Notes of Ch 6 Geomorphic Processes| Class 11th Geography

Why earth is uneven?

• The earth’s crust is dynamic. It moved and moves vertically and horizontally.

• The differences in the internal forces operating from within the earth which built up the crust have been responsible for the variations in the outer surface of the crust.

• The earth’s surface is being continuously subjected to external forces induced basically by energy.

• 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.

Geomorphic Processes

• 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.

• Diastrophism and volcanism are endogenic geomorphic processes.

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

• 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 level. Geomorphic processes and geomorphic agents especially exogenic, unless stated separately, are one and the same.

• 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 on the movement of all surface material on earth.

• 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

• 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. 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.

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

• It include geological phenomena and processes that originate externally to the Earth's surface.

• They are genetically related to the atmosphere, hydrosphere and biosphere, and therefore to processes of weathering, erosion, transportation, deposition, denudation etc.

• 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.

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

• Different types of rocks with differences in their structure offer varying resistances to various geomorphic processes.

Weathering

• 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 materials in order to reduce them to fragmental state.

• Weathering is defined as mechanical disintegration and chemical decom position of rocks through the actions of various elements of weather and climate.

• 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. Climate is of particular importance. Not only weathering processes differ from climate to climate, but also the depth of the weathering mantle

• There are three major groups of weathering processes:
(i) Chemical;
(ii) Physical or mechanical;
(iii) 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 .

• 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 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. So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.

Physical Weathering Processes

• Physical or mechanical weathering processes depend on some applied forces. The applied forces could be:
(i) gravitational forces such as over burden pressure, load and shearing stress;
(ii) expansion forces due to temperature changes, crystal growth or animal activity;
(iii) 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.

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. Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.

Special Effects of Weathering

Exfoliation

• Exfoliation is a result but not a process. Flaking off of more or less curved sheets of shells from over rocks or bedrock results in smooth and rounded surfaces.

• Exfoliation can occur due to expansion and contraction induced by temperature changes.

• Exfoliation domes and tors result due to unloading and thermal expansion respectively.

Significance of Weathering

• 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.

• Biomes and bio- diversity is basically a result of forests (vegetation) and forests depend upon the depth of weathering mantles.

• 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, aluminium, copper etc., which are of great importance for the national economy.
Weathering is an important process in the formation of soils.

• 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 what is called enrichment.

Mass Movements

• 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.

• Gravity exerts its force on all matter, both bedrock and the products of weathering. So, 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 do not come under erosion though there is a shift (aided by gravity) of materials from one place to another.

• Several activating causes precede mass movements. They are:
(i) Removal of support from below to materials above through natural or artificial means;
(ii) Increase in gradient and height of slopes;
(iii) Overloading through addition of materials naturally or by artificial filling;
(iv) Overloading due to heavy rainfall, saturation and lubrication of slope materials;
(v) Removal of material or load from over the original slope surfaces;
(vi) Occurrence of earthquakes, explosions or machinery;
(vii) Excessive natural seepage;
(viii) Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks;
(ix) Indis- criminate removal of natural vegetation.

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

Landslides

• These are rapid and perceptible movements. dry materials are found.

• The size and shape of the materials are depending on the nature of the rock, degree of weathering,
steepness of slope.

• 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.

• Sliding of individual rock masses down bedding, joint or fault surfaces is rockslide.

• Rock fall is free falling of rock blocks over any steep slope keeping itself away from the slope.

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.

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

• 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

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

• 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.

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

Process of Soil Formation

• Weathering is basic process for soil formation.

• The weathered material is transported and decomposed due to bacteria lichens and moss.

• The dead remains increases the humus of the soil. minor grasses and ferns can grow. Bushes,
trees also grow. plants roots and burrowing animals help the soil formation.

Soil Forming Factors

• Parent material
• Topography
• Climate
• Biological activity
• Time

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.

Topography

• It is a passive control factor.
• 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 favourable.

• Soils over flat areas may develop a thick layer of clay with good accumulation of organic matter giving the soil dark colour.

Climate

• It is an active factor in soil formation.

• Climatic elements are:
(i) moisture in terms of its intensity, frequency and duration of precipitation - evaporation and humidity;
(ii) temperature in terms of seasonal and diurnal variations

• Precipitation increases the biological activity.

• Excess of water helps to transport the dissolved particles to downward (eluviation).

• Deposition of these particles is called ‘Illuviation’.

• Heavy rainfall removes the calcium, magnesium, sodium, potasium along with silica.

• Removal of silica is called desilication.

• 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.

• In tropical climates, under moderate rainfall conditions calcium carbonate nodules are
formed.

Biological activity

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

• 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.

• 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 legumenace plantsants, temites, rodents, earthworms change the chemical composition of the soil.

Time

• Important controlling factor of soil formation.

• Longer the time, thicker the soil layers.

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