The level above which there is perpetual snow cover is called the snowline. The snowline varies with altitude and latitude. In the polar region it is at sea-level; in East Africa it is at 5000 m; in the northern hemisphere it is lower on the shady north-facing side of a mountain than the southfacing side. When the accumulation of snow in a region increases year by year, it gradually turns into ice by its own weight.
Masses of ice that cover large areas of a continent are called ice s~ets, and those which occupy mountain valleys are called valley glaciers. Today ice sheets occur in Antarctica and Greenland, while valley glaciers are found in the Himalayas, Andes, Alps and Rockies. The period when the high latitudes were buried under ice sheets is known as the Ice Age. With the melting of the ice at the end of the Ice Age, enormous quantities of water were set free. Some of these formed lakes, examples being the Great Lakes of America...and the lakes of Finland.
A glacier is defined as a mass of ice that moves under the influence of gravity along a confined course away from its source area. However, the movement is not of the glacier as a whole. Throughout the glacier bits of ice are melting, tric,kling down-valley and then turning back into ice the whole time. This means that within the glacier there is a gradual down-valley movement.
Glacial erosion consists of two processes: (i) plucking or the tearing away of blocks of rock which have become frozen into the base and sides of a glacier, and (ii) abrasion or the wearing away of rocks beneath a glacier by the scouring action of the rocks embedded in the glacier.
The erosional features produced by glaciers include the cirque. A cirque or corrie originates as a small hollow where snow accumulates. The snow becomes compacted to glacial- ice, forming a cirque glacier, and eventually flows downslope under the influence of gravity. The characteristic shape of the cirque is a result. of the freeze-thaw erosion on the headwall and the rotational slip of ice withi.,n the concave floor of the hollow, which is widened and deepened by plucking. Many cirques contain small circular lakes called tams. Sometimes corries develop on adjacent slopes and only a knife-edge ridge, called an arete, separates them.
If a glacier extending down a valley enters a part of the valley which is wider than the rest, the glacier ice spreads out to fill the valley; this causes the upper layers of the ice to crack along lines parallel to the valley sides. These cracks are very deep and are called crevasses. As the amount of ice in a valley increases, the power to erode by a valley glacier also increases. This results in the glacier deepening, straightening and widening a river valley.
The overdeepening of the valley gives it a characteristic U shape. Hanging valleys are another common feature ii1. areas that have been glaciated. These are tributary valleys that lie above the main valley and are separated from it by steep slopes down which streams may flow as a waterfall or a series of rapids. (Hanging valleys may also form during the retreat of a coastline under rapid erosion.)
Certain features are produced by glacial deposits. A valley glacier carries a large amount of rock waste called moraine. The moraine forming along the sides of a glacier is called lateral moraine; that along the front of a glacier is called terminal moraine; that at the bottom of a glacier is the ground moraine. When two glaciers join together, their inner lateral moraines coalesce to give a medial moraine. Terminal moraine material is carried down-valley by the melt waters issuing from the glacier's snout (front) and is deposited as a layer called an outwash plain. One of the most conspicuous features of lowlands which have been glaciated by ice sheets is the widespread morainic deposits. Because of the numerous boulders in the clay these are called boulder clay deposits.
The deposits are sometimes several hundred metres thick and their surface is marked by long rounded hills, called drumlins. Large blocks of rock of a material, quite different to that of the rocks of the region, often occur in areas which lay under ice sheets. These blocks are known as erratics. Rivers and streams occur inside most glaciers and these are heavily loaded with rock debris. As an ice front retreats the rivers build up ridge-like deposits called eskers. They develop on top of the boulder clay deposits. Roche moutonnees are another feature produced by glacial deposition.
Thursday, October 29, 2009
TYPES OF COASTS
Coastal regions may be either submerged or uplifted by changes in land or sea levels. Thus, coasts are either submerged or emerged types. When a highland coast is submerged the lower parts of its river valleys become flooded. These submerged parts of the valley are called rias. Rias are common in S.W. England, S.W. Ireland, and N.W. Spain. Due to submergence the coast becomes indented and the tips of the headlands may be turned into islands. Longitudinal coasts are formed when a highland coast whose valleys are parallel to the coast is submerged. Some of the valleys are flooded and the separating mountain ranges become chains of islands. These valleys are sometimes called sounds. This type of coast occurs in Yugoslavia and along parts of the Pacific coasts of North and South America.
When glaciated highland coasts become submerged, the flooded lower parts of the valleys are called fiords. During glaciation the river valleys become widened and deepened. After the glaciers have disappeared and the sea has risen, the steep-sided valleys are drowned. The water inside the fiord is much deeper than it is at the entrance of the fiord. Fiords have steeper sides and deeper water than rias. All the fiord coasts lie in the belt of prevailing westerly winds and are on the western sides of land masses. It was in these regions that vast amounts of ice accumulated in the Ice Age. Some of the best examples of fiord coasts lie in Chile, South Island of New Zealand, Greenland, Norway and British Columbia. Both rias and fiords often provide good natural harbours.
A rise in a sea level along a lowland coast causes the sea to penetrate inland along the river valleys. The flooded parts of the valleys are called estuaries. When a part of the continental shelf emerges from the sea it forms a coastal plain. Such emerged lowland coasts have no bays or headlands and deposition takes place in the shallow water offshore, producing off-shore bars, lagoons, spits and beaches. The development of ports is difficult in such areas.
There are some coasts marked by coral formation. Coral is a limestone rock made up of the skeletons of tiny marine organisms called coral polyps. Polyps form below' the level of low tide as they cannot grow outside water. They
. thrive in sunlit, clear salt water down to a depth of about 55 metres in sea temperatures of about 21°C. Extensive coral formations develop between 300N and 300S, especially on the eastern sides of land masses where warm currents flow near to the coasts. Coral masses are often called reefs, of which there are three kinds. A fringing reef is a narrow coral platform separated from the coast by a lagoon which may disappear at low water. The surface of the platform is usually flat or slightly concave and its outer edge drops away steeply to the surrounding sea floor. A barrier reef is a wide coral platform separated from the coast by a wide, deep lagoon. The Great Barrier Reef off the east coast of Australia is famous. Barrier reefs also occur around islands forming a continuous ring of coral. An atoll is a circular or horseshoe shaped coral reef, enclosing a lagoon within it.
When glaciated highland coasts become submerged, the flooded lower parts of the valleys are called fiords. During glaciation the river valleys become widened and deepened. After the glaciers have disappeared and the sea has risen, the steep-sided valleys are drowned. The water inside the fiord is much deeper than it is at the entrance of the fiord. Fiords have steeper sides and deeper water than rias. All the fiord coasts lie in the belt of prevailing westerly winds and are on the western sides of land masses. It was in these regions that vast amounts of ice accumulated in the Ice Age. Some of the best examples of fiord coasts lie in Chile, South Island of New Zealand, Greenland, Norway and British Columbia. Both rias and fiords often provide good natural harbours.
A rise in a sea level along a lowland coast causes the sea to penetrate inland along the river valleys. The flooded parts of the valleys are called estuaries. When a part of the continental shelf emerges from the sea it forms a coastal plain. Such emerged lowland coasts have no bays or headlands and deposition takes place in the shallow water offshore, producing off-shore bars, lagoons, spits and beaches. The development of ports is difficult in such areas.
There are some coasts marked by coral formation. Coral is a limestone rock made up of the skeletons of tiny marine organisms called coral polyps. Polyps form below' the level of low tide as they cannot grow outside water. They
. thrive in sunlit, clear salt water down to a depth of about 55 metres in sea temperatures of about 21°C. Extensive coral formations develop between 300N and 300S, especially on the eastern sides of land masses where warm currents flow near to the coasts. Coral masses are often called reefs, of which there are three kinds. A fringing reef is a narrow coral platform separated from the coast by a lagoon which may disappear at low water. The surface of the platform is usually flat or slightly concave and its outer edge drops away steeply to the surrounding sea floor. A barrier reef is a wide coral platform separated from the coast by a wide, deep lagoon. The Great Barrier Reef off the east coast of Australia is famous. Barrier reefs also occur around islands forming a continuous ring of coral. An atoll is a circular or horseshoe shaped coral reef, enclosing a lagoon within it.
LANDFORMS MADE BY WAVE ACTION
Wave erosion has three aspects. (i) Corrosive action involves the hurling of pebbles and sand against the base of a cliff by breaking waves; this causes undercutting and rock break-up. (ii) Hydraulic action involves water thrown against a cliff face by breaking waves causing air in cracks and crevices to become suddenly compressed; when the water retreats the air expands, often explosively, causing the rocks to shatter. (ill) Attrition involves the breaking up of the boulders and pebbles dashed against the shore into finer particles.
Cliffs are steep or vertical rock faces formed by waves undercutting the rock. The rocks of some cliffs are in layers which slope landwards. In other cliffs the rock layers slope sea wards and blocks of rock loosened by erosion easily fall into the sea. The cliffs are often very steep and overhanging. A cave develops along a line of weakness at the base of a cliff which has been subjected to prolonged wave action. If a joint extends from the end of the cave to the top of the cliff, this becomes enlarged in time and finally opens out on the cliff top to form a blow hole. Caves which develop on either side of a headland such that they ultimately join together, give rise to a natural arch. When the arch collapses, the end of the headland stands up as a stack.
The depositional features of waves are beaches, spits and bars,.and mud flats. Beaches usually lie between high and low water levels. Material which is eroded from a coast may be carried along the coast as a spit; this is likely to happen along indented coasts or coasts broken by river mouths. A spit is a low, narrow ridge of pebbles or sand joined to the land at one end with the other end terminating in the sea.
A bar is very similar to a spit. A bay-bar grows right across a bay; such bay-bars are called nehrungs along the coast of Poland. When a bar links an island to the mainland it is called a tom bolo. Tides tend to deposit fine silts along gently shelved coasts, especially in bays and estuaries. The deposition of these silts together, perhaps with river alluvium, results in the building up of a platform of mud called a mud flat. Salt tolerant plants soon colonise the mud flat which in time becomes a swamp or marshland. In tropical lands these mud flats often become mangrove swamps.
Cliffs are steep or vertical rock faces formed by waves undercutting the rock. The rocks of some cliffs are in layers which slope landwards. In other cliffs the rock layers slope sea wards and blocks of rock loosened by erosion easily fall into the sea. The cliffs are often very steep and overhanging. A cave develops along a line of weakness at the base of a cliff which has been subjected to prolonged wave action. If a joint extends from the end of the cave to the top of the cliff, this becomes enlarged in time and finally opens out on the cliff top to form a blow hole. Caves which develop on either side of a headland such that they ultimately join together, give rise to a natural arch. When the arch collapses, the end of the headland stands up as a stack.
The depositional features of waves are beaches, spits and bars,.and mud flats. Beaches usually lie between high and low water levels. Material which is eroded from a coast may be carried along the coast as a spit; this is likely to happen along indented coasts or coasts broken by river mouths. A spit is a low, narrow ridge of pebbles or sand joined to the land at one end with the other end terminating in the sea.
A bar is very similar to a spit. A bay-bar grows right across a bay; such bay-bars are called nehrungs along the coast of Poland. When a bar links an island to the mainland it is called a tom bolo. Tides tend to deposit fine silts along gently shelved coasts, especially in bays and estuaries. The deposition of these silts together, perhaps with river alluvium, results in the building up of a platform of mud called a mud flat. Salt tolerant plants soon colonise the mud flat which in time becomes a swamp or marshland. In tropical lands these mud flats often become mangrove swamps.
LANDFORMS MADE BY WIND ACTION
LANDFORMS MADE BY WIND ACTION
Wind action is very striking in arid and semi-arid regions. Wind erosion consists of abrasion which breaks up rocks and produces rock pedestals, zeugens, yardangs, and inselbergs, and deflation which blows away rock waste and thus lowers desert surface producing depressions. Wind deposition gives rise to dunes, made of sand, and loess, made of desert dust.
Various kinds of desert surfaces are recognised. A
sandy desert, called erg in the Sahara and koum in Turkey, is an undulating plain of sand produced by wind action. A stony desert, called reg in Algeria and serir in Libya and Egypt, has its surface covered with boulders, angular pebbles and gravel which have been produced by diurnal temperature changes. Rocky desert, called hamada in the Sahara, is characterised by bare rock surface formed by deflation. Badlands develop in semi-desert regions mainly as a result of water erosion produced by violent rain storn.J. The land is broken by extensive gullies and ravines which are separated by steep-sided ridges.
A desert area which has a surface layer of hard rock underlain by soft rock develops a 'ridge and furrow' landscape under wind action. The ridges are called zeugens. Bands of hard and soft rocks which lie parallel to the prevailing winds in a desert region develop another 'ridge and furrow' pattern. The belts of hard rock stand up as rocky ribs in fantastic shapes: they are called yardangs. They
are common in Asian deserts and the Atacama Desert. '
Some depressions produced by wind deflation reach down to the water table; a swamp or an .oasis then develops. In some desert regions erosion has removed all the original surface except for isolated pieces which stand up as roundtopped masses called inselbergs. They are common in the Kalahari Desert, parts of Algeria, and Western Australia.
