Geomorphic Processes PDF

CHAPTER GEOMORPHIC PROCESSES d he A fter learning about how the earth was forces continuously elevat...

CHAPTER GEOMORPHIC PROCESSES d he A fter learning about how the earth was forces continuously elevate or build up parts born, how it evolved its crust and other of the earth’s surface and hence the exogenic inner layers, how its crustal plates processes fail to even out the relief variations is moved and are moving, and other information of the surface of the earth. So, variations remain on earthquakes, the forms of volcanism and as long as the opposing actions of exogenic and about the rocks and minerals the crust is endogenic forces continue. In general terms, bl composed of, it is time to know in detail about the endogenic forces are mainly land building the surface of the earth on which we live. Let forces and the exogenic processes are mainly us start with this question. pu land wearing forces. The surface of the earth is sensitive. Humans depend on it for their Why is the surface of the earth uneven? sustenance and have been using it extensively and intensively. So, it is essential to understand be T The earth’s crust is dynamic. You are well its nature in order to use it effectively without re aware that it has moved and moves vertically disturbing its balance and diminishing its o R and horizontally. Of course, it moved a bit faster potential for the future. Almost all organisms in the past than the rate at which it is moving contribute to sustain the earth’s environment. now. The differences in the internal forces However, humans have caused extensive tt E operating from within the earth which built up damage to the environment through over use the crust have been responsible for the of resources. Use we must, but must also leave variations in the outer surface of the crust. The it potential enough to sustain life through the C earth’s surface is being continuously subjected future. Most of the surface of the earth had and to external forces induced basically by energy has been shaped over very long periods of time (sunlight). Of course, the internal forces are still (hundreds and thousands of years) and no N active though with different intensities. That because of its use and misuse by humans its means, the earth’s surface is being potential is being diminished at a fast rate. If continuously subjected to by external forces the processes which shaped and are shaping originating within the earth’s atmosphere and the surface of the earth into varieties of forms © by internal forces from within the earth. The (shapes) and the nature of materials of which external forces are known as exogenic forces it is composed of, are understood, precautions and the internal forces are known as endogenic can be taken to minimise the detrimental effects forces. The actions of exogenic forces result in of human use and to preserve it for posterity. wearing down (degradation) of relief/elevations and filling up (aggradation) of basins/ GEOMORPHIC PROCESSES depressions, on the earth’s surface. The You would like to know the meaning of phenomenon of wearing down of relief geomorphic processes. The endogenic and variations of the surface of the earth through exogenic forces causing physical stresses and erosion is known as gradation. The endogenic chemical actions on earth materials and  46 FUNDAMENTALS OF PHYSICAL GEOGRAPHY bringing about changes in the configuration ENDOGENIC PROCESSES of the surface of the earth are known as The energy emanating from within the earth is geomorphic processes. Diastrophism and the main force behind endogenic geomorphic volcanism are endogenic geomorphic processes. This energy is mostly generated by processes. These have already been discussed radioactivity, rotational and tidal friction and in brief in the preceding unit. Weathering, mass primordial heat from the origin of the earth. wasting, erosion and deposition are exogenic This energy due to geothermal gradients and geomorphic processes. These exogenic heat flow from within induces diastrophism processes are dealt with in detail in this chapter. and volcanism in the lithosphere. Due to d Any exogenic element of nature (like water, variations in geothermal gradients and heat flow ice, wind, etc.,) capable of acquiring and from within, crustal thickness and strength, he transporting earth materials can be called a the action of endogenic forces are not uniform geomorphic agent. When these elements of and hence the tectonically controlled original nature become mobile due to gradients, they crustal surface is uneven. remove the materials and transport them over is slopes and deposit them at lower level. Diastrophism Geomorphic processes and geomorphic agents All processes that move, elevate or build up especially exogenic, unless stated separately, portions of the earth’s crust come under bl are one and the same. diastrophism. They include: (i) orogenic A process is a force applied on earth processes involving mountain building materials affecting the same. An agent is a pu through severe folding and affecting long and mobile medium (like running water, moving ice narrow belts of the earth’s crust; (ii) epeirogenic masses, wind, waves and currents etc.) which processes involving uplift or warping of large removes, transports and deposits earth parts of the earth’s crust; (iii) earthquakes be T materials. Running water, groundwater, involving local relatively minor movements; glaciers, wind, waves and currents, etc., can re (iv) plate tectonics involving horizontal