GEOMORPHIC PROCESSES

Why Earth Is Uneven  ?

1. Due to internal and external forces earth is changing its surface conditions. The earth crust is always dynamic. It moves vertically and horizontally.  

2. The phenomenon of wearing down of relief variations of the surface of the earth through erosion is known as gradation.  

3. The endogenic forces always elevate parts of the earth’s surface and hence the exogenic processes fail to even out the relief variations of the surface of the earth. So, variations remain as long as there is difference between endogenic and exogenic forces.  

Geomorphic Processes  

1. 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 geomorphic processes.  

i). Diastrophism and volcanism are endogenic geomorphic processes.  

ii). Weathering, mass wasting, erosion and deposition are exogenic geomorphic processes.  

2. Any exogenic element of nature (like water, ice, wind, etc.,) capable of acquiring and transporting earth materials can be called a geomorphic agent. When these elements of nature become mobile due to gradients, they remove the materials and transport them over slopes and deposit them at lower levels.   

3. Gravity besides being a directional force activating all downslope movements of matter also causes stresses on the earth’s materials. Indirect gravitational stresses activate wave and tide induced currents and winds. Without gravity and gradients there would be no mobility and hence no erosion, transportation and deposition are possible. So, gravitational stresses are as important as the other geomorphic processes. Gravity is the force that is keeping us in contact with the surface and it is the force that switches the movement of all surface material on earth.  

4. All the movements either within the earth or on the surface of the earth occur due to gradients - from higher levels to lower levels, from high pressure to low pressure areas etc. 

Endogenic Processes  

1. The energy generating from within the earth is the main force behind the endogenic geomorphic processes.  

2. The energy generated due to radioactivity, rotational force, tidal friction, primordial heat from the origin of the earth.  

3. Diastrophism And volcanism are due to geothermal gradients and heat flow from within the earth. Crustal thickness, strength, action of endogenic forces are due to variations in geothermal gradients and heat flow are uneven.  

Diastrophism  

All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. They include:  

i) Orogenic processes involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust.

ii) Epeirogenic processes involving uplift or warping of large parts of the earth’s crust.

iii) Earthquakes involving local relatively minor movements.

iv) Plate tectonics involving horizontal movements of crustal plates.  

Orogeny is a mountain building process whereas epeirogeny is continental building process. Through the processes of orogeny, epeirogeny, earthquakes and plate tectonics, there can be faulting and fracturing of the crust. All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks. 

Volcanism  

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.  

 Exogenic Processes  

1. The exogenic processes derive their energy from atmosphere determined by the ultimate energy from the sun and also the gradients created by tectonic factors. 

2. 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. 

3. The shear stresses result in angular displacement or slippage.  

4. Molecular stresses that may be caused by a number of factors amongst which temperature changes, crystallization and melting are the most common.  

5. 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, and erosion is development of stresses in the body of the earth materials.  

6. All the exogenic geomorphic processes are covered under a general term, denudation (means uncover). Weathering, mass wasting/movements, erosion and transportation are included in denudation.  

7. The differences on the surface of the earth though originally related to the crustal evolution continue to exist in some form or the other due to differences in the type and structure of earth materials, differences in geomorphic processes and in their rates of operation. 

8. Some of the exogenic geomorphic process are: Weathering, mass wasting, erosion and deposition.

Weathering  

1. Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.  

2. Weathering is action of elements of weather and climate over earth materials. There are a number of processes within weathering which act either individually or together to affect the earth mater in order to reduce them to fragmental state.  

3. As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.  

4. Weathering processes are conditioned by many complex geological, climatic, topographic and vegetative factors. Climate is of particular importance. Not only weathering processes differ from climate to climate, but also the depth of the weathering mantle.  

5. There are three major groups of weathering processes: Chemical; Physical or mechanical; Biological weathering processes.  

 Chemical Weathering  

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 along with heat must be present to speed up all chemical reactions.   

Solution  

i). When substances are dissolved in acids or water, then the water or acid with dissolved substances is called a solution. This process includes the removal of solids in solution and depends upon the solubility of a mineral in weak acids or water.    

ii). Many solids disintegrate and mix up as a suspension in water as they come in contact with water. Some of the soluble rock-forming minerals like sulphates, nitrates, and potassium, etc. are affected by this process. Hence, these minerals are simply leached out without leaving any remains in rainy climates and accumulate in dry regions.    

iii). Minerals like calcium magnesium bicarbonate and calcium carbonate present in limestone are soluble in water containing carbonic acid and are transported away in water as a solution.    

iv). Carbon dioxide for med by decomposing organic matter along with soil water significantly assists in this reaction.    

v). Sodium chloride is also a rock-forming mineral and is vulnerable to this process of solution.  

vi). Carbonation, oxidation and Hydration go hand in hand and accelerate the weathering process.  

 Carbonation  

Carbonation is the reaction of bicarbonate and carbonate with minerals. It is a general process helping the fragmentation of feldspars and carbonate minerals. Carbon dioxide from the soil and atmospheric air is absorbed by water to form carbonic acid that acts like weak acid. Magnesium carbonates and Calcium carbonates are dissolved in carbonic acid. These are removed in a solution without leaving any residue resulting in cave formation.  