There are two types of sand dunes. A barchan is a crescent shaped sand dune, the horns of which point away from the direction of the dominant wind; the leeward slope is relatively steep and the windward slope gentle. This asymmetry is due to eddies being set up by the prevailing wind blowing over the crest of the dune. Barchans migrate as grains of sand are blown up the windward slope and roll down the leeward slope. The best examples are found in the Sahara and Turkey. A seif dune forms when a cross wind develops to the prevailing wind and the corridors between the dunes are swept clear of sand by this wind.
Some of the fine particles blown out of deserts by the winds are deposited on land where they accumulate to form loess. Loess is friable and easily eroded by rivers. There are extensive deposits of loess in northern China formed of the desert soil from the Gobi Desert. The loess deposits of central Europe were probably formed in the last Ice Age when the out-blowing winds carried fine glacial dust from the ice sheets of northern Europe. Loess deposits are unusually fertile. They are also used for building.
As the edges of desert and semi-desert highlands get pushed back by erosion and weathering, a gently sloping platform develops; this is called a pediment.
Wind action is very striking in arid and semi-arid regions. Wind erosion consists of abrasion which breaks up rocks and produces rock pedestals, zeugens, yardangs, and inselbergs, and deflation which blows away rock waste and thus lowers desert surface producing depressions. Wind deposition gives rise to dunes, made of sand, and loess, made of desert dust.
Various kinds of desert surfaces are recognised. A
sandy desert, called erg in the Sahara and koum in Turkey, is an undulating plain of sand produced by wind action. A stony desert, called reg in Algeria and serir in Libya and Egypt, has its surface covered with boulders, angular pebbles and gravel which have been produced by diurnal temperature changes. Rocky desert, called hamada in the Sahara, is characterised by bare rock surface formed by deflation. Badlands develop in semi-desert regions mainly as a result of water erosion produced by violent rain storn.J. The land is broken by extensive gullies and ravines which are separated by steep-sided ridges.
A desert area which has a surface layer of hard rock underlain by soft rock develops a 'ridge and furrow' landscape under wind action. The ridges are called zeugens. Bands of hard and soft rocks which lie parallel to the prevailing winds in a desert region develop another 'ridge and furrow' pattern. The belts of hard rock stand up as rocky ribs in fantastic shapes: they are called yardangs. They
are common in Asian deserts and the Atacama Desert. '
Some depressions produced by wind deflation reach down to the water table; a swamp or an .oasis then develops. In some desert regions erosion has removed all the original surface except for isolated pieces which stand up as roundtopped masses called inselbergs. They are common in the Kalahari Desert, parts of Algeria, and Western Australia.
There are two types of sand dunes. A barchan is a crescent shaped sand dune, the horns of which point away from the direction of the dominant wind; the leeward slope is relatively steep and the windward slope gentle. This asymmetry is due to eddies being set up by the prevailing wind blowing over the crest of the dune. Barchans migrate as grains of sand are blown up the windward slope and roll down the leeward slope. The best examples are found in the Sahara and Turkey. A seif dune forms when a cross wind develops to the prevailing wind and the corridors between the dunes are swept clear of sand by this wind.
Some of the fine particles blown out of deserts by the winds are deposited on land where they accumulate to form loess. Loess is friable and easily eroded by rivers. There are extensive deposits of loess in northern China formed of the desert soil from the Gobi Desert. The loess deposits of central Europe were probably formed in the last Ice Age when the out-blowing winds carried fine glacial dust from the ice sheets of northern Europe. Loess deposits are unusually fertile. They are also used for building.
As the edges of desert and semi-desert highlands get pushed back by erosion and weathering, a gently sloping platform develops; this is called a pediment.
KARST OR LIMESTONE TOPOGRAPHY
KARST OR LIMESTONE TOPOGRAPHY
In dry regions, mainly in highlands composed of limestone-like rocks on a large scale, 'karst' topography is caused by the movement of underground water as an agent of gradation. It is so named after a province of Yugoslavia on the Adriatic Sea coast where such fOrmations are most noticeable. Karst is a region of well-jomted carboniferous limestone in which carbonation is the dominant weathering process. Carbonation in the karst region produces features such as sink hole, swallow holes, and caves or caverns (a large cave is called a cavern). The most striking features of caves are stalactites and stalagmites.
Sink Holes Sink hole is a funnel-shaped depression which has an average depth of three to nine metres and in area, it may vary from one square metre to more. In the limestone plateau of Kentucky in the USA, the number of sink holes is well over 60,000.
Swallow Holes Swallow holes are cylindrical in shape lying underneath the sink hole. These holes swallow the sub-surface streams which may re-appear from rock openings.
Caves or Caverns A cave is an underground chamber that is accessible from the surface. Caves are most frequently found in cliffs along coasts and in limestone areas. In limestone regions, caves are the result of the rock being dissolved through carbonation by underground streams. The water seeps through the roof of the caverns in the form of a continuous chain of drops. A portion of the drop hangs on the roof and on the evaporation of water, a small deposit of limestone is left behind contributing to the formation of a stalactite, growing downwards from the roof. The remaining portion of the drop falls on the floor of the cavern. This also evaporates and forms a stalagmite, rising upwards from the floor.
Stalactites and stalagmites often meet to merge into a column.
In dry regions, mainly in highlands composed of limestone-like rocks on a large scale, 'karst' topography is caused by the movement of underground water as an agent of gradation. It is so named after a province of Yugoslavia on the Adriatic Sea coast where such fOrmations are most noticeable. Karst is a region of well-jomted carboniferous limestone in which carbonation is the dominant weathering process. Carbonation in the karst region produces features such as sink hole, swallow holes, and caves or caverns (a large cave is called a cavern). The most striking features of caves are stalactites and stalagmites.
Sink Holes Sink hole is a funnel-shaped depression which has an average depth of three to nine metres and in area, it may vary from one square metre to more. In the limestone plateau of Kentucky in the USA, the number of sink holes is well over 60,000.
Swallow Holes Swallow holes are cylindrical in shape lying underneath the sink hole. These holes swallow the sub-surface streams which may re-appear from rock openings.
Caves or Caverns A cave is an underground chamber that is accessible from the surface. Caves are most frequently found in cliffs along coasts and in limestone areas. In limestone regions, caves are the result of the rock being dissolved through carbonation by underground streams. The water seeps through the roof of the caverns in the form of a continuous chain of drops. A portion of the drop hangs on the roof and on the evaporation of water, a small deposit of limestone is left behind contributing to the formation of a stalactite, growing downwards from the roof. The remaining portion of the drop falls on the floor of the cavern. This also evaporates and forms a stalagmite, rising upwards from the floor.
Stalactites and stalagmites often meet to merge into a column.
ARTESIAN WELL
ARTESIAN WELL
A special type of well in which water rises automatically under the pressure of a column of water to the ground surface, either through a natural or man-made hole, is known as an artesian well. The name artesian is derived from the province of Artois in France, where the first well of this type was sunk. Artesian wells occur in regions which fulfil certain conditions: a layer of permeable rock between two impermeable rock layers; synclinal or tilted rock structure; exposure of permeable rock to the ground sUrface; and sufficient amount of rainfall. The biggest area 0( artesian well in the world is Great Artesian Basin of Australia.
A special type of well in which water rises automatically under the pressure of a column of water to the ground surface, either through a natural or man-made hole, is known as an artesian well. The name artesian is derived from the province of Artois in France, where the first well of this type was sunk. Artesian wells occur in regions which fulfil certain conditions: a layer of permeable rock between two impermeable rock layers; synclinal or tilted rock structure; exposure of permeable rock to the ground sUrface; and sufficient amount of rainfall. The biggest area 0( artesian well in the world is Great Artesian Basin of Australia.
Landforms Made By Groundwater
Landforms Made By Groundwater The water that occurs below the surface of the earth is called subsurface water. Ground water is that part of subsurface water which fully saturates the pore spaces of the rock or its overburden and which behaves in response to the gravitational force. It is contained in the soil and underlying rock. Ground water may be derived from rain water that has percolated down or from water that has been trapped within the rock during its formation.
The water percolates down to collect above the impermeable layers of rocks, and eventually all the pore spaces above this layer become saturated with water forming the ground water zone. The underground water and the run-off water on the surface mutually affect each other. Underground water may be meteoric water, from precipitation; primary or juvenile water having its source in chemical changes deep inside the earth; connate water, remnant of ancient seas; and magmatic water, from the action of volcanic heat or water-containing rocks at great depths. Water from surface sources cannot be as naturally suitable and as economically exploitable as groundwater.
Compared to surface water supplies, ground water in most cases has a constant composition and temperature and is free from turbidity, objectionable colours and pathogenic organisms. Thus it requires very little treatment.
The rain-water or snow-melt that neither runs off along the surface nor evaporates but sinks into the ground is known as underground water. Springs, artesian wells, geysers, oasis, swamp, marsh, bogs, karst, sink hole, and caves (stalactites and stalagmites) are examples of landforms made by groundwater.
The water percolates down to collect above the impermeable layers of rocks, and eventually all the pore spaces above this layer become saturated with water forming the ground water zone. The underground water and the run-off water on the surface mutually affect each other. Underground water may be meteoric water, from precipitation; primary or juvenile water having its source in chemical changes deep inside the earth; connate water, remnant of ancient seas; and magmatic water, from the action of volcanic heat or water-containing rocks at great depths. Water from surface sources cannot be as naturally suitable and as economically exploitable as groundwater.
Compared to surface water supplies, ground water in most cases has a constant composition and temperature and is free from turbidity, objectionable colours and pathogenic organisms. Thus it requires very little treatment.
The rain-water or snow-melt that neither runs off along the surface nor evaporates but sinks into the ground is known as underground water. Springs, artesian wells, geysers, oasis, swamp, marsh, bogs, karst, sink hole, and caves (stalactites and stalagmites) are examples of landforms made by groundwater.
Landforms Made By River Action
Landforms Made By River Action Rivers are one
the greatest sculpturing agents at work in humid regior In its youthful stage the river flows turbulently in a narr01 steep-sided valley whose floors are broken by pot hol, and waterfalls. A youthful valley is 'V' -shaped, with steE gradient. The water of a fast-flowing river swirls if its bE is uneven. The pebbles carried by a swirling river C1 circular depressions in the river bed. These graduall deepen and are called pot holes. Much larger but simil; depressions form at the base of a waterfall; these are callE
plunge pools. Interlocking spurs are another feature of youthful valley.
Some valleys have very steep sides and are bot
. narrow and deep; these are called gorges. A gorge is ofte formed when a waterfall retreats upstream. One of the most famous gorges formed in this way lies below tl1 Victoria Falls. A gorge will also form when a riVE maintains its course across a belt of country which is bein uplifted. The Indus, the Brahmaputra and the headwateJ of the Ganga have cut deep gorges in the Himalayas. , huge gorge is called a canyon, and it usually occurs in dr regions where large rivers are actively eroding verticall and where weathering of the valley sides is minimum.
Valleys of the mature stage haye the shape of an ope 'V' in cross-section. The gradient is more gentle, river bend are more pronounced, spurs are removed by lateral erosio: and their remains form a line of bluffs on each side of th valley floor.
Active deposition starts taking place on the conve Danks of meanders during maturity. After the stage maturing is reached, the river begins to overflow its bank and it deposits fine silts and muds on the valley floor. Thi is the final stage in the growth of a flood plain. Meander are pronounced and cut-ofts develop and produce ox-bolA lakes. The river builds up its banks with alluvium (thi banks are called ievees). The river thus flows betweeJ pronounced banks and above the level of the flood plain In course of time, river erosion, transportation, and depo sition turn the original surface into an almost level plaiI which is called a pleneplain.
A delta is formed at the mouth of a river where it deposits more material than can be carried away, as the speed of the river is reduced by the time it enters a sea or lake. Also, fine clay particles carried in suspension in the river coagulate in the presence of salt water and get deposited. Deltas can form on the shores of tidal seas, e.g., River Colorado. Any river, irrespective of its stage of development, can form a delta. The river must have a large load, obviously having active erosion in the upper valley. There are three basic types of delta: (i) Arcuate: This delta is composed of coarse sediments such as gravel and sand and is triangular in shape; e.g., Nile, Ganga, Indus, Mekong, Hwang-ho, Niger; (ii) Bird's Foot or Digitate, composed of very fine sediments called silt, the river channel divides into a few distributaries only and these maintain clearly defined channels across the delta. The Mississippi Delta is an example. This type occurs in seas which have few currents and tides to disturb the sediments; (ii) Estuarine: This delta develops at the mouth of submerged rivers; e.g. the deltas of the Ob, Elbe and the Vistula.
In the first stage, deposition divides the river into several distributaries. Spits and bars rise and lagoons are formed. (Lagoons are shallow stretches of water separated from the sea by a barrier such as a spit.) In the next stage the lagoons begin to get filled in with sediments, and they become swampy. The delta begins to assume a more solid appearance. In the third stage, the old part of the delta becomes colonised by plants. As a delta grows larger, the old parts merge imperceptibly with the flood plain, and they no longer have the appearance of a delta.