o R be called geomorphic agents. movements of crustal plates. In the process of orogeny, the crust is Do you think it is essential to distinguish severely deformed into folds. Due to epeirogeny, tt E geomorphic agents and geomorphic there may be simple deformation. Orogeny is processes? a mountain building process whereas epeirogeny is continental building process. C Gravity besides being a directional force Through the processes of orogeny, epeirogeny, activating all downslope movements of matter earthquakes and plate tectonics, there can be also causes stresses on the earth’s materials. faulting and fracturing of the crust. All these no N Indirect gravitational stresses activate wave and processes cause pressure, volume and tide induced currents and winds. Without temperature (PVT) changes which in turn gravity and gradients there would be no induce metamorphism of rocks. mobility and hence no erosion, transportation © and deposition are possible. So, gravitational Epeirogeny and orogeny, cite the stresses are as important as the other differences. 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 Volcanism all surface material on earth. All the movements Volcanism includes the movement of molten either within the earth or on the surface of the rock (magma) onto or toward the earth’s earth occur due to gradients — from higher surface and also formation of many intrusive levels to lower levels, from high pressure to low and extrusive volcanic forms. Many aspects of pressure areas etc. volcanism have already been dealt in detail  GEOMORPHIC PROCESSES 47 under volcanoes in the Unit II and under processes and their respective driving forces. igneous rocks in the preceding chapter in this It should become clear from this chart that for unit. each process there exists a distinct driving force or energy. What do the words volcanism and As there are different climatic regions on volcanoes indicate? the earth’s surface owing to thermal gradients created by latitudinal, seasonal and land and water spread variations, the exogenic EXOGENIC PROCESSES geomorphic processes vary from region to d The exogenic processes derive their energy region. The density, type and distribution of from atmosphere determined by the ultimate vegetation which largely depend upon energy from the sun and also the gradients he created by tectonic factors. Why do you think that the slopes or gradients are created by tectonic factors? is Gravitational force acts upon all earth materials having a sloping surface and tend to bl produce movement of matter in down slope direction. Force applied per unit area is called stress. Stress is produced in a solid by pushing Figure 6.1 : Denudational processes and their driving forces pu or pulling. This induces deformation. Forces acting along the faces of earth materials are shear stresses (separating forces). It is this precipitation and temperature exert influence indirectly on exogenic geomorphic processes. be T stress that breaks rocks and other earth Within different climatic regions there may be materials. The shear stresses result in angular local variations of the effects of different climatic re displacement or slippage. Besides the elements due to altitudinal differences, aspect o R gravitational stress earth materials become variations and the variation in the amount of subjected to molecular stresses that may be insolation received by north and south facing caused by a number of factors amongst which slopes as compared to east and west facing tt E temperature changes, crystallisation and slopes. Further, due to differences in wind melting are the most common. Chemical velocities and directions, amount and kind of processes normally lead to loosening of bonds C precipitation, its intensity, the relation between between grains, dissolving of soluble minerals precipitation and evaporation, daily range of or cementing materials. Thus, the basic reason temperature, freezing and thawing frequency, that leads to weathering, mass movements, and no N depth of frost penetration, the geomorphic erosion is development of stresses in the body processes vary within any climatic region. of the earth materials. As there are different climatic regions on What is the sole driving force behind all the earth’s surface the exogenic geomorphic the exogenic processes? © processes vary from region to region. Temperature and precipitation are the two Climatic factors being equal, the intensity important climatic elements that control of action of exogenic geomorphic processes various processes. depends upon type and structure of rocks. The All the exogenic geomorphic processes are term structure includes such aspects of rocks covered under a general term, denudation. The as folds, faults, orientation and inclination of word ‘denude’ means to strip off or to uncover. beds, presence or absence of joints, bedding Weathering, mass wasting/movements, erosion planes, hardness or softness of constituent and transportation are included in denudation. minerals, chemical susceptibility of mineral The flow chart (Figure 6.1) gives the denudation constituents; the permeability or impermeability  48 FUNDAMENTALS OF PHYSICAL GEOGRAPHY etc. Different types of rocks with differences in their structure offer varying resistances to various geomorphic processes. A particular rock may be resistant to one process and non- resistant to another. And, under varying climatic conditions, particular rocks may exhibit different degrees of resistance to geomorphic processes and hence they