Hydration  

Hydration is the chemical addition of water. Minerals take up  water and enlarge. This enlargement causes an increase in the volume of the material itself or rock. This process is long and reversible, sustained recurrence of this process causes fatigue in the rocks. This may lead to their disintegration of rocks.  

Oxidation And Reduction  

i). In weathering, oxidation denotes a mixture of a mineral with oxygen to form hydroxides or oxides.   

ii). Oxidation happens where there is ready access to the oxygenated waters and atmosphere. The minerals commonly involved in this process are manganese, Sulphur, iron, etc. In the process of oxidation, rock fragmentation happens due to the disturbance caused by adding of oxygen. 

iii). Red color of iron upon oxidation turns to yellow or brown. When oxidized minerals are positioned in a situation where oxygen is absent, reduction occurs. Such circumstances exist commonly below the water table, waterlogged ground and in areas of stagnant water. Red color of iron upon reduction turns to greenish or bluish grey. These weathering processes are interconnected.  

Physical Weathering Processes  

Physical or mechanical weathering processes are influenced by some applied forces. The applied forces are:    

i) Gravitational forces like shearing stress, load, and overburden pressure.   

ii) Expansion forces due to crystal growth, animal activity or temperature variations.  

iii) Water pressures regulated by drying and wetting cycles.    

Many of these forces are applied both at the surface and within different earth materials leading to rock breakage. Most of the physical weathering processes are caused by pressure release and thermal expansion.  

  

Unloading and Expansion  

i) Elimination of covering rock load because of sustained erosion causes vertical pressure release with the result that the upper layers of the rock enlarge producing fragmentation of rock masses.   

ii) Fractures will occur roughly parallel to the ground surface. In areas of curved ground surface, arched fractures incline to create massive sheets or exfoliation slabs of rock.  

iii) Exfoliation sheets resulting from expansion due to pressure release and unloading may measure hundreds or even thousands of metres in horizontal extent. Big, smooth rounded domes are called exfoliation domes.  

Temperature Changes And Expansion  

i) Several minerals in rocks possess their own limits of contraction and expansion.   

ii) With an upsurge in temperature, all minerals enlarge and thrust against its neighbor and as temperature drops, a corresponding shrinkage takes place.

iii) Due to diurnal changes in the temperatures, this internal movement among the mineral grains of the superficial layers of rocks takes place repeatedly. This process is effective in high elevations and arid climates where diurnal temperature variations are extreme.  

  

Freezing Thawing And Frost Weathering  

i) Frost weathering happens due to development of ice within openings and cracks of rocks during recurrent cycles of melting and freezing. This process is effective at high elevations in mid-latitudes where melting and freezing is frequently recurrent.   

ii) Glacial regions are subject to frost wedging every day.  

iii) In this course, the rate of freezing is significant. Hasty freezing of water causes its high pressure and rapid expansion. The resulting expansion affects joints, cracks and small intergranular fractures to become wider and wider till the rock breaks apart.  

  

Salt Weathering  

Salt crystallization is most effective of all salt-weathering processes. Salts in rocks enlarge due to hydration, crystallization and thermal action. Various salts like sodium, barium, calcium, potassium, and magnesium, have an inclination to enlarge. Enlargement of these salts relies on temperature and their thermal properties. High-temperature ranges between 30 and 50 degrees Celsius of surface temperatures in deserts support such salt expansion. Salt crystals in the adjacent surface pores cause splitting of single grains within rocks, which ultimately drop. This process of dropping of individual grains may result in granular disintegration or granular foliation. 

Biological Activity And Weathering  

i) 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 chemical attack and assists in the penetration of moisture and air.  

ii) Decaying plant and animal matter help in the production of humic , carbonic and other acids which enhance decay and solubility of some elements.  

iii) Algae utilize mineral nutrients for growth and help in the concentration of iron and manganese oxides. 

  

Special Effects Of Weathering 

Exfoliation  : (i) Exfoliation is a result but not a process. (ii) Flaking off of more or less curved sheets of shells from over rocks or bedrock results in smooth and rounded surfaces. (iii) Exfoliation can occur due to expansion and contraction induced by temperature changes. (iv) Exfoliation domes and tors result due to unloading and thermal expansion respectively.  

Significance Of Weathering    (i) Weathering processes are responsible for breaking down the rocks into smaller fragments and preparing the way for formation of not only regolith and soils, but also erosion and mass movements.  (ii) Biomes and bio- diversity is basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.  (iii) Erosion cannot be significant if the rocks are not weathered. That means, 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, aluminum, copper etc., which are of great importance for the national economy. (iv) Enrichment : When rocks undergo weathering, some materials are removed through chemical or physical leaching by groundwater and thereby the concentration of remaining (valuable) materials increases. Without such a weathering taking place, the concentration of the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is called enrichment.  

Mass Movement  

I). These movements transfer the mass of rock debris down the slopes under the direct influence of gravity. That means, air, water or ice do not carry debris with them from place to place but on the other hand the debris may carry with it air, water or ice. 

