- Geo means earth, Morphic means change. Thus in a literal meaning geomorphic processes are methods in which the shape earth crust undergoes change over the years.
- The endogenic and exogenic forces causing physical stresses and chemical actions on earth materials and bringing about changes in the configuration of the surface of the earth are known as geomorphic processes.
Why is the surface of the earth uneven?
- The earth’s crust is dynamic. It has moved and moves vertically and horizontally.
- The earth’s surface is being continuously subjected to by external forces originating within the earth’s atmosphere and by internal forces from within the earth.
- Due to variations in geothermal gradients and heat flow from within, crustal thickness and strength, the action of endogenic forces are not uniform and hence the tectonically controlled original crustal surface is uneven.
- The differences in the internal forces operating from within the earth which built up the crust have been responsible for creating new relief features like mountain ranges, uplift or depression make folding and faulting, volcanic disturbances modify the landscape
- The internal forces are known as endogenic forces.
- The endogenic forces are mainly land building forces.
- Diastrophism and volcanism are endogenic geomorphic processes.
- The earth’s surface is being continuously subjected to external forces induced basically by energy (sunlight).
- The actions of exogenic forces result in wearing down (degradation) of relief/elevations and filling up (aggradation) of basins/depressions, on the earth’s surface. The phenomenon of wearing down of relief variations of the surface of the earth through erosion is known as gradation.
- The external forces are known as exogenic forces.
- The exogenic processes are mainly land wearing forces.
- Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes.
- An agent is a mobile medium (like running water, moving ice masses, wind, waves and currents etc.) which removes, transports and deposits earth materials.
- Running water, groundwater, glaciers, wind, waves and currents, etc., can be called geomorphic agents.
- When these elements of nature become mobile due to gradients, they remove the materials and transport them over slopes and deposit them at lower level.
- The energy emanating from within the earth is the main force behind endogenic geomorphic processes.
- This energy is mostly generated by radioactivity, rotational and tidal friction and primordial heat from the origin of the earth.
- This energy due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.
All processes that move, elevate or build up portions of the earth’s crust come under diastrophism.
- Orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust;
- Epeirogenic processes involving uplift or warping of large parts of the earth’s crust; Epeirogeny is continental building process.
- Earthquakes involving local relatively minor movements;
- Plate tectonics involving horizontal movements of crustal plates.
All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks.
Volcanism includes the movement of molten rock (magma) onto or toward the earth’s surface and also formation of many intrusive and extrusive volcanic forms.
- The exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors.
- Gravitational force acts upon all earth materials having a sloping surface and tend to produce movement of matter in down slope direction.
- Force applied per unit area is called stress. Stress is produced in a solid by pushing or pulling. This induces deformation.
- Forces acting along the faces of earth materials are shear stresses (separating forces). It is this stress that breaks rocks and other earth materials. The shear stresses result in angular displacement or slippage.
- The earth materials become subjected to molecular stresses that may be caused by a number of factors amongst which temperature changes, crystallisation and melting are the most common.
- Chemical processes normally lead to loosening of bonds between grains, dissolving of soluble minerals or cementing materials.
- Thus, the basic reason that leads to weathering, mass movements, erosion and deposition is development of stresses in the body of the earth materials.
Why the exogenic geomorphic processes vary from region to region?
- As there are different climatic regions on the earth’s surface .Temperature and precipitation are the two important climatic elements that control various processes.
- Within different climatic regions there may be
- Local variations of the effects of different climatic elements due to altitudinal differences
- The variation in the amount of insolation received by north and south facing slopes as compared to east and west facing slopes.
- Due to differences in wind velocities and directions, amount and kind of precipitation, its intensity, the relation between precipitation and evaporation, daily range of temperature, freezing and thawing frequency, depth of frost penetration, the geomorphic processes vary within any climatic region.
- Exogenic geomorphic processes depend upon type and structure of rocks (The term structure includes such aspects of rocks as folds, faults, orientation and inclination of beds, presence or absence of joints, bedding planes, hardness or softness of constituent minerals, chemical susceptibility of mineral constituents; the permeability or impermeability)
- All the exogenic 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 flow chart gives the denudation precipitation and temperature exerts influence indirectly on exogenic geomorphic processes.
- Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate over earth materials.
- As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
There are three major groups of weathering processes:
- Chemical Weathering Processes
- Physical or Mechanical Weathering Processes
- Biological Weathering Processes
Chemical Weathering Processes
A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface and/or soil water and other acids.
Water and air (oxygen and carbon dioxide) along with heat must be present to speed up all chemical reactions.
