CHAPTER 8 Landforms and Cycle of Erosion
- Fluvial landforms and Cycle of Erosion – Deposition Landforms and Erosional Landforms.
- Glacial landforms and Cycle of Erosion
- Marine landforms and Cycle of Erosion
- Arid landforms and Cycle of Erosion
- Karst landforms and Cycle of Erosion
CHAPTER 8 PART 1
Fluvial Landforms and Cycle of Erosion
- The landforms created as a result of degradational action (erosion) or aggradational work (deposition) of running water are called fluvial landforms.
- The fluvial processes may be divided into three physical phases – erosion, transportation and deposition.
Various Aspects of Fluvial Erosive Action
- Corrasion or abrasion == solid river load striking against rocks and wearing them down.
- Hydration == force of running water wearing down rocks.
- Attrition == river load particles striking, colliding against each other and breaking down in the process.
- Downcutting == Erosion in vertical direction (downcutting leads to valley deepening) or
- Lateral erosion == Erosion in horizontal direction, especially the walls of the stream.
- Corrosion == Chemical action that leads to weathering.
River Valley Formation
- The extended depression on ground through which a stream flows throughout its course is called a river valley.
- At different stages of the erosional cycle the valley acquires different profiles.
- At a young stage, the valley is deep, narrow with steep wall-like sides and a convex slope. The erosional action here is characterized by predominantly vertical downcutting The profile of valley here is typically ‘V’ shaped.
- A deep and narrow ‘V’ shaped valley is also referred to as gorge and may result due to downcutting erosion and because of recession of a waterfall. Most Himalayan rivers pass through deep gorges (at times more than 500 metres deep) before they descend to the plains.
- An extended form of gorge is called a The Grand Canyon of the Colorado river in Arizona (USA) runs for 483 km and has a depth of 2.88 km.
- A tributary valley lies above the main valley and is separated from it by a steep slope down which the stream may flow as a waterfall or a series of rapids.
- As the cycle attains maturity, the lateral erosion becomes prominent and the valley floor flattens out. The valley profile now becomes typically ‘U’ shaped with a broad base and a concave slope.
- Young rivers (A) close to their source tend to be fast-flowing, high-energy environments with rapid headward erosion, despite the hardness of the rock over which they may flow.
- Steep-sided “V-shaped’ valleys, waterfalls, and rapids are characteristic features.
- Mature rivers (B) are lower-energy systems. Erosion takes place on the outside of bends, creating looping meanders in the soft alluvium of the river plain. Deposition occurs on the inside of bends and on the river bed.
- At a river’s mouth (C), sediment is deposited as the velocity of the river slows. As the river becomes shallower more deposition occurs, forming islands and braiding the main channel into multiple, narrower channels.
- As the sediment is laid down, the actual mouth of the river moves away from the source into the sea or lake, forming a delta.
- Head ward erosion == Erosion at the origin of a stream channel, which causes the origin to move back away from the direction of the stream flow, and so causes the stream channel to lengthen.
- A waterfall is simply the fall of an enormous volume of water from a great height.
- They are mostly seen in youth stage of river.
- Relative resistance of rocks, relative difference in topographic reliefs, fall in the sea level and related rejuvenation, earth movements etc. are responsible for the formation of waterfalls.
- For example, Jog orGersoppa falls on Sharavati (a tributary of Cauveri) has a fall of 260 metres.
- The kettle-like small depressions in the rocky beds of the river valleys are called pot holes which are usually cylindrical in shape.
- Potholing or pothole-drilling is the mechanism through which the grinding tools (fragments of rocks, e.g. boulders and angular rock fragments) when caught in the water eddies or swirling water start dancing in a circular manner and grind and drill the rock beds of the valleys like a drilling machine.
- They thus form small holes which are gradually enlarged by the repetition of the said mechanism. The potholes go on increasing in both diameter and depth.
- Stepped benches along the river course in a flood plain are called terraces.
- Terraces represent the level of former valley floors and remnants of former (older) flood plains.
- Gulley is an incised water-worn channel, which is particularly common in semi-arid areas.
- It is formed when water from overland-flows down a slope, especially following heavy rainfall, is concentrated into rills, which merge and enlarge into a gulley.
