GEOMORPHIC PROCESSES

C H A P T E R

After learning about how the earth wasborn, how it evolved its crust and otherinner layers, how its crustal plates

moved and are moving, and other informationon earthquakes, the forms of volcanism andabout the rocks and minerals the crust iscomposed of, it is time to know in detail aboutthe surface of the earth on which we live. Letus start with this question.

Why is the surface of the earth uneven?

The earth’s crust is dynamic. You are wellaware that it has moved and moves verticallyand horizontally. Of course, it moved a bit fasterin the past than the rate at which it is movingnow. The differences in the internal forcesoperating from within the earth which built upthe crust have been responsible for thevariations in the outer surface of the crust. Theearth’s surface is being continuously subjectedto external forces induced basically by energy(sunlight). Of course, the internal forces are stillactive though with different intensities. Thatmeans, the earth’s surface is beingcontinuously subjected to by external forcesoriginating within the earth’s atmosphere andby internal forces from within the earth. Theexternal forces are known as exogenic forcesand the internal forces are known as endogenicforces. The actions of exogenic forces result inwearing down (degradation) of relief/elevationsand filling up (aggradation) of basins/depressions, on the earth’s surface. Thephenomenon of wearing down of reliefvariations of the surface of the earth througherosion is known as gradation. The endogenic

forces continuously elevate or build up partsof the earth’s surface and hence the exogenicprocesses fail to even out the relief variationsof the surface of the earth. So, variations remainas long as the opposing actions of exogenic andendogenic forces continue. In general terms,the endogenic forces are mainly land buildingforces and the exogenic processes are mainlyland wearing forces. The surface of the earth issensitive. Humans depend on it for theirsustenance and have been using it extensivelyand intensively. So, it is essential to understandits nature in order to use it effectively withoutdisturbing its balance and diminishing itspotential for the future. Almost all organismscontribute to sustain the earth’s environment.However, humans have caused extensivedamage to the environment through over useof resources. Use we must, but must also leaveit potential enough to sustain life through thefuture. Most of the surface of the earth had andhas been shaped over very long periods of time(hundreds and thousands of years) andbecause of its use and misuse by humans itspotential is being diminished at a fast rate. Ifthe processes which shaped and are shapingthe surface of the earth into varieties of forms(shapes) and the nature of materials of whichit is composed of, are understood, precautionscan be taken to minimise the detrimental effectsof human use and to preserve it for posterity.

GEOMORPHIC PROCESSES

You would like to know the meaning ofgeomorphic processes. The endogenic andexogenic forces causing physical stresses andchemical actions on earth materials and

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bringing about changes in the configurationof the surface of the earth are known asgeomorphic processes. Diastrophism andvolcanism are endogenic geomorphicprocesses. These have already been discussedin brief in the preceding unit. Weathering, masswasting, erosion and deposition are exogenicgeomorphic processes. These exogenicprocesses are dealt with in detail in this chapter.

Any exogenic element of nature (like water,ice, wind, etc.,) capable of acquiring andtransporting earth materials can be called ageomorphic agent. When these elements ofnature become mobile due to gradients, theyremove the materials and transport them overslopes and deposit them at lower level.Geomorphic processes and geomorphic agentsespecially exogenic, unless stated separately,are one and the same.

A process is a force applied on earthmaterials affecting the same. An agent is amobile medium (like running water, moving icemasses, wind, waves and currents etc.) whichremoves, transports and deposits earthmaterials. Running water, groundwater,glaciers, wind, waves and currents, etc., canbe called geomorphic agents.

Do you think it is essential to distinguishgeomorphic agents and geomorphicprocesses?

Gravity besides being a directional forceactivating all downslope movements of matteralso causes stresses on the earth’s materials.Indirect gravitational stresses activate wave andtide induced currents and winds. Withoutgravity and gradients there would be nomobility and hence no erosion, transportationand deposition are possible. So, gravitationalstresses are as important as the othergeomorphic processes. Gravity is the force thatis keeping us in contact with the surface and itis the force that switches on the movement ofall surface material on earth. All the movementseither within the earth or on the surface of theearth occur due to gradients — from higherlevels to lower levels, from high pressure to lowpressure areas etc.

ENDOGENIC PROCESSES

The energy emanating from within the earth isthe main force behind endogenic geomorphicprocesses. This energy is mostly generated byradioactivity, rotational and tidal friction andprimordial heat from the origin of the earth.This energy due to geothermal gradients andheat flow from within induces diastrophismand volcanism in the lithosphere. Due tovariations in geothermal gradients and heat flowfrom within, crustal thickness and strength,the action of endogenic forces are not uniformand hence the tectonically controlled originalcrustal surface is uneven.