A river and its tributaries drain an area, which is called a 'river basin'. Its boundary formed by the crest line of the surrounding highland is the watershed of the basin. A river system usually develops a patten which is related to the general structure of the basin. A dendritic pattern develops in a region made of rocks which offer the same resistance to erosion and which has a uniform structure. A trellis drainage pattern develops in a region made up of alternate belts of hard and soft rocks which all dip in the same direction and which lie at right angles to the general slope, down which the river flows. A radial pattern develops on a dome or volcanic cone. The river flows outward, forming a pattern like the spokes of a wheel.
A river' at any stage of its development from youth to old age may be rejuvenated, and a young valley may occur in an old landscape. Where the river crosses from the original flood plain to the new flood plain, there may be a waterfall or rapids: this point is called knickpoint.
the greatest sculpturing agents at work in humid regior In its youthful stage the river flows turbulently in a narr01 steep-sided valley whose floors are broken by pot hol, and waterfalls. A youthful valley is 'V' -shaped, with steE gradient. The water of a fast-flowing river swirls if its bE is uneven. The pebbles carried by a swirling river C1 circular depressions in the river bed. These graduall deepen and are called pot holes. Much larger but simil; depressions form at the base of a waterfall; these are callE
plunge pools. Interlocking spurs are another feature of youthful valley.
Some valleys have very steep sides and are bot
. narrow and deep; these are called gorges. A gorge is ofte formed when a waterfall retreats upstream. One of the most famous gorges formed in this way lies below tl1 Victoria Falls. A gorge will also form when a riVE maintains its course across a belt of country which is bein uplifted. The Indus, the Brahmaputra and the headwateJ of the Ganga have cut deep gorges in the Himalayas. , huge gorge is called a canyon, and it usually occurs in dr regions where large rivers are actively eroding verticall and where weathering of the valley sides is minimum.
Valleys of the mature stage haye the shape of an ope 'V' in cross-section. The gradient is more gentle, river bend are more pronounced, spurs are removed by lateral erosio: and their remains form a line of bluffs on each side of th valley floor.
Active deposition starts taking place on the conve Danks of meanders during maturity. After the stage maturing is reached, the river begins to overflow its bank and it deposits fine silts and muds on the valley floor. Thi is the final stage in the growth of a flood plain. Meander are pronounced and cut-ofts develop and produce ox-bolA lakes. The river builds up its banks with alluvium (thi banks are called ievees). The river thus flows betweeJ pronounced banks and above the level of the flood plain In course of time, river erosion, transportation, and depo sition turn the original surface into an almost level plaiI which is called a pleneplain.
A delta is formed at the mouth of a river where it deposits more material than can be carried away, as the speed of the river is reduced by the time it enters a sea or lake. Also, fine clay particles carried in suspension in the river coagulate in the presence of salt water and get deposited. Deltas can form on the shores of tidal seas, e.g., River Colorado. Any river, irrespective of its stage of development, can form a delta. The river must have a large load, obviously having active erosion in the upper valley. There are three basic types of delta: (i) Arcuate: This delta is composed of coarse sediments such as gravel and sand and is triangular in shape; e.g., Nile, Ganga, Indus, Mekong, Hwang-ho, Niger; (ii) Bird's Foot or Digitate, composed of very fine sediments called silt, the river channel divides into a few distributaries only and these maintain clearly defined channels across the delta. The Mississippi Delta is an example. This type occurs in seas which have few currents and tides to disturb the sediments; (ii) Estuarine: This delta develops at the mouth of submerged rivers; e.g. the deltas of the Ob, Elbe and the Vistula.
In the first stage, deposition divides the river into several distributaries. Spits and bars rise and lagoons are formed. (Lagoons are shallow stretches of water separated from the sea by a barrier such as a spit.) In the next stage the lagoons begin to get filled in with sediments, and they become swampy. The delta begins to assume a more solid appearance. In the third stage, the old part of the delta becomes colonised by plants. As a delta grows larger, the old parts merge imperceptibly with the flood plain, and they no longer have the appearance of a delta.
A river and its tributaries drain an area, which is called a 'river basin'. Its boundary formed by the crest line of the surrounding highland is the watershed of the basin. A river system usually develops a patten which is related to the general structure of the basin. A dendritic pattern develops in a region made of rocks which offer the same resistance to erosion and which has a uniform structure. A trellis drainage pattern develops in a region made up of alternate belts of hard and soft rocks which all dip in the same direction and which lie at right angles to the general slope, down which the river flows. A radial pattern develops on a dome or volcanic cone. The river flows outward, forming a pattern like the spokes of a wheel.
A river' at any stage of its development from youth to old age may be rejuvenated, and a young valley may occur in an old landscape. Where the river crosses from the original flood plain to the new flood plain, there may be a waterfall or rapids: this point is called knickpoint.
EXFOLIATION
Aptly called 'onion weathering', exfoliation is a process of physical weathering in which the outer layers of a rock split off into thin sheets or scales. It has been suggested that it is caused by the alternate expansion and contraction resulting from a wide diurnal range of temperature in arid regions, which would lead to stresses in the rock. However, it is now thought that the presence of water may be important. in the process. Screes are mounds of angular rock particles weathered from the rocky masses and which collect round their bases. Exfoliation domes are to be seen in Khasi Hills, Meghalaya. The large domal hill in Tiruchirapalli, too, is an example of exfoliation effect
EROSION
EROSION
Erosion refers to the disintegration of rocks which lie exposed to what are called the agents of erosion, Le., gravity, running water, wind, and moving ice, and, on the coasts, waves, tides and currents.
Landforms Made By Rain Action Rain action is an aspect of erosion as it involves movement. It is most marked in semi-arid regions because these have little or no vegetation and the rains, though infrequent, are torrential. It produces the following features.
A gully is an incised water-worn channel. Overland flow down a slope, following heavy rainfall, is concentrated into rills, which merge and enlarge into the gully.
When rain falls on slopes made of clay and boulders, the clay is rapidly removed except where the boulders form a protection. When this happens, columns of clay capped by boulders develop. These an~ called earth pillars.
Erosion refers to the disintegration of rocks which lie exposed to what are called the agents of erosion, Le., gravity, running water, wind, and moving ice, and, on the coasts, waves, tides and currents.
Landforms Made By Rain Action Rain action is an aspect of erosion as it involves movement. It is most marked in semi-arid regions because these have little or no vegetation and the rains, though infrequent, are torrential. It produces the following features.
A gully is an incised water-worn channel. Overland flow down a slope, following heavy rainfall, is concentrated into rills, which merge and enlarge into the gully.
When rain falls on slopes made of clay and boulders, the clay is rapidly removed except where the boulders form a protection. When this happens, columns of clay capped by boulders develop. These an~ called earth pillars.
PHYSICAL OR MECHANICAL WEATHERING
PHYSICAL OR MECHANICAL WEATHERING Mechanical weathering is the physical disintegration of a rock into smaller particles. It takes place without changing the rock's chemical composition. Although it is most rapid in sedimentary rocks, yet it does not spare even the harder granite and the marble. Several factors are responsible for mechanical weathering.
Temperature Mechanical weathering is common in deserts, cold or hot, under the influence of rapid changes in daily temperature. In deserts, rocks are exposed to the blazing sun during the day and are intensely heated. This results in expansion of outer layers. At nightfall, the temperature drops rapidly and the outer layers contract more rapidly than the interior, setting up internal stresses. The continuous expansion and contraction for several years cause the rocks to crack and split.
, Repeated Wetting and Drying Repeated wetting and drying of the surface layers of the rock results in development of stresses. Stresses so produced cause surface to split off. When rocks are wetted, the outer layers absorb a certain amount of moisture and expand. When they dry, this moisture evaporates and they quickly shrink. Weathering by repeated wetting and - drying takes place especially in tropical regions such as Malaysia.
Frost Action In temperate latitudes or areas of cold climate, the alternate freezing and melting of water inside the cracks in rocks split them into fragments called frost. The conversion of water into frost or ice increase the volume of water. This phenomenon, also known as frost weathering, develops a strong force in widening the crevices in rock by physical destruction over a period of time. The magnitude of frost action is indicated by a continual increase in the formation of series over the mountain sides.
Biotic Factors The rocks are also destroyed by plants', and animals' activities. The long and tenacious root fibres of the plants work down into the cracks of rock. The burrowing by earthworms, ants, rats, etc. makes channels through the rocks and contributes to their destruction. The quarrying, mining, deforestation and indiscriminate cultivation of land by man are other contributing factors. Such biological actions may be physical or chemical in nature.
Temperature Mechanical weathering is common in deserts, cold or hot, under the influence of rapid changes in daily temperature. In deserts, rocks are exposed to the blazing sun during the day and are intensely heated. This results in expansion of outer layers. At nightfall, the temperature drops rapidly and the outer layers contract more rapidly than the interior, setting up internal stresses. The continuous expansion and contraction for several years cause the rocks to crack and split.
, Repeated Wetting and Drying Repeated wetting and drying of the surface layers of the rock results in development of stresses. Stresses so produced cause surface to split off. When rocks are wetted, the outer layers absorb a certain amount of moisture and expand. When they dry, this moisture evaporates and they quickly shrink. Weathering by repeated wetting and - drying takes place especially in tropical regions such as Malaysia.
Frost Action In temperate latitudes or areas of cold climate, the alternate freezing and melting of water inside the cracks in rocks split them into fragments called frost. The conversion of water into frost or ice increase the volume of water. This phenomenon, also known as frost weathering, develops a strong force in widening the crevices in rock by physical destruction over a period of time. The magnitude of frost action is indicated by a continual increase in the formation of series over the mountain sides.
Biotic Factors The rocks are also destroyed by plants', and animals' activities. The long and tenacious root fibres of the plants work down into the cracks of rock. The burrowing by earthworms, ants, rats, etc. makes channels through the rocks and contributes to their destruction. The quarrying, mining, deforestation and indiscriminate cultivation of land by man are other contributing factors. Such biological actions may be physical or chemical in nature.
CHEMICAL WEATHERING
WEATHERING
The term 'weathering' means the weakening, breaking-up, rotting and the disintegration of rocks at or near the earth's surface. It starts as soon as a rock is exposed to the influence of weather. The disintegrated material, the products or the results of weathering, do not involve any motion except the falling down of the material by the force of gravity. Weathering is of two kinds: (i) chemical weathering, and (ii) physical or mechanical weathering.
CHEMICAL WEATHERING Chef!Ucal weathering is the basic process by which denudation take place. (The general wearing away of the land surface by external agencies or forces is known as denudation or degradation.) Chemical weathering is the extremely slow and gradual decomposition of rocks due to exposure to air and water. Air and water, main agents of chemical weathering, contain chemical elements which set up chemical reactions in the surface layers of exposed rocks. Such reactions may weaken or entirely dissolve certain constituents of the rock, thus loosening the other crystals and weakening the whole surface. The chemical reactions between rock and water are rapid if both the temperature and moisture are high as found in humid tropics.
There are four major chemical weathering processes. (a) Solution Solution is the most potent weathering
process in limestone regions because the rain water attacks and dissolves the calcium carbonate of which the rock is chiefly formed. However, limestone is not the only rock to suffer from solution. All rocks are subject to solution to some extent, although the process is much slower. The rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure. Sedimentary rocks have pore spaces in which air and water. can lodge and thus attack the rock. The density of joints or cracks in the rock is also crucial to weathering. This factor is very clearly seen in weathering of granite in Malaysia.
(b) Oxidation Oxidation is the reaction of oxygen in air or water with minerals in the rock. This results in decomposition of the rock and it starts crumbling.
(c) Carbonation The rain-water containscarbon~dioxide in solution. It has an acidic effect and it reacts with rocks to form new chemical substances. This is the process of carbonation and is noticed in lower humid latitudes.
(d) Decomposition by Organic Acids Bacteria present in rocks produce acids which, when dissolved in water, help to' speed up the weathering of the underlying rocks.
The term 'weathering' means the weakening, breaking-up, rotting and the disintegration of rocks at or near the earth's surface. It starts as soon as a rock is exposed to the influence of weather. The disintegrated material, the products or the results of weathering, do not involve any motion except the falling down of the material by the force of gravity. Weathering is of two kinds: (i) chemical weathering, and (ii) physical or mechanical weathering.
CHEMICAL WEATHERING Chef!Ucal weathering is the basic process by which denudation take place. (The general wearing away of the land surface by external agencies or forces is known as denudation or degradation.) Chemical weathering is the extremely slow and gradual decomposition of rocks due to exposure to air and water. Air and water, main agents of chemical weathering, contain chemical elements which set up chemical reactions in the surface layers of exposed rocks. Such reactions may weaken or entirely dissolve certain constituents of the rock, thus loosening the other crystals and weakening the whole surface. The chemical reactions between rock and water are rapid if both the temperature and moisture are high as found in humid tropics.
