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Exogenetic Earth Movements
The natural forces which help in levelling of the earth’s surface by the process of
degradation, transport and aggradation are called exogenetic agents.
Exogenetic Movements
The surface of the earth is ever changing by the endogenetic and exogenetic
forces. The exogenetic movements on the earth’s surface gradually try to level
the uneven surface of the earth. The agents of weathering and erosion are
continuously involved in undoing the changes created by the endogenetic
movements of the earth.
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Geomorphic Processes
The endogenetic and exogenetic forces that bring about changes in the
configuration of the surface of the earth through physical and chemical actions
on earth materials are known as geomorphic processes.
Diastrophism and volcanism are endogenetic geomorphic processes.
Weathering, mass wasting, erosion and deposition are exogenetic geomorphic
processes.
Geomorphic agents
Any Exogenetic element of nature (like water, ice, wind, etc.,) capable of
removing, transporting and depositing earth materials can be called a
geomorphic agent.
Running water, groundwater, glaciers, wind, waves and currents, etc., can be
called geomorphic agents.
Exogenetic Geomorphic Processes
The exogenetic processes derive their energy from atmosphere determined by
the ultimate energy from the sun and also the gradients created by tectonic
factors.
As explained above, Weathering, mass wasting, erosion and deposition are
exogenetic geomorphic processes.
All the exogenetic geomorphic processes are covered under a general term,
denudation. The word ‘denude’ means to strip off or to uncover.
Weathering, mass wasting/movements, erosion and transportation are included
in denudation.
The basic reason that leads to weathering, mass movements and erosion is the
development of stresses in the body of the earth materials.
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Force applied per unit area is called stress. Stress is produced in a solid by
pushing or pulling which induces deformations in the body.
Different kinds of stress are produced in the earth materials viz. sheer stress,
gravitational stress, molecular stress etc.
Exogenetic geomorphic processes are greatly influenced by climatic elements
such as temperature and precipitation. Hence, the exogenetic geomorphic
processes vary from region to region.
Exogenetic geomorphic processes also depend upon type and structure of
rocks. As different types of rocks offer varying resistance to various geomorphic
processes.
Weathering
Weathering is the action of elements of weather and climate over earth
materials.
Weathering is defined as mechanical disintegration and chemical
decomposition of rocks through the actions of various elements of weather and
climate.
Weathering processes are responsible for breaking down the rocks into smaller
fragments
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Erosion cannot be significant if the rocks are not weathered. That means,
weathering aids mass wasting and erosion.
Weathering is a static process as very little, or no motion of materials takes
place in weathering, it is an in-situ or on-site process.
Weathering processes are conditioned by many complex geological, climatic,
topographic and vegetative factors.
Types of the weathering process
There are three major groups of weathering processes. However, very rarely does
any one of these processes ever operate completely by itself. In reality, the
weathering process involves combinations of all three types of weathering
processes. These processes are:-
1. Chemical weathering processes
2. Physical or Mechanical weathering processes
3. Biological weathering processes
Chemical weathering processes
Chemical weathering causes rocks to decompose or dissolve and reduce them
to a fine clastic state through chemical reactions by oxygen, water or acids.
The mineral contained in the rocks undergo chemical changes when they get in
contact with atmospheric air and water.
Presence of Water, air (oxygen and carbon dioxide) and high-temperature help
in speeding up the weathering process.
Types of the chemical weathering process
There are different weathering process related to chemical action viz. hydration,
carbonation and oxidation. These weathering processes are interrelated and go
hand in hand and hasten the weathering process.
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Solution
When something is dissolved in water or acids, the water or acid with dissolved
contents is called solution.
This process involves removal of solids in solution
The process of weathering through solution depends upon the solubility of a
mineral in water or weak acids.
Soluble rock-forming minerals like nitrates, sulphates, and potassium etc. are
easily leached out without leaving any residue in the rainy climate.
Carbonation
Carbonation is the reaction of carbonate and bicarbonate with minerals
It is a common process helping the breaking down of feldspars and carbonate
minerals.
It takes place in rocks containing carbonates of calcium, sodium, magnesium,
potassium etc. when they come in touch with rainwater which contains
dissolved carbon dioxide.
Hydration
Hydration is the chemical addition of water.
Many rock minerals swell and contract during wetting and drying and a
repetition of this process results in their disintegration.
Salts in pore spaces undergo rapid and repeated hydration and help in rock
fracturing.
Oxidation
In weathering, oxidation means a combination of a mineral with oxygen to form
oxides or hydroxides.
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Oxidation occurs where there is ready access to the atmosphere and
oxygenated waters.
In the process of oxidation, rock breakdown occurs due to the disturbance
caused by the addition of oxygen
The minerals most commonly involved in this process are iron, manganese,
sulphur etc.
Reduction
When oxidised minerals are placed in an environment where oxygen is absent,
reduction takes place.
Such conditions usually exist below the water table, in areas of stagnant water
and waterlogged ground.
Physical Weathering Processes
The disintegration of rocks by some applied forces is called physical or
mechanical weathering.
These applied forces could be due to the action of gravity, heat and water.
Many of these forces are applied both at the surface and within different earth
materials leading to rock fracture
Most of the physical weathering processes are caused by thermal expansion
and pressure release.
These processes are small and slow but can cause great damage to the rocks
because of continued fatigue the rocks suffer due to the repetition of
contraction and expansion.