Diastrophism

All processes that move, elevate or build upportions of the earth’s crust come underdiastrophism. They include: (i) orogenicprocesses involving mountain buildingthrough severe folding and affecting long andnarrow belts of the earth’s crust; (ii) epeirogenicprocesses involving uplift or warping of largeparts of the earth’s crust; (iii) earthquakesinvolving local relatively minor movements;(iv) plate tectonics involving horizontalmovements of crustal plates.

In the process of orogeny, the crust isseverely deformed into folds. Due to epeirogeny,there may be simple deformation. Orogeny isa mountain building process whereasepeirogeny is continental building process.Through the processes of orogeny, epeirogeny,earthquakes and plate tectonics, there can befaulting and fracturing of the crust. All theseprocesses cause pressure, volume andtemperature (PVT) changes which in turninduce metamorphism of rocks.

Epeirogeny and orogeny, cite thedifferences.

Volcanism

Volcanism includes the movement of moltenrock (magma) onto or toward the earth’ssurface and also formation of many intrusiveand extrusive volcanic forms. Many aspects ofvolcanism have already been dealt in detail

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GEOMORPHIC PROCESSES 47

processes and their respective driving forces.It should become clear from this chart that foreach process there exists a distinct driving forceor energy.

As there are different climatic regions onthe earth’s surface owing to thermal gradientscreated by latitudinal, seasonal and land andwater spread variations, the exogenicgeomorphic processes vary from region toregion. The density, type and distribution ofvegetation which largely depend upon

under volcanoes in the Unit II and underigneous rocks in the preceding chapter in thisunit.

What do the words volcanism andvolcanoes indicate?

EXOGENIC PROCESSES

The exogenic processes derive their energyfrom atmosphere determined by the ultimateenergy from the sun and also the gradientscreated by tectonic factors.

Why do you think that the slopes orgradients are created by tectonic factors?

Gravitational force acts upon all earthmaterials having a sloping surface and tend toproduce movement of matter in down slopedirection. Force applied per unit area is calledstress. Stress is produced in a solid by pushingor pulling. This induces deformation. Forcesacting along the faces of earth materials areshear stresses (separating forces). It is thisstress that breaks rocks and other earthmaterials. The shear stresses result in angulardisplacement or slippage. Besides thegravitational stress earth materials becomesubjected to molecular stresses that may becaused by a number of factors amongst whichtemperature changes, crystallisation andmelting are the most common. Chemicalprocesses normally lead to loosening of bondsbetween grains, dissolving of soluble mineralsor cementing materials. Thus, the basic reasonthat leads to weathering, mass movements, anderosion is development of stresses in the bodyof the earth materials.

As there are different climatic regions onthe earth’s surface the exogenic geomorphicprocesses vary from region to region.Temperature and precipitation are the twoimportant climatic elements that controlvarious processes.

All the exogenic geomorphic processes arecovered under a general term, denudation. Theword ‘denude’ means to strip off or to uncover.Weathering, mass wasting/movements, erosionand transportation are included in denudation.The flow chart (Figure 6.1) gives the denudation

precipitation and temperature exert influenceindirectly on exogenic geomorphic processes.Within different climatic regions there may belocal variations of the effects of different climaticelements due to altitudinal differences, aspectvariations and the variation in the amount ofinsolation received by north and south facingslopes as compared to east and west facingslopes. Further, due to differences in windvelocities and directions, amount and kind ofprecipitation, its intensity, the relation betweenprecipitation and evaporation, daily range oftemperature, freezing and thawing frequency,depth of frost penetration, the geomorphicprocesses vary within any climatic region.

What is the sole driving force behind allthe exogenic processes?

Climatic factors being equal, the intensityof action of exogenic geomorphic processesdepends upon type and structure of rocks. Theterm structure includes such aspects of rocksas folds, faults, orientation and inclination ofbeds, presence or absence of joints, beddingplanes, hardness or softness of constituentminerals, chemical susceptibility of mineralconstituents; the permeability or impermeability

Figure 6.1 : Denudational processes and theirdriving forces

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etc. Different types of rocks with differences intheir structure offer varying resistances tovarious geomorphic processes. A particularrock may be resistant to one process and non-resistant to another. And, under varyingclimatic conditions, particular rocks mayexhibit different degrees of resistance togeomorphic processes and hence they operateat differential rates and give rise to differencesin topography. The effects of most of theexogenic geomorphic processes are small andslow and may be imperceptible in a short timespan, but will in the long run affect the rocksseverely due to continued fatigue.