- The ravines of Chambal Valley in Central India and the Chos of Hoshiarpur in Punjab are examples of gulleys.
- A meander is defined as a pronounced curve or loop in the course of a river channel.
- The outer bend of the loop in a meander is characterized by intensive erosion and vertical cliffs and is called the cliff-slope side. This side has a concave slope.
- The inner side of the loop is characterized by deposition, a gentle convex slope, and is called the slip-off side.
- Morphologically, the meanders may be wavy, horse-shoe type or ox-bow/ bracelet type.
- Sometimes, because of intensive erosion action, the outer curve of a meander gets accentuated to such an extent that the inner ends of the loop come close enough to get disconnected from the main channel and exist as independent water bodies. These water bodies are converted into swamps in due course of time.
- In the Indo-Gangetic plains, southwards shifting of Ganga has left many ox-bow lakes to the north of the present course of the Ganga.
Peneplane (Or peneplain)
- This refers to an undulating featureless plain punctuated with low-lying residual hills of resistant rocks. It is considered to be an end product of an erosional cycle.
- Peneplain, gently undulating (wave like), almost featureless plain that, in principle, would be produced by fluvial erosion that would, in the course of geologic time, reduce the land almost to baselevel (sea level), leaving so little gradient that essentially no more erosion could occur.
- The typical shape of a river course as it completes its erosional cycle is referred to as the drainage pattern of a stream.
- A drainage pattern reflects the structure of basal rocks, resistance and strength, cracks or joints and tectonic irregularity, if any.
Dendric or Pinnate
- This is an irregular tree branch shaped
- Examples: Indus, Godavari, Mahanadi, Cauvery, Krishna.
- In this type of pattern the short subsequent streams meet the main stream at right angles, and differential erosion through soft rocks paves the way for tributaries.
- Examples: Seine and its tributaries in Paris basin (France).
- The main stream bends at right angles and the tributaries join at right angles creating rectangular patterns.
- This pattern has a subsequent origin (subsequent drainage – you will study this in Indian drainage systems). Example: Colorado river (USA).
- The tributaries join the main stream at acute angles.
- This pattern is common in Himalayan foothill regions.
- The tributaries seem to be running parallel to each other in a uniformly sloping region.
- Example: rivers of lesser Himalayas
- The tributaries from a summit follow the slope downwards and drain down in all directions.
- Examples: streams of Saurashtra region and the Central French Plateau, Mt. Kilimanjaro etc..
- When the upland has an outer soft stratum, the radial streams develop subsequent tributaries which try to follow a circular drainage around the summit.
- Example: Black Hill streams of South Dakota.
- In a low lying basin the streams converge from all sides.
- Examples: streams of Ladakh, Tibet, and the Baghmati and its tributaries in Nepal.
Fluvial Depositional Landforms
- Fluvial Depositional Landforms are landforms created by the depositional activity of rivers.
- The depositional action of a stream is influenced by stream velocity and the volume of river load.
- The decrease in stream velocity reduces the transporting power of the streams which are forced to leave some load to settle down.
- Increase in river load is effected through accelerated rate of erosion in the source catchment areas consequent upon deforestation.
- Various landforms resulting from fluvial deposition are as follows:
Alluvial Fans and Cones
- When a stream leaves the mountains and comes down to the plains, its velocity decreases due to a lower gradient.
- As a result, it sheds a lot of material, which it had been carrying from the mountains, at the foothills.
- This deposited material acquires a conical shape and appears as a series of continuous fans. These are called alluvial fans.
- Such fans appear throughout the Himalayan foothills in the north Indian plains.
- These are narrow ridges of low height on both sides of a river, formed due to deposition action of the stream, appearing as natural embankments.
- These act as a natural protection against floods but a breach in a levee causes sudden floods in adjoining areas, as it happens in the case of the Hwang Ho river of China.
- A delta is a tract of alluvium at the mouth of a river where it deposits more material than can be carried away.
- The river gets divided into distributaries which may further divide and rejoin to form a network of channels.
A delta is formed by a combination of two processes:
- load-bearing capacity of a river is reduced as a result of the check to its speed as it enters a sea or lake, and
- clay particles carried in suspension in the river coagulate in the presence of salt water and are deposited.