There are four major chemical weathering processes. (a) Solution Solution is the most potent weathering
process in limestone regions because the rain water attacks and dissolves the calcium carbonate of which the rock is chiefly formed. However, limestone is not the only rock to suffer from solution. All rocks are subject to solution to some extent, although the process is much slower. The rate at which solution takes place is affected not only by the mineral composition of the rock but also by its structure. Sedimentary rocks have pore spaces in which air and water. can lodge and thus attack the rock. The density of joints or cracks in the rock is also crucial to weathering. This factor is very clearly seen in weathering of granite in Malaysia.
(b) Oxidation Oxidation is the reaction of oxygen in air or water with minerals in the rock. This results in decomposition of the rock and it starts crumbling.
(c) Carbonation The rain-water containscarbon~dioxide in solution. It has an acidic effect and it reacts with rocks to form new chemical substances. This is the process of carbonation and is noticed in lower humid latitudes.
(d) Decomposition by Organic Acids Bacteria present in rocks produce acids which, when dissolved in water, help to' speed up the weathering of the underlying rocks.
EXTERNAL FORCES
As we have seen in the pervious chapter, the earth's crust is undergoing geological changes caused by the internal processes (earth movements, earthquakes, volcanoes, etc.), which create new relief features, such as mountains, plateaus and plains. Meanwhile, there are external processes that are working vigorously to wear away the surface. The interaction of the internal and external processes gives rise to the great diversity of present-day landforms.
The external forces that produce physical features-Le., those forces which operate on earth's surface-cause them by (i) denudation or (ii) deposition. The forces of denudation are weathering and erosion. The forces of deposition are water, ice, wind and living organisms.
The external forces that produce physical features-Le., those forces which operate on earth's surface-cause them by (i) denudation or (ii) deposition. The forces of denudation are weathering and erosion. The forces of deposition are water, ice, wind and living organisms.
VOLCANIC LANDFORMS
VOLCANIC LANDFORMS There are several landforms that are associated with volcanoes. Important among them include:
Batholith Batholith or bathylith is a very large domeshaped intrusion of magma, typically several kilometres in depth and extending over hundreds of square kilometres. It is usually composed of acid rocks, such as granite and diorite, and is always associated with an area where mountain-building has taken place. Examples of batholiths include Dartmoor, Devon, and the Moume Mountains in Northern Ireland.
Laccolith Laccolith or laccolite is a large dome-like mass of igneous rock (magma) that was intruded along a bedding plane in a sedimentary sequence of rocks. In the well-developed laccoliths, the base tends to be relatively flat so that the resulting intrusion has a lens shape. When a number of laccoliths are stacked one above another from a single intrusion, they are termed a cedar-tree laccolith.
Sill Sill is a horizontal intrusion of magma along a bedding plane between two rock layers. When cooled, magma forms a -tabular sheet more or less parallel to the surrounding layers of rock. The best known example in the British Isles is the Great Whin Sill in Northern England which is composed of dolerite.
Dyke When a mass of magma cuts across the bedding planes and forms a wall-like structure, it is termed a dyke. Dykes tend to occur in large numbers, known as swarms, e.g., on. the coast of Isle of Arran, and on the Isle of Mull, Scotland. Some dykes when exposed on the surface, stand as ridges or escarpments. In ridges, the side with gentle slope is termed dip, while the side with steeper slope is termed scarp.
Hot Spring Hot or thermal spring is a spring of hot water that flows out of the ground heated by volcanic areas, e.g., at Bath, Avon. The water that flows from hot springs often contains a large proportion of dissolved minerals, which may be deposited as basins or terraces around the hot springs. Iceland and New Zealand have thousands of hot springs.
Geysers Geysers are fountains of hot water and superheated steam that may spout up to a height of 150 feet from the earth beneath. The phenomena are associated with a thermal or volcanic region in which the water below is being heated beyond boiling point. Almost all the world's geysers are confined to three major areas; Iceland, the Rotorua district of North Island (New Zealand), and Yellowstone Park of USA. The world's best known geyser is perhaps 'Old Faithful' in Yellowstone National Park, Wyoming, which erupts at regular intervals - every 63 minutes on the average. In 1904, the Waimangu Geyser in New Zealand erupted to a height of about 457 m, higher than the world's tallest building, the Sears Tower (445 m) in Chicago.
Batholith Batholith or bathylith is a very large domeshaped intrusion of magma, typically several kilometres in depth and extending over hundreds of square kilometres. It is usually composed of acid rocks, such as granite and diorite, and is always associated with an area where mountain-building has taken place. Examples of batholiths include Dartmoor, Devon, and the Moume Mountains in Northern Ireland.
Laccolith Laccolith or laccolite is a large dome-like mass of igneous rock (magma) that was intruded along a bedding plane in a sedimentary sequence of rocks. In the well-developed laccoliths, the base tends to be relatively flat so that the resulting intrusion has a lens shape. When a number of laccoliths are stacked one above another from a single intrusion, they are termed a cedar-tree laccolith.
Sill Sill is a horizontal intrusion of magma along a bedding plane between two rock layers. When cooled, magma forms a -tabular sheet more or less parallel to the surrounding layers of rock. The best known example in the British Isles is the Great Whin Sill in Northern England which is composed of dolerite.
Dyke When a mass of magma cuts across the bedding planes and forms a wall-like structure, it is termed a dyke. Dykes tend to occur in large numbers, known as swarms, e.g., on. the coast of Isle of Arran, and on the Isle of Mull, Scotland. Some dykes when exposed on the surface, stand as ridges or escarpments. In ridges, the side with gentle slope is termed dip, while the side with steeper slope is termed scarp.
Hot Spring Hot or thermal spring is a spring of hot water that flows out of the ground heated by volcanic areas, e.g., at Bath, Avon. The water that flows from hot springs often contains a large proportion of dissolved minerals, which may be deposited as basins or terraces around the hot springs. Iceland and New Zealand have thousands of hot springs.
Geysers Geysers are fountains of hot water and superheated steam that may spout up to a height of 150 feet from the earth beneath. The phenomena are associated with a thermal or volcanic region in which the water below is being heated beyond boiling point. Almost all the world's geysers are confined to three major areas; Iceland, the Rotorua district of North Island (New Zealand), and Yellowstone Park of USA. The world's best known geyser is perhaps 'Old Faithful' in Yellowstone National Park, Wyoming, which erupts at regular intervals - every 63 minutes on the average. In 1904, the Waimangu Geyser in New Zealand erupted to a height of about 457 m, higher than the world's tallest building, the Sears Tower (445 m) in Chicago.
PLAINS
PLAINS A plain is a relatively flat and low-lying land surface with least difference between its highest and lowest points. The plains are usually lowlands, seldom rising more than a few hundred feet above sea level. Some of them may be smooth, while others are slightly rolling. Plains can be placed according to their position and surface relief but are better classified on the basis of their mode of formation.
They are sub-divided into structural, erosional and depositional plains.
1. Structural Plains These are the structurally depressed areas of the world, that make up some of the most extensive natural lowlands on the earth's surface. They are formed by horizontally-bedded rocks, relatively undisturbed by the crustal movement of the earth. Examples include the Russian Platform, the Great Plains of USA, and the central lowlands of Australia.
2. Erosional Plains Erosional plains are formed when an elevated tract of land, such as a mountain, a hill or a plateau, is worn down to a plain by the process of erosion. Rain, rivers, ice and wind help to smooth out the irregularities of the earth's surface, and in terms of millions of years, even high mountains can be reduced to low undulating plains. Examples include northern Canada, northern Europe and western Africa (all ice-eroded plains) and parts of Sahara in Africa (wind-eroded plain).
3. Depositional Plains These are plains formed by the deposition of materials brought by various agents of transportation, such as rivers, glaciers, wind and sea waves. A list of different types of depositional plains and their depositional agents is given below.
The Indo-Ganga Plains in India, the Hwang Ho Plains of North China, the Po River Plains or Lombardy in North Italy and that of Nile River are examples of some of the great alluvial plains. Plains of north-western Eurasia and that of Ladakh to the east of Shyok River and north of Chang Chenmo River are examples of plains made by glacial depositon. Plains in West Rajasthan, Turkmenistan and north-western China represent loesses.
They are sub-divided into structural, erosional and depositional plains.
1. Structural Plains These are the structurally depressed areas of the world, that make up some of the most extensive natural lowlands on the earth's surface. They are formed by horizontally-bedded rocks, relatively undisturbed by the crustal movement of the earth. Examples include the Russian Platform, the Great Plains of USA, and the central lowlands of Australia.
2. Erosional Plains Erosional plains are formed when an elevated tract of land, such as a mountain, a hill or a plateau, is worn down to a plain by the process of erosion. Rain, rivers, ice and wind help to smooth out the irregularities of the earth's surface, and in terms of millions of years, even high mountains can be reduced to low undulating plains. Examples include northern Canada, northern Europe and western Africa (all ice-eroded plains) and parts of Sahara in Africa (wind-eroded plain).
3. Depositional Plains These are plains formed by the deposition of materials brought by various agents of transportation, such as rivers, glaciers, wind and sea waves. A list of different types of depositional plains and their depositional agents is given below.
The Indo-Ganga Plains in India, the Hwang Ho Plains of North China, the Po River Plains or Lombardy in North Italy and that of Nile River are examples of some of the great alluvial plains. Plains of north-western Eurasia and that of Ladakh to the east of Shyok River and north of Chang Chenmo River are examples of plains made by glacial depositon. Plains in West Rajasthan, Turkmenistan and north-western China represent loesses.
PLATEAUS
PLATEAUS A plateau is an elevated area generally in contrast to the nearby areas. It has a large area on its top unlike a mountain and has an extensively even or undulating surface. The rocks of the plateaus are layered with sandstones, shales and limestones.
According to their mode of formation and their physical appearance, plateaus may be grouped into the following types:
1. Tectonic Plateaus These are formed by earth movements which cause uplift, and are normally of a considerable size, and fairly uniform altitude. They are also called continental plateaus. Their heights vary from 600-1,500 metres. Plateaus of Brazil, South Africa, West Australia, Chhota Nagpur and Shillong are examples of continental plateaus.
2. Intermontane Plateaus When plateaus are enclosed by fold mountains, they are known as intermontane plateaus, e.g., Tibetan Plateau (between the Himalayas and the Kunlun), Bolivian Plateau (between two ranges of the Andes), etc.
3. Piedmont Plateaus Situated at the foot of a mountain, piedmont plateaus are bounded on the opposite side by a plain or an ocean. The plateau of Malwa in India, those of Patagonia in Argentina and the Appalachian in the USA are some of the examples of piedmont plateaus. These are also called the plateaus of denudation because areas which were formerly high have now been reduced in elevation by various agents of erosion.
According to their mode of formation and their physical appearance, plateaus may be grouped into the following types:
1. Tectonic Plateaus These are formed by earth movements which cause uplift, and are normally of a considerable size, and fairly uniform altitude. They are also called continental plateaus. Their heights vary from 600-1,500 metres. Plateaus of Brazil, South Africa, West Australia, Chhota Nagpur and Shillong are examples of continental plateaus.
2. Intermontane Plateaus When plateaus are enclosed by fold mountains, they are known as intermontane plateaus, e.g., Tibetan Plateau (between the Himalayas and the Kunlun), Bolivian Plateau (between two ranges of the Andes), etc.
3. Piedmont Plateaus Situated at the foot of a mountain, piedmont plateaus are bounded on the opposite side by a plain or an ocean. The plateau of Malwa in India, those of Patagonia in Argentina and the Appalachian in the USA are some of the examples of piedmont plateaus. These are also called the plateaus of denudation because areas which were formerly high have now been reduced in elevation by various agents of erosion.
OROGENESIS
As mentioned earlier, orogenesis is the process by which mountains are formed. Orogenic movements result in the thrusting, folding and faulting that form the major mountain ranges. This occurs when two continents collide and the sediments between them are intensely deformed into linear mountain ranges. Volcanic activity and earthquakes are closely associated with orogenesis.
Since the dawn of geological time, no less than nine orogenic movements have taken place. Some of them occurred in Pre-Cambrian times between 600-3,500 million years ago. The three more recent orogenies are:
(i) The Caledonian About 320 million years ago, the Caledonian orogenies raised the mountains of Scandinavia and Scotland. This orogenies is represented in North America.
(ii) The Hercynian During the Hercynian earth movements, which occurred about 240 million years ago, ranges such as the Ural Mountains, the Pennines and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America were formed.
(iii) The Alpine The Alpine movement occurred about 30 million years ago. Young fold mountain ranges were buckled up and overthrust on a gigantic scale. Being the most recently formed, these ranges, such as the Alps, the Himalayas, the Andes and the Rockies are the loftiest and the most imposing.
Since the dawn of geological time, no less than nine orogenic movements have taken place. Some of them occurred in Pre-Cambrian times between 600-3,500 million years ago. The three more recent orogenies are:
(i) The Caledonian About 320 million years ago, the Caledonian orogenies raised the mountains of Scandinavia and Scotland. This orogenies is represented in North America.
(ii) The Hercynian During the Hercynian earth movements, which occurred about 240 million years ago, ranges such as the Ural Mountains, the Pennines and Welsh Highlands in Britain, the Harz Mountains in Germany, the Appalachians in America were formed.
