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.
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