operate at differential rates and give rise to differences d in topography. The effects of most of the exogenic geomorphic processes are small and slow and may be imperceptible in a short time he span, but will in the long run affect the rocks severely due to continued fatigue. Finally, it boils down to one fact that the differences on the surface of the earth though Figure 6.2 : Climatic regimes and depth of weathering mantles (adapted and modified from Strakhov, 1967) is 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 Activity bl materials, differences in geomorphic processes and in their rates of operation. Mark the latitude values of different Some of the exogenic geomorphic processes climatic regimes in Figure 6.2 and compare the details. pu have been dealt in detail here. WEATHERING There are three major groups of weathering be T Weathering is action of elements of weather and processes : (i) chemical; (ii) physical or mechanical; (iii) biological weathering processes. re climate over earth materials. There are a Very rarely does any one of these processes ever o R number of processes within weathering which act either individually or together to affect the operate completely by itself, but quite often a earth materials in order to reduce them to dominance of one process can be seen. tt E fragmental state. Chemical Weathering Processes Weathering is defined as mechanical A group of weathering processes viz; solution, C disintegration and chemical decom- carbonation, hydration, oxidation and position of rocks through the actions of reduction act on the rocks to decompose, various elements of weather and climate. dissolve or reduce them to a fine clastic state no N through chemical reactions by oxygen, surface As very little or no motion of materials and/or soil water and other acids. Water and takes place in weathering, it is an in-situ or air (oxygen and carbon dioxide) along with on-site process. heat must be present to speed up all chemical © reactions. Over and above the carbon dioxide Is this little motion which can occur present in the air, decomposition of plants and sometimes due to weathering synonymous animals increases the quantity of carbon with transportation? If not, why? dioxide underground. These chemical Weathering processes are conditioned by reactions on various minerals are very much many complex geological, climatic, topographic similar to the chemical reactions in a laboratory. and vegetative factors. Climate is of particular importance. Not only weathering processes Solution differ from climate to climate, but also the depth When something is dissolved in water or acids, of the weathering mantle (Figure 6.2). the water or acid with dissolved contents is  GEOMORPHIC PROCESSES 49 called solution. This process involves removal Many clay minerals swell and contract during of solids in solution and depends upon wetting and drying and a repetition of this solubility of a mineral in water or weak acids. process results in cracking of overlying On coming in contact with water many solids materials. Salts in pore spaces undergo rapid and repeated hydration and help in rock disintegrate and mix up as suspension in fracturing. The volume changes in minerals water. Soluble rock forming minerals like due to hydration will also help in physical nitrates, sulphates, and potassium etc. are weathering through exfoliation and granular affected by this process. So, these minerals are disintegration. d easily leached out without leaving any residue in rainy climates and accumulate in dry Oxidation and Reduction he regions. Minerals like calcium carbonate and In weathering, oxidation means a combination calcium magnesium bicarbonate present in of a mineral with oxygen to form oxides or limestones are soluble in water containing hydroxides. Oxidation occurs where there is carbonic acid (formed with the addition of ready access to the atmosphere and is carbon dioxide in water), and are carried away oxygenated waters. The minerals most in water as solution. Carbon dioxide produced commonly involved in this process are iron, by decaying organic matter along with soil manganese, sulphur etc. In the process of bl water greatly aids in this reaction. Common oxidation rock breakdown occurs due to the salt (sodium chloride) is also a rock forming disturbance caused by addition of oxygen. Red colour of iron upon oxidation turns to brown mineral and is susceptible to this process of pu or yellow. When oxidised minerals are placed solution. in an environment where oxygen is absent, reduction takes place. Such conditions exist be T Carbonation usually below the water table, in areas of Carbonation is the reaction of carbonate and stagnant water and waterlogged ground. Red re o R bicarbonate with minerals and is a common colour of iron upon reduction turns to greenish process helping the breaking down of or bluish grey. These weathering processes are inter- feldspars and carbonate minerals. Carbon tt E related. Hydration, carbonation and oxidation dioxide from the atmosphere and soil air is go hand in hand and hasten the weathering absorbed by water, to form carbonic acid that process. C acts as a weak acid. Calcium carbonates and magnesium carbonates are dissolved in Can we give iron rusting as an example carbonic acid and are removed in a solution no N of oxidation? How essential is water in without leaving any residue resulting in cave chemical weathering processes? Can formation. chemical weathering processes dominate in water scarce hot deserts? Hydration © Hydration is the chemical addition of water. Minerals take up water and expand; this Physical Weathering Processes expansion causes an increase in the volume of Physical or mechanical weathering processes the material itself or rock. Calcium sulphate depend on some applied forces. The applied takes in water and turns to gypsum, which is forces could be: (i) gravitational forces such as more unstable than calcium sulphate. This overburden pressure, load and shearing stress; process is reversible and long, continued (ii) expansion forces due to temperature repetition of this process causes fatigue in the changes, crystal growth or animal activity; rocks and may lead to their disintegration. (iii) water pressures controlled by wetting and  50 FUNDAMENTALS OF PHYSICAL GEOGRAPHY drying cycles. Many of these forces are applied temperatures, this internal movement among both at the surface and within different earth the mineral grains of the superficial layers of materials leading to rock fracture. Most of the rocks takes place regularly. This process is physical weathering processes are caused by most effective in dry climates and high thermal expansion and pressure release. These elevations where diurnal temperature changes processes are small and slow but can cause are drastic. As has been mentioned earlier great damage to the rocks because of though these movements are very small they continued fatigue the rocks suffer due to make the rocks weak due to continued fatigue. repetition of contraction and expansion. The surface layers of the rocks tend to expand d more than the rock at depth and this leads to Unloading and Expansion the formation of stress within the rock resulting he in heaving and fracturing parallel to the Removal of overlying rock load because of surface. Due to differential heating and continued erosion causes vertical pressure resulting expansion and contraction of surface release with the result that the upper layers of layers and their subsequent exfoliation from the rock expand producing disintegration of is the surface results in smooth rounded surfaces rock masses. Fractures will develop roughly in rocks. In rocks like granites, smooth parallel to the ground surface. In areas of surfaced and rounded small to big boulders bl curved ground surface, arched fractures tend called tors form due to such exfoliation. to produce massive sheets or exfoliation slabs of rock. Exfoliation sheets resulting from pu What is the difference between exfoliation expansion due to unloading and pressure domes and exfoliated tors? release may measure hundreds or even thousands of metres in horizontal extent. Large, be T smooth rounded domes called exfoliation Freezing, Thawing and Frost Wedging domes (Figure 6.3) result due to this process. re Frost weathering occurs due to growth of ice o R within pores and cracks of rocks during repeated cycles of freezing and melting. This process is most effective at high elevations in tt E mid-latitudes where freezing and melting is often repeated. Glacial areas are subject to frost wedging daily. In this process, the rate of C freezing is important. Rapid freezing of water causes its sudden expansion and high pressure. The resulting expansion affects joints, cracks no N and small inter granular fractures to become wider and wider till the rock breaks apart. Figure 6.3 : A large exfoliation dome in granite rock Salt Weathering © near bhongir (Bhuvanagiri) town in Andhra Pradesh Salts in rocks expand due to thermal action, Temperature Changes and Expansion hydration and crystallisation. Many salts like calcium, sodium, magnesium, potassium and Various minerals in rocks possess their own barium have a tendency to expand. Expansion limits of expansion and contraction. With rise of these salts depends on temperature and in temperature, every mineral expands and their thermal properties. High temperature pushes against its neighbour and as ranges between 30 and 50 oC of surface temperature falls, a corresponding contraction temperatures in deserts favour such salt takes place. Because of diurnal changes in the expansion. Salt crystals in near-surface pores  GEOMORPHIC PROCESSES 51 cause splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may 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 d gypsum crystals in desert areas heave up overlying layers of materials and with the result he polygonal cracks develop all over the heaved surface. With salt crystal growth, chalk breaks Fig.6.4 : Exfoliation (Flacking) and granular down most readily, followed by limestone, disintegration sandstone, shale, gneiss and granite etc. temperature changes. Exfoliation domes and is tors result due to unloading and thermal BIOLOGICAL ACTIVITY AND WEATHERING expansion respectively. Biological weathering is contribution to or bl removal of minerals and ions from the SIGNIFICANCE OF WEATHERING weathering environment and physical changes Weathering processes are responsible for due to growth or movement of organisms. pu breaking down the rocks into smaller Burrowing and wedging by organisms like fragments and preparing the way for formation earthworms, termites, rodents etc., help in of not only regolith and soils, but also erosion exposing the new surfaces to chemical attack and mass movements. Biomes and bio- be T and assists in the penetration of moisture and diversity is basically a result of forests re air. Human beings by disturbing vegetation, (vegetation) and forests depend upon the depth o R ploughing and cultivating soils, also help in of