II). Gravity exerts its force on all matter, both bedrock and the products of weathering. So, weathering is not a prerequisite for mass movement though it aids mass movements.   

III). Mass movements are very active over weathered slopes rather than over un weathered materials. Mass movements do not come under erosion though there is a shift (aided by gravity) of material from one place to another.  

IV). Several activating causes precede mass movements. They are: 

(a) Removal of support from below to materials above through natural or artificial means; 

(b) Increase in gradient and height of slopes; 

(c) Overloading through addition of materials naturally or by artificial filling.

(d) Overloading due to heavy rainfall, saturation and lubrication of slope materials.

(e) Removal of material or load from over the original slope surfaces.  

(f) Occurrence of earthquakes, explosions or machinery.

(g) Excessive natural seepage. 

(h) Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks.

(i) Indies - criminate removal of natural vegetation.   

V). Heave (heaving up of soils due to frost growth and other causes), flow and slide are the three forms of movements.  

VI). Mass movement can be grouped under two major classes. They are: slow movements and rapid movements.  

Slow Movement  

i) Movement of materials is extremely slow and imperceptible except through                           extended observation. Materials involved can be soil or rock debris. Creep is one type under this category which can occur on moderately steep, soil covered slopes.  

ii) 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.   

iii) Solifluction which involves slow downslope flowing soil mass or fine grained rock debris saturated or lubricated with water. This process is quite common in moist temperate areas where surface melting of deeply frozen ground and long continued rain respectively, occur frequently. 

iv) When the upper portions get saturated and when the lower parts are impervious to water percolation, flowing occurs in the upper parts.  

Rapid Movement :  

I) 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.  

II) 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. Mudflows can cause great destruction to human habitations.  

III) 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.  

IV) Landslides : 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 And Deposition  

i) 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.  

ii) 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.  

iii) The erosion and transportation of earth materials is brought about by wind, running water, glaciers, wind and waves and ground water.  

iv) Deposition is a consequence of erosion. The erosional agents loose their velocity and hence energy on gentler slopes and the materials carried by them start to settle themselves.  

  

Soil Formation  

1. Soil is the collection of natural bodies on the earth’s surface containing living matter and supporting or capable of supporting plants.  

2. Soil is a dynamic material in which many chemical, biological, and physical activities go on constantly. It is the result of decay, it is also a medium of growth. It is changing and developing body. Characteristics are changing from season to season.  

3. Too cold, too hot, and dry areas biological activity stops. organic matter increases when leaves and decompose.  

  

Process Of Soil Formation  

1. Weathering is the basic process of soil formation.  

2. The weathered material is transported and decomposed due to bacteria lichens and mosses.  

3. The dead remains of organisms and plants help in humus accumulation. Mior grasses and ferns may grow. Bushes and trees also grow. Plant roots and burrowing animals helps the soil formation.  

  

Soil Forming Patterns  

Five basic factors control the formation of soils:  

i) Parent material,   

ii) Topography,  

iii) Climate,  

iv) Biological activity,  

v) Time.   

 In fact soil forming factors act in union and affect the action of one another.  

  

I) Parent Material  

It is a passive control factor in soil formation. It can be any in-situ 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.  

Nature and rate of weathering and depth of weathering mantle are important considerations under parent materials.  

  

II) Topography  

(a) It is a passive control factor. 

(b) Soils will be thin on steep slopes and thick over flat upland areas. Over gentle slopes where erosion is slow and percolation of water is good, soil formation is very favorable.  

(c) Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark color.  

 

III) Climate  

(a) It is an active factor in soil formation. Climatic elements are:   Moisture in terms of its intensity, frequency and duration of precipitation - evaporation and humidity,  Temperature in terms of seasonal and diurnal variations, temperature in terms of seasonal and diurnal variations.  

(b) Precipitation increases the chemical and biological activities.   

(c) Excess of water helps in the downward transportation of soil components through the soil (eluviation) and deposits the same down below (illuviation).  

(d) Heavy rainfall removes the calcium, magnesium, sodium, potassium along with silica. Removal of silica is called desalination.  

(e) In dry areas excess of evaporation leads to deposition of salts on the surface of the soil. These salt layers are called ‘hard pans’ in the hot deserts.  

(f) In tropical climates, under moderate rainfall conditions calcium carbonate nodules are formed.  

  

  

IV) Biological Activity  

a) Plants and animals add organic matter to the soil. also helps in moisture retention. 

b) Dead plants add humus to the soil In humid areas, the bacterial activity is higher than cold areas. As a result undecomposed material is found in cold areas.  

(c) In hot areas bacteria fix the nitrogen in the soil which is used by the plants Rhizobium is the bacteria fix the nitrogen in the soil and live in the roots of leguminous plants, termites, rodents, earthworms change the chemical composition of the soil.  

  

V) Time  

a) Important controlling factor of soil formation.  

b) Longer the time, thicker the soil layers.  

c) No specific length of time in absolute terms can be fixed for soils to develop and mature. 

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