128207931 1 Factors Controlling Landform Development Sanjeev
FACTORS CONTROLLING LANDFORM DEVELOPMENT Definition of ‘Landform’: ●
“Landform is the function of Structure, Process and Time, i.e
--- W.M. Davis “Landform is the function of process, geomaterial and changes through time”, i.e.
--- K.J. Gregory “Landform is the ratio of the rate of upliftment by the endogenetic processes and the degradation by the exogenetic processes.” --- W. Penck
Meaning of ‘Landform’: ● Landform refers to the 3-dimensional morphological features of the earth characterised by the shape, size and dimensions, which are basically the product of i. Endogenetic and exogenetic forces [older view], ii. Slopes or a combination of slopes [recent view]. ● Thus, landforms are the manifestation of various slope types (determined by geomorphic processes acting on them) and the geomaterials / structure. Meaning of ‘Controlling Factors’: ● They are not involved in the creation or destruction of the landforms, but they determine the rate at which such destruction or deformation or formation will take place. ● The controlling factors determine the magnitude, amplitude, dimension, shape and size of the landforms. ● There are about 10 different types of main controlling factors, which have many sub-factors:
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“Geological structure is a dominant controlling factor in the evolution of landforms and is reflected in them”. --- W. D. Thornbury The structure, as a rule, is older than the landscape, and provides a base for the operation of the various gradational processes which give rise to new sequential landforms. If the rocks are folded, they are more resistant to weathering and erosion because the molecular strength due to compression and cementing is high. The rocks, which have more fractures, joints, cracks, rifts, fissures, etc. are more prone to erosion because they become permeable and water can seep through them easily. Normally, less resistant rocks are more rapidly eroded and give birth to lowland while resistant rocks produce bold topography due to less erosion. Thus hardness plays key role. In permeable rocks, water is able to penetrate underground and the rocks become liable to denudation with comparative ease. Highly permeable rocks disfavours erosion as they allow more efficient percolation of water and hence form high relief topography. In some instances, porosity can promote and enhance permeability. Some rocks, like limestone, are soluble and dissolve easily in water. The solution of underground rocks leads to the formation of underground gullies and caverns. With regards to texture, a coarse-grained rock can weather more quickly than a finegrained rock, as the later possess a better inter-locking structure between their crystals. For igneous group of rocks, basic rocks weather more rapidly than acid rocks. Sedimentary rocks vary considerably in their resistance to weathering. Although metamorphic rocks generally present more resistance to erosion than do their sedimentary and igneous counterparts, it is not easy to identify a seperate class of distinctly metamorphic landforms. Arrangement of rocks means disposition of rock beds mainly of sedimentary rocks due to the deformation process like isostatic, tectonic and orogenetic movements and gets deformed into folded, faulted, domed and homoclinical structures. Different fault types produce, after erosion, landforms of varying characteristics. Domed structure results either due to upwarping of crustal surface effected by diastrophic force or due to the intrusion of magma into surficial rocks. Homoclinal structures involve both hard and soft rocks, and so these are subjected to differential erosion with the result of rivers forming their valleys along the soft rocks. The horizontal structures are subjected to differential erosion and give birth to step-like scraps and bench topography (structural benches). Chemical composition determines the nature of chemical weathering of rocks, which in-turn determines the resultant landforms. Dark-coloured minerals are more susceptible to weathering than light-coloured minerals as the later reflect most of the sun’s rays and heat less rapidly than the former. Rocks having larger crystal become well-jointed rocks. But, once exposed, they are prone to mechanical weathering. Rocks having vertical strata are easily loosened and broken down due to the temperature changes, frost action, water & wind action. Conversely, rocks having horizontal beds are more compact and are less affected by the mechanisms of disintegration & decomposition. Thus, structure influences not only the major outline of the relief, but even micro-relief features and landforms bear the imprint of the rocks and their character & stratification. But, geological structure in itself is a controlling factor only in a limited sense & its influence cannot be wholly seperated from the influence of processes in the evolution of landforms.