- The finest particles are carried farthest to accumulate as bottom-set beds. Depending on the conditions under which they are formed, deltas can be of many types.
Arcuate or Fan-shaped (Curved)
- This type of delta results when light depositions give rise to shallow, shifting distributaries and a general fan-shaped profile. Examples: Nile, Ganga, Indus.
Bird’s Foot Delta (Elongated)
- This type of delta emerges when limestone sediment deposits do not allow downward seepage of water.
- The distributaries seem to be flowing over projections of these deposits which appear as a bird’s foot.
- The currents and tides are weak in such areas and the number of distributaries lesser as compared to an arcuate delta. Example: Mississippi river.
- Sometimes the mouth of the river appears to be submerged. This may be due to a drowned valley because of a rise in sea level.
- Here fresh water and the saline water get mixed. When the river starts ‘filling its mouth’ with sediments, mud bars, marshes and plains seem to be developing in it.
- These are ideal sites for fisheries, ports and industries because estuaries provide access to deep water, especially if protected from currents and tides. Example: Hudson estuary.
- This is a pointed delta formed generally along strong coasts and is subjected to strong wave action. There are very few or no distributaries in a cuspate delta.
- Example: Tiber river on west coast of Italy.
High-constructive deltas – Elongate and Lobate Delta
- Develops when fluvial action and depositional process dominate the system.
- Elongate delta is represented by the bird-foot delta of the Mississippi River.
- Lobate delta is represented by the Godavari River.
Lobate: Shaped like a lobe.
Godavari – Lobate
Krishna – Arcuate
Kaveri – Quadrilateral
Nile, Indus, Ganga-Brahmaputra – Arcuate
All the above are more or less the same kind (arcuate) of deltas.
- Both of these types have a large sediment supply that tend to disperse sediment along the shoreline.
- A lobate delta (a sub type of fan shaped delta) is formed if the river water is as dense as the seawater (precipitation or coagulation of river sediments occur immediately and hence the delta is not elongated).
- A bird-boot delta (elongated delta) is formed when the river water is lighter than sea water (precipitationor coagulation of river sediments can occur at a distance from shore and hence the delta is elongated).
- Shoreline energy is high and much of the sediment delivered by the river is reworked by wave action or currents before it is finally deposited.
- Deltas formed by rivers such as the Nile and the Rhône have been classified as wave-dominated.
- In this class of high-destructive delta, sediment is finally deposited as arcuate sand barriers near the mouth of the river.
CHAPTER 8 PART 2
Karst Landforms and Cycle of Erosion
- Karst is a landscape which is underlain by limestone which has been eroded by dissolution, producing towers, fissures, sinkholes, etc.
- It is so named after a province of Yugoslavia on the Adriatic sea coast where such formations are most noticeable.
- Karst topography is a landscape formed from the dissolution of soluble rocks such aslimestone, dolomite, and gypsum.
- It is characterized by underground drainage systems with sinkholes, caves etc..
Conditions Essential for Full Development of Karst Topography
- Presence of soluble rocks, preferably limestone at the surface or sub-surface level.
- These rocks should be dense, highly jointed and thinly bedded.
- This is an underground cave formed by water action by various methods in a limestone or chalk area.
- When a part of the cavern collapses the portion which keeps standing forms an arch.
Sink Hole/Swallow Hole
- Sink holes are funnel-shaped depressions having an average depth of three to nine metres.
- These holes are developed by enlargement of the cracks found in such rocks, as a result of continuous solvent action of the rainwater.
- The surface streams which sink disappear underground through swallow holes.
- When a number of adjoining sink holes collapse, they form an open, broad area called a karst window.
- In a valley, the water often gets lost through cracks and fissures in the bed. These are called sinking creeks, and if their tops are open, they are called bogas.
Stalactite and Stalagmite
- The water containing limestone in solution, seeps through the roof in the form of a continuous chain of drops.
- A portion of the roof hangs on the roof and on 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 to the floor. This also evaporates, leaving behind a small deposit of limestone aiding the formation of a stalagmite, thicker and flatter, rising upwards from the floor.
- Sometimes, stalactite and stalagmite join together to form a complete pillar known as the column.
