the following geomorphic features: beaches, deltas,
flood plains, and glacial moraines.
Other Fundamental concept of geomorphology
· Time and history:
landforms evolve through time. The age of the structure, length of time one or
more processes have been in effect, and the sequences of geologic events all
play a role in the evolution of a landscape.
· Response:
Landforms are part of a system that reacts and changes to external forces.
·
Related concepts:
· Lag time:
The time it takes for a landform to change in respond to a new set of
conditions
· Relict landform:
A landform formed under a previous condition. (e.g.
·
Equilibrium (self-regulation, homeostasis)
· Perception of
equilibrium state is a function of time:
·
Static Equilibrium: no perceived change
· Steady State
Equilibrium: Fluctuation about a mean
· Dynamic
Equilibrium: Fluctuation about a moving average
· Dynamic
Metastable Equilibrium: Fluctuation about a moving average marked by
discontinuities
·
Feedback:
·
negative feedback: reduces or alters
·
positive feedback : enhances or exacerbates
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Evolution of Geomorphic Theory and the introduction of important concepts
· Catastrophism: 18th
and 19th centuries: Landscape had an innate permanence changed only by
catastrophic events.
· Uniformitarianism (Hutton, 1785;
Playfair, 1802, Illustrations of the Huttonian Theory of the Earth, Lyell, 1830,
Principles of Geology:(cf. Darwin, 1859, Origin of the Species).
· The present
landscape can be explained by processes now observable.
· Geologic
exploration by post Civil War geologist, western US--These geologists laid
the ground work for
· C Gilbert
(1843-1918) introduced the concept of self-regulating equilibrium landforms,
such as graded streams. [G. C. Gilbert, 1914, Report on the Geology of
the
· John Wessley Powell (1834-1902)--descriptive
classification of streams; concept of base level; elaborated on the progressive
erosion of mountain ranges
·
Glaciation, Louis Agassiz, 1840, Etudes sur les Glaciers--made popular
the theory of continental glaciation
·
Landscape evolution (Historical approaches)
· William Morris Davis
(1850-1934): cycles of erosion
· Walther Penck:
The relative rates of processes (e.g. rate of uplift vs rate of denudation)
controls landscape morphology (1894)
· Morphometrics:
The application of statistics and mathematics to the analysis of landforms
· e.g.Horton, 1954,
Erosional development of streams and their drainage basins: GSA Bulletin, v.
82, p. 275-370.
·
Systems approach:
· Modern geomorphologist
view a landform assemblage as an intricate system that can be studied by
analyzing the variables or components that compose it.
· The forces
producing change (e.g. energy), the materials upon which the forces act (and
their resistance to change), and the processes by which the change is produced
are all considered.
·
Viewing landscapes as fractals:
· Fractals: Objects
exhibiting increasing detail with increasing magnification. Fractals are self
similar in that the pattern viewed under magnification is similar to that of
the whole.
· All landscapes
have fractal elements.--That's why a scale is important
· Examples:
drainage networks, coastlines, sedimentary layers, etc.
Weathering is the breakdown and alteration of rocks and minerals
at or near the Earth's surface into products that are more in equilibrium with
the conditions found in this environment. Most rocks and minerals
are formed deep within the Earth's crust where temperatures and pressures
differ greatly from the surface. Because the physical and chemical nature of
materials formed in the Earth's interior are characteristically in
disequilibrium with conditions occurring on the surface. Because of this
disequilbrium, these materials are easily attacked, decomposed, and eroded by
various chemical and physical surface processes.
Weathering is the first step for a number of other geomorphic and
biogeochemical processes. The products of weathering are a major source of
sediments for erosion and deposition. Many types of sedimentary
rocks are composed of particles that have been weathered, eroded,
transported, and terminally deposited in basins. Weathering also contributes to
the formation of soil by providing
mineral particles like sand, silt, and clay. Elements
and compounds extracted from the rocks and minerals by weathering
processes supply nutrients for plant uptake. The fact that the oceans are saline
in the result of the release of ion salts from rock and minerals on the
continents, leaching and runoff transport these ions from land to the ocean basins
where they accumulate in the seawater. In conclusion, weathering is a process
that is fundamental to many other aspects of the hydrosphere, lithosphere,
and biosphere.
There are three broad categories of mechanisms for weathering: chemical,
physical and biological.
Products of Weathering
The process of weathering can result in the following three outcomes on rocks
and minerals:
(1). The complete loss of
particular atoms or compounds from the weathered surface.
(2). The addition of specific
atoms or compounds to the weathered surface.
(3). A breakdown of one mass
into two or more masses, with no chemical change in the mineral or rock.
Chemical Weathering
Chemical weathering involves the
alteration of the chemical and mineralogical composition of the weathered
material. A number of different processes can result in chemical weathering.
The most common chemical weathering processes are hydrolysis, oxidation,
reduction, hydration, carbonation, and solution.
Hydrolysis is the weathering
reaction that occurs when the two surfaces of water and compound meet. It
involves the reaction between mineral ions and the ions of water
(OH- and H+), and results in the decomposition of the rock surface by
forming new compounds, and by increasing the pH of the solution involved
through the release of the hydroxide ions.
Oxidation is the reaction
that occurs between compounds and oxygen.
Reduction is simply the
reverse of oxidation, and is thus caused by the addition of one or more
electrons producing a more stable compound.
Hydration involves the rigid attachment of H+
and
Carbonation is the reaction of
carbonate and bicarbonate ions with minerals. The formation of
carbonates usually takes place as a result of other chemical processes.
Carbonation is especially active when the reaction environment is abundant with
carbon dioxide. The formation of carbonic acid, a product of carbon dioxide and
water, is important in the solution of carbonates and the decomposition of
mineral surfaces because of its acidic nature.
Water and the ions
it carries as it moves through and around rocks and minerals can further the
weathering process. Geomorphologists call this phenomena solution. The
effects of dissolved carbon dioxide and hydrogen ions in water have already
been mentioned, but solution also entails the effects of a number of other
dissolved compounds on a mineral or rock surface. Molecules can mix in solution
to form a great variety of basic and acidic decompositional compounds. The most
important factor affecting all of the above mentioned chemical weathering
processes is climate. Climatic conditions control the rate of weathering that
takes place by regulating the catalysts of moisture and temperature.