(iii) The Alpine The Alpine movement occurred about 30 million years ago. Young fold mountain ranges were buckled up and overthrust on a gigantic scale. Being the most recently formed, these ranges, such as the Alps, the Himalayas, the Andes and the Rockies are the loftiest and the most imposing.
RIFT VALLEY
Rift Valley is a flat-bottomed valley formed by the sinking of the ground between two nearly parallel faults or two parallel series of steep faults. Valley sides are steep and follow the fault lines, which also form the edge of the mountain masses. There are conflicting views as to the origin of rift valleys, including tension in the earth's crust, compression, or the cracking of a crustal dome along the crest. Currently, it is believed that rift valleys result from tectonic plates moving apart at constructive plate margins.
The Great Rift Valley of Africa, which stretches from Mozambique in the south to Syria in the north, is the most well known rift va!ley. It has a total length of 6,440 km. The Rhine Valley (between the Vosges and Black Forest mountains) and Narmada Valley (between the Vindhya and Satpura ranges) are other examples of rift valleys.
The Great Rift Valley of Africa, which stretches from Mozambique in the south to Syria in the north, is the most well known rift va!ley. It has a total length of 6,440 km. The Rhine Valley (between the Vosges and Black Forest mountains) and Narmada Valley (between the Vindhya and Satpura ranges) are other examples of rift valleys.
LANDFORMS FORMED BY INTERNAL FORCES
MOUNTAINS According to the Penguin Dictionary, mountain is a portion of the land surface rising considerably above the surrounding country either as a single eminence or in a range or chain. Some authorities regard eminences above 600 m (2,000 feet) as mountains, those below being referred to as hills.
Mountains make up a large portion of the earth's surface. They are formed in the process of orogenesis. Based on their mode of formation, four main types of mountains can be distinguished.
1. Fold Mountains Fold mountains are formed under compression in which the sedimentary rock strata are squeezed into a succession of folds. Folds are the buckling of once horizontal rock strata. Horizontal compression results in the formation of upfolds called anticlines and downfolds called synclines.
Fold mountains are the most widespread. They are of two types: young fold mountains and old fold mountains. Young fold mountains have been formed relatively recently and are higher than the old fold mountains. They have pointed peaks and rugged features like steeper slopes and deeper valleys. Examples include the Andes, the Rockies, the Alps and the Himalayas. Old fold mountains have been formed long ago. They have rounded peaks and gentler slopes. The Appalachians, the Urals and the Aravalis are good examples of the old fold mountains.
2. Block Mountains When the earth's crust bends,
folding occurs, but when it cracks, faulting takes place. Faulting may be caused by tension or compression, forces which lengthen or shorten the earth's crust, causing a section of it to subside or to rise above the surrounding level. The land between the two parallel faults either rises, forming block mountains or horsts, or subsides into a depression termed as a rift valley or graben.
The Arabian Peninsula, the Sinai Peninsula, the Vosgus (France), Black Forest mountains (Germany), and Salt Range in Pakistan are cited as typical examples of block mountains. In India, the Vindhyas and the Satpuras are examples of block mountains.
3. Volcanic Mountains Volcanic mountains are, in fact, volcanoes which are built up from material ejected from fissures in the earth's crust. These materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud. Volcanic mountains are often called mountains of accumulation, as these are formed by the accumulation of volcanic material. They are common in the Circum-Pacific belt and include such volcanic peaks as Mt Fuji (Japan), Mt Mayon (the Philippine), Mt Merapi (Sumatra), Mt Agung (Indonesia) and Mt Catopaxi (Ecuador).
4. Residual Mountains Residual mountains owe their present form due to erosion by different agencies. That is why they are also known as relict mountain or mountain of
circumdenudation. The residual mountains stand alone in the surrounding area reduced in height. Examples of residual mountains include Mt Manodnock (USA), and the Nilgiri,
Mountains make up a large portion of the earth's surface. They are formed in the process of orogenesis. Based on their mode of formation, four main types of mountains can be distinguished.
1. Fold Mountains Fold mountains are formed under compression in which the sedimentary rock strata are squeezed into a succession of folds. Folds are the buckling of once horizontal rock strata. Horizontal compression results in the formation of upfolds called anticlines and downfolds called synclines.
Fold mountains are the most widespread. They are of two types: young fold mountains and old fold mountains. Young fold mountains have been formed relatively recently and are higher than the old fold mountains. They have pointed peaks and rugged features like steeper slopes and deeper valleys. Examples include the Andes, the Rockies, the Alps and the Himalayas. Old fold mountains have been formed long ago. They have rounded peaks and gentler slopes. The Appalachians, the Urals and the Aravalis are good examples of the old fold mountains.
2. Block Mountains When the earth's crust bends,
folding occurs, but when it cracks, faulting takes place. Faulting may be caused by tension or compression, forces which lengthen or shorten the earth's crust, causing a section of it to subside or to rise above the surrounding level. The land between the two parallel faults either rises, forming block mountains or horsts, or subsides into a depression termed as a rift valley or graben.
The Arabian Peninsula, the Sinai Peninsula, the Vosgus (France), Black Forest mountains (Germany), and Salt Range in Pakistan are cited as typical examples of block mountains. In India, the Vindhyas and the Satpuras are examples of block mountains.
3. Volcanic Mountains Volcanic mountains are, in fact, volcanoes which are built up from material ejected from fissures in the earth's crust. These materials include molten lava, volcanic bombs, cinders, ashes, dust and liquid mud. Volcanic mountains are often called mountains of accumulation, as these are formed by the accumulation of volcanic material. They are common in the Circum-Pacific belt and include such volcanic peaks as Mt Fuji (Japan), Mt Mayon (the Philippine), Mt Merapi (Sumatra), Mt Agung (Indonesia) and Mt Catopaxi (Ecuador).
4. Residual Mountains Residual mountains owe their present form due to erosion by different agencies. That is why they are also known as relict mountain or mountain of
circumdenudation. The residual mountains stand alone in the surrounding area reduced in height. Examples of residual mountains include Mt Manodnock (USA), and the Nilgiri,
SOME DISASTROUS VOLCANOES
SOME DISASTROUS VOLCANOES In the history of mankind, perhaps the most disastrous eruptions were those of Mt Krakatau, Mt Vesuvius, Mt Pelee, Mt St. Helens and
Mt Pinatubo.
The greatest volcanic explosion known to men is perhaps that of Mt. Krakatau in August 1883. Krakatau is a small volcanic island in the Sunda Straits, midway between Java and Sumatra. The 1883 explosion's intensity could be gauged from the fact that it could be heard in Australia, almost 3,000 miles away. Though Krakatau itself was not inhabited and nobody was killed by the lava flows, the vibration set up enormous waves over 100 feet high which drowned 36,000 people in the coastal districts of Indonesia.
Mt Vesuvius, standing 4,000 feet above the Bay of Naples, erupted violently on August 24, 79 AD. The City of Pompeii and the City of Herculaneum were the worst affected. Almost the entire population of the two cities was buried alive.
The eruption of Mt PeIee of the West Indies in May 1902 was the most catastrophic of modem times. St. Pierre, the capital of Martinique, lying on the path of the lava, was completely destroyed within minutes. The entire population of 30,000 was killed almost instantly.
Mt St. Helens in the United States erupted, after 123 years of inactivity, in 1980, blowing away the entire summit.
Mt Pinatubo in the Philippines erupted violently in June 1991 after six centuries of dormancy.
Distribution of Volcanoes in the World Volcanoes are located in a fairly clearly-defined pattern around the world, closely related to regions that have been intensely folded or faulted. They occur along coastal mountain ranges, as off-shore islands and in the midst of oceans, but there are few in the interiors of continents. The greatest concentration is probably that in the Circum-Pacific region, popularly termed the Pacific Ring of Fire. This region accounts for two-thirds of the world's volcanoes. It is said that there are almost 100 active volcanoes in the Philippines, 40 in the Andes, 35 in Japan, and more then 70 in Indonesia.
The Atlantic coasts have comparatively few active volcanoes but many dormant or extinct volcanoes. Volcanoes of the Mediterranean region are mainly associated with the Alpine folds, e.g., Vesuvius, Etna, Stromboli, etc. The Himalayas have no active volcano at all.
In Africa, some volcanoes are found along the East African Rift Valley, e.g., Mt Kilimanjaro and Mt Kenya, both probably extinct. The only active volcano of West Africa is Mt Cameroon.
Mt Pinatubo.
The greatest volcanic explosion known to men is perhaps that of Mt. Krakatau in August 1883. Krakatau is a small volcanic island in the Sunda Straits, midway between Java and Sumatra. The 1883 explosion's intensity could be gauged from the fact that it could be heard in Australia, almost 3,000 miles away. Though Krakatau itself was not inhabited and nobody was killed by the lava flows, the vibration set up enormous waves over 100 feet high which drowned 36,000 people in the coastal districts of Indonesia.
Mt Vesuvius, standing 4,000 feet above the Bay of Naples, erupted violently on August 24, 79 AD. The City of Pompeii and the City of Herculaneum were the worst affected. Almost the entire population of the two cities was buried alive.
The eruption of Mt PeIee of the West Indies in May 1902 was the most catastrophic of modem times. St. Pierre, the capital of Martinique, lying on the path of the lava, was completely destroyed within minutes. The entire population of 30,000 was killed almost instantly.
Mt St. Helens in the United States erupted, after 123 years of inactivity, in 1980, blowing away the entire summit.
Mt Pinatubo in the Philippines erupted violently in June 1991 after six centuries of dormancy.
Distribution of Volcanoes in the World Volcanoes are located in a fairly clearly-defined pattern around the world, closely related to regions that have been intensely folded or faulted. They occur along coastal mountain ranges, as off-shore islands and in the midst of oceans, but there are few in the interiors of continents. The greatest concentration is probably that in the Circum-Pacific region, popularly termed the Pacific Ring of Fire. This region accounts for two-thirds of the world's volcanoes. It is said that there are almost 100 active volcanoes in the Philippines, 40 in the Andes, 35 in Japan, and more then 70 in Indonesia.
The Atlantic coasts have comparatively few active volcanoes but many dormant or extinct volcanoes. Volcanoes of the Mediterranean region are mainly associated with the Alpine folds, e.g., Vesuvius, Etna, Stromboli, etc. The Himalayas have no active volcano at all.
In Africa, some volcanoes are found along the East African Rift Valley, e.g., Mt Kilimanjaro and Mt Kenya, both probably extinct. The only active volcano of West Africa is Mt Cameroon.
VOLCANO ACTION
VOLCANO ACTION Rocks below the crust have a very high temperature, but the great" pressure upon these keeps them in a semi-solid state. If the pressure weakens (as happens when faulting or folding takes place) then some of the rocks become liquid-called magma.. This magma forces its way into cracks of the crust.
Within the crust volcanic features are bathoiith, a large mass of magma which often forms the root of a mountain and is made of granite; sill, a sheet of magma lying along the bedding plane-some may give rise to waterfalls and rapids when crossed by rivers; dyke, a wall-like feature formed when a mass of magma cuts across the bed~ing planes. Some dykes when exposed on the surface resist erosion and stand up as ridges or escarpments.
Sometimes magma reaches the surface through a vent (hole) or a fissure (crack). When magma reaches the surface, it is called lava. If lava comE;s through a vent, it builds up a volcano (cone-shaped mound) and if it emerges through a fissure, it builds up a lava platform or lava flow.
Fluid lavas give rise to gently sloping cones, e.g., Mauna Loa (Hawaii). Viscous lava gives rise to steeply sloping cones. Sometimes very viscous lava forms a spine or plug. Cones are made of ash and cinders. A composite cone is made of alternate layers of lava and ash. Sometimes explosive eruptions are so violent that the whole top of the volcano sinks into the magma beneath the vent. A huge crate called caldera later marks the site of the volcano. A caldera may become the site of a lake, e.g., Laka Toba (Sumatra) and Crater Lake (USA).
Volcanoes pass through three stages. In the active stage eruptions are frequent (Mt Etna in Italy, Cotopaxi in Ecuador). In the dormant (sleeping) stage eruptions become infrequent (e.g. Mt Vesuvius, Italy). This is followed by a long period of inactivity. Volcanoes which have not erupted in historic tim~s are called extinct (Mt Aconcagua in the Argentine Andes).
Within the crust volcanic features are bathoiith, a large mass of magma which often forms the root of a mountain and is made of granite; sill, a sheet of magma lying along the bedding plane-some may give rise to waterfalls and rapids when crossed by rivers; dyke, a wall-like feature formed when a mass of magma cuts across the bed~ing planes. Some dykes when exposed on the surface resist erosion and stand up as ridges or escarpments.
Sometimes magma reaches the surface through a vent (hole) or a fissure (crack). When magma reaches the surface, it is called lava. If lava comE;s through a vent, it builds up a volcano (cone-shaped mound) and if it emerges through a fissure, it builds up a lava platform or lava flow.