Finally, it boils down to one fact that thedifferences on the surface of the earth thoughoriginally related to the crustal evolutioncontinue to exist in some form or the other dueto differences in the type and structure of earthmaterials, differences in geomorphic processesand in their rates of operation.

Some of the exogenic geomorphic processeshave been dealt in detail here.

WEATHERING

Weathering is action of elements of weather andclimate over earth materials. There are anumber of processes within weathering whichact either individually or together to affect theearth materials in order to reduce them tofragmental state.

Weathering is defined as mechanicaldisintegration and chemical decom-position of rocks through the actions ofvarious elements of weather and climate.

As very little or no motion of materialstakes place in weathering, it is an in-situ oron-site process.

Is this little motion which can occursometimes due to weathering synonymouswith transportation? If not, why?

Weathering processes are conditioned bymany complex geological, climatic, topographicand vegetative factors. Climate is of particularimportance. Not only weathering processesdiffer from climate to climate, but also the depthof the weathering mantle (Figure 6.2).

Figure 6.2 : Climatic regimes and depth of weatheringmantles (adapted and modified from Strakhov, 1967)

Activity

Mark the latitude values of differentclimatic regimes in Figure 6.2 andcompare the details.

There are three major groups of weatheringprocesses : (i) chemical; (ii) physical ormechanical; (iii) biological weathering processes.Very rarely does any one of these processes everoperate completely by itself, but quite often adominance of one process can be seen.

Chemical Weathering Processes

A group of weathering processes viz; solution,carbonation, hydration, oxidation andreduction act on the rocks to decompose,dissolve or reduce them to a fine clastic statethrough chemical reactions by oxygen, surfaceand/or soil water and other acids. Water andair (oxygen and carbon dioxide) along withheat must be present to speed up all chemicalreactions. Over and above the carbon dioxidepresent in the air, decomposition of plants andanimals increases the quantity of carbondioxide underground. These chemicalreactions on various minerals are very muchsimilar to the chemical reactions in a laboratory.

Solution

When something is dissolved in water or acids,the water or acid with dissolved contents is

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called solution. This process involves removalof solids in solution and depends uponsolubility of a mineral in water or weak acids.On coming in contact with water many solidsdisintegrate and mix up as suspension inwater. Soluble rock forming minerals likenitrates, sulphates, and potassium etc. areaffected by this process. So, these minerals areeasily leached out without leaving any residuein rainy climates and accumulate in dryregions. Minerals like calcium carbonate andcalcium magnesium bicarbonate present inlimestones are soluble in water containingcarbonic acid (formed with the addition ofcarbon dioxide in water), and are carried awayin water as solution. Carbon dioxide producedby decaying organic matter along with soilwater greatly aids in this reaction. Commonsalt (sodium chloride) is also a rock formingmineral and is susceptible to this process ofsolution.

Carbonation

Carbonation is the reaction of carbonate andbicarbonate with minerals and is a commonprocess helping the breaking down offeldspars and carbonate minerals. Carbondioxide from the atmosphere and soil air isabsorbed by water, to form carbonic acid thatacts as a weak acid. Calcium carbonates andmagnesium carbonates are dissolved incarbonic acid and are removed in a solutionwithout leaving any residue resulting in caveformation.

Hydration

Hydration is the chemical addition of water.Minerals take up water and expand; thisexpansion causes an increase in the volume ofthe material itself or rock. Calcium sulphatetakes in water and turns to gypsum, which ismore unstable than calcium sulphate. Thisprocess is reversible and long, continuedrepetition of this process causes fatigue in therocks and may lead to their disintegration.

Many clay minerals swell and contract duringwetting and drying and a repetition of thisprocess results in cracking of overlyingmaterials. Salts in pore spaces undergo rapidand repeated hydration and help in rockfracturing. The volume changes in mineralsdue to hydration will also help in physicalweathering through exfoliation and granulardisintegration.

Oxidation and Reduction

In weathering, oxidation means a combinationof a mineral with oxygen to form oxides orhydroxides. Oxidation occurs where there isready access to the atmosphere andoxygenated waters. The minerals mostcommonly involved in this process are iron,manganese, sulphur etc. In the process ofoxidation rock breakdown occurs due to thedisturbance caused by addition of oxygen. Redcolour of iron upon oxidation turns to brownor yellow. When oxidised minerals are placedin an environment where oxygen is absent,reduction takes place. Such conditions existusually below the water table, in areas ofstagnant water and waterlogged ground. Redcolour of iron upon reduction turns to greenishor bluish grey.

These weathering processes are inter-related. Hydration, carbonation and oxidationgo hand in hand and hasten the weatheringprocess.