Physical Weathering
Physical weathering is the breakdown
of mineral or rock material by entirely mechanical methods brought about by a
variety of causes. Some of the forces originate within the rock or mineral,
while others are applied externally. Both of these stresses lead to strain and
the rupture of the rock. The processes that may cause mechanical rupture are abrasion,
crystallization, thermal insolation, wetting and drying,
and pressure release.
Abrasion occurs when some
force causes two rock surfaces to come together causing mechanical wearing or
grinding of their surfaces. Collision between rock surfaces normally occurs
through the erosional transport of material by wind, water, or ice.
crystallization can cause the
necessary stresses needed for the mechanical rupturing of rocks and minerals.
insolation weathering, The physical breakdown of rock by their expansion
and contraction due to diurnal temperature changes is one of the most keenly
debated topics in rock weathering research. Known as insolation weathering,
it is the result of the physical inability of rocks to conduct heat well. This
inability to conduct heat results in differential rates of expansion and
contraction. Thus, the surface of the rock expands more than its interior, and
this stress will eventually cause the rock to rupture. Differential expansion
and contraction may also be due to the variance in the colors of mineral grains
in rock. Dark colored grains, because of their absorptive properties, will
expand much more than light colored grains. Therefore, in a rock peppered with
many different colored grains, rupturing can occur at different rates at the
various mineral boundaries.
Alternate wetting and
drying of rocks, sometimes known as slaking, can be a very important
factor in weathering. Slaking occurs by the mechanism of "ordered
water", which is the accumulation of successive layers of water molecules
in between the mineral grains of a rock. The increasing thickness of the water
pulls the rock grains apart with great tensional stress.
Pressure release of rock can cause
physical weathering due to unloading. The majority of igneous rocks were
created deep under the Earth's surface at much higher pressures and
temperatures. As erosion brings these rock formations to the surface,
they become subjected to less and less pressure. This unloading of pressure
causes the rocks to fracture horizontally with an increasing number of
fractures as the rock approaches the Earth's surface. Spalling, the vertical
development of fractures, occurs because of the bending stresses of unloaded
sheets across a three dimensional plane.
Biological Weathering
Biological weathering involves the
disintegration of rock and mineral due to the chemical and/or physical agents
of an organism. The types of organisms that can cause weathering range from bacteria
to plants to animals.
Biological weathering involves
process that can be either chemical and physical in character. Some of the more
important processes are:
1. Simple breaking of
particles, by the consumption of soils particles by animals. Particles can also
fracture because of animal burrowing or by the pressure put forth by growing
roots.
2. Movement and mixing of
materials. Many large soil organisms cause the movement of soil particles. This
movement can introduce the materials to different weathering processes found at
distinct locations in the soil profile.
3. Simple chemical processes
like solution can be enhanced by the carbon dioxide produced by respiration.
Carbon dioxide mixing with water forms carbonic acid.
4. The complex chemical effects
that occur as a result of chelation. Chelation is a biological process
where organism produce organic substances, known as chelates, that have
the ability to decompose minerals and rocks by the removal of metallic cations.
5. Organism can influence the
moisture regime in soils and therefore enhance weathering. Shade from aerial
leaves and stems, the presence of roots masses, and humus all act to
increase the availability of water in the soil profile. Water is a necessary
component in several physical and chemical weathering processes.
6. Organisms can influence the
pH of the soil solution. Respiration from plant roots releases
carbon dioxide. If the carbon dioxide mixes with water carbonic acid is formed
which lowers soil pH. Cation exchange reactions by which plants absorb
nutrients from the soil can also cause pH changes. The absorption processes
often involves the exchange of basic cations for hydrogen ions.
Generally, the higher the concentration of hydrogen ions the more acidic
a soil becomes.
Erosion is defined as the
removal of soil, sediment,
regolith, and rock
fragments from the landscape. Most landscapes show obvious evidence of erosion.
Erosion is responsible for the creation of hills and valleys. It removes
sediments from that were once glaciated, shapes the shorelines of lakes and
coastlines, and transports material downslope from elevated sites. In order for
erosion to occur three processes must take place: detachment, entrainment
and transport. Erosion also requires a medium to move material. Wind,
water, and ice are the mediums primarily responsible for erosion. Finally, the
process of erosion stops when the transported particles fall out of the
transporting medium and settle on a surface. This process is called deposition.
Energy of Erosion
The energy for erosion comes from
several sources. Mountain building creates a disequilibrium within the Earth's
landscape because of the creation of relief. Gravity acts to vertically move
materials of higher relief to lower elevations to produce an equilibrium.
Gravity also acts on the mediums of erosion to cause them to flow to base
level.
Solar radiation and its influence
on atmospheric processes is another source of energy for erosion. Rainwater has
a kinetic energy imparted to it when it falls from the atmosphere.
Snow has potential energy when it is deposited in higher elevations.
This potential energy can be converted into the energy of motion when the snow
is converted into flowing glacial ice. Likewise, the motion of air because of
differences in atmospheric pressure can erode surface material when velocities
are high enough to cause particle entrainment.
The Erosion Sequence
Erosion can be seen as a
sequence of three events: detachment, entrainment, and transport.
These three processes are often closely related and sometimes not easy
distinguished between each other. A single particle may undergo detachment,
entrainment, and transport many times.
Detachment
Erosion begins with the detachment
of a particle from surrounding material. Sometimes detachment requires the
breaking of bonds which hold particles together. Many different types of bonds
exist each with different levels of particle cohesion. Some of the strongest
bonds exist between the particles found within igneous rocks. In these
materials, bonds are derived from the growth of mineral crystals during
cooling. In sedimentary rocks, bonds are weaker and are mainly caused by the
cementing effect of compounds such as iron oxides, silica, or calcium. The
particles found in soils are held together by even weaker bonds which result
from the cohesion effects of water and the electro-chemical bonds found in clay
and particles of organic matter.
