Landforms and Their Evolution PDF

Landforms and their Evolution Landforms and their Evolution A small to medium-sized part of the surface of the earth is called a landform and several related landforms together are known as landscapes. Each landform is a result of the action of certain geomorphic processes (endogenic and exogenic)...

Landforms and their Evolution Landforms and their Evolution A small to medium-sized part of the surface of the earth is called a landform and several related landforms together are known as landscapes. Each landform is a result of the action of certain geomorphic processes (endogenic and exogenic) and agents (rainfall, wind, glacier, waves). Each landform has its own physical shape, size, nature and once formed, these landforms may change over time due to the continued action of geomorphic processes and agents.  The evolution of landforms refers to the stages of transformation of one landform to another or the transformation of individual landforms once they are formed. Therefore, a landform goes through stages of development – youth, maturity and old age.  Erosion and deposition are the two important aspects in the evolution of landforms. Most of the geomorphic processes are imperceptible (unobservable as they are very slow and long processes).  The evolution of landforms depends on geomorphic agents i.e., running water, groundwater, glaciers, waves and winds. These agents reduce landmasses through erosion and also develop some landforms through deposition. Both erosion and deposition change the surface of the earth. Running Water  Running water is considered the most important geomorphic agent responsible for the degradation of the land surface. Running water can exist as streams, rivers in valleys or in the form of overland flow on the general land surface as a sheet.  The vigorous and youthful rivers (running water) flowing over steep slopes mostly give rise to the erosional landforms.  With time, streams lose their velocity due to continued erosion, turn gentler and thereby facilitate active deposition.  The gentler the river channels in slope/gradient, the greater the deposition. So the depositional forms are mainly associated with rivers flowing over medium to gentler slopes.  When the stream beds turn gentler due to continued erosion, downward cutting becomes less dominant and lateral (sideways) erosion of banks increases which reduces the hills and valleys to plains.  Overland flow leads to sheet erosion. This flow may follow narrow to wide paths depending upon the irregularities of the land surface. The flowing water takes along with it minor or major quantities of materials from the land surface and gradually small and narrow rills are formed. These rills gradually develop into long and wide gullies, which further deepen, lengthen, widen and unite to form a network of valleys.  Rills →→→→ Gullies →→→→Valleys  During the early stages, down cutting dominates. In the middle stages, lateral erosion (sideways) becomes intense and gradually the valley sides get reduced to lower slopes. The divides between drainage basins are also lowered and they become completely flattened, leaving a low land of faint relief with some low resistant remnants called monadnocks (isolated blocks of rocks) standing out here and there. This type of plain forming as a result of stream erosion is called a peneplain (an almost plain). The characteristics of each of the stages of landscapes developing in running water regimes may be summarised as follows: Youth  Also called the mountain stage. The velocity and speed of the stream is very high because the slope is steep. Vertical erosion takes place and V-shaped valleys are formed here.  Streams are few during this stage with poor integration.  The valleys have no floodplains or narrow flood plains along trunk streams.  Stream divides are broad and flat with marshes, swamps and lakes.  Waterfalls and rapids may exist where local hard rock bodies are exposed. Mature or Middle Stage  During this stage, there are many streams with good integration.  Lateral erosion is dominant here due to which widening of the valley occurs. The volume of the river water increases and the slope of the river is moderate.  Streams may flow in meanders confined within the valley.  Swamps and marshes of the youth disappear and the stream divide turns sharp.  Waterfalls and rapids disappear. Old or Lower Stage  Streams meander freely over vast floodplains showing natural levees, ox-bow lakes, etc.  Divides are broad with swamps, marshes and lakes.  The depth of the river is shallow here.  