weathering mantles. Erosion cannot be mixing and creating new contacts between air, significant if the rocks are not weathered. That water and minerals in the earth materials. means, weathering aids mass wasting, erosion tt E Decaying plant and animal matter help in the and reduction of relief and changes in production of humic, carbonic and other acids landforms are a consequence of erosion. which enhance decay and solubility of some Weathering of rocks and deposits helps in the C elements. Plant roots exert a tremendous enrichment and concentrations of certain pressure on the earth materials mechanically valuable ores of iron, manganese, aluminium, copper etc., which are of great importance for no N breaking them apart. the national economy. Weathering is an SPECIAL EFFECTS OF WEATHERING important process in the formation of soils. Exfoliation When rocks undergo weathering, some © This has already been explained under materials are removed through chemical physical weathering processes of unloading, or physical leaching by groundwater and thermal contraction and expansion and salt thereby the concentration of remaining weathering. Exfoliation is a result but not a (valuable) materials increases. Without process. Flaking off of more or less curved such a weathering taking place, the sheets of shells from over rocks or bedrock concentration of the same valuable results in smooth and rounded surfaces material may not be sufficient and (Figures 6.3; 6.4). Exfoliation can occur due economically viable to exploit, process and to expansion and contraction induced by refine. This is what is called enrichment.  52 FUNDAMENTALS OF PHYSICAL GEOGRAPHY MASS MOVEMENTS the three forms of movements. Figure 6.5 shows the relationships among different types of mass These movements transfer the mass of rock movements, their relative rates of movement debris down the slopes under the direct and moisture limits. 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. The movements of mass may range from slow to rapid, d affecting shallow to deep columns of materials and include creep, flow, slide and fall. Gravity exerts its force on all matter, both bedrock and he 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 is over unweathered materials. Mass movements are aided by gravity and no geomorphic agent like running water, bl glaciers, wind, waves and currents participate in the process of mass movements. That means Figure 6.5 : Relationships among different types of mass movements, their relative rates of movement mass movements do not come under erosion pu and moisture limits (after Whitehead, 2001) though there is a shift (aided by gravity) of materials from one place to another. Materials Mass movements can be grouped under over the slopes have their own resistance to two major classes: (i) slow movements; be T disturbing forces and will yield only when force (ii) rapid movements. is greater than the shearing resistance of the re Slow Movements o R materials. Weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping Creep is one type under this category which beds, vertical cliffs or steep slopes, abundant can occur on moderately steep, soil covered tt E precipitation and torrential rains and scarcity slopes. Movement of materials is extremely of vegetation etc., favour mass movements. slow and imperceptible except through Several activating causes precede mass extended observation. Materials involved can C movements. They are : (i) removal of support be soil or rock debris. Have you ever seen fence from below to materials above through natural posts, telephone poles lean downslope from or artificial means; (ii) increase in gradient and their vertical position and in their linear no N height of slopes; (iii) overloading through alignment? If you have, that is due to the creep addition of materials naturally or by artificial effect. Depending upon the type of material involved, several types of creep viz., soil creep, filling; (iv) overloading due to heavy rainfall, talus creep, rock creep, rock-glacier creep etc., saturation and lubrication of slope materials; © can be identified. Also included in this group (v) removal of material or load from over the is solifluction which involves slow downslope original slope surfaces; (vi) occurrence of flowing soil mass or fine grained rock debris earthquakes, explosions or machinery; saturated or lubricated with water. This process (vii) excessive natural seepage; (viii) heavy is quite common in moist temperate areas drawdown of water from lakes, reservoirs and where surface melting of deeply frozen ground rivers leading to slow outflow of water from and long continued rain respectively, occur under the slopes or river banks; (ix) indis- frequently. When the upper portions get criminate removal of natural vegetation. saturated and when the lower parts are Heave (heaving up of soils due to frost impervious to water percolation, flowing occurs growth and other causes), flow and slide are in the upper parts.  