Physical, chemical, and biological weathering act to weaken the
particle bonds found in rock materials. As a result, weathered materials are
normally more susceptible than unaltered rock to the forces of detachment. The
agents of erosion can also exert their own forces of detachment upon the surface
rocks and soil through the following mechanisms:
Plucking: ice freezes onto
the surface, particularly in cracks and crevices, and pulls fragments out from
the surface of the rock.
Cavitation: intense erosion
due to the surface collapse of air bubbles found in rapid flows of water. In
the implosion of the bubble, a micro-jet of water is created that travels with
high speeds and great pressure producing extreme stress on a very small area of
a surface. Cavitation only occurs when water has a very high velocity, and
therefore its effects in nature are limited to phenomenon like high waterfalls.
Raindrop impact: the force of a
raindrop falling onto a soil or weathered rock surface is often sufficient to
break weaker particle bonds. The amount of force exerted by a raindrop in a
function of the terminal velocity and mass of the raindrop.
Abrasion: the excavation of
surface particles by material carried by the erosion agent. The effectiveness
of this process is related to the velocity of the moving particles, their mass,
and their concentration at the eroding surface. Abrasion is very active in
glaciers where the particles are firmly held by ice. Abrasion can also occur
from the particles held in the erosional mediums of wind and water.
Entrainment
Entrainment is the process of
particle lifting by the agent of erosion. In many circumstances, it is hard to
distinguish between entrainment and detachment. There are several forces that
provide particles with a resistance to this process. The most important force is
frictional resistance. Frictional resistance develops from the interaction
between the particle to its surroundings. A number of factors increase
frictional resistance, including: gravity, particle slope angle relative
to the flow direction of eroding medium, particle mass, and surface
roughness.
Entrainment also has to
overcome the resistance that occurs because of particle cohesive bonds. These
bonds are weakened by weathering or forces created by the erosion agent (abrasion,
plucking, raindrop impact, and cavitation).
Entrainment Forces
The main force reponsible for
entrainment is fluid drag. The strength of fluid drag varies with the
mass of the eroding medium (water is 9000 times more dense than air) and its
velocity. Fluid drag causes the particle to move because of horizontal force
and vertical lift. Within a medium of erosion, both of these forces are
controlled by velocity. Horizontal force occurs from the push of the agent
against the particle. If this push is sufficient to overcome friction and the
resistance of cohesive bonds, the particle moves horizontally. The vertical
lift is produced by turbulence or eddies within the flow that push the particle
upward. Once the particle is lifted the only force resisting its transport is
gravity as the forces of friction, slope angle, and cohesion are now
non-existent. The particle can also be transported at velocities lower than the
entrainment velocities because of the reduction in forces acting on it.
Many hydrologists and
geomorphologists require a mathematical model to predict levels of entrainment,
especially in stream environments. In these highly generalized models, the
level of particle entrainment is relative to particle size and the velocity of
the medium of erosion. These quantitative models can be represented
graphically. On these graphs, the x-axis represents the log of particle
diameter, and the y-axis the log of velocity. The relationship between these
two variables to the entrainment of particles is described by a curve, and not
by a straight line.
The critical entrainment
velocity curve suggests that particles below a certain size are just as
resistant to entrainment as particles with larger sizes and masses. Fine silt and
clay particles tend to have higher resistance to entrainment because of the
strong cohesive bonds between particles. These forces are far stronger than the
forces of friction and gravity.
Transport
Once a particle is entrained, it tends to move as long as the velocity
of the medium is high enough to transport the particle horizontally. Within the
medium, transport can occur in four different ways:
·
Suspension is where the particles are carried by the medium without touching
the surface of their origin. This can occur in air, water, and ice.
·
Saltation is where the particle moves from the surface to the medium in
quick continuous repeated cycles. The action of returning to the surface
usually has enough force to cause the entrainment of new particles. This
process is only active in air and water.
·
Traction is the movement of particles by rolling, sliding, and shuffling
along the eroded surface. This occurs in all erosional mediums.
·
Solution is a transport mechanism that occurs only in aqueous
environments. Solution involves the eroded material being dissolve and carried
along in water as individual ions.
Particle weight, size, shape, surface configuration, and medium type are
the main factors that determine which of these processes operate.
Deposition
The erosional transport of material through the landscape is
rarely continuous. Instead, we find that particles may undergo repeated cycles
of entrainment, transport, and deposition. Transport
depends on an appropriate balance of forces within the transporting medium. A
reduction in the velocity of the medium, or an increase in the resistance of
the particles may upset this balance and cause deposition. Reductions in
competence can occur in a variety of ways. Velocity can be reduced locally by
the sheltering effect of large rocks, hills, stands of vegetation or other
obstructions. Normally, competence changes occur because of large scale
reductions in the velocity of flowing medium. For wind, reductions in velocity
can be related to variations in spatial heating and cooling which create
pressure gradients and wind. In water, lower velocities can be caused by
reductions in discharge or a change in the grade of the stream. Glacial flows
of ice can become slower if precipitation input is reduced or when the ice
encounters melting. Deposition can also be caused by particle precipitation
and flocculation. Both of these processes are active only in water.
Precipitation is a process where dissolved ions become solid because of changes
in the temperature or chemistry of the water. Flocculation is a chemical
process where salt causes the aggregation of minute clay particles into larger
masses that are too heavy to remain suspended.
STREAM FLOW AND FLUVIAL PROCESSES
Streams alter the
Earth's landscape through the movement of water and sediment
Streams are powerful erosive
agents moving material from their bed and banks.
In mountainous regions, stream erosion often produces deep channels
and canyons.
Streams also deposit vast amounts of sediment on the terrestrial landscape and
within lakes
and ocean basins.Geomorphologists
often view streams as systems.
The stream system, like almost all environmental systems, is open to both
inputs and outputs of various types of materials.
The Long Profile of Streams
The topographic long profile or grade of an
average stream is
concave-upwards. At their headwaters, the grade of a
stream is usually steep. As streams get closer to sea-level, the angle of the
grade becomes more gently sloping. Near the mouth of the stream, the
grade becomes almost flat. The grade of a stream develops over thousands and sometimes
millions of years. It is an equilibrium process that attempts to reduce
topographic bumps in the long profile through erosion and deposition.
Stream Discharge
The flow of water through a stream channel is called stream
discharge. In most countries, it is measured in cubic meters per second.