Most of the landscape is at or slightly above sea level. Erosional Landforms The following are some of the major erosional landforms: 1. Valleys – The valleys made by rivers are erosional landforms. Small and narrow rills gradually develop into long and wide gullies; these gullies further deepen, widen and lengthen to give rise to valleys. Depending upon dimensions and shape, there are various types of valleys like V-shaped valleys, canyons and gorges. 1. Gorge – Gorge is formed due to active down-cutting of the valleys. A gorge is a narrow and deep river valley which has steep slopes. A gorge is almost equal in width at its top as well as at its bottom. Gorges usually form in hard rocks. 2. Canyon– A canyon is characterised by steep step-like side slopes and may be as deep as a gorge. A canyon is wider at its top than at its bottom. Canyons usually form in horizontal bedded sedimentary rocks. The Great Canyon of Colorado River in the state of Arizona, U.S.A is the largest canyon in the world. The canyon of Gandikota, situated on the Pennar river in Andhra Pradesh is known as the Great Canyon of India. 3. V-shaped Valley – Due to the steep slope and large volume of water, the river cuts its bed vertically forming a narrow and deep river valley. This is called a v-shaped valley.  2. Potholes – Small depressions in the rocky beds of the river valleys are called potholes. They are cylindrical in shape. Potholes are generally formed in coarse-grained rocks such as sandstones and granites. 3. Plunge Pool – It is a deep depression in a stream bed at the base of a waterfall. It is created by erosional forces of falling water at the base of a waterfall. 4. Incised or Entrenched Meanders – Streams flowing over gentler slopes undergo active lateral erosion and develop sinuous or meandering courses. Meandering is commonly found over floodplains and delta plains where slopes are gentle. But very deep and wide meanders can also be found cut in hard rocks. Such meanders are called incised or entrenched meanders. 5. River Terraces – The narrow step like flat surfaces on either side of the valley floor are called river terraces. They represent the level of former valley floors. Depositional Landforms The major depositional landforms are as follows:  Alluvial fans  Alluvial fans are often found at the foot of mountain ranges. When the streams flow over mountainous slopes, they usually carry a very coarse load. This load is too heavy for the streams to be carried over gentler slopes and gets deposited as a broad low to high cone-shaped deposit known as an alluvial fan. An alluvial fan can be defined as a fan-shaped deposit of gravel, sand and other smaller particles of sediments.  With time, the streams which flow over these fans change their course forming many channels called distributaries.  In humid regions, alluvial fans with low cones and gentler slopes are formed. In arid and semi-arid regions, alluvial fans appear as high cones with steep slopes.  Delta  The Delta is found in the old stage of a river. It is a triangular-shaped landform made up of alluvial deposition in the mouth of the river.  Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification. The coarsest materials settle first and the finer ones like clays and silts are carried out into the sea.   Krishna River Delta  Floodplain  Floodplain is a major landform of river deposition. A floodplain is a flat area of land adjacent to a river. It stretches from the bank of its channel to the base of the enclosing valley walls which experience flooding during the period of high discharge.  A river bed made of river deposits is the active floodplain and the floodplain which lies above the bank is an inactive floodplain.  Natural Levees and Point bars  Both are associated with floodplains.  Natural levees are formed along the banks of large rivers. These are linear, low and parallel ridges of coarse deposits along the river banks, usually cut into individual mounds.  Point bars are also called meander bars. They occur on the concave side of meanders of large rivers. These are formed by sediments deposited in a linear arrangement by flowing waters along the banks. They contain a mixed size of sediments and are almost uniform in profile and width.   Meanders  Loop-like channel patterns called meanders develop over flood and delta plains. Meander is a type of channel pattern.  Meanders are the result of both erosional and depositional processes. They are typical landforms of the middle and lower course of a river.  Meanders are formed because:  Water flowing over gentler slopes have a tendency to work laterally on banks.  The unconsolidated nature of alluvial deposits makes up the banks with many irregularities which can be used by water exerting pressure laterally.  Coriolis force acts on the fluid water, deflecting it like it deflects the wind.  In meanders, there is active deposition along the concave bank and erosion along the convex bank.  