- This process involves removal of solids in solution and depends upon solubility of a mineral in water or weak acids.
- On coming in contact with water many solids disintegrate and mix up as suspension in water.
- Soluble rock forming minerals like nitrates, sulphates, and potassium etc. are affected by this process.
- Minerals like calcium carbonate and calcium magnesium bicarbonate present in limestones are soluble in water containing carbonic acid (formed with the addition of carbon dioxide in water), and are carried away in water as solution.
- Common salt (sodium chloride) is also a rock forming mineral and is susceptible to this process of solution.
- Carbonation is the reaction of carbonate and bicarbonate with minerals and is a common process helping the breaking down of feldspars and carbonate minerals.
- Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid.
- Calcium carbonates and magnesium carbonates are dissolved in carbonic acid and are removed in a solution without leaving any residue resulting in cave formation.
- Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock.
- Calcium sulphate takes in water and turns to gypsum, which is more unstable than calcium sulphate. This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration.
- Many clay minerals swell and contract during wetting and drying and a repetition of this process results in cracking of overlying materials.
- Salts in pore spaces undergo rapid and repeated hydration and help in rock fracturing.
- The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration.
- 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. The minerals most commonly involved in this process are iron, manganese, sulphur etc.
- In the process of oxidation rock breakdown occurs due to the disturbance caused by addition of oxygen.
- Red colour of iron upon oxidation turns to brown or yellow.
- When oxidised minerals are placed in an environment where oxygen is absent, reduction takes place.
- Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- Red colour of iron upon reduction turns to greenish or bluish grey.
Physical Weathering Processes
Physical or mechanical weathering processes depend on some applied forces. The applied forces could be:
- Gravitational forces such as overburden pressure, load and shearing stress
- Expansion forces due to temperature changes, crystal growth or animal activity
- Water pressures controlled by wetting and drying cycles.
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.
- Unloading and Expansion
- Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses.
- Fractures will develop roughly parallel to the ground surface. In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock.
- Exfoliation sheets resulting from expansion due to unloading and pressure release may measure hundreds or even thousands of metres in horizontal extent.
- Temperature Changes and Expansion:
- With rise in temperature, every mineral expands and pushes against its neighbour and as temperature falls, a corresponding contraction takes place.
- Because of diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place regularly.
- This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.
- The surface layers of the rocks tend to expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface.
- In rocks like granites, smooth surfaced and rounded small to big boulders called tors form due to exfoliation.
- Freezing, Thawing and Frost Wedging:
- Frost weathering occurs due to 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. Glacial areas are subject to frost wedging daily.
- The resulting expansion affects joints, cracks and small inter granular fractures to become wider and wider till the rock breaks apart.
- Salt Weathering:
- Salts in rocks expand due to thermal action, hydration and crystallisation.
- Many salts like calcium, sodium, magnesium, potassium and barium have a tendency to expand.
- Expansion of these salts depends on temperature and their thermal properties. High temperature ranges in deserts favour such salt expansion.
- Salt crystals in near-surface pores cause splitting of individual grains within rocks, which eventually fall off, which result in granular disintegration or granular foliation.
- Salt crystallisation is most effective of all salt-weathering processes. In areas with alternating wetting and drying conditions salt crystal growth is favoured and the neighbouring grains are pushed aside.
- Sodium chloride and gypsum crystals in desert areas heave up overlying layers of materials and with the result polygonal cracks develop all over the heaved surface.
- With salt crystal growth, chalk breaks down most readily, followed by limestone, sandstone, shale, gneiss and granite etc.
BIOLOGICAL ACTIVITY AND WEATHERING
- Biological weathering is contribution to or removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms.
- Burrowing and wedging by organisms like earthworms, termites, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air.
- Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials.
- Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements.
- Algae utilise mineral nutrients for growth and help in concentration of iron and manganese oxides.
- Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
SIGNIFICANCE OF WEATHERING
- Weathering is an important process in the formation of soils.
- Biomes and biodiversity is basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.
- Weathering aids mass wasting, erosion and reduction of relief and changes in landforms are a consequence of erosion.
- Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc., which are of great importance for the national economy.
- These movements transfer the mass of rock debris down the slopes under the direct influence of gravity.
- The movements of mass may range from slow to rapid, affecting shallow to deep columns of materials and include creep, flow, slide and fall.
- Gravity exerts its force on all matter, both bedrock and the products of weathering.
- Weathering is not a pre-requisite for mass movement though it aids mass movements. Mass movements are very active over weathered slopes rather than over unweathered materials.