The following equation defines stream discharge mathematically:Q = V x W x D
where Q is the discharge, V is the velocity, W
is the average width and D is the
average depth of the flow. Stream discharge varies over both time and space.
Discharge normally increases downstream as more water enters the stream channel
from runoff and groundwater flow. Discharge
varies temporally because of chaotic behavior of its inputs like precipitation
and snow melt. As discharge increases corresponding changes in velocity,
channel depth and width are made within the stream system. Of the three
variables that change within the stream system with an increase in discharge,
velocity is the least responsive.
Velocity and Turbulence
Because of frictional drag, stream velocity is at a maximum at
the center of the channel near the surface and a minimum near the bed
and banks.
The dynamics of stream flow is primarily influenced by friction,
channel topography and channel shape. Within a stream channel, three types flow
can be observed:
·
Laminar flow - water flow in the stream is not altered in its direction.
Water flows as parallel molecular streams.
·
Turbulent flow - water flows as discrete eddies and vortices.
Caused by channel topography and friction.
·
Helical flow - spiral flow in a stream. Caused by channel shape. Meandering
channels cause this type of flow. Helical flow has an important role in sediment
transport and deposition, and in the creation of point bars.
Finally, flow is not always contained within the stream channel. During
periods of high stream discharge overbank flow may occur. Overbank flow
or flooding involves the spilling of water over the stream's banks and
onto the floodplain.
Sediment Transport
All streams carry sediment. Most of the sediment found in a
stream has been washed into the channel from surface runoff. Sediment is
also added from the erosion of the stream channel bed and banks.
The quantity of sediment in a stream varies temporally due to changes in
discharge. Normally, as discharge and velocity increase, the amount of
sediment being carried by the stream rises correspondingly.
Stream Channel Types
Within a single stream we can often recognize three different channel
types. These unique channel types develop in response to changes in stream
velocity, sediment texture, and stream grade.
Channels located in the
upper reaches of many streams tend to be narrow with flow moving at high
velocities . The high flow velocities found in these streams are the result of
a steep grade and gravity. Within these stream systems, erosion is a
very active process as the channel tries to adjust itself to the topography of
the landscape. Deposition occurs primarily during periods of low flow.
As a result, floodplain deposits are very limited, and the stream bed
is very transient and shallow.
Streams with high sediment
loads that encounter a sudden reduction in flow velocity generally have a braided
channel type .This type of stream channel often occurs further down the stream
profile where the grade changes from being steep to gently sloping. In a braided
stream, the main channel divides into a number of smaller, interlocking or
braided channels.
Meandering channels form
where streams are flowing over a relatively flat landscape with a broad
floodplain .
Stream Channel Features
Within the stream channel are a variety of sedimentary beds and
structures. Many of these features are dependent upon the complex interaction
between stream velocity and sediment size.
Streams carrying coarse
sediments develop sand and gravel bars. These types of bars seen often
in braided streams which are common in elevated areas . Bars develop in
braided streams because of reductions in discharge. Two conditions often
cause the reduction in discharge: reduction in the gradient of the stream
and/or the reduction of flow after a precipitation event or spring melting of
snow and ice.
Point bars develop where
stream flow is locally reduced because of friction and reduced water depth. In
a meandering stream, point bars tend to be common on the inside of a
channel bend.
In straight streams, bar-like
deposits can form in response to the thalweg and helical flow.
In a straight channel stream, bars form in the regions of
the stream away from the thalweg. Riffles, another type of coarse
deposit, develop beneath the thalweg in locations where the faster flow moves
vertically up in the channel. Between the riffles are scoured pools
where material is excavated when the zone of maximum stream velocity approaches
the stream's bed.
Dunes and ripples
are the primary sedimentary features in streams whose channel is composed
mainly of sand and silt. Dunes are about 10 or more centimeters in height and
are spaced a meter or more apart. They are common in streams with higher
velocities. Ripples are only a few centimeters in height and spacing, and are
found in slow moving streams with fine textured beds. Both of these features
move over time, migrating down stream. Material on the gently sloping stoss-side
of these features rolls and jumps up the slope under the influence of water
flow.
The Floodplain
Alongside stream channels
are relatively flat areas known as floodplains . Floodplains develop
when streams over-top their levees spreading discharge and
suspended sediments over the land surface during floods. Levees
are ridges found along the sides of the stream channel composed of sand or
gravel. Levees are approximately one half to four times the channel width in
diameter. Upon retreat of the flood waters, stream velocities are reduced
causing the deposition of alluvium. Repeated flood cycles over time can
result in the deposition of many successive layers of alluvial material.
Floodplain deposits can raise the elevation of the stream bed. This
process is called aggradation.
Floodplains can also contain
sediments deposited from the lateral migration of the river channel. This
process is common in both braided and meandering channels. Braided
channels produce horizontal deposits of sand during times of reduced discharge.
In meandering streams, channel migration leads to the vertical
deposition of point bar deposits. Both braided and meandering channel
deposits are more coarse than the materials laid down by flooding.
A number of other geomorphic
features can be found on the floodplain. Intersecting the levees are narrow
gaps called crevasses. These features allow for the movement of water to
the floodplain and back during floods. Topographical depressions are
found scattered about the floodplain. Depressions contain the some of the
finest deposits on the floodplain because of their elevation. Oxbow lakes
are the abandoned channels created when meanders are cut off from the rest of
the channel because of lateral stream erosion.
Alluvial Fans and Deltas
Streams flowing into
standing water normally create a delta . A delta is body of sediment
that contains numerous horizontal and vertical layers. Deltas are created when
the sediment load carried by a stream is deposited because of a sudden
reduction in stream velocity. The surface of most deltas is marked by small
shifting channels that carry water and sediments away from the main river
channel. These small channels also act to distribute the stream's sediment load
over the surface of the delta. Some deltas, like the
Most deltas contain three
different types of deposits: foreset, topset and bottomset
beds. Foreset beds make up the main body of deltas. They are deposited at
the outer edge of the delta. Steeper angles develop in finer sediments. On top
of the foreset beds are the nearly horizontal topset beds. These beds
are of varying grain sizes and are formed from deposits of the small shifting
channels found on the delta surface. In front and beneath the foreset beds are
the bottomset beds. These beds are composed of fine silt and clay.