Can we give iron rusting as an exampleof oxidation? How essential is water inchemical weathering processes? Canchemical weathering processes dominatein water scarce hot deserts?

Physical Weathering Processes

Physical or mechanical weathering processesdepend on some applied forces. The appliedforces could be: (i) gravitational forces such asoverburden pressure, load and shearing stress;(ii) expansion forces due to temperaturechanges, crystal growth or animal activity;(iii) water pressures controlled by wetting and

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drying cycles. Many of these forces are appliedboth at the surface and within different earthmaterials leading to rock fracture. Most of thephysical weathering processes are caused bythermal expansion and pressure release. Theseprocesses are small and slow but can causegreat damage to the rocks because ofcontinued fatigue the rocks suffer due torepetition of contraction and expansion.

Unloading and Expansion

Removal of overlying rock load because ofcontinued erosion causes vertical pressurerelease with the result that the upper layers ofthe rock expand producing disintegration ofrock masses. Fractures will develop roughlyparallel to the ground surface. In areas ofcurved ground surface, arched fractures tendto produce massive sheets or exfoliation slabsof rock. Exfoliation sheets resulting fromexpansion due to unloading and pressurerelease may measure hundreds or eventhousands of metres in horizontal extent. Large,smooth rounded domes called exfoliationdomes (Figure 6.3) result due to this process.

temperatures, this internal movement amongthe mineral grains of the superficial layers ofrocks takes place regularly. This process ismost effective in dry climates and highelevations where diurnal temperature changesare drastic. As has been mentioned earlierthough these movements are very small theymake the rocks weak due to continued fatigue.The surface layers of the rocks tend to expandmore than the rock at depth and this leads tothe formation of stress within the rock resultingin heaving and fracturing parallel to thesurface. Due to differential heating andresulting expansion and contraction of surfacelayers and their subsequent exfoliation fromthe surface results in smooth rounded surfacesin rocks. In rocks like granites, smoothsurfaced and rounded small to big boulderscalled tors form due to such exfoliation.

What is the difference between exfoliationdomes and exfoliated tors?

Freezing, Thawing and Frost Wedging

Frost weathering occurs due to growth of icewithin pores and cracks of rocks duringrepeated cycles of freezing and melting. Thisprocess is most effective at high elevations inmid-latitudes where freezing and melting isoften repeated. Glacial areas are subject to frostwedging daily. In this process, the rate offreezing is important. Rapid freezing of watercauses its sudden expansion and high pressure.The resulting expansion affects joints, cracksand small inter granular fractures to becomewider and wider till the rock breaks apart.

Salt Weathering

Salts in rocks expand due to thermal action,hydration and crystallisation. Many salts likecalcium, sodium, magnesium, potassium andbarium have a tendency to expand. Expansionof these salts depends on temperature andtheir thermal properties. High temperatureranges between 30 and 50oC of surfacetemperatures in deserts favour such saltexpansion. Salt crystals in near-surface pores

Figure 6.3 : A large exfoliation dome in granite rocknear bhongir (Bhuvanagiri) town in Andhra Pradesh

Temperature Changes and Expansion

Various minerals in rocks possess their ownlimits of expansion and contraction. With risein temperature, every mineral expands andpushes against its neighbour and astemperature falls, a corresponding contractiontakes place. Because of diurnal changes in the

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cause splitting of individual grains withinrocks, which eventually fall off. This process offalling off of individual grains may result ingranular disintegration or granular foliation.

Salt crystallisation is most effective of allsalt-weathering processes. In areas withalternating wetting and drying conditions saltcrystal growth is favoured and the neighbouringgrains are pushed aside. Sodium chloride andgypsum crystals in desert areas heave upoverlying layers of materials and with the resultpolygonal cracks develop all over the heavedsurface. With salt crystal growth, chalk breaksdown most readily, followed by limestone,sandstone, shale, gneiss and granite etc.

BIOLOGICAL ACTIVITY AND WEATHERING

Biological weathering is contribution to orremoval of minerals and ions from theweathering environment and physical changesdue to growth or movement of organisms.Burrowing and wedging by organisms likeearthworms, termites, rodents etc., help inexposing the new surfaces to chemical attackand assists in the penetration of moisture andair. Human beings by disturbing vegetation,ploughing and cultivating soils, also help inmixing and creating new contacts between air,water and minerals in the earth materials.Decaying plant and animal matter help in theproduction of humic, carbonic and other acidswhich enhance decay and solubility of someelements. Plant roots exert a tremendouspressure on the earth materials mechanicallybreaking them apart.