As meanders grow into deep loops, they may cut off due to erosion at inflection points resulting in ox- bow lakes.   Groundwater In this section, we discuss the role of groundwater in the erosion of landmasses and the evolution of landforms.  The result of the work of groundwater cannot be seen in all types of rocks. Physical or mechanical removal of materials by moving groundwater is insignificant in developing landforms.  In rocks like dolomite, limestone which are rich in calcium carbonate, the groundwater as well as surface water through the chemical action of solution and precipitation deposition form varieties of landforms. The processes of solution and deposition are active on limestones or dolomites which occur either exclusively or interbedded with other rocks.  Any limestone, dolomite or gypsum region showing typical landforms produced by the action of groundwater through the process of solution and deposition is called Karst topography after the typical topography developed in the limestone rock of the Karst region in the Balkans adjacent to the Adriatic sea. The Karst topography is characterised by erosional and depositional landforms. Erosional Landforms due to Groundwater The following are the erosional landforms formed due to the action of groundwater –  1. Sinkhole – A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom. The area of these sinkholes may vary from a few square metres to a hectare and the depth ranges from less than half a metre to thirty metres or more. When a sinkhole is formed solely through the process of solution, it is called a solution sink. Quite often, sinkholes are covered up with soil mantle and appear as shallow water pools. 2. Doline/Collapse sinks – When the bottom of the sinkhole forms the roof of a void or cave underground, it might collapse leaving a large hole opening into a void or a cave below which is called collapse sinks. The term doline is sometimes used for collapse sinks. 3. Uvala – When a series of smaller sinkholes coalesce into a compound sinkhole, it is called uvala or valley sinks. 4. Lappies – These are irregular grooves and ridges formed when most of the surfaces of limestone are removed by a solution process. 5. Caves – Caves normally have an opening through which streams are discharged. Caves having an opening at both ends are called tunnel caves. Cave formation is prominent in areas where there are alternating beds of rocks (sandstone, shales, quartzite) with dolomites or limestone in between or in areas where limestones are massive, dense and occur as thick beds. Depositional Landforms due to Groundwater The following are the depositional landforms formed within the limestone caves: 1. Stalactite – Stalactites are calcium carbonate deposits hanging as icicles. They are usually broad at the base and taper towards the free ends. The stalactites that stretch towards the sides are known as helactites. 2. Stalagmite – Stalagmites are calcium carbonate deposits that grow upwards from the cave floor. Stalagmites may take the shape of a column, a disc with either a smooth, rounded building end or a miniature crater-like depression. The stalactite and stalagmite eventually fuse to give rise to columns and pillars of varying diameters. Glaciers The term glacier comes from the French word “glace” meaning ice. A glacier is a huge mass of ice moving as sheets. When the ice sheets move over the land, it is called a continental glacier, and if a vast sheet of ice is spread over the plains at the foot of mountains, it is called a piedmont glacier. The mountain and valley glaciers flow down the slopes of mountains in broad trough-like valleys. The movement of glaciers is mainly due to the gravitational force. The movement of glaciers is slow, it could be a few centimetres to a few metres a day or even less or more.  Over 96% of the glaciers occur in Antarctica and Greenland. The world’s largest glacier is the Lambert Glacier in Antarctica.  Sometimes, the surface of the glacier develops cracks known as crevasses. These are deep fissures of variable width in the surface of a glacier.  A glacier during its lifetime creates various landforms due to erosional and depositional activities. Erosion takes place by the action of plucking and abrasion. Due to the movement of glaciers, debris gets removed, divides get lowered and eventually the slope gets reduced to such an extent that glaciers stop moving, leaving only a mass of low hills and vast outwash plains along with other depositional features. Erosional Landforms due to Glaciers Cirques  These are the most common landforms in glaciated mountains and are often found at the heads of glacial valleys.  