GEOMORPHIC PROCESSES 53 Rapid Movements discontinuities in the rock, the degree of weathering and the steepness of the slope. These movements are mostly prevalent in Depending upon the type of movement of humid climatic regions and occur over gentle materials several types are identified in this to steep slopes. Movement of water-saturated category. clayey or silty earth materials down low-angle Slump is slipping of one or several units of terraces or hillsides is known as earthflow. rock debris with a backward rotation with Quite often, the materials slump making step- respect to the slope over which the movement like terraces and leaving arcuate scarps at their takes place (Figure 6.6). Rapid rolling or sliding heads and an accumulation bulge at the toe. d When slopes are steeper, even the bedrock especially of soft sedimentary rocks like shale he or deeply weathered igneous rock may slide downslope. Another type in this category is mudflow. In the absence of vegetation cover and with is heavy rainfall, thick layers of weathered materials get saturated with water and either slowly or rapidly flow down along definite bl channels. It looks like a stream of mud within a valley. When the mudflows emerge out of Figure 6.6 : Slumping of debris with backward rotation channels onto the piedmont or plains, they can pu be very destructive engulfing roads, bridges of earth debris without backward rotation of and houses. Mudflows occur frequently on the mass is known as debris slide. Debris fall is slopes of erupting or recently erupted volcanoes. nearly a free fall of earth debris from a vertical be T Volcanic ash, dust and other fragments turn or overhanging face. Sliding of individual rock into mud due to heavy rains and flow down as re masses down bedding, joint or fault surfaces tongues or streams of mud causing great o R is rockslide. Over steep slopes, rock sliding is destruction to human habitations. very fast and destructive. Figure 6.7 shows A third type is the debris avalanche, which landslide scars over steep slopes. Slides occur is more characteristic of humid regions with tt E as planar failures along discontinuities like or without vegetation cover and occurs in bedding planes that dip steeply. Rock fall is narrow tracks on steep slopes. This debris free falling of rock blocks over any steep slope C avalanche can be much faster than the keeping itself away from the slope. Rock falls mudflow. Debris avalanche is similar to snow occur from the superficial layers of the rock avalanche. no N In Andes mountains of South America and the Rockies mountains of North America, there are a few volcanoes which erupted during the last decade and very © devastating mudflows occurred down their slopes during eruption as well as after eruption. Landslides These are relatively rapid and perceptible movements. The materials involved are relatively dry. The size and shape of the Figure 6.7 : Landslide scars in Shiwalik Himalayan ranges detached mass depends on the nature of near river Sarada at India-Nepal border, Uttar Pradesh  54 FUNDAMENTALS OF PHYSICAL GEOGRAPHY face, an occurrence that distinguishes it from erosion it is not a pre-condition for erosion to rockslide which affects materials up to a take place. Weathering, mass-wasting and substantial depth. erosion are degradational processes. It is erosion that is largely responsible for Between mass wasting and mass continuous changes that the earth’s surface is movements, which term do you feel is undergoing. As indicated in Figure 6.1, most appropriate? Why? Can solifluction denudational processes like erosion and be included under rapid flow movements? transportation are controlled by kinetic energy. Why it can be and can’t be? The erosion and transportation of earth d materials is brought about by wind, running water, glaciers, waves and ground water. Of In our country, debris avalanches and he these the first three agents are controlled by landslides occur very frequently in the climatic conditions. They represent three states Himalayas. There are many reasons for this. One, the Himalayas are tectonically of matter —gaseous (wind), liquid (running active. They are mostly made up of water) and solid (glacier) respectively. is sedimentary rocks and unconsolidated and semi-consolidated deposits. The Can you compare the three climatically slopes are very steep. Compared to the controlled agents? Himalayas, the Nilgiris bordering bl Tamilnadu, Karnataka, Kerala and the The erosion can be defined as “application Western Ghats along the west coast are of the kinetic energy associated with the agent relatively tectonically stable and are pu to the surface of the land along which it moves”. mostly made up of very hard rocks; but, Kinetic energy is computed as KE = 1/2 mv2 still, debris avalanches and landslides occur though not as frequently as in the where ‘m’ is the mass and ‘v’ is the velocity. be T Himalayas, in these hills. Why? Many Hence the energy available to perform work will slopes are steeper with almost vertical depend on the mass of the material and the re cliffs and escarpments in the Western velocity with which it is moving. Obviously then o R Ghats and Nilgiris. Mechanical weathering you will find that though the glaciers move at due to temperature changes and ranges very low velocities due to tremendous mass are is pronounced. They receive heavy more effective as the agents of erosion and wind, tt E amounts of rainfall over short periods. being in gaseous state, is less effective. So, there is almost direct rock fall quite The work of the other two agents of erosion- frequently in these places along with C waves and ground water is not controlled by landslides and debris avalanches. climate. In case of waves it is the location along the interface of litho and hydro sphere — no N coastal region — that will determine the work EROSION AND DEPOSITION of waves, whereas the work of ground water is Erosion involves acquisition and transportation determined more by the lithological character of rock debris. When massive rocks break into of the