Fluid lavas give rise to gently sloping cones, e.g., Mauna Loa (Hawaii). Viscous lava gives rise to steeply sloping cones. Sometimes very viscous lava forms a spine or plug. Cones are made of ash and cinders. A composite cone is made of alternate layers of lava and ash. Sometimes explosive eruptions are so violent that the whole top of the volcano sinks into the magma beneath the vent. A huge crate called caldera later marks the site of the volcano. A caldera may become the site of a lake, e.g., Laka Toba (Sumatra) and Crater Lake (USA).
Volcanoes pass through three stages. In the active stage eruptions are frequent (Mt Etna in Italy, Cotopaxi in Ecuador). In the dormant (sleeping) stage eruptions become infrequent (e.g. Mt Vesuvius, Italy). This is followed by a long period of inactivity. Volcanoes which have not erupted in historic tim~s are called extinct (Mt Aconcagua in the Argentine Andes).
LANDSLIDES IN INDIA
The Himalayas are prone to landslides, especially during the monsoon months, from June to October. The types of landslides include block slumping, debris fall, debris slide, rock fall, rotational slip and slumping.
The pressure of population and the economic exploitation of the mountain region have been major causes for landslides. Turning forest land into orchards (apple growing being a lucrative activity), the increased construction and road building activities, and grazing by cattle are some of the activities that have led to increased chances of landslides. Factors such as deforestation by the timber industry and shifting agriculture have also contributed to the removal of valuable vegetation cover, leading to soil erosion and frequent landslides. Efforts are, however, being made to lessen the impact of landslides.
Of late, several thematic maps depicting geology, slope, drainage, land use, relief and landslide hazard comprising about 2,500 sq km of Alaknanda valley from Devaprayag to Nandaprayag have already been prepared. A criterion for zoning for landslide hazard has also been developed by the Central Building Research Institute (CBRI). These maps are useful because they enable the concerned authorities to take decisions on techno-economic feasibility of land use, geographical location of dams, construction of bridges and housing complexes, alignment of roads, and in undertaking suitable measures to combat hazards and preserve the ecology of the Himalayas. An innovative and costeffective technology for designing and building rigid masonry retaining walls characterised by reinforced backfill has been developed. The new expertise has been successfully tested by constructing a retaining wall, 11 metre high, located on the Hardwar-Badrinath road in collaboration with the Border Road Organisation. The CBRI has taken up a project related to the engineering behaviour of joints, discontinuities, slip surfaces and shear zones with specific emphasis on landslides and hazard assessment.
Engineering methods such as building underground wells or tunnels and surface channels by pumping out groundwater are useful in preventing landslides. Since the methods of checkinb landslides are prohibitively expensive, it seems to be more rational to concentrate on the prevention of the consequences of landslides. Prior knowledge of landslide may enable authorities to evacuate people before the loss of lives and property; but this requires vigilance, forecasting and constant monitoring.
The pressure of population and the economic exploitation of the mountain region have been major causes for landslides. Turning forest land into orchards (apple growing being a lucrative activity), the increased construction and road building activities, and grazing by cattle are some of the activities that have led to increased chances of landslides. Factors such as deforestation by the timber industry and shifting agriculture have also contributed to the removal of valuable vegetation cover, leading to soil erosion and frequent landslides. Efforts are, however, being made to lessen the impact of landslides.
Of late, several thematic maps depicting geology, slope, drainage, land use, relief and landslide hazard comprising about 2,500 sq km of Alaknanda valley from Devaprayag to Nandaprayag have already been prepared. A criterion for zoning for landslide hazard has also been developed by the Central Building Research Institute (CBRI). These maps are useful because they enable the concerned authorities to take decisions on techno-economic feasibility of land use, geographical location of dams, construction of bridges and housing complexes, alignment of roads, and in undertaking suitable measures to combat hazards and preserve the ecology of the Himalayas. An innovative and costeffective technology for designing and building rigid masonry retaining walls characterised by reinforced backfill has been developed. The new expertise has been successfully tested by constructing a retaining wall, 11 metre high, located on the Hardwar-Badrinath road in collaboration with the Border Road Organisation. The CBRI has taken up a project related to the engineering behaviour of joints, discontinuities, slip surfaces and shear zones with specific emphasis on landslides and hazard assessment.
Engineering methods such as building underground wells or tunnels and surface channels by pumping out groundwater are useful in preventing landslides. Since the methods of checkinb landslides are prohibitively expensive, it seems to be more rational to concentrate on the prevention of the consequences of landslides. Prior knowledge of landslide may enable authorities to evacuate people before the loss of lives and property; but this requires vigilance, forecasting and constant monitoring.
METHODS TO MINIMISE DAMAGE
R.U. Cooke (1984) and W.J. Kochelman (1986) have proposed some methods for reducing the landslide hazard.
1. Avoidance One way is to avoid landslides by
controlling the location, timing and nature of development. The measures include:
. bypassing unstable areas;
. putting restrictions on land use;
. mapping of hazard-prone areas and land use zoning; . acquiring and restructuring of public property;
. spreading social awareness among people;
. disclosing the nature of hazard to prospective prop
erty buyers;
. promoting insurance against hazard;
. giving financial assistance such as loans, tax credits,
etc., to promote the reduction of the hazard.
2. Reducing sheear stress One could reduce shear stress: . limit or reduce angles of slope, cut and fill;
. limit or reduce unit lengths of slope;
. remove unstable material.
3. Reducing shear stress and augmenting shear resistance
This could be achieved through an improved drainage system which involves
. improving surface drainage that covers terrace drains
and other drains;
. improving subsurface drainage;
. controlling unsustainable agriculture.
4. Increasing shear resistance This would be through . retaining structures such as cribs or building retaining
walls;
. adoption of engineering methods by piling, tie-rods,
anchors etc.;
. building hard surface e.g., .concrete surface;
. controlling fill compaction.
1. Avoidance One way is to avoid landslides by
controlling the location, timing and nature of development. The measures include:
. bypassing unstable areas;
. putting restrictions on land use;
. mapping of hazard-prone areas and land use zoning; . acquiring and restructuring of public property;
. spreading social awareness among people;
. disclosing the nature of hazard to prospective prop
erty buyers;
. promoting insurance against hazard;
. giving financial assistance such as loans, tax credits,
etc., to promote the reduction of the hazard.
2. Reducing sheear stress One could reduce shear stress: . limit or reduce angles of slope, cut and fill;
. limit or reduce unit lengths of slope;
. remove unstable material.
3. Reducing shear stress and augmenting shear resistance
This could be achieved through an improved drainage system which involves
. improving surface drainage that covers terrace drains
and other drains;
. improving subsurface drainage;
. controlling unsustainable agriculture.
4. Increasing shear resistance This would be through . retaining structures such as cribs or building retaining
walls;
. adoption of engineering methods by piling, tie-rods,
anchors etc.;
. building hard surface e.g., .concrete surface;
. controlling fill compaction.
TYPES OF LANDSLIDES
Landslides are extremely complicated and varied nomena. They differ in terms of sliding, flowing, creel toppling or speed of movement so markedly that
extremely difficult to combine all these diagnostic pheJ ena into a standard taxonomy. Classifications of lands have been attempted by T.H. Nilsen (1979), R.J. E (19'73), AJ. Nemcock (1972), AW. Skempton and
Hutchinson (1964), and D.J. Varnes (1978). The sel advanced by Varnes has received widest acceptance
1. Rotational slide It is a classic form of land: Some cases produce multiple regressive phenomena when continued instability produces new head carps to develop progressively up the slope.
2. Translational slide It involves relatively flat, planar movement following the surface. This type of movement is found in bedding planes made of sedimentary or metamorphic rocks dipping in the direction of slope.
3. Roto-translational slide It is a complex type where a combination of slip along a circular arc and a flat plane is found.
4. Soil-slab failure In this case, a slab of saturated regolith is converted into a thick liquid. So the speed of landslide accelerates to as high as 10m/sec.
5. Debris slide or avalanche It occurs in surface deposits of granular materials. The surface of rupture is almost parallel to the inclination of bedrock.
6. Debris flow It occurs when debris is saturated with water. When rigid solid also fallsalongwith the sliding mass, the phenomenon is called plug flow.
7. Falls These take place through air; for example,
jointed weathered rock falls from vertical cliffs.
8. Topples After detachment from cliffs the outward
rotation of angular blocks and rock columns cause toppling.
9. Mudflow It contains 20 to 80 per cent fine sediments saturated with water. Friction is caused by viscous movement that generates enough power to carry even large boulders.
10. Soil creep It is the least destructive of landslide phenomena. Creep is slow and superficial.
P.E. Kent (1966) proposed a hypothesis based on fluidisation of rock mass. He said that accumulated stress within rock particles causes compression of air in the pore spaces. This results in a fast-moving stream of debris. A. Heim (1932) held elasto-mechanical collisions responsible for landslides. His emphasis was on exchange of stresses between solid particles rather than fluids.
extremely difficult to combine all these diagnostic pheJ ena into a standard taxonomy. Classifications of lands have been attempted by T.H. Nilsen (1979), R.J. E (19'73), AJ. Nemcock (1972), AW. Skempton and
Hutchinson (1964), and D.J. Varnes (1978). The sel advanced by Varnes has received widest acceptance
1. Rotational slide It is a classic form of land: Some cases produce multiple regressive phenomena when continued instability produces new head carps to develop progressively up the slope.
2. Translational slide It involves relatively flat, planar movement following the surface. This type of movement is found in bedding planes made of sedimentary or metamorphic rocks dipping in the direction of slope.
3. Roto-translational slide It is a complex type where a combination of slip along a circular arc and a flat plane is found.
4. Soil-slab failure In this case, a slab of saturated regolith is converted into a thick liquid. So the speed of landslide accelerates to as high as 10m/sec.
5. Debris slide or avalanche It occurs in surface deposits of granular materials. The surface of rupture is almost parallel to the inclination of bedrock.
6. Debris flow It occurs when debris is saturated with water. When rigid solid also fallsalongwith the sliding mass, the phenomenon is called plug flow.
7. Falls These take place through air; for example,
jointed weathered rock falls from vertical cliffs.
8. Topples After detachment from cliffs the outward
rotation of angular blocks and rock columns cause toppling.
9. Mudflow It contains 20 to 80 per cent fine sediments saturated with water. Friction is caused by viscous movement that generates enough power to carry even large boulders.
10. Soil creep It is the least destructive of landslide phenomena. Creep is slow and superficial.
P.E. Kent (1966) proposed a hypothesis based on fluidisation of rock mass. He said that accumulated stress within rock particles causes compression of air in the pore spaces. This results in a fast-moving stream of debris. A. Heim (1932) held elasto-mechanical collisions responsible for landslides. His emphasis was on exchange of stresses between solid particles rather than fluids.
FACTORS RESPONSIBLE FOR LANDSLIDES
FACTORS RESPONSIBLE FOR LANDSLIDES
Slope instability may be caused by removal of lateral or underlying support mainly by river erosion and road cuts, landfill dumping, faulting, tectonic movement or the creation of artificial slopes by constructional activities.
Weathering involves rock disintegration, causing weakening of soil and decreased resistance to shearing. A significant cause of landslide is related to increased water infiltration which causes saturation of soil. It may be due to ploughing or poor organisation of drainage on a sloping area that has undergone modification due to deforestation and urbanisation. Pore water pressure is increased by soil saturation which results in a positive ff on the slope.
Landslides due to slumping may oc due to construction of settlement built filled up land that suffers from poor cc paction or engineering. In forests, tilT harvesting may negatively affect slope
bility. Tractors, in generaL cause imme damage as runoff follows the wheelin
Apart from the above-mentioned for the causes of slope failure may be dis guished as (i) immediate causes suer vibrations, earthquake tremors, heavy] cipitation and freezing and thawing; and long-term causes such as the slow
progressive steepening of the slope.
R.u. Cooke and J.e. Doornkamp (1' suggested a few factors that contribut, landslides.
(i) Factors leading to accelerated sl stress. surcharge i.e., loading of the crest ~f slopes \ an additional load;
. undermining of slope;
. lateral pressure exerted on cracks due to factors
freezing.
(ii) Factors that cause reduced sllear strength .c acteristic of some soil particles like clay to swell and sh alternatively in wet and dry periods;
. rock structure such as faults, joints, bedding E . pore-pressure effects;
. drying and desiccation;
. loss of capillary action;
. crumbling soil structure that leads to reduced csion in soil.
According to Cooke and Doornkamp, the proces movement which follows planes is called shear. ApF forces are called stresses. Slope failure takes place as a CI of sheu stresses operational along straight or curved s planes.
Strain is the deformation caused by movement.It is the result of shear stresses it is called shear strain. amount of resistance offered by the slope to move] is measured by the strength of the slope. The compo of this which is directed against shear stresses is tel the shear strength.
Slope instability may be caused by removal of lateral or underlying support mainly by river erosion and road cuts, landfill dumping, faulting, tectonic movement or the creation of artificial slopes by constructional activities.
Weathering involves rock disintegration, causing weakening of soil and decreased resistance to shearing. A significant cause of landslide is related to increased water infiltration which causes saturation of soil. It may be due to ploughing or poor organisation of drainage on a sloping area that has undergone modification due to deforestation and urbanisation. Pore water pressure is increased by soil saturation which results in a positive ff on the slope.