A glacier cuts these cirques while moving down the mountainous slopes. They are long, deep and wide troughs or basins with steep concave to vertically dropping high walls at its head as well as sides.  When the glacier disappears, very often, a lake of water can be seen within the cirques which are called cirque or tarn lakes. There can be two or more cirques one leading into another down below in a stepped sequence. Horns and Aretes  Horns are formed by the headward erosion of the cirque walls. When three or more radiating glaciers cut headward and their cirques meet, high, sharp-pointed and steep-sided peaks called horns are formed.  The highest peak in the Himalayas – Everest and the highest peak in the Alps – Matterhorn are formed by headward erosion of radiating cirques.  Due to the progressive erosion, the divides between cirque side walls or headwalls get narrow and form serrated or saw-toothed edges sometimes called aretes with a very sharp crest and a zig-zag outline.  Erosional and depositional landforms due to glaciers Glacial Valleys/Troughs  Since glacial mass is heavy and slow-moving, erosional activity is uniform in all directions. A steep-sided curved bottom valley has a U-shaped profile. The U-shaped valley is a typical glacial feature.  When tributary glaciers are unable to cut as deeply as the main glacier, hanging valleys are formed. These are at higher levels than the main valley and appear hanging over the main valley. They enter the main valley at some height.  Very deep glacial troughs filled with seawater and making up shorelines (in high altitude) are called fjords/fiords. Fjords are common in Norway, New Zealand and Greenland. Read more on glacial erosional landforms in the link. Depositional Landforms due to Glaciers The melting glaciers deposit unassorted coarse and fine debris called glacial till. The rock fragments in the till are mostly angular to sub-angular in shape. Streams of water formed by the melting of glaciers carry some amount of rock debris (relatively fine) and deposit it. Such glacio-fluvial deposits are called outwash deposits. The outward deposits are roughly stratified and assorted (unlike till deposits) and rock fragments in them are somewhat rounded at their edges. The following are the depositional landforms commonly found in glaciated regions: Moraines  Moraines are the deposits of glacial till. These debris fields exist in places where glaciers have moved through in the past. There are various types of moraines and these are classified based on their location.   Terminal moraines – These are long ridges of debris deposited at the end (toe) of the glaciers.  Lateral moraines –  These ridges of deposits run parallel to the sides of a glacier.  The lateral moraine may join a terminal moraine forming a horse-shoe shaped ridge.  There can be many lateral moraines on either side in a glacial valley.  These moraines partly or fully owe their origin to glacio-fluvial waters pushing up materials/debris to the sides of glaciers.  Ground moraines – These are glacial deposits (irregular sheets of till) formed on the floor of a glacial valley.  Medial moraines –  The moraines formed in the centre of the glacial valley and flanked by lateral moraines are called medial moraines.  Sometimes medial moraines are difficult to distinguish from ground moraines.  These are formed in the middle of two glaciers. They are the result of the merging of two lateral moraines which continue as medial moraines.  Recessional moraines –  The recessional moraines run across the landscape behind a terminal moraine.  These are formed when the receding glacier pauses at certain places for a long time before moving further. Types of Moraines Eskers With the melting of glaciers in summer, the water flows on the surface of ice or moves down along the margins or even moves down through the holes in the ice. These waters flow as streams in a channel under the ice. These streams flow over the ground (not in a valley cut in the ground) with ice forming its banks. Boulders and blocks, as well as rock debris carried into this stream, settle in the valley of ice beneath the glacier and when the ice melts it is seen as a sinuous ridge called esker. Outwash Plains When the glacier reaches its lowest point and melts, it leaves behind a layered deposition of rock debris, sand, clay, gravel, etc. This layered surface is called an outwash plain. Drumlins These are oval-shaped ridge-like depositions of glacial till with some masses of gravel and sand. They are usually 1 km in length and approximately 30 m in height. The end of the drumlins facing the glacier is called the stoss end which is steeper and the other end is called the tail. Drumlins give an indication of the direction of glacier movement. Glacial landscapes with various depositional landforms Waves and Currents Coastal processes are the most dynamic and therefore, most destructive. The changes along the coasts are mainly due to waves. When waves break, the water is thrown with great force onto the shore which results in great churning of sediments on the sea bottom. The constant impact of breaking waves drastically affects the coasts. Tsunami waves and storm waves cause far-reaching changes in a short span of time than normal breaking waves. The coastal landforms also depend upon: 1. the configuration of land and seafloor. 