CHAPTER 8 PART 3
Marine Landforms and Cycle of Erosion
- Sea waves, aided by winds, currents, tides and storms carry on the erosional and depositional processes.
- The erosive work of the sea depends upon size and strength of waves, slope, height of the shore between low and high tides, shape of the coast, composition of rocks, depth of water, human activity etc.
- The wave pressure compresses the air trapped inside rock fissures, joints, faults, etc. forcing it to expand and rupture the rocks along weak points. This is how rocks undergo weathering under wave action.
- Waves also use rock debris as instruments of erosion (glaciers are quite good at this). These rock fragments carried by waves themselves get worn down by striking against the coast or against one another.
- The solvent or chemical action of waves is another mode of erosion, but it is pronounced only in case of soluble rocks like limestone and chalk.
Marine Erosional Landforms
- These are narrow, deep indentations (a deep recess or notch on the edge or surface of something) carved due to headward erosion (downcutting) through vertical planes of weakness in the rocks by wave action.
- With time, further headward erosion is hindered by lateral erosion of chasm mouth, which itself keeps widening till a bay is formed.
- When the sea waves strike against a cliff, the cliff gets eroded (lateral erosion) gradually and retreats.
- The waves level out the shore region to carve out a horizontal plane or a wave-cut platform.
- The bottom of the cliff suffers the maximum intensive erosion by waves and, as a result, a notch appears at this position.
- Shoreline marked by a steep bank (escarpment, scarp).
- Differential erosion by sea waves through a rock with varying resistance across its structure produces arched caves in rocks called sea caves.
- When waves from opposite directions strike a narrow wall of rock, differential erosion of the rock leaves a bridge like structure called Sea arch.
- When a portion of the sea arch collapses, the remaining column-like structure is called a stack, skarry or chimney rock.
- If the fluvial erosion of a stream at the shore doesn’t match the retreat of the sea, the rivers appear to be hanging over the sea. These river valleys are called hanging valleys.
Blow Holes or Spouting Horns
- The burst of water through a small hole on a sea cave due to the compression of air in the cave by strong waves. They make a peculiar noise.
Plane of Marine Erosion/Peneplain
- The eroded plain left behind by marine action is called a plain of marine erosion. If the level difference between this plain and the sea level is not much, the agents of weathering convert it into a peneplain.
Marine Depositional Landforms
- This is the temporary covering of rock debris on or along a wave-cut platform.
- Currents and tidal currents deposit rock debris and sand along the coast at a distance from the shoreline.
- The resultant landforms which remain submerged are called bars.
- The enclosed water body so created is called a
- It is the overwater counterpart of a bar.
Spit and Hook
- A spit is a projected deposition joined at one end to the headland, with the other end free in the sea.
- The mode of formation is similar to a bar or barrier.
- A shorter spit with one end curved towards the land is called a
- Sometimes, islands are connected to each other by a bar called tombolo.
- The boundary between the coast (the part of the land adjoining or near the sea) and the shore (the land along the edge of a sea) is known as the coastline.
Coastlines can be divided into the following classes:
- Coastline of Emergence
- Coastline of Submergence
- Neutral coastline
- Compound coastline
- Fault coastline
- Coastline are modified either due to rise or fall in sea levels or upliftment or subsidence of land, or both.
Coastlines of Emergence
- These are formed either by an uplift of the land or by the lowering of the sea level.
- Bars, spits, lagoons, salt marshes, beaches, sea cliffs and arches are the typical features.
- The east coast of India, especially its south-eastern part (Tamil Nadu coast), appears to be a coast of emergence.
- The west coast of India, on the other hand, is both emergent and submergent. The northern portion of the coast is submerged as a result of faulting and the southern portion, that is the Kerala coast, is an example of an emergent coast.
- Coramandal coast == Tamil Nadu Coast== Coastline of emergence
- Malabar coast == Kerala Coast == Coastline of emergence
- Konkan coast == Maharashtra and Goa Coast == Coastline of submergence.
Coastlines of Submergence
- A submerged coast is produced either by subsidence of land or by a rise in sea level.
- Ria, fjord, Dalmatian and drowned lowlands are its typical features.
