CHAPTER GEOMORPHIC PROCESSES 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 Ainner layers, how its crustal plates processes fail to even out the relief variations 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, 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. 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 The earth’s crust is dynamic. You are well its nature in order to use it effectively without aware that it has moved and moves vertically disturbing its balance and diminishing its 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 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 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 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 2021-22 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 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, 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 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 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 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 materials. Running water, groundwater, involving local relatively minor movements; glaciers, wind, waves and currents, etc., can (iv) plate tectonics involving horizontal 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, geomorphic agents and geomorphic there may be simple deformation. Orogeny is processes? a mountain building process whereas epeirogeny is continental building process. 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 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 2021-22 GEOMORPHIC PROCESSES 47 under volcanoes in the Unit II and under driving forces. It should become clear from igneous rocks in the preceding chapter in this this chart that for each process there exists a unit. distinct driving force or energy. As there are different climatic regions owing What do the words volcanism and to variations in thermal gradients created by volcanoes indicate? latitudinal, seasonal, and land and water spread on the surface of the earth, the exogenic geomorphic processes vary from region to EXOGENIC PROCESSES region. The density, type and distribution of The exogenic processes derive their energy vegetation which largely depend upon from atmosphere determined by the ultimate precipitation and temperature also exert energy from the sun and also the gradients created by tectonic factors. Why do you think that the slopes or gradients are 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 Figure 6.1 : Denudational processes and their by pushing or pulling. This induces driving forces deformation. Forces acting along the faces of influence indirectly on exogenic geomorphic earth materials are shear stresses (separating processes. Within different climatic regions forces). It is this stress that breaks rocks and there may be local variations of the effects of other earth materials. The shear stresses different climatic elements due to altitudinal result in angular displacement or slippage. differences, aspect variations and the variation Besides the gravitational stress earth in the amount of insolation received by north materials become subjected to molecular and south facing slopes as compared to east stresses that may be caused by a number of and west facing slopes. Further, due to factors amongst which temperature changes, differences in wind velocities and directions, crystallisation and melting are the most amount and kind of precipitation, its intensity, common. Chemical processes normally lead the relation between precipitation and to loosening of bonds between grains, evaporation, daily range of temperature, dissolving of soluble minerals or cementing freezing and thawing frequency, depth of frost materials. Thus, the basic reason that leads penetration, the geomorphic processes vary to weathering, mass movements, and erosion within any climatic region. is development of stresses in the body of the earth materials. What is the sole driving force behind all Temperature and precipitation are the the exogenic processes? two important climatic elements that control various processes. Climatic factors being equal, the intensity All the exogenic geomorphic processes of action of exogenic geomorphic processes are covered under a general term, depends upon type and structure of rocks. The denudation. The word ‘denude’ means to term structure includes such aspects of rocks strip off or to uncover. Weathering, mass as folds, faults, orientation and inclination of wasting/movements, erosion and beds, presence or absence of joints, bedding transportation are included in denudation.