2. whether the coast is advancing (emerging) or retreating (submerging) landward. Types of Coasts Assuming the sea level to be constant, two types of coasts are considered to explain the concept of evolution of coastal landforms: 1. High rocky coasts – submerged coasts 2. Low, smooth and gently sloping sedimentary coasts – emerged coasts High Rocky Coasts  Along the high rocky coasts, erosion features dominate and shores don’t show any depositional landform initially. The rivers appear to have been drowned with a highly irregular coastline. The coastline is highly indented with the extension of water into land where glacial valleys (fjords) are present. The hillsides drop off sharply into the water.  Along high coasts, waves break with high force against the land, shaping the hillsides into cliffs. With constant hitting by waves, the cliffs recede leaving a wave-cut platform in front of the sea cliff. Waves gradually minimise the irregularities along the shore.  The materials which fall off from the cliff, gradually break into smaller fragments and become rounded. These fragments get deposited in the off-shore. After a considerable period of time, with more addition of material to this deposit in the off-shore, a wave built terrace would develop in front of the wave-cut terrace.  As the erosion along the coast takes place, a good supply of material becomes available to longshore currents and waves to deposit them as beaches and as bars, which are long ridges of sand and/or shingle parallel to the coast in the nearshore zone.  Bars are submerged and when they emerge above water, they are called barrier bars.  A barrier bar that gets keyed up to the headland of a bay is called a spit.  When the barrier bars and spits form at the mouth of a bay and block it, a lagoon is formed.  These lagoons get gradually filled up by sediments from the land giving rise to a coastal plain. Low Sedimentary Coasts  Along low sedimentary coasts, the rivers appear to extend their length by building coastal plains and deltas.  The coastline appears smooth with occasional incursions of water in the form of lagoons and tidal creeks.  Depositional features dominate over erosional ones.  When waves break over a low sedimentary coast, the sediments in the bottom get churned and move readily to build bars, spits, barrier bars and lagoons. Lagoons would eventually turn into swamps which would subsequently turn into coastal plains.  Tsunami waves and storms cause drastic changes irrespective of the supply of sediments. Large rivers that carry huge sediments build deltas along low sedimentary coasts.  The east coast of India is a low sedimentary coast and depositional forms dominate the east coast. The west coast is a high rocky retreating coast and erosional forms dominate the west coast. Erosional Landforms  When erosion is the dominant shore process, wave-cut cliffs and terraces are two forms that are usually found.   At the foot of the cliffs, there may be a gentle sloping or flat platform covered by rock debris. Such platforms which occur at elevations above the average height of waves are called wave-cut terraces.  The lashing of the waves against the base of the cliff and the rock debris that gets hit against the cliff along with lashing waves create hollows and these hollows deepen and widen to form sea caves.  A 90 m long sea cave is found on the Loliem beach in Canacona in Goa. The world’s most extensive cave is the 1.5 km long Matainaka cave in New Zealand.  The resistant masses of rock that appear as rocks standing isolated as small islands just off the shore, originally parts of a cliff or hill, are called sea stacks.  All the above features are temporary and eventually disappear because of wave erosion giving rise to narrow coastal plains, and with a rush of deposits from over the land behind may get covered up by alluvium or may get covered up by shingle or sand to form a wide beach. Depositional Landforms 1. Beach 1. It is an elongated stretch of sand, pebbles, gravels, etc. deposited along the coast. Most of the sediments making up the beaches comes from land carried by the streams and rivers or from wave erosion. The sandy beach which appears so permanent may be reduced to a very narrow strip of coarse pebbles in some other season, so these beaches are temporary features. 