Bottom set beds are formed when the finest material is carried out to sea by
stream flow.
An alluvial fan is a
large fan-shaped deposit of sediment on which a braided stream flows
over . Alluvial fans develop when streams carrying a heavy load reduce their
velocity as they emerge from mountainous terrain to a nearly horizontal plain.
The fan is created as braided streams shift across the surface of this feature
depositing sediment and adjusting their course. The image below shows several
alluvial fans that formed because of a sudden change in elevation.
COASTAL AND MARINE PROCESSES AND LANDFORMS
The various landforms of coastal areas are almost exclusively the result
of the action of ocean waves. Wave action creates some of the
world's most spectacular erosional landforms. Where wave energy
is reduced depositional landforms, like beaches, are created.
Properties of Waves
The source of energy
for coastal erosion and sediment transport is wave
action. A wave possesses potential energy as a result of its position
above the wave trough, and kinetic energy caused by the motion of
the water within the wave. This wave energy is generated by the frictional
effect of winds moving over the ocean surface. The higher the wind speed and
the longer the fetch, or distance of open water across which the wind
blows and waves travel, the larger the waves and the more energy they therefore
possess. It is important to realize that moving waves do not move the water
itself forward, but rather the waves impart a circular motion to the individual
molecules of water. If you have ever gone fishing in a boat on the ocean or a
large lake you will have experienced this phenomenon. As a moving wave passes
beneath you, the boat rises and falls but does not move any distance across the
water body.
Waves posses several
measurable characteristics including length and height. Wavelength is defined as the horizontal
distance from wave crest to wave crest,
while wave height is the vertical
difference between the wave's trough and crest. The time taken for successive
crests to pass a point is called the wave period and remains almost constant
despite other changes in the wave. The length of a wave (L) is equal to the
product of the wave period (P) and the velocity of the wave (V): L
= V · P
Long open-ocean waves or swells
travel faster than short, locally generated sea waves. They also have longer
wave periods and this is how they are distinguished from the short sea waves on
reaching the coast. Long swells which have traveled hundreds of kilometres may
have wave periods of up to 20 seconds. Smaller sea waves have wave periods of 5
to 8 seconds.
Where ocean depths are greater
than the length of the waves, the wave motion does not extend to the ocean
floor and therefore remains unaffected by the floor. As the ocean depth falls
below half the wavelength, the wave motion becomes increasingly affected by the
bottom. As the depth of water decreases the wave height increases rapidly and
the wavelength decreases rapidly. Thus, the wave becomes more and more peaked
as it approaches the shore, finally curling over as a breaker and
breaking on the shore. As the wave breaks, its potential energy is
converted into kinetic energy, providing a large amount of energy for the wave
to do work along the shoreline. If you have ever watched waves breaking
on a shore you may have observed that the waves appear to climb out of the
water and also catch up to one another segment off the headland. As a result of
this process, headlands are usually sites of intense erosion while
embayments are usually sites of sediment deposition. Given enough time
wave erosion will tend to create a smooth coastline.
Wave Refraction
Waves are subject to a
reorientation, or wave refraction of their direction of travel as
they approach the coast. Where oblique waves approach a straight shore, the
frictional drag exerted by the sea floor turns the waves to break nearly
parallel to the shore. On an indented coast the situation is more complex
Erosion, Transportation, and Deposition Along Coasts
A number of mechanical and
chemical effects produce erosion of rocky shorelines by waves. Depending on the
geology of the coastline, nature of wave attack, and long-term changes in
sea-level as well as tidal ranges, erosional landforms such as wave-cut
notches, sea cliffs and even unusual landforms such as caves,
sea arches, and sea stacks can form.
Transportation by waves and
currents is necessary in order to move rock particles eroded from one part of a
coastline to a place of deposition elsewhere. One of the most important
transport mechanisms results from wave refraction. Since waves rarely break
onto a shore at right angles, the upward movement of water onto the beach (swash)
occurs at an oblique angle. However, the return of water (backwash) is
at right angles to the beach, resulting in the net movement of beach material
laterally. This movement is known as beach drift . The endless cycle of
swash and backwash and resulting beach drift can be observed on all beaches.
Frequently, backwash and rip
currents cannot remove water from the shore zone as fast as it is piled up
there by waves. As a result, there is a buildup of water that results in the
lateral movement of water and sediment just offshore in a direction with the
waves. The currents produced by the laterial movement of water are known as longshore
currents. The movement of sediment is known as longshore drift,
which is distinct from the beach drift described earlier which operates on land
at the beach. The combined movement of sediment via longshore drift and beach
drift is known as littoral drift.
Tidal currents along coasts can
also be effective in moving eroded material. While incoming and outgoing tides
produce currents in opposite directions on a daily basis, the current in one
direction is usually stronger than in the other resulting in a net one-way
transport of sediment. Longshore drift, longshore currents, and tidal currents
in combination determine the net direction of sediment transport and areas of
deposition.
Many kinds of depositional
landforms are possible along coasts depending on the configuration of the
original coastline, direction of sediment transport, nature of the waves, and
shape and steepness of the offshore underwater slope. Some common depositional
forms are spits, bayhead beaches, barrier beaches or bay-mouth
bars, tombolos , and cuspate forelands.
GLACIERS
Introduction
Glaciers have played an important role in the shaping of landscapes in the
middle and high latitudes and in alpine environments. Their ability to erode
soil and rock, transport sediment, and deposit
sediment is extraordinary. During the last glacial period
more than 50 million square kilometers of land surface were geomorphically
influenced by the presence of glaciers.
Occurrence and Types of Glaciers
Today, glacial ice covers
about 10 % of the Earth's land surface. During the height of the Pleistocene,
ice sheets probably covered about 30 %. Currently, the most extensive
continental glaciers are found in
Glaciers can be classified
according to size. Continental glaciers are the largest, with surface
coverage in the order of 5 million square kilometers.