region. If the rocks are permeable and © smaller fragments through weathering and soluble and water is available only then karst any other process, erosional geomorphic topography develops. In the next chapter we agents like running water, groundwater, shall be dealing with the landforms produced glaciers, wind and waves remove and by each of these agents of erosion. transport it to other places depending upon Deposition is a consequence of erosion. The the dynamics of each of these agents. Abrasion erosional agents loose their velocity and hence by rock debris carried by these geomorphic energy on gentler slopes and the materials agents also aids greatly in erosion. By erosion, carried by them start to settle themselves. In relief degrades, i.e., the landscape is worn other words, deposition is not actually the work down. That means, though weathering aids of any agent. The coarser materials get  GEOMORPHIC PROCESSES 55 deposited first and finer ones later. By of the weathered material) which is the basic deposition depressions get filled up. The same input for soil to form. First, the weathered erosional agents viz., running water, glaciers, material or transported deposits are colonised wind, waves and groundwater act as by bacteria and other inferior plant bodies like aggradational or depositional agents also. mosses and lichens. Also, several minor What happens to the surface of the earth organisms may take shelter within the mantle due to erosion and deposition is elaborated in and deposits. The dead remains of organisms the next chapter on landforms and their and plants help in humus accumulation. Minor evolution. grasses and ferns may grow; later, bushes and d trees will start growing through seeds brought There is a shift of materials in mass in by birds and wind. Plant roots penetrate he movements as well as in erosion from one down, burrowing animals bring up particles, place to the other. So, why can’t both be mass of material becomes porous and sponge- treated as one and the same? Can there like with a capacity to retain water and to permit be appreciable erosion without rocks the passage of air and finally a mature soil, a undergoing weathering? complex mixture of mineral and organic is products forms. SOIL FORMATION Is weathering solely responsible for soil bl Soil and Soil Contents formation? If not, why? You see plants growing in soils. You play in pu the ground and come into contact with soil. Pedology is soil science. A pedologist is a You touch and feel soil and soil your clothes soil-scientist. while playing. Can you describe it? be T A pedologist who studies soils defines soil as a collection of natural bodies on the earth’s Soil-forming Factors re surface containing living and/or dead matter o R and supporting or capable of supporting plants. Five basic factors control the formation of soils: Soil is a dynamic medium in which many (i) parent material; (ii) topography; (iii) climate; chemical, physical and biological activities go (iv) biological activity; (v) time. In fact soil tt E on constantly. Soil is a result of decay, it is also forming factors act in union and affect the the medium for growth. It is a changing and action of one another. developing body. It has many characteristics C that fluctuate with the seasons. It may be Parent Material alternatively cold and warm or dry and moist. Parent material is a passive control factor in Biological activity is slowed or stopped if the no N soil becomes too cold or too dry. Organic matter soil formation. Parent materials can be any in- increases when leaves fall or grasses die. The situ or on-site weathered rock debris (residual soil chemistry, the amount of organic matter, soils) or transported deposits (transported the soil flora and fauna, the temperature and soils). Soil formation depends upon the texture © the moisture, all change with the seasons as (sizes of debris) and structure (disposition of well as with more extended periods of time. individual grains/particles of debris) as well That means, soil becomes adjusted to as the mineral and chemical composition of the conditions of climate, landform and vegetation rock debris/deposits. and will change internally when these Nature and rate of weathering and depth of controlling conditions change. weathering mantle are important considerations under parent materials. There may be Process of Soil Formation differences in soil over similar bedrock and dissimilar bedrocks may have similar soils Soil formation or pedogenesis depends first on above them. But when soils are very young weathering. It is this weathering mantle (depth and have not matured these show strong links  56 FUNDAMENTALS OF PHYSICAL GEOGRAPHY with the type of parent rock. Also, in case of climates and in areas with intermediate some limestone areas, where the weathering precipitation conditions, calcium carbonate processes are specific and peculiar, soils will nodules (kanker) are formed. show clear relation with the parent rock. Temperature acts in two ways — increasing or reducing chemical and biological activity. Topography Chemical activity is increased in higher Topography like parent materials is another temperatures, reduced in cooler temperatures passive control factor. The influence of (with an exception of carbonation) and stops topography is felt through the amount of in freezing conditions. That is why, tropical soils d exposure of a surface covered by parent with higher temperatures show deeper profiles materials to sunlight and the amount of and in the