Types of the physical weathering process
Unloading
The process of unloading involves removal of overlying rock load because of
continued erosion
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Unloading causes a release of vertical pressure on the rock resulting in
expansion of upper layers which further results in disintegration of rock masses
Due to disintegration fractures are developed in the rock mass, roughly parallel
to ground surface
In areas of a curved ground surface, rock fractures tend to produce Large, and
smooth rounded domes called exfoliation domes
An exfoliation dome
Temperature Changes
With the rise in temperature, every mineral expands, and as the temperature
falls, a corresponding contraction takes place.
Because of diurnal changes in the temperatures, there is a regular internal
movement among the mineral grains
These regular movements make the rocks weak due to continued fatigue and
cause fracture and further disintegration of rock masses
This process is most effective in dry climates and high elevations where diurnal
temperature changes are drastic.
Frost Weathering
Frost weathering occurs due to the growth of ice within pores and cracks of
rocks during repeated cycles of freezing and melting.
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This process is most effective at high elevations in mid-latitudes where freezing
and melting is often repeated.
Rapid freezing of water causes its sudden expansion which causes joints, cracks
and small inter granular fractures to become wider and wider till the rock
breaks apart.
Salt Weathering
Many salts in rocks like calcium, sodium, magnesium, potassium expand due to
thermal action, hydration and crystallisation.
Salt weathering causes splitting of individual grains within rocks, which
eventually fall off.
This process of falling off of individual grains may result in granular
disintegration or foliation.
Salt weathering is common in desert areas due to high-temperature ranges
Biological Weathering Processes
This refers to disintegration and decomposition of rock masses due to growth or
movement of organisms.
Burrowing and wedging by organisms like earthworms, termites, rodents etc.,
help in exposing the new surfaces to moisture and chemical attack causing their
decomposition.
Human activities like ploughing and cultivating cause mixing of air, water in the
minerals, thereby aiding in weathering process.
Decaying plant and animal matter help in the production of humic, carbonic and
other acids which enhance decay and solubility of some rocks.
Plant roots penetrate the cracks in the rocks and exert tremendous pressure on
the earth materials mechanically breaking them apart.
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Mass Movements
These movements transfer the mass of rock debris down the slopes under the
direct influence of gravity.
The debris may carry with it air, water or ice.
The process of weathering aids in mass movements. Mass movements are very
active over weathered slopes rather than over unweathered materials.
No geomorphic agent like running water, glaciers, wind, waves and currents
participate in the process of mass movements.
Mass movements are aided by gravity
Mass movement is also aided by weak unconsolidated materials, thinly bedded
rocks, faults, steep slopes, abundant precipitation and torrential rains and
scarcity of vegetation etc.
Classification of Mass Movements
Heave (heaving up of soils due to frost growth and other causes), flow and slide
are the three forms of movements. The figure, given below shows the
relationships among different types of mass movements, their relative rates of
movement and moisture limits.
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Types of Mass Movements
Mass movements can be grouped into two major classes:
1. Slow movements
2. Rapid Movements
Slow movements
Creep
Creep refers to the movement of materials which is extremely slow and
imperceptible in normal conditions
Creep, generally occur on moderately steep, soil-covered slopes.
Depending upon the type of material involved, several types of creep viz., soil
creep, talus creep, rock creep, rock-glacier creep etc., can be identified.
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Creep
Solifluction
Solifluction refers to slow downslope flowing soil mass or fine-grained rock
debris saturated or lubricated with water.
This process is quite common in moist temperate areas
Rapid Movements
Earthflow
Earthflow refers to the movement of water-saturated clayey or silty earth
materials down steep slopes
These movements are most prevalent in humid climatic regions and occur over
gentle to steep slopes.
Mudflow
In the region of sparse vegetation and heavy rainfall, thick layers of weathered
materials get saturated with water and flow down along definite channels.
It looks like a stream of mud within a valley.
Mudflows frequently occur on the slopes of erupting or recently erupted
volcanoes.
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Mudflows can cause great destruction to human habitations
Avalanche
This is also a type of debris flow.
Debris avalanche can be much faster than the mudflow.
Debris avalanche is similar to snow avalanche.
It is more characteristic of humid regions with or without vegetation cover
It occurs in narrow tracks on steep slopes.
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Debris Avalanche
Landslide
Landslide involves relatively rapid and perceptible movements of the rock mass.
The materials involved are relatively dry.
The size and shape of the detached mass in the landslide depends on the nature
of discontinuities in the rock, the degree of weathering and the steepness of the
slope
Depending upon the type of movement, a landslide can take place either
by slump involving back rotation with respect to the slope or by rapid rolling or
sliding of earth debris without backward rotation, referred to as debris slide.
Similarly, sliding down of individual rock masses is referred to as the rock slide.
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Landslide
Erosion
Erosion involves acquisition and transportation of rock debris.
Erosion results in degradation of the surface relief i.e. wearing down of the
landscape.
It is erosion that is largely responsible for continuous changes that the earth’s
surface is undergoing.
When massive rocks break into smaller fragments through weathering and any
other process, erosional geomorphic agents like running water, groundwater,
glaciers, wind and waves remove and transport it to other places
Abrasion by rock debris carried by these geomorphic agents also aids greatly in
erosion.
Thus, weathering aids erosion, but it is not a pre-condition for erosion to take
place.
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Deposition
Deposition is a consequence of erosion.
Gradually, the erosional agents lose their velocity and hence, the materials
carried by them start to settle themselves.
The coarser materials get deposited first and finer ones later.
By deposition, depressions get filled up.
The same erosional agents, viz. running water, glaciers, wind, waves and
groundwater act as aggradational or depositional agents also.
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