SPECIAL EFFECTS OF WEATHERING

Exfoliation

This has already been explained underphysical weathering processes of unloading,thermal contraction and expansion and saltweathering. Exfoliation is a result but not aprocess. Flaking off of more or less curvedsheets of shells from over rocks or bedrockresults in smooth and rounded surfaces(Figures 6.3; 6.4). Exfoliation can occur dueto expansion and contraction induced by

temperature changes. Exfoliation domes andtors result due to unloading and thermalexpansion respectively.

SIGNIFICANCE OF WEATHERING

Weathering processes are responsible forbreaking down the rocks into smallerfragments and preparing the way for formationof not only regolith and soils, but also erosionand mass movements. Biomes and bio-diversity is basically a result of forests(vegetation) and forests depend upon the depthof weathering mantles. Erosion cannot besignificant if the rocks are not weathered. Thatmeans, weathering aids mass wasting, erosionand reduction of relief and changes inlandforms are a consequence of erosion.Weathering of rocks and deposits helps in theenrichment and concentrations of certainvaluable ores of iron, manganese, aluminium,copper etc., which are of great importance forthe national economy. Weathering is animportant process in the formation of soils.

When rocks undergo weathering, somematerials are removed through chemicalor physical leaching by groundwater andthereby the concentration of remaining(valuable) materials increases. Withoutsuch a weathering taking place, theconcentration of the same valuablematerial may not be sufficient andeconomically viable to exploit, process andrefine. This is what is called enrichment.

Fig.6.4 : Exfoliation (Flacking) and granulardisintegration

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the three forms of movements. Figure 6.5 showsthe relationships among different types of massmovements, their relative rates of movementand moisture limits.

Figure 6.5 : Relationships among different types ofmass movements, their relative rates of movement

and moisture limits (after Whitehead, 2001)

MASS MOVEMENTS

These movements transfer the mass of rockdebris down the slopes under the directinfluence of gravity. That means, air, water orice do not carry debris with them from place toplace but on the other hand the debris maycarry with it air, water or ice. The movementsof mass may range from slow to rapid,affecting shallow to deep columns of materialsand include creep, flow, slide and fall. Gravityexerts its force on all matter, both bedrock andthe products of weathering. So, weathering isnot a pre-requisite for mass movement thoughit aids mass movements. Mass movements arevery active over weathered slopes rather thanover unweathered materials.

Mass movements are aided by gravity andno geomorphic agent like running water,glaciers, wind, waves and currents participatein the process of mass movements. That meansmass movements do not come under erosionthough there is a shift (aided by gravity) ofmaterials from one place to another. Materialsover the slopes have their own resistance todisturbing forces and will yield only when forceis greater than the shearing resistance of thematerials. Weak unconsolidated materials,thinly bedded rocks, faults, steeply dippingbeds, vertical cliffs or steep slopes, abundantprecipitation and torrential rains and scarcityof vegetation etc., favour mass movements.

Several activating causes precede massmovements. They are : (i) removal of supportfrom below to materials above through naturalor artificial means; (ii) increase in gradient andheight of slopes; (iii) overloading throughaddition of materials naturally or by artificialfilling; (iv) overloading due to heavy rainfall,saturation and lubrication of slope materials;(v) removal of material or load from over theoriginal slope surfaces; (vi) occurrence ofearthquakes, explosions or machinery;(vii) excessive natural seepage; (viii) heavydrawdown of water from lakes, reservoirs andrivers leading to slow outflow of water fromunder the slopes or river banks; (ix) indis-criminate removal of natural vegetation.

Heave (heaving up of soils due to frostgrowth and other causes), flow and slide are

Mass movements can be grouped undertwo major classes: (i) slow movements;(ii) rapid movements.

Slow Movements

Creep is one type under this category whichcan occur on moderately steep, soil coveredslopes. Movement of materials is extremelyslow and imperceptible except throughextended observation. Materials involved canbe soil or rock debris. Have you ever seen fenceposts, telephone poles lean downslope fromtheir vertical position and in their linearalignment? If you have, that is due to the creepeffect. Depending upon the type of materialinvolved, several types of creep viz., soil creep,talus creep, rock creep, rock-glacier creep etc.,can be identified. Also included in this groupis solifluction which involves slow downslopeflowing soil mass or fine grained rock debrissaturated or lubricated with water. This processis quite common in moist temperate areaswhere surface melting of deeply frozen groundand long continued rain respectively, occurfrequently. When the upper portions getsaturated and when the lower parts areimpervious to water percolation, flowing occursin the upper parts.