Mountain or alpine glaciers
are the smallest type of glacier. These glaciers can range in size from a small
mass of ice occupying a cirque to a much larger system filling a
mountain valley. Some mountain glaciers are even found in the tropics. The
merger of many alpine glaciers creates the third type of glacier, piedmont
glaciers . Piedmont glaciers are between several thousand to several tens
of thousands of square kilometers in size.
Growth of Glaciers
Ice that makes up glaciers originally fell on its surface as snow.
To become ice, this snow underwent modifications that caused it to become more
compact and dense. Glacial ice has a density of about 850 kilograms per cubic
meter. The density of snow ranges from about 50 to 300 kilograms per cubic
meter (the density of fresh water is approximately 1000 kilograms per cubic
meter). After the snow falls, the crystals can be reduced by the effects of melting
and sublimation. Scientists call this process ablation. For most
glaciers, ablation is a phenomena dominant in the summer months. The snow also
undergoes physical compaction through melting and refreezing. At first, these
processes cause the original snowflakes to be transformed into small round
crystals. This partly melted, compressed snow is called névé. Névé
has a density exceeding 500 kilograms per cubic meter. If the névé survives the
ablation that occurs during the summer months it is called firn. When
this process happens year after year, a number of layers of firn can
accumulate. Accumulation then causes a further increase in density, modifying
the firn into glacier ice, as the lower layers of firn are compressed by the
weight of the layers above. On average, the transformation of névé into glacial
ice may take 25 to 100 years.
Glacier Movement
To be called a glacier, a mass
of ice must be capable of motion. Glacial movement occurs when the growing ice
mass becomes too heavy to maintain its rigid shape and begins to flow by
plastic deformation. In most mountain glaciers, flow of ice begins with
accumulations of snow and ice greater than 20 meters.
Flow rates within the various
regions of a glacier are not uniform. From directly above, the middle of the
glacier appears to flow with the greatest speed . At the margins of glacier,
surface movement is slowed down because of the frictional effects of the valley
wall. Looking at the glacier in cross-section, we notice that the bottom of the
glacier also moves slowly, once again, because of the influence of frictional
forces .
In the upper reaches of the
glacier, ice and snow accumulate in a broad basin formed by the effects of physical
weathering and erosion. As the glacier proceeds to move downslope,
the flow lines of the ice begin to converge because of the narrowing of the
valley. This convergence causes a compression of the ice flow in central
section of the glacier. At the terminal end of the glacier, flow lines spread
out as the ice is no long constricted by valley walls.
The velocity of flow of
glacier ice is influenced by a variety of factors. Some of the more important
factors are the gradient of the valley floor, the temperature and thickness of
the ice, and the constriction caused by the valley walls.
Glacier Mass Balance
Scientists often view glaciers
as systems that are influenced by a number of inputs and outputs.
The main inputs to the glacial system are water, in the form of snow,
and eroded sediments that are picked up by the moving ice. Water leaves
the glacial system when ice is converted into water or vapor. Sediment is
deposited at the base of the glacier as till and at its terminal end as moraines
or materials reworked by glaciofluvial processes.
We can also use a systems
approach to help us understand why glaciers expand and shrink, and advance and retreat.
This type of modeling is referred to as glacier mass balance. The
mass balance of a glacier involves two main components: accumulation of
snow in the glacier's zone of accumulation and the ablation of
ice in the zone of ablation The zone of accumulation occurs in the upper
reaches of the glacier where yearly additions of snow exceed losses due to melting,
evaporation, and sublimation. The surface of this zone is covered
by snow throughout the year. Below the zone of accumulation is the zone of
ablation. In this zone, the losses of snow and ice from melting, evaporation, and
sublimation are greater than the additions. The line that separates these two
areas is called the firn limit or snow line.
Additions to the glacial
system occur in the zone of accumulation where snow is converted into
glacial ice over time. This ice then flows downslope into the zone of ablation.
In the zone of ablation, losses occur from the glacier from the melting,
evaporation, and sublimation of solid and liquid forms of water.
The firn limit marks the separation point between the two zones. Above
the firn limit snow is able to survive the summer season.
Forward flow of glacial ice is
controlled by gravity and the accumulation of snow in the zone of
accumulation. If losses due to ablation are identical to
accumulations, the glacier will appear to be standing still in spite of the
fact that the ice is actually moving forward. Advance of the glacier's terminus
occurs when net accumulation is greater than net ablation.
Ideally, the rate of flow and
movement of the glacier's terminus should be controlled by net accumulation
and net ablation. This relationship, however, is not temporally immediated. In
many glaciers, there are significant time lags between one year's net
accumulation and net ablation and the corresponding movement of the glacier.
Sometimes this lag can be in the order of several decades when the glaciers are
quite large. Some glaciers can experience a rapid forward surges as
great as 10 to 20 meters per day. This phenomenon is believed to be caused by
large inputs of snow sometime in the past.
Eventually, all glacier ice is
lost in the zone of ablation by the processes of melting, evaporation,
and sublimation. Another process that can remove mass from a glacier is calving.
This process occurs in glaciers whose terminus reaches large bodies of water.
Calving involves the separation of portions of the glacier ice into the water
body. Many icebergs enter the oceans of the world from the calving of the
Today most glaciers are
retreating because of the general warming of global temperatures since the
beginning of this century. This indicates that the mass balances of these
glaciers are negative because of less snow accumulating or higher levels of
ablation. During the Little Ice Age, when global temperatures were
cooler than present, many glaciers over much of the world made strong advances.
Glacial Erosion
Two major erosional
processes occur at the base of a glacier. At the base of a glacier,
large amounts of rock and sediments are incorporated into the
glacier's ice. The material in the ice then acts as an abrasive agent
when combined with glacier movement. This process is known as scouring.
Scouring creates a variety of features. The most conspicuous feature of scouring
is striations . Striations appear as scratches of various size on rock
surfaces. In some cases, abrasion can polish the surface of some rock types
smooth. This geomorphic feature is known as glacial polish. The abrasive
action of scouring also produces a fine clay-sized sediment that
is often transported away from the glacier by meltwater. As a result of
this process, glacial meltwater can have a light, cloudy appearance, and is
called glacial milk.
The second major erosional
process that occurs at the base of a glacier is plucking.
