The natural forces which help in levelling of the earth's surface by the process of degradation, transport and aggradation are called exogenetic agents.
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.
- 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.
- 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.
- 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 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
- 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:-
- Chemical weathering processes
- Physical or Mechanical weathering processes
- 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.
- 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 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 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.
- In weathering, oxidation means a combination of a mineral with oxygen to form oxides or hydroxides.
- 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.
- 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
- The process of unloading involves removal of overlying rock load because of continued erosion
- 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
- 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 occurs due to the growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
- 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.
- 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.
- 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.
Types of Mass Movements
Mass movements can be grouped into two major classes:
- Slow movements
- Rapid Movements
- 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.
- 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
- 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.
- 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.
- Mudflows can cause great destruction to human habitations
- 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.
- 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.
- 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.
- 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.