II. Climate: ● “Each climatic type produces its own characteristic assemblage of landforms”. --- Richthofen, Passarge, Jenson, Walther, Thorbecke and Sapper ● Climate affects the relative importance of the different weathering processes by controlling rates of operation. Climatic elements like precipitation, temperature and the related weathering processes have been given greater emphasis as a modifier of the landforms.
For e.g., in a desert environment all the rock-types (limestone, granite, basalt, sandstone, etc.) ultimately wear down into sand and the structural control has a limited stage and time after which the climatic forces reduce all the features and amalgamate into a sandy desert. Also, same type of rocks responds differently in different climatic conditions. For e.g.sandstone, which gets eroded in the humid regions while stand still in arid regions.
III. Stage: ● “As the different erosional agencies act on the earth’s surface, there is produced a sequence of landforms having distinctive characteristics at the successive stages of their development”. --- W. D. Thornbury ● Stage is a unit of time or a measurement of time, which is characterised by its energy level, entropy, intensity, and magnitude of the process. ● Each stage of the erosion cycle is characterised by distinctive landforms with the help of which it is possible to identify the stage of evolution of landscape. ● There is a direct relationship between the stage of development and the character of landforms. The stages of development of landforms, however, do not have equal life span. IV. Time: ● “Landscape is the function of time and space”. ● ● ●
--- Schumm and Lichty Time is a highly debatable concept in the evolution of landform as it does not show direct implication and it is not even a process or a mechanism of change. But with time, every object or phenomena in the nature must decay. This is the golden and eternal law. Generally, no perceptible change may occur in the morphological features during short period of time. But sometime, the response is instantaneous.
V. Topography: ● Slope has a direct relationship with the rate of erosion and weathering. ● If the slope is steeper, the intensity of weathering is greater. But, over the horizontal slopes, more depositional work can be seen. ● For example, the steeper slopes along the western coast of India have led to greater incidences of landslides and rockfall in the western Ghats. ● The amount of available relief and degree of dissection are both bound to affect slope form. High relief increases erosion, which wears down the topography. VI. Gravity: ● The gravity transport also brings substantial changes on the crust of the earth. ● Mass movement is the downslope transfer of material through the direct action of gravity. ● The process can be rapid like landslide, or imperceptibly slow, as creep of soil down the gentle slope of gravity-covered field. ● Rock material may move under the influence of gravity either as a movement of weathering products down a slope, or as mass movement of rock along joint planes or bedding planes. VII. Biota: ● If a landform / region is vegetated, soil erosion is less and geomaterials are more compact. ● But, the roots of the plants and trees exert pressure on the rocks and break them apart. ● The microorganisms associated with the roots of plants and trees encourage decomposition and disintegration of rocks through physico-biochemical weathering. ● Humans have emerged as the most coercive force bringing modification in crustal features. VIII. Space: ● Spatial scale has much significance in controlling the rate and mechanism of operation of processes and their responses (landforms) as the areal coverages of study areas change.
There has always been shift in the selection of ideal geomorphic unit having specific areal coverage for the study of landforms and geomorphic processes with varying viewpoints and objectives.
IX. Process: ● “Geomorphic processes leave their distinctive imprints upon landforms and each geomorphic process develops its own landfroms and each geomorphic process develops its own characteristic assemblage of landforms.” --- W. D. Thornbury ● The word ‘process’ includes the actions of all the endogenetic (internal) and exogenetic (external) forces. Process is significant in the evolution of landforms. ● The endogenetic forces cause vulcanicity and diastrophism, and produce irregularites on the earth’s surface by building mountains, plateaus, hills and undulating topography. ● On the other hand, there are exogenetic forces, which tend to level down the earth’s surface. The agents of these external processes of denudation are running water, groundwater, glaciers, wind and sea waves. X. Rate of Endogenetic Forces: ● Landforms originate mainly by the endogenetic forces but the rate at which it operate is important in shaping their morphogenetic characteristics. ● Landforms are the product of the ratio between the rate of endogenetic forces and the rate of denudation. The relief content of a landform is determined by the rate at which these forces operate.