Landslides due to slumping may oc due to construction of settlement built filled up land that suffers from poor cc paction or engineering. In forests, tilT harvesting may negatively affect slope
bility. Tractors, in generaL cause imme damage as runoff follows the wheelin
Apart from the above-mentioned for the causes of slope failure may be dis guished as (i) immediate causes suer vibrations, earthquake tremors, heavy] cipitation and freezing and thawing; and long-term causes such as the slow
progressive steepening of the slope.
R.u. Cooke and J.e. Doornkamp (1' suggested a few factors that contribut, landslides.
(i) Factors leading to accelerated sl stress. surcharge i.e., loading of the crest ~f slopes \ an additional load;
. undermining of slope;
. lateral pressure exerted on cracks due to factors
freezing.
(ii) Factors that cause reduced sllear strength .c acteristic of some soil particles like clay to swell and sh alternatively in wet and dry periods;
. rock structure such as faults, joints, bedding E . pore-pressure effects;
. drying and desiccation;
. loss of capillary action;
. crumbling soil structure that leads to reduced csion in soil.
According to Cooke and Doornkamp, the proces movement which follows planes is called shear. ApF forces are called stresses. Slope failure takes place as a CI of sheu stresses operational along straight or curved s planes.
Strain is the deformation caused by movement.It is the result of shear stresses it is called shear strain. amount of resistance offered by the slope to move] is measured by the strength of the slope. The compo of this which is directed against shear stresses is tel the shear strength.
LANDSLIDES
LANDSLIDES
Landslides cause severe loss of life, injury, damage to property, destruction of communication networks and loss of precious soil and land. Although the occurrence of land~lides is declining all over the world due to greater scientific understanding and public awareness, in many areas the mounting pressure of population at the base of slopes, canyons and unstable borders of plateau have led to an increase in dangers due to landslides. Landslides are universal phenomena, but more than being 'natural hazards', they are induced by human activity.
M.A Carson and M.J. Kirkby (1972) divided hill slopes into (i) weathering-limited slopes and (ii) transport-limited slopes. In the former case, rock disintegrates in situ, whereas, in the latter case, slopes are covered by thick soil or disintegrated rock materials, known as regolith. Due to the presence of regolith, transport-limited slopes experience frequent landslides.
The term, 'landslide' encompasses falling, toppling, sliding, flowing and subsidence of soil and rock materials und~r the strong influence of gravity and other f\lctors. Some geomorphologists thus prefer to use the term mass movement instead of landslides. The resultant landforms produced by mass movements are termed mass wasting. Mass movement occurs when the slope gradient exceeds its threshold angle of stability.
Landslides cause severe loss of life, injury, damage to property, destruction of communication networks and loss of precious soil and land. Although the occurrence of land~lides is declining all over the world due to greater scientific understanding and public awareness, in many areas the mounting pressure of population at the base of slopes, canyons and unstable borders of plateau have led to an increase in dangers due to landslides. Landslides are universal phenomena, but more than being 'natural hazards', they are induced by human activity.
M.A Carson and M.J. Kirkby (1972) divided hill slopes into (i) weathering-limited slopes and (ii) transport-limited slopes. In the former case, rock disintegrates in situ, whereas, in the latter case, slopes are covered by thick soil or disintegrated rock materials, known as regolith. Due to the presence of regolith, transport-limited slopes experience frequent landslides.
The term, 'landslide' encompasses falling, toppling, sliding, flowing and subsidence of soil and rock materials und~r the strong influence of gravity and other f\lctors. Some geomorphologists thus prefer to use the term mass movement instead of landslides. The resultant landforms produced by mass movements are termed mass wasting. Mass movement occurs when the slope gradient exceeds its threshold angle of stability.
EARTHQUAKES IN INDIA
India falls quite prominently on the global seismic belt which runs in an east-west direction and is called the 'Alpine-Himalayan belt'. The main seismic zone lies along the 'main boundary fault' which runs from the Hindukush mountains in the west to Sadiya (Assam) in the east and bends southwards while passing through the Andaman and Nicobar Islands on its way to the Indonesian archipelago.
The unique geographical location of the Indian subcontinent bordered by the Himalayan belt (which is part of the Tethys geosyncline) between the Gondwanaland and Laurasian plate is held to be responsible for the unstable geological nature of the Indian subcontinent in general. According to the scientists of the Wadia Institute of Himalayan Geology, the frequent earthquakes in the Himalayas can be attributed to the northward movement of the Indian landmass for the last 80 million years. Such northward plate movement builds up stress and releases itself as energy from the earth's interiors. The recent instances of earthquakes at Jabalpur and Killary suggest that the frequency of earthquake has shifted from quake-prone areas to the so-called stable landmasses. Although some scientists attribute such earthquakes to reservoirs, others believe the root cause of earthquakes to lie in the unique geological structure of the country.
Geologically, the country is formed of several sequences of rock units, which in a vast region of peninsular India display the most ancient Archaean rocks. Geologists are still not clear whether the Indian subcontinent is made up of smaller plates merged together or there is a single massive block dissected by faults, joints or lineations. Since the Archaean rock strata dates back to 2.5 billion years, it forms the base rock over which other rock layers exist. Since the base rock is not visible, it is not subject to study and analysis regarding its physical and chemical status. Due to continued denudation, the Himalayas are also rising to maintain equilibrium. On the other hand, sediments regularly accumulated in the Bay of Bengal and the Arabian Sea put an enormous load on the oceanfloor. This phenomenon, in turn, is believed to exert pressure on the mainland too. The oceanic ridges and other complex structural features of the Indian Ocean may also have influenced the neighbouring landmass in a significant manner.
Based on seismic data and different geological and geophysical parameters, India is divided into five seismic zones. Of the five seismic zones, zone five is the most active region and zone one shows least seismic activity.
The entire north-eastern region falls in zone five. In fact, in the last 100 years, as many as five major earthquakes with a magnitude of 7.0 occurred in this regionAssam in July 1918, July 1930 and October 1943, Arunachal Pradesh-China border in August 1950 and ManipurMyanmar border in August 1988.
The high level of seismicity in this zone results from the fact that this belt is the line along which the Indian plate (or pristine Gondwanaland) meets the Eurasian plate in accordance with the terms of the theory of plate tectonics. This being a convergent edge, the Indian plate is thrusting underneath the Eurasian plate at a speed of 5 cm per year. This movement gives rise to a tremendous stress which keeps accumulating in the rocks (just as a compressed spring conserves stress) and is released from time to time in the form of earthquakes. Since a lot of energy is released through these seismic tremors, the earthquakes are asso
ciated with kuch devastation and loss of life and property.
Thus, a relatively young Himalayan region which has not yet attained stability has witnessed many mqjor earthquakes in the recent past.
Besides th~ north-east, zone five includes parts of Jammu and Kashmir, Himachal Pradesh, the hills of Uttaranchal, Rann of Kutch (which includes Bhuj) in Gujarat, northern Bihar and the Andaman and Nicobar islands.
One of the reasons for this regiQn being prone to earthquake is the presence of the young-fold Himalayan mountains here which have frequent tectonic movements.
Zone four which is the next most active region of seismic activity covers Sikkiin, Delhi, remaining parts of Jammu and Kashmir, Himachal Pradesh, Bihar, the rest of Uttaranchal, the northern parts of Uttar Pradesh and West Bengal, parts of Gujarat and small portions of Maharashtra near' the west coast.
In the last ten years, Uttar Pradesh (areas now in Uttaranchal), Maharashtra and Madhya Pradesh have had a number of severe earthquakes. These include the devastating Uttarkashi (now in Uttaranchal) earthquake of 6.6 magnitude in October 1991, Latur-Osmanabad (Maharashtra) quake in September 1993, Jabalpur (Madhya Pradesh) in May 1997 and Chamoli (Utta,'anchal) in March 1999-all of a magnitude of over 6.
Zone three comprises Kerala, Goa, Lakshadweep, remaining parts of Uttar Pradesh and West Bengal, parts of Punjab, Rajasthan, Maharashtra, Madhya Pradesh, Orissa, Andhra Pradesh and Karnataka.
The remaining states with lesser known activity fall in zones one and two.
The Seismic Zones map ot' India in circulation needs
revision, say experts; more and more areas, hitherto not on the map, are proving susceptible to earthquakes of greater magnitude than attributed to them at present.
The unique geographical location of the Indian subcontinent bordered by the Himalayan belt (which is part of the Tethys geosyncline) between the Gondwanaland and Laurasian plate is held to be responsible for the unstable geological nature of the Indian subcontinent in general. According to the scientists of the Wadia Institute of Himalayan Geology, the frequent earthquakes in the Himalayas can be attributed to the northward movement of the Indian landmass for the last 80 million years. Such northward plate movement builds up stress and releases itself as energy from the earth's interiors. The recent instances of earthquakes at Jabalpur and Killary suggest that the frequency of earthquake has shifted from quake-prone areas to the so-called stable landmasses. Although some scientists attribute such earthquakes to reservoirs, others believe the root cause of earthquakes to lie in the unique geological structure of the country.
Geologically, the country is formed of several sequences of rock units, which in a vast region of peninsular India display the most ancient Archaean rocks. Geologists are still not clear whether the Indian subcontinent is made up of smaller plates merged together or there is a single massive block dissected by faults, joints or lineations. Since the Archaean rock strata dates back to 2.5 billion years, it forms the base rock over which other rock layers exist. Since the base rock is not visible, it is not subject to study and analysis regarding its physical and chemical status. Due to continued denudation, the Himalayas are also rising to maintain equilibrium. On the other hand, sediments regularly accumulated in the Bay of Bengal and the Arabian Sea put an enormous load on the oceanfloor. This phenomenon, in turn, is believed to exert pressure on the mainland too. The oceanic ridges and other complex structural features of the Indian Ocean may also have influenced the neighbouring landmass in a significant manner.
Based on seismic data and different geological and geophysical parameters, India is divided into five seismic zones. Of the five seismic zones, zone five is the most active region and zone one shows least seismic activity.
The entire north-eastern region falls in zone five. In fact, in the last 100 years, as many as five major earthquakes with a magnitude of 7.0 occurred in this regionAssam in July 1918, July 1930 and October 1943, Arunachal Pradesh-China border in August 1950 and ManipurMyanmar border in August 1988.
The high level of seismicity in this zone results from the fact that this belt is the line along which the Indian plate (or pristine Gondwanaland) meets the Eurasian plate in accordance with the terms of the theory of plate tectonics. This being a convergent edge, the Indian plate is thrusting underneath the Eurasian plate at a speed of 5 cm per year. This movement gives rise to a tremendous stress which keeps accumulating in the rocks (just as a compressed spring conserves stress) and is released from time to time in the form of earthquakes. Since a lot of energy is released through these seismic tremors, the earthquakes are asso
ciated with kuch devastation and loss of life and property.
Thus, a relatively young Himalayan region which has not yet attained stability has witnessed many mqjor earthquakes in the recent past.
Besides th~ north-east, zone five includes parts of Jammu and Kashmir, Himachal Pradesh, the hills of Uttaranchal, Rann of Kutch (which includes Bhuj) in Gujarat, northern Bihar and the Andaman and Nicobar islands.
One of the reasons for this regiQn being prone to earthquake is the presence of the young-fold Himalayan mountains here which have frequent tectonic movements.
Zone four which is the next most active region of seismic activity covers Sikkiin, Delhi, remaining parts of Jammu and Kashmir, Himachal Pradesh, Bihar, the rest of Uttaranchal, the northern parts of Uttar Pradesh and West Bengal, parts of Gujarat and small portions of Maharashtra near' the west coast.
In the last ten years, Uttar Pradesh (areas now in Uttaranchal), Maharashtra and Madhya Pradesh have had a number of severe earthquakes. These include the devastating Uttarkashi (now in Uttaranchal) earthquake of 6.6 magnitude in October 1991, Latur-Osmanabad (Maharashtra) quake in September 1993, Jabalpur (Madhya Pradesh) in May 1997 and Chamoli (Utta,'anchal) in March 1999-all of a magnitude of over 6.
Zone three comprises Kerala, Goa, Lakshadweep, remaining parts of Uttar Pradesh and West Bengal, parts of Punjab, Rajasthan, Maharashtra, Madhya Pradesh, Orissa, Andhra Pradesh and Karnataka.
The remaining states with lesser known activity fall in zones one and two.
The Seismic Zones map ot' India in circulation needs
revision, say experts; more and more areas, hitherto not on the map, are proving susceptible to earthquakes of greater magnitude than attributed to them at present.
Earthquakes
Earthquakes are violent tremors of the earth's crust which originate naturally and below the surface sending out a series of shock waves.