Plucking is the process of particle detachment by moving glacial ice. In this
process, basal ice freezes in rock surface cracks. As the main body of the
glacial ice moves material around the ice in the cracks is pulled and plucked
out. The intensity of the plucking process is greatest on the lee-side
of rock mounds. When combined with glacial abrasion, the action of plucking on
rock mounds produces a unique asymmetrical feature known as roche moutonnee.
Roche moutonnee are smooth on the side of ice advancement and steep and jagged
on the opposite side.
Glaciers generally flow
over the land surface along a path of least resistance. The flow of an alpine
glacier into a valley, causes the glacier to rapidly advance producing a
swollen tongue of ice at the glacier's snout, known as a lobe. As
the lobe moves down the valley it often encounters the lobes of other
glaciers from connecting valleys. The glacier grows in size with addition of
the flow of connected sub-valleys.
A number of distinct erosional
features can be observed in mountainous regions that have experienced the
effects of glaciation. Much of this erosion is exerted on the bottoms and sides
of alpine valleys that guide the flow of glaciers. This erosion causes the
bottom and the sides of any glaciated valley to become both wider and deeper
over time. Glacial erosion also results in a change in the valley's
cross-sectional shape. Glacial valleys tend to have a pronounced U-shape that
contrasts sharply with V-shape valley created by stream erosion. Small
adjoining feeder valleys entering a large valley in a glaciated mountainous
region tend to have their floors elevated some distance above the level of the
main valley's floor. Geomorphologist call this landform a hanging valley.
Hanging valleys develop because larger, more massive glaciers create more
erosion and deeper valleys. Many hanging valleys are also the sites of
sensational waterfalls.
Some of the other features
associated with glacier erosion in alpine regions are cirques, horns,
and arêtes. Cirques are the bowl shaped depressions found at the
head of glacial valleys. For most alpine glaciers, cirques are the areas in the
alpine valleys where snow first accumulated and was modified into glacial ice.
The glaciers that occupy cirques are called cirque glaciers. Horns
are pyramidal peaks that form when several cirques chisel a mountain from three
or more sides. The most famous horn is the
Talus and other foot-slope
deposits are also common in a glaciated valley. Because of the enhancement of freeze-thaw
processes bedrock in alpine areas is weathered by the growth of ice
crystals. This type of weathering shatters the bedrock into sharp angular
fragments that accumulate at the bottom of rock slopes as talus. Much of the
debris carried by an alpine glacier comes from valley sides where talus
accumulates.
The erosional landforms
produced by continental glaciers are usually less obvious than those
created by alpine glaciers. Like alpine glaciers, the movement of continental
glaciers followed topographic trends found in the landscape. Continental ice
sheets were very thick, between 1000 to 3000 meters. The mass of these glaciers
covered all but the highest features and had extremely strong erosive power.
Much of the
Glacial Deposition
A large part of the surface of
a glacier is covered with a coating of sediment and rock debris. This is
especially prevalent near the snout of the glacier, where most of the ice has
been lost to ablation and sediment is left behind. Sediment is added to glacial
ice in two ways. Large quantities of sediment are picked up by abrasion
and plucking at the base of the ice. In alpine areas, sediment is added
to the surface of the glacier from the valley walls through various types of mass
movement. Much of the debris that is added to the ice of the glacier is
eventually delivered to the snout because of the continual forward flow of
glacial ice. From the snout this material can be placed directly from the ice
or it can be deposited through the action of flowing meltwater.
Geomorphologists call the later deposits glaciofluvial deposits.
The technical term used to describe material deposited by the ice is called till
or moraine. All glacial deposits are by and large known as glacial
drift.
Till is a heterogeneous
combination of unstratified sediments ranging in size from large boulders
to minute particles of clay. When till is deposited along the edge of a
glacier it tends to form irregular hills and mounds known as moraines. A
terminal moraine is a deposit that mark, the farthest advance of a glacier.
Moraine deposits created during halts in the retreat of the glacier are
called recessional moraines. The debris that falls from valley
side slopes can be concentrated in a narrow belt and cause a deposit known as a
lateral moraine .When two glaciers flow together, two lateral moraines can
merge to form an interior belt of debris, called a medial moraine). A till
plain is a large, relatively flat plain of till that forms when a sheet of
ice becomes detached from the main body of the glacier and melts in place.
Sometimes the sediments in a till plain can contain large boulders. If these boulders
are transported a great distance from their place of origin, they are called erratics
Glaciofluvial deposits are
generally quite stratified and less assorted in particle size. Outwash
deposits are formed when sand is eroded, transported, and deposited by
meltwater streams from the glacier's snout and nearby till deposits to areas in
front of the glacier. Outwash plain develops when there are a great
number of meltwater streams depositing material ahead of the glacier.
Glaciofluvial deposits are also directly in front of the glacier. Where water
rich in sediment flows off the snout of the ice, a conical-shaped pile of
sediment, known as a kame, can be deposited. Many kames are often found
on or at the edge of moraines.
Glaciers can also contain
sinuous flows of meltwater that occur in ice tunnels at the base of the ice.
The beds of these sub-surface glacial streams are composed of layers of sand
and gravel. When the ice melts from around the meltwater tunnels, the
beds of sand and gravel are deposited on the Earth's surface as long twisting
ridges known as eskers.
When glaciers are rapidly retreating,
numerous blocks of ice can become detached from the main body of the glacier.
If glacial drift is then placed around the ice, a depression on the surface
called a kettle hole can be created when the ice melts ,
Kettle holes are commonly found on moraine and outwash plain deposits.
Large kettle holes that reach below the water table can form into lakes.
Glacial retreat also creates hill
shaped deposits of till known as a drumlins . Drumlins often occur in
large congregations across areas of
EOLIAN PROCESSES AND LANDFORMS
Introduction
Eolian landforms are found in
regions of the Earth where erosion and deposition by wind are the
dominant geomorphic forces shaping the face of the landscape. Regions
influenced by wind include most of the dry climates of the Earth .According to
the Köppen Climate
Classification System , this would include regions of the world
that are classified as arid deserts (BW)
and semiarid steppe (BS). Wind can also
cause erosion and deposition in environments where sediments have been recently
deposited or disturbed. Such environments include lake and ocean coastline beaches,
alluvial fans, and farmland where topsoil has been disturbed by
cultivation.