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Rapid Movements

These movements are mostly prevalent inhumid climatic regions and occur over gentleto steep slopes. Movement of water-saturatedclayey or silty earth materials down low-angleterraces or hillsides is known as earthflow.Quite often, the materials slump making step-like terraces and leaving arcuate scarps at theirheads and an accumulation bulge at the toe.When slopes are steeper, even the bedrockespecially of soft sedimentary rocks like shaleor deeply weathered igneous rock may slidedownslope.

Another type in this category is mudflow.In the absence of vegetation cover and withheavy rainfall, thick layers of weatheredmaterials get saturated with water and eitherslowly or rapidly flow down along definitechannels. It looks like a stream of mud withina valley. When the mudflows emerge out ofchannels onto the piedmont or plains, they canbe very destructive engulfing roads, bridgesand houses. Mudflows occur frequently on theslopes of erupting or recently erupted volcanoes.Volcanic ash, dust and other fragments turninto mud due to heavy rains and flow down astongues or streams of mud causing greatdestruction to human habitations.

A third type is the debris avalanche, whichis more characteristic of humid regions withor without vegetation cover and occurs innarrow tracks on steep slopes. This debrisavalanche can be much faster than themudflow. Debris avalanche is similar to snowavalanche.

In Andes mountains of South Americaand the Rockies mountains of NorthAmerica, there are a few volcanoes whicherupted during the last decade and verydevastating mudflows occurred downtheir slopes during eruption as well asafter eruption.

Landslides

These are relatively rapid and perceptiblemovements. The materials involved arerelatively dry. The size and shape of thedetached mass depends on the nature of

discontinuities in the rock, the degree ofweathering and the steepness of the slope.Depending upon the type of movement ofmaterials several types are identified in thiscategory.

Slump is slipping of one or several units ofrock debris with a backward rotation withrespect to the slope over which the movementtakes place (Figure 6.6). Rapid rolling or sliding

of earth debris without backward rotation ofmass is known as debris slide. Debris fall isnearly a free fall of earth debris from a verticalor overhanging face. Sliding of individual rockmasses down bedding, joint or fault surfacesis rockslide. Over steep slopes, rock sliding isvery fast and destructive. Figure 6.7 showslandslide scars over steep slopes. Slides occuras planar failures along discontinuities likebedding planes that dip steeply. Rock fall isfree falling of rock blocks over any steep slopekeeping itself away from the slope. Rock fallsoccur from the superficial layers of the rock

Figure 6.6 : Slumping of debris with backward rotation

Figure 6.7 : Landslide scars in Shiwalik Himalayan rangesnear river Sarada at India-Nepal border, Uttar Pradesh

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face, an occurrence that distinguishes it fromrockslide which affects materials up to asubstantial depth.

Between mass wasting and massmovements, which term do you feel ismost appropriate? Why? Can solifluctionbe included under rapid flow movements?Why it can be and can’t be?

In our country, debris avalanches andlandslides occur very frequently in theHimalayas. There are many reasons forthis. One, the Himalayas are tectonicallyactive. They are mostly made up ofsedimentary rocks and unconsolidatedand semi-consolidated deposits. Theslopes are very steep. Compared to theHimalayas, the Nilgiris borderingTamilnadu, Karnataka, Kerala and theWestern Ghats along the west coast arerelatively tectonically stable and aremostly made up of very hard rocks; but,still, debris avalanches and landslidesoccur though not as frequently as in theHimalayas, in these hills. Why? Manyslopes are steeper with almost verticalcliffs and escarpments in the WesternGhats and Nilgiris. Mechanical weatheringdue to temperature changes and rangesis pronounced. They receive heavyamounts of rainfall over short periods.So, there is almost direct rock fall quitefrequently in these places along withlandslides and debris avalanches.

EROSION AND DEPOSITION

Erosion involves acquisition and transportationof rock debris. When massive rocks break intosmaller fragments through weathering andany other process, erosional geomorphicagents like running water, groundwater,glaciers, wind and waves remove andtransport it to other places depending uponthe dynamics of each of these agents. Abrasionby rock debris carried by these geomorphicagents also aids greatly in erosion. By erosion,relief degrades, i.e., the landscape is worndown. That means, though weathering aids

erosion it is not a pre-condition for erosion totake place. Weathering, mass-wasting anderosion are degradational processes. It iserosion that is largely responsible forcontinuous changes that the earth’s surface isundergoing. As indicated in Figure 6.1,denudational processes like erosion andtransportation are controlled by kinetic energy.The erosion and transportation of earthmaterials is brought about by wind, runningwater, glaciers, waves and ground water. Ofthese the first three agents are controlled byclimatic conditions. They represent three statesof matter —gaseous (wind), liquid (runningwater) and solid (glacier) respectively.