frozen tundra regions soils contain he surface and sub-surface drainage over and largely mechanically broken materials. through the parent materials. Soils will be thin Biological Activity on steep slopes and thick over flat upland areas. Over gentle slopes where erosion is slow The vegetative cover and organisms that occupy and percolation of water is good, soil formation the parent materials from the beginning and also is is very favourable. Soils over flat areas may at later stages help in adding organic matter, develop a thick layer of clay with good moisture retention, nitrogen etc. Dead plants accumulation of organic matter giving the soil provide humus, the finely divided organic matter bl dark colour. In middle latitudes, the south of the soil. Some organic acids which form facing slopes exposed to sunlight have different during humification aid in decomposing the conditions of vegetation and soils and the north pu minerals of the soil parent materials. facing slopes with cool, moist conditions have Intensity of bacterial activity shows up some other soils and vegetation. differences between soils of cold and warm climates. Humus accumulates in cold climates be T Climate as bacterial growth is slow. With undecomposed re Climate is an important active factor in soil organic matter because of low bacterial activity, o R formation. The climatic elements involved in soil layers of peat develop in sub-arctic and tundra development are : (i) moisture in terms of its climates. In humid tropical and equatorial intensity, frequency and duration of climates, bacterial growth and action is intense tt E precipitation - evaporation and humidity; and dead vegetation is rapidly oxidised leaving (ii) temperature in terms of seasonal and very low humus content in the soil. Further, diurnal variations. bacteria and other soil organisms take gaseous C Precipitation gives soil its moisture content nitrogen from the air and convert it into a which makes the chemical and biological chemical form that can be used by plants. This activities possible. Excess of water helps in the process is known as nitrogen fixation. no N downward transportation of soil components Rhizobium, a type of bacteria, lives in the root through the soil (eluviation) and deposits the nodules of leguminous plants and fixes nitrogen same down below (illuviation). In climates like beneficial to the host plant. The influence of large wet equatorial rainy areas with high rainfall, animals like ants, termites, earthworms, rodents © not only calcium, sodium, magnesium, etc., is mechanical, but, it is nevertheless potassium etc. but also a major part of silica is important in soil formation as they rework the removed from the soil. Removal of silica from soil up and down. In case of earthworms, as the soil is known as desilication. In dry climates, they feed on soil, the texture and chemistry of because of high temperature, evaporation the soil that comes out of their body changes. exceeds precipitation and hence ground water is brought up to the surface by capillary action Time and in the process the water evaporates leaving Time is the third important controlling factor behind salts in the soil. Such salts form into a in soil formation. The length of time the soil crust in the soil known as hardpans. In tropical forming processes operate, determines  GEOMORPHIC PROCESSES 57 maturation of soils and profile development. A Is it necessary to separate the process of soil becomes mature when all soil-forming soil formation and the soil forming control processes act for a sufficiently long time factors? developing a profile. Soils developing from recently deposited alluvium or glacial till are Why are time, topography and parent considered young and they exhibit no horizons material considered as passive control or only poorly developed horizons. No specific factors in soil formation? length of time in absolute terms can be fixed for soils to develop and mature. d EXERCISES he 1. Multiple choice questions. (i) Which one of the following processes is a gradational process? (a) Deposition (c) Volcanism is (b) Diastrophism (d) Erosion (ii) Which one of the following materials is affected by hydration process? bl (a) Granite (c) Quartz (b) Clay (d) Salts pu (iii) Debris avalanche can be included in the category of: (a) Landslides (c) Rapid flow mass movements be T (b) Slow flow mass movements (d) Subsidence re 2. Answer the following questions in about 30 words. o R (i) It is weathering that is responsible for bio-diversity on the earth. How? (ii) What are mass movements that are real rapid and perceptible? List. tt E (iii) What are the various mobile and mighty exogenic geomorphic agents and what is the prime job they perform? C (iv) Is weathering essential as a pre-requisite in the formation of soils? Why? 3. Answer the following questions in about 150 words. no N (i) “Our earth is a playfield for two opposing groups of geomorphic processes.” Discuss. (ii) Exogenic geomorphic processes derive their ultimate energy from the sun’s heat. Explain. © (iii) Are physical and chemical weathering processes independent of each other? If not, why? Explain with examples. (iv) How do you distinguish between the process of soil formation and soil- forming factors? What is the role of climate and biological activity as two important control factors in the formation of soils? Project Work Depending upon the topography and materials around you, observe and record climate, possible weathering process and soil contents and characteristics.