The chief cause of the earthquake shocks is the sudden slipping of rock formations along faults and fractures in the earth's crust. This happens due to constant change in volume and density of rocks due to intense temperature and pressure in the earth's interior. Some quakes originate at depths as great as several hundred kilometres and in such cases the tremors are too weak to reach the surface or cause much damage. The actual shifting of the land at the time of an earthquake occurs only in a narrow zone on either side of the faultline. In such a case, the main zone of shock and consequent destruction is linear because the vibrations originate in the line of. fracture. A sudden slipping of even five to fifteen metres along a line of fracture 80 to several hundred kilometres long can cause a very severe earthquake. Volcanic activity also can cause an earthquake but the earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth's crust. Some minor earthquakes are caused by the collapse of roofs of cavities, mines or tunnels.
The place of origin of an earthquake inside the earth is called its focus.
The point on the earth's surface vertically above the focus is called epicentre. On the earth's surface, the maximum damage is caused at the epicentre.
The vibrations of earthquakes which can be felt by human beings last from a few seconds to several minutes. Generally, the greater the intensity of the shocks, the longer they last. The average duration of shocks of sufficient intensity to produce much damage is perhaps from one to two minutes.
Earthquake waves travel ordinarily at the rate of about 5 to 8 km per second through the outer part of the crust but travel faster with depth.
An isoseismic line is an imaginary line connecting all points on the surface of the earth where the intensity of shaking produced by earthquake waves is the same.
Magnitude and intensity are the two ways in which a quake's strength is generally expressed. The magnitude is a measure that depends on the seismic energy radiated by the quake as recorded on seismographs. The intensity, in turn, is a measure that depends on the damage caused by the quake. It does not have a mathematical basis but is based on observed effects.
A quake's magnitude is usually measured in terms of the Rich~er scale. The Richter scale is logarithmic, Le., the difference between magnitude 4 and 5 is one-tenth of that between 5 and 6.
For measurement of the intensity of an earthquake, the Modified Mercalli Intensity Scale is used.
Aftershocks are earthquakes that often occur during the. days and months that follow some larger quake. Aftershocks occur in the same general region as the main shock and are believed to be the re~;ult of minor readjustment of stress at places in the fault zones.
Seismic waves spread out from the seismic focus. The waves are of three main types. P (primary, or push) waves are compressional, and can pass through any medium: they are the first waves to be recorded on a seismogram. S (secondary or shake)
waves are distortional waves: they cannot be transmitted by liquids. L (surface or long) waves travel along the surface of the earth and are recorded after the P and 5 waves. Earthquakes can cause vertical and horizontal displacement of parts of the crust. They can cause the raising or covering of parts of the sea-floor, and landslides. Earthquakes occur when there is sudden displacement of rock strata along lines of weakness in the earth's crust and also during volcanic eruption.
The two main areas of earthquakes in the world are (i) around the Pacific Ocean along a belt of volcanoes known as the ring of fire, and (ii) from the middle of Asia through the Mediterranean Sea to West Indies; the former is the more active of the two.
The chief cause of the earthquake shocks is the sudden slipping of rock formations along faults and fractures in the earth's crust. This happens due to constant change in volume and density of rocks due to intense temperature and pressure in the earth's interior. Some quakes originate at depths as great as several hundred kilometres and in such cases the tremors are too weak to reach the surface or cause much damage. The actual shifting of the land at the time of an earthquake occurs only in a narrow zone on either side of the faultline. In such a case, the main zone of shock and consequent destruction is linear because the vibrations originate in the line of. fracture. A sudden slipping of even five to fifteen metres along a line of fracture 80 to several hundred kilometres long can cause a very severe earthquake. Volcanic activity also can cause an earthquake but the earthquakes of volcanic origin are generally less severe and more limited in extent than those caused by fracturing of the earth's crust. Some minor earthquakes are caused by the collapse of roofs of cavities, mines or tunnels.
The place of origin of an earthquake inside the earth is called its focus.
The point on the earth's surface vertically above the focus is called epicentre. On the earth's surface, the maximum damage is caused at the epicentre.
The vibrations of earthquakes which can be felt by human beings last from a few seconds to several minutes. Generally, the greater the intensity of the shocks, the longer they last. The average duration of shocks of sufficient intensity to produce much damage is perhaps from one to two minutes.
Earthquake waves travel ordinarily at the rate of about 5 to 8 km per second through the outer part of the crust but travel faster with depth.
An isoseismic line is an imaginary line connecting all points on the surface of the earth where the intensity of shaking produced by earthquake waves is the same.
Magnitude and intensity are the two ways in which a quake's strength is generally expressed. The magnitude is a measure that depends on the seismic energy radiated by the quake as recorded on seismographs. The intensity, in turn, is a measure that depends on the damage caused by the quake. It does not have a mathematical basis but is based on observed effects.
A quake's magnitude is usually measured in terms of the Rich~er scale. The Richter scale is logarithmic, Le., the difference between magnitude 4 and 5 is one-tenth of that between 5 and 6.
For measurement of the intensity of an earthquake, the Modified Mercalli Intensity Scale is used.
Aftershocks are earthquakes that often occur during the. days and months that follow some larger quake. Aftershocks occur in the same general region as the main shock and are believed to be the re~;ult of minor readjustment of stress at places in the fault zones.
Seismic waves spread out from the seismic focus. The waves are of three main types. P (primary, or push) waves are compressional, and can pass through any medium: they are the first waves to be recorded on a seismogram. S (secondary or shake)
waves are distortional waves: they cannot be transmitted by liquids. L (surface or long) waves travel along the surface of the earth and are recorded after the P and 5 waves. Earthquakes can cause vertical and horizontal displacement of parts of the crust. They can cause the raising or covering of parts of the sea-floor, and landslides. Earthquakes occur when there is sudden displacement of rock strata along lines of weakness in the earth's crust and also during volcanic eruption.
The two main areas of earthquakes in the world are (i) around the Pacific Ocean along a belt of volcanoes known as the ring of fire, and (ii) from the middle of Asia through the Mediterranean Sea to West Indies; the former is the more active of the two.
MERCALLI SCALE GRADATION OF EARTHQUAKES
I Most people do not notice, animals may be uneasy,
can be detected by a seismograph.
II Hanging objects sway back and forth.
III Many people feel the movement, parked cars may
rock.
IV Doors, windows, and shelves may rattle, people
indoors can feel movement.
V Light furniture moves, pictures fall off walls, objects
fall from shelves.
VI Nearly everyone feels movement, light furniture falls over, windows may crack.can be detected by a seismograph.
II Hanging objects sway back and forth.
III Many people feel the movement, parked cars may
rock.
IV Doors, windows, and shelves may rattle, people
indoors can feel movement.
V Light furniture moves, pictures fall off walls, objects
fall from shelves.
Some people fall over, walls may crack.
Heavy furniture falls over, some walls crumble. Many people panic, some buildings collapse, dams crack.
X Railway lines are bent, most buildings are damaged,
roads crack.
XI Bridges collapse, buried pipes break, most buildings
collapse.
XII All man-made structures are destroyed.
RICHTER SCALE GRADATION OF EARTHQUAKES
Less than 2.0 'Micro' earthquakes; generally not felt, but recorded
2.0-2.9 Potentially perceptible
3.0-3.9 Felt by some
4.0-4.9 Felt by most
5.0-5.9 'Moderate' earthquakes; damaging shocks
6.0-6.9 'Large' earthquakes; destructive in populated regions
7.0-7.9 'Major' earthquakes; inflict serious damages
Greater than 8.0 'Great' earthquakes; cause extensive de
struction near epicentre
PLATE TECTONICS
A. Wegener, in 1915, first gave the term 'continental drift', which implied that parts of the crust are capable of horizontal movements round the globe, causing the continents to slowly change their positions in relation to one another. The body of theory now called plate tectonics, put forward by A. Holmes and others, embodies concepts which explain- the distribution and origin of many relief features.
The basic principle of plate tectonics is simple-the lithosphere of the earth is considered to be divided into lithospheric plates, of which there are six major plates and many smaller blocks within or between each major plate. Each plate is capable of moving over the asthenosphere, carrying oceanic and continental crust alike. These plates move as single independent bodies-they move past each other or collide. At plate boundaries, major tectonic landforms are created.
The areas of collision may be made up of three elements: deep trenches in the ocean floor; arc-like rows of volcanic iSlands; and mountain ranges. Through geological time, as these plates drifted apart, vast areas of ocean basins came into existence, with thin basaltic crust. At the same time the processes of sedimentation, orogeny and igneous intrusion acting over the subduction zones (plate descended in the mantle) have gradually created thick continental crust with its granitic upper layer. Thus, the continents have evolved and increased in size over a 3-4 million period.
The process of drifting, however, continues, which is observable by phenomena like continued upthrust of the Andes' mountains, the widening of the Atlantic Ocean and the northward shift of coastal California.
The basic principle of plate tectonics is simple-the lithosphere of the earth is considered to be divided into lithospheric plates, of which there are six major plates and many smaller blocks within or between each major plate. Each plate is capable of moving over the asthenosphere, carrying oceanic and continental crust alike. These plates move as single independent bodies-they move past each other or collide. At plate boundaries, major tectonic landforms are created.
The areas of collision may be made up of three elements: deep trenches in the ocean floor; arc-like rows of volcanic iSlands; and mountain ranges. Through geological time, as these plates drifted apart, vast areas of ocean basins came into existence, with thin basaltic crust. At the same time the processes of sedimentation, orogeny and igneous intrusion acting over the subduction zones (plate descended in the mantle) have gradually created thick continental crust with its granitic upper layer. Thus, the continents have evolved and increased in size over a 3-4 million period.
The process of drifting, however, continues, which is observable by phenomena like continued upthrust of the Andes' mountains, the widening of the Atlantic Ocean and the northward shift of coastal California.
Earth Movements
The face of the earth is constantly being reshaped by internal forces, such as earth movements, volcanoesr earthquakes and landslides, and external forces, such as river, rain water, glacier, wind and sea waves. In this chapter, we will discuss landforms formed due to activities of earth's internal forces.
EARTH MOVEMENTS
The powerful internal forces operating from within the crust are called earth movements. Such movements may be slow and sudden. Earth movements are classified into tectonic movements, vertical movements and horizontal movements.
Tectonic Movements The earth movements which bring about vast changes on the earth's surface are called tectonic movements (see box). The concentration of great internal forces within the earth raises local areas upwards or cause them sinking downwards. Tectonic movements are divided into sudden movements and slow or secular movements.
Sudden Movements These are commonly noticed during an earthquake.
Slow or Secular Movements These movements continue much longer as compared to our life span. The periodical advance and retreat of continental glaciers and ice caps because of global changes in climate are said to have caused them. These movements are relative to each other, the land advancing against sea is termed a negative movement and the sea advancing on land is known as positive movement.
Vertical Movements Responsible for a rise or a fall of a portion of the earth surface, vertical movements of the earth do not disturb the horizontality of the strata as they were originally laid down. Vertical movements cause uplift and subsidence. When a part of the earth's crust rises in relation to surrounding portions, it is known as uplift. Conversely, when the sinking of a part of the earth's crust, relative to the surrounding portions takes place, it is called subsidence. These earth movements on a large scale build up continents and plateaus.
Horizontal Movements These movements are responsible for greatly disturbing the horizontal arrangement of layers of rock. They involve both the forces of compression and tension. Tension is the puIling force. Compression is a force that pushes against a body from directly opposite sides. The tension is responsible for breaking of rock layers with their subsequent sliding or displacement. It is termed as the formation of a 'fault'. The compression leads to the bending of horizontal layers of deep sediments into a shape known as a 'fold'. These two phenomena of folding and faulting lead to the building up of mountains.
EARTH MOVEMENTS
The powerful internal forces operating from within the crust are called earth movements. Such movements may be slow and sudden. Earth movements are classified into tectonic movements, vertical movements and horizontal movements.
Tectonic Movements The earth movements which bring about vast changes on the earth's surface are called tectonic movements (see box). The concentration of great internal forces within the earth raises local areas upwards or cause them sinking downwards. Tectonic movements are divided into sudden movements and slow or secular movements.
Sudden Movements These are commonly noticed during an earthquake.
Slow or Secular Movements These movements continue much longer as compared to our life span. The periodical advance and retreat of continental glaciers and ice caps because of global changes in climate are said to have caused them. These movements are relative to each other, the land advancing against sea is termed a negative movement and the sea advancing on land is known as positive movement.
Vertical Movements Responsible for a rise or a fall of a portion of the earth surface, vertical movements of the earth do not disturb the horizontality of the strata as they were originally laid down. Vertical movements cause uplift and subsidence. When a part of the earth's crust rises in relation to surrounding portions, it is known as uplift. Conversely, when the sinking of a part of the earth's crust, relative to the surrounding portions takes place, it is called subsidence. These earth movements on a large scale build up continents and plateaus.
Horizontal Movements These movements are responsible for greatly disturbing the horizontal arrangement of layers of rock. They involve both the forces of compression and tension. Tension is the puIling force. Compression is a force that pushes against a body from directly opposite sides. The tension is responsible for breaking of rock layers with their subsequent sliding or displacement. It is termed as the formation of a 'fault'. The compression leads to the bending of horizontal layers of deep sediments into a shape known as a 'fold'. These two phenomena of folding and faulting lead to the building up of mountains.
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