Can you compare the three climaticallycontrolled agents?

The erosion can be defined as “applicationof the kinetic energy associated with the agentto the surface of the land along which it moves”.Kinetic energy is computed as KE = 1/

2 mv2

where ‘m’ is the mass and ‘v’ is the velocity.Hence the energy available to perform work willdepend on the mass of the material and thevelocity with which it is moving. Obviously thenyou will find that though the glaciers move atvery low velocities due to tremendous mass aremore effective as the agents of erosion and wind,being in gaseous state, is less effective.

The work of the other two agents of erosion-waves and ground water is not controlled byclimate. In case of waves it is the location alongthe interface of litho and hydro sphere —coastal region — that will determine the workof waves, whereas the work of ground water isdetermined more by the lithological characterof the region. If the rocks are permeable andsoluble and water is available only then karsttopography develops. In the next chapter weshall be dealing with the landforms producedby each of these agents of erosion.

Deposition is a consequence of erosion. Theerosional agents loose their velocity and henceenergy on gentler slopes and the materialscarried by them start to settle themselves. Inother words, deposition is not actually the workof any agent. The coarser materials get

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deposited first and finer ones later. Bydeposition depressions get filled up. The sameerosional agents viz., running water, glaciers,wind, waves and groundwater act asaggradational or depositional agents also.

What happens to the surface of the earthdue to erosion and deposition is elaborated inthe next chapter on landforms and theirevolution.

There is a shift of materials in massmovements as well as in erosion from oneplace to the other. So, why can’t both betreated as one and the same? Can therebe appreciable erosion without rocksundergoing weathering?

SOIL FORMATION

Soil and Soil Contents

You see plants growing in soils. You play inthe ground and come into contact with soil.You touch and feel soil and soil your clotheswhile playing. Can you describe it?

A pedologist who studies soils defines soilas a collection of natural bodies on the earth’ssurface containing living and/or dead matterand supporting or capable of supporting plants.

Soil is a dynamic medium in which manychemical, physical and biological activities goon constantly. Soil is a result of decay, it is alsothe medium for growth. It is a changing anddeveloping body. It has many characteristicsthat fluctuate with the seasons. It may bealternatively cold and warm or dry and moist.Biological activity is slowed or stopped if thesoil becomes too cold or too dry. Organic matterincreases when leaves fall or grasses die. Thesoil chemistry, the amount of organic matter,the soil flora and fauna, the temperature andthe moisture, all change with the seasons aswell as with more extended periods of time.That means, soil becomes adjusted toconditions of climate, landform and vegetationand will change internally when thesecontrolling conditions change.

Process of Soil Formation

Soil formation or pedogenesis depends first onweathering. It is this weathering mantle (depth

of the weathered material) which is the basicinput for soil to form. First, the weatheredmaterial or transported deposits are colonisedby bacteria and other inferior plant bodies likemosses and lichens. Also, several minororganisms may take shelter within the mantleand deposits. The dead remains of organismsand plants help in humus accumulation. Minorgrasses and ferns may grow; later, bushes andtrees will start growing through seeds broughtin by birds and wind. Plant roots penetratedown, burrowing animals bring up particles,mass of material becomes porous and sponge-like with a capacity to retain water and to permitthe passage of air and finally a mature soil, acomplex mixture of mineral and organicproducts forms.

Is weathering solely responsible for soilformation? If not, why?

Pedology is soil science. A pedologist is asoil-scientist.

Soil-forming Factors

Five basic factors control the formation of soils:(i) parent material; (ii) topography; (iii) climate;(iv) biological activity; (v) time. In fact soilforming factors act in union and affect theaction of one another.

Parent Material

Parent material is a passive control factor insoil formation. Parent materials can be any in-situ or on-site weathered rock debris (residualsoils) or transported deposits (transportedsoils). Soil formation depends upon the texture(sizes of debris) and structure (disposition ofindividual grains/particles of debris) as wellas the mineral and chemical composition of therock debris/deposits.

Nature and rate of weathering and depth ofweathering mantle are important considerationsunder parent materials. There may bedifferences in soil over similar bedrock anddissimilar bedrocks may have similar soilsabove them. But when soils are very youngand have not matured these show strong links

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with the type of parent rock. Also, in case ofsome limestone areas, where the weatheringprocesses are specific and peculiar, soils willshow clear relation with the parent rock.