2. Praia da Casino beach in Brazil is the world’s longest beach. Marina beach, Chennai is the second longest beach in the world. 2. Bars – The bar is a stretch of sand deposition off the shoreline. 3. Barrier Bar – The further addition of sand to the off-shore bar results in a barrier bar. 4. Spits – When the barrier bars get keyed up to one end of the bay, spits are formed. Spits may also get attached to headlands/hills. 5. Lagoon – The bars, barriers and spits formed at the mouth of the bay gradually extend leaving only a small opening of the bay into the sea and the bay eventually develop into a lagoon. The lagoon is eventually replaced by a wide coastal plain. These lagoons get filled up gradually by sediments coming from the land or from the beach itself (aided by wind). 6. Tombolo – A tombolo is a bar connecting an island with the coast.  Depositional features of waves Wind The wind is the main geomorphic agent in the arid region.  The desert floors being dry and barren heat up very fast. They heat up the air directly above them resulting in upward movements in the hot light air with turbulence, and any obstruction in its path sets up whirlwinds, eddies, downdrafts and updrafts. Winds also move along the desert floors with great speed and the obstruction on their path results in turbulence. There are also storm winds which are very destructive.  Winds cause deflation, abrasion and impact. Deflation involves the lifting and removal of dust and smaller particles from the surface of rocks. In the transportation process, sand and silt act as effective tools to abrade the land surface. The impact is the sheer force of momentum which occurs when sand is blown into or against a rock surface.  The rainfall is scarce in arid regions, however, torrential rains come down in a short period of time. The desert rocks which are devoid of any vegetation get exposed to chemical and mechanical weathering due to drastic diurnal changes in temperature, decay faster and the torrential rains remove the weathered debris easily. Thus the weathered materials in the deserts are moved not only by wind but also by rain (sheet wash). The general mass erosion is carried mainly by sheet floods and the wind moves fine particles. Erosional Landforms due to Wind  Pediplains – When the high relief structures in desserts are reduced to low featureless plains by the activities of wind, they are called as pediplains.  Mushroom, Table and Pedestal rocks – In deserts, a greater amount of sand and rock particles are transported close to the ground by the winds which cause more erosion in the lower part of the rock than the top. These results in the formation of rock pillars shaped like a mushroom with narrow pillars with broad top surfaces. Sometimes, the top surface is broad like a tabla top and quite often, the remnants stand out like pedestals. 1. Mushroom Rock   Deflation hollows – When deflation causes a shallow depression by persistent movement of winds, they are called deflation hollows.  Playas – In basins with mountains and hills around and along, the drainage is towards the centre of the basin and due to gradual deposition of sediment from basin margins a nearly level plain forms at the centre of the basin. When there is sufficient water, this plain is covered up by a shallow water body and such types of shallow lakes are termed “playas”. In playas, water is retained for a short duration only (evaporation) and quite often the playas contain a good deposition of salts. The playa plain covered up by salts is called alkali flats. Depositional Landforms due to Wind Sand Dunes– Dry hot deserts are good places for sand dune formation. According to the shape of a sand dune, there are varieties of sand forms: 1. Barchans – Barchans are one of the classic desert landforms. These are crescent shaped dunes with points or wings directed away from the wind direction. 2. Parabolic dunes – These are reversed barchans with wind direction being the same. These are formed when sand surfaces are partially covered with vegetation. 3. Seif dunes – These are similar to barchans but have only one wing or point. These are formed when there is a shift in wind conditions. The long wings of seif can grow very long and high. 4. Longitudinal dunes – These appear as long ridges of considerable length but are low in height. These are found when the supply of sand is poor and wind direction is constant. 5. Transverse dunes – These dunes are formed when the wind direction is constant and the source of sand is an elongated feature at right angles to the wind direction. Various types of sand dunes, arrows indicate wind direction