Unlike streams, wind has the
ability to transport sediment up-slope as well as down-slope. The relative
ability of wind to erode materials is is slight when compared to the other
major erosional mediums, water and ice. Ice and water can have greater erosive
power primarily because of their greater density. Water is about 800
times more dense than air (density of air is 1.29 kg m-3, while the
density of water is 1000 kg m-3). This physical difference limits
the size of particles wind can move. The power of wind to erode surface
particles is controlled primarily by two factors: wind velocity and surface
roughness. Erosive force increases exponentially with increases in wind velocity.
For example, a velocity increase from 2 to 4 meters per second causes an eight-fold
increase in erosive capacity, while an increase in wind speed from 2 to 10
meters per second generates a 125-fold increase in erosional force.
Consequently, fast winds are capable of causing much more erosion than slow
winds.
At ground level, the roughness
of the surface plays an important role in controlling the nature of wind
erosion. Boulders, trees, buildings, shrubs, and even small plants like grass
and herbs can increase the frictional roughness of the surface and reduce wind
velocity. Vegetation can also reduce the erosional effects of wind by binding
soil particles to roots. Thus, as a general rule, the areas that show
considerable amounts of wind erosion are open locations with little or no plant
cover.
Threshold and Terminal Fall Velocities
Threshold velocity can be defined as velocity required to entrain
a particle of a particlular size. In general, the larger the particle, the
higher the threshold velocity required to move it. This law can sometimes be
broken when clay sized particles are involved in the entraiment process.
Clay particles have a general tendency to
become cohesively bonded to each other. This aggregation results in the
clumping of several particles into a mass of much larger size. As a result, the
threshold velocity required to entrain clay is can as great as the wind speed
required to move grains of sand. Silt is usually the easiest type
of particle to be entrain by wind.
Terminal fall velocity can be defined as velocity at which a particle
being transported by wind or water falls out and is deposited on the
ground surface.
Sand Transport
Three different processes are
responsible for the transport of sediment by wind.
- Wind erosion of surface
particles begins when air velocities reach about 4.5 meters per second. A
rolling motion called traction or creep (the later term should
not to be confused with soil creep) characterizes this first movement of
particles. In strong winds, particles as large as small pebbles can move
through traction.
- The second type of wind
sediment transport involves particles being lifted off the ground, becoming
suspended in the air, and then returning to the ground surface several
centimeters downwind. This type of transport is called saltation, and
this process accounts for 75 to 80 percent of the sediment transport in dry
land environments. Saltating particles are also responsible for sending
additional sediment into transport. When a falling particle strikes the ground
surface, part of its force of impact is transferred to another particle causing
it to become airborne.
- Small sized particles like
silt and clay have the ability to be lifted well above the zone of saltation
during very strong winds and can be carried in suspension thousands of
meters into the air and hundreds of kilometers downwind.
Erosional Landforms
When the force of wind is
concentrated on a particular spot in the landscape, erosion can carve out a pit
known as a deflation hollow. Deflation hollows range in size from a few
meters to a hundred meters in diameter, and may develop over several days or a
couple of seasons. Much larger depressions are also found in the arid regions
throughout the world. These broad, shallow depressions, called pans, can
cover thousands of square kilometers.
In some dry climate areas,
persistent winds erode all sediments the size of sand and smaller
leaving pebbles and larger particles on the ground surface. Surfaces
loaded with such particles are called desert pavement or reg and
sometimes resemble a worn, polished cobblestone street surface.
Depositional Landforms
Sand dunes are the most
noticeable landforms produced by wind erosion and deposition of sediment. The
largest dune fields are found in the
Sand Dune Formation
Sand dunes form in
environments that favor the deposition of sand.Deposition occurs
in areas where a pocket of slower moving air forms next to much faster moving
air. Such pockets typically form behind obstacles like the leeward sides
of slopes. As the fast air slides over the calm zone, saltating grains fall out
of the air stream and accumulate on the ground surface.
Dunes first begin their life
as a stationary pile of sand that forms behind some type of vertical obstacle.
However, when they reach a certain size threshold continued growth may also be
associated with active surface migration. In a migrating dune, grains of sand
are transported by wind from the windward to the leeward side and
begin accumulating just over the crest. When the upper leeward slope reaches an
angle of about 30-34 degrees the accumulating pile becomes unstable, and small
avalanches begin to occur, moving sand to the lower part of the leeward slope.
As a result of this process, the dune migrates over the ground as sand is eroded
from one side and deposited on the other. This process also causes the
appearance of the dune to take on a wave shape. Active movement of sand
particles across the dune causes windward slope to become shallow, while the
leeward slope maintains a steep slope or slip-face.
The velocity of the wind above
the ground surface determines the height of a dune. The maximum height is
variable but usually falls in the range of 10 to 25 meters. In most cases, dune
height is a function of surface friction. Height growth stops when friction can
no longer slow the wind flowing over the dune to a point where deposition
occurs. The tallest sand dunes in the world are found in
Desert Dunes
Desert sand dunes occur in an
amazing diversity of forms: Barchan, Transverse,
Parabolic, Barchanoid Ridge, Longitudinal, Seif, Star Dune, Dome, Reversing.
Coastal Dunes
Active sand dune formation is
also found on the coasts of the continents. Coastal dunes form when there is a
large supply of beach sand and strong winds blowing from sea to shore. The
beach area must also be wide and sufficiently influenced by wave action to keep
it free of plants.
Many coastal dune deposits
develop in association with blowouts in ridges of beach sand. Blowouts
are small saucer shaped depressions where there is a deposit of sand at the upwind
end of the feature. As wind erosion continues, the deposit grows and begins to
migrate inland forming a parabolic sand dune. The flanks of these dunes tend to
be more stable and are often colonized by plants like dune grass, sea oats, and
sand cherry. This colonization by plants re-inforces the stability of the
dune's flanks.
Loess Deposits
Loess is another major
deposit created by wind. Less visible than sand dunes, loess is found over
large areas of the Earth . It is also important for humans because it creates
very fertile soils. Large deposits of loess exist in northeastern