- Mass movements are aided by gravity and no geomorphic agent like running water, glaciers, wind, waves and currents participate in the process of mass movements.
- Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation and torrential rains and scarcity of vegetation etc., favour mass movements.
- Several activating causes precede mass movements.
- Removal of support from below to materials above through natural or artificial means
- Increase in gradient and height of slopes
- Overloading through addition of materials naturally or by artificial filling
- Overloading due to heavy rainfall, saturation and lubrication of slope materials
- Earthquakes, explosions or machinery
- Excessive natural seepage
- Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks
- Indiscriminate removal of natural vegetation.
Mass movements can be grouped under three major classes:
- Creep is one type under this category which can occur on moderately steep, soil covered slopes.
- Movement of materials is extremely slow and imperceptible except through extended observation. Materials involved can be soil or rock debris.
- 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 involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water.
- This process is common in moist temperate areas.
- These movements are mostly prevalent in humid climatic regions and occur over gentle to steep slopes.
- Movement of water-saturated clayey or silty earth materials down low-angle terraces or hillsides is known as earthflow.
- Quite often, the materials slump making step like terraces and leaving arcuate scarps at their heads and an accumulation bulge at the toe.
- When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale or deeply weathered igneous rock may slide downslope.
- Another type in this category is mudflow.
- In the absence of vegetation cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flows down along definite channels.
- Mudflows occur frequently on the slopes of erupting or recently erupted volcanoes.
- Volcanic ash, dust and other fragments turn into mud due to heavy rains and flow down as tongues or streams of mud causing great destruction to human habitations.
- A third type is the debris avalanche,
- It is more characteristic of humid regions with or without vegetation cover and occurs in narrow tracks on steep slopes.
- This debris avalanche can be much faster than the mudflow.
- Landslides are relatively rapid and perceptible movements. The materials involved are relatively dry.
- The size and shape of the detached mass 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 of materials several types are identified in this category.
- Slump is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place .
- Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide.
- Debris fall is nearly a free fall of earth debris from a vertical or overhanging face. Sliding of individual rock masses down bedding, joint or fault surfaces is rockslide.
EROSION AND DEPOSITION
- Erosion involves acquisition and transportation of rock debris. 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 depending upon the dynamics of each of these agents.
- Abrasion by rock debris carried by these geomorphic agents also aids greatly in erosion. By erosion, relief degrades, i.e., the landscape is worn down.
- It is erosion that is largely responsible for continuous changes that the earth’s surface is undergoing.
- Denudational processes like erosion and transportation are controlled by kinetic energy.
- The erosion and transportation of earth materials is brought about by wind, running water, glaciers, waves and ground water.
- Of these the first three agents are controlled by climatic conditions.
- A pedologist who studies soils defines soil as a collection of natural bodies on the earth’s
- surface containing living matter and supporting or capable of supporting plants.
- Soil is a result of decay, it is also the medium for growth. It is a changing and developing body.
- The soil chemistry, the amount of organic matter, the soil flora and fauna, the temperature and the moisture, all change with the seasons as well as with more extended periods of time.
- That means, soil becomes adjusted to conditions of climate, landform and vegetation and will change internally when these controlling conditions change.
- Pedology is soil science. A pedologist is a soil-scientist.
Process of Soil Formation
Soil formation or pedogenesis depends first on weathering. It is this weathering mantle (depth of the weathered material) which is the basic input for soil to form.
Soil forming factors act in union and affect the action of one another.
Five basic factors control the formation of soils:
- Parent material is a passive control factor in soil formation.
- Parent materials can be any insitu or on-site weathered rock debris (residual soils) or transported deposits (transported soils).
- Soil formation depends upon the texture (sizes of debris) and structure (disposition of individual grains/particles of debris) as well as the mineral and chemical composition of the rock debris/deposits.
- Topography like parent materials is another passive control factor.
- The influence of topography is felt through the amount of exposure of a surface covered by parent materials to sunlight and the amount of surface and sub-surface drainage over and through the parent materials.
- Steep slopes:
- Soils will be thin on steep slopes.
- Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favourable.
- Soils are thick over flat upland areas.
- Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark colour.
- In middle latitudes, the south facing slopes exposed to sunlight have different conditions of vegetation and soils and the north facing slopes with cool, moist conditions have some other soils and vegetation.
- Climate is an important active factor in soil formation.
- The climatic elements involved in soil development are :
- Moisture in terms of its intensity, frequency and duration of precipitation - evaporation and humidity.
- Temperature in terms of seasonal and diurnal variations.
- Excess of water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (ill