Topography

Topography like parent materials is anotherpassive control factor. The influence oftopography is felt through the amount ofexposure of a surface covered by parentmaterials to sunlight and the amount ofsurface and sub-surface drainage over andthrough the parent materials. Soils will be thinon steep slopes and thick over flat uplandareas. Over gentle slopes where erosion is slowand percolation of water is good, soil formationis very favourable. Soils over flat areas maydevelop a thick layer of clay with goodaccumulation of organic matter giving the soildark colour. In middle latitudes, the southfacing slopes exposed to sunlight have differentconditions of vegetation and soils and the northfacing slopes with cool, moist conditions havesome other soils and vegetation.

Climate

Climate is an important active factor in soilformation. The climatic elements involved in soildevelopment are : (i) moisture in terms of itsintensity, frequency and duration ofprecipitation - evaporation and humidity;(ii) temperature in terms of seasonal anddiurnal variations.

Precipitation gives soil its moisture contentwhich makes the chemical and biologicalactivities possible. Excess of water helps in thedownward transportation of soil componentsthrough the soil (eluviation) and deposits thesame down below (illuviation). In climates likewet equatorial rainy areas with high rainfall,not only calcium, sodium, magnesium,potassium etc. but also a major part of silica isremoved from the soil. Removal of silica fromthe soil is known as desilication. In dry climates,because of high temperature, evaporationexceeds precipitation and hence ground wateris brought up to the surface by capillary actionand in the process the water evaporates leavingbehind salts in the soil. Such salts form into acrust in the soil known as hardpans. In tropical

climates and in areas with intermediateprecipitation conditions, calcium carbonatenodules (kanker) are formed.

Temperature acts in two ways — increasingor reducing chemical and biological activity.Chemical activity is increased in highertemperatures, reduced in cooler temperatures(with an exception of carbonation) and stopsin freezing conditions. That is why, tropical soilswith higher temperatures show deeper profilesand in the frozen tundra regions soils containlargely mechanically broken materials.

Biological Activity

The vegetative cover and organisms that occupythe parent materials from the beginning and alsoat later stages help in adding organic matter,moisture retention, nitrogen etc. Dead plantsprovide humus, the finely divided organic matterof the soil. Some organic acids which formduring humification aid in decomposing theminerals of the soil parent materials.

Intensity of bacterial activity shows updifferences between soils of cold and warmclimates. Humus accumulates in cold climatesas bacterial growth is slow. With undecomposedorganic matter because of low bacterial activity,layers of peat develop in sub-arctic and tundraclimates. In humid tropical and equatorialclimates, bacterial growth and action is intenseand dead vegetation is rapidly oxidised leavingvery low humus content in the soil. Further,bacteria and other soil organisms take gaseousnitrogen from the air and convert it into achemical form that can be used by plants. Thisprocess is known as nitrogen fixation.Rhizobium, a type of bacteria, lives in the rootnodules of leguminous plants and fixes nitrogenbeneficial to the host plant. The influence of largeanimals like ants, termites, earthworms, rodentsetc., is mechanical, but, it is neverthelessimportant in soil formation as they rework thesoil up and down. In case of earthworms, asthey feed on soil, the texture and chemistry ofthe soil that comes out of their body changes.

Time

Time is the third important controlling factorin soil formation. The length of time the soilforming processes operate, determines

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maturation of soils and profile development. Asoil becomes mature when all soil-formingprocesses act for a sufficiently long timedeveloping a profile. Soils developing fromrecently deposited alluvium or glacial till areconsidered young and they exhibit no horizonsor only poorly developed horizons. No specificlength of time in absolute terms can be fixedfor soils to develop and mature.

Is it necessary to separate the process of

soil formation and the soil forming control

factors?

Why are time, topography and parent

material considered as passive control

factors in soil formation?

EXERCISES

1. Multiple choice questions.

(i) Which one of the following processes is a gradational process?

(a) Deposition (c) Volcanism

(b) Diastrophism (d) Erosion

(ii) Which one of the following materials is affected by hydration process?

(a) Granite (c) Quartz

(b) Clay (d) Salts

(iii) Debris avalanche can be included in the category of:

(a) Landslides (c) Rapid flow mass movements

(b) Slow flow mass movements (d) Subsidence

2. Answer the following questions in about 30 words.

(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.

(iii) What are the various mobile and mighty exogenic geomorphic agents andwhat is the prime job they perform?

(iv) Is weathering essential as a pre-requisite in the formation of soils? Why?

3. Answer the following questions in about 150 words.

(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’sheat. Explain.

(iii) Are physical and chemical weathering processes independent of eachother? 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 twoimportant control factors in the formation of soils?

Project Work

Depending upon the topography and materials around you, observe and recordclimate, possible weathering process and soil contents and characteristics.

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