- When a region is dissected by streams into a system of valleys and divides, submergence produces a highly irregular shoreline called ria coastline.
- The coast of south-west Ireland is a typical example of ria coastline.
- Some coastal regions have been heavily eroded by glacial action and the valley glacier troughs have been excavated below sea level.
- After the glaciers have disappeared, a fjord coastline emerges.
- These coasts have long and narrow inlets with very steep sides.
- The fjord coasts of Norway are a typical example.
- The Dalmatian coasts result by submergence of mountain ridges with alternating crests and troughs which run parallel to the sea coast.
- The Dalmatian coast of Yugoslavia is a typical example.
- A drowned lowland coast is low and free from indentations, as it is formed by the submergence of a low-lying area.
- It is characterized by a series of bars running parallel to the coast, enclosing lagoons.
- The Baltic coast of eastern Germany is an example of this type of coastline.
- These are coastlines formed as a result of new materials being built out into the water.
- The word ‘neutral’ implies that there need be no relative change between the level of sea and the coastal region of the continent.
- Neutral coastlines include the alluvial fan shaped coastline, delta coastline, volcano coastline and the coral reef coastline.
- Such coastlines show the forms of two of the previous classes combined, for example, submergence followed by emergence or vice versa.
- The coastlines of Norway and Sweden are examples of compound coastlines.
- Such coastlines are unusual features and result from the submergence of a downthrown block along a fault, such that the uplifted block has its steep side (or the faultline) standing against the sea forming a fault coastline.
CHAPTER 8 PART 4
Glacial Landforms and Cycle of Erosion
- A glacier is a moving mass of ice at speeds averaging few meters a day.
- Types of Glaciers:continental glaciers, ice caps, piedmont glaciers and valley glaciers.
- The continental glaciers are found in the Antarctica and in Greenland. The biggest continental ice sheet in
- Ice caps are the covers of snow and ice on mountains from which the valley or mountain glaciers originate.
- The piedmont glaciers form a continuous ice sheet at the base of mountains as in southern Alaska.
- The valley glaciers, also known as Alpine glaciers, are found in higher regions of the Himalayas in our country and all such high mountain ranges of the world.
- The largest of Indian glaciers occur in the Karakoram range, viz. Siachen (72 km), while Gangotri in Uttar Pradesh (Himalayas) is 25.5 km long.
- A glacier is charged with rock debris which are used for erosional activity by moving ice.
- A glacier during its lifetime creates various landforms which may be classified into erosional and depositional landforms.
Glacial Erosional Landforms
- Hollow basin cut into a mountain ridge.
- It has steep sided slope on three sides, an open end on one side and a flat bottom.
- When the ice melts, the cirque may develop into a tarn lake.
- Original stream-cut valley, further modified by glacial action.
- It is a ‘U’ Shaped Valley. It at mature stage of valley formation.
- Since glacial mass is heavy and slow moving, erosional activity is uniform – horizontally as well as vertically.
- A steep sided and flat bottomed valley results, which has a ‘U’ shaped profile.
- Formed when smaller tributaries are unable to cut as deeply as bigger ones and remain ‘hanging’ at higher levels than the main valley as discordant tributaries.
- A valley carved out by a small tributary glacier that joins with a valley carved out by a much larger glacier.
- Steep-sided, sharp-tipped summit with the glacial activity cutting into it from two
- Ridge that acquires a ‘horn’ shape when the glacial activity cuts it from more than two sides.
- Steep-sided narrow entrance-like feature at the coast where the stream meets the coast.
- Fjords are common in Norway, Greenland and New Zealand.
Glacial Depositional Landforms
- When the glacier reaches its lowest point and melts, it leaves behind a stratified deposition material, consisting of rock debris, clay, sand, gravel etc. This layered surface is called till plain or an outwash plain.
- Winding ridge of un-assorted depositions of rock, gravel, clay etc. running along a glacier in a till plain.
- The eskers resemble the features of an embankment and are often used for making roads.
- Broken ridges or un-assorted depositions looking like hump in a till plain.
- Inverted boat-shaped deposition in a till plain caused by deposition.
- Formed when the deposited material in a till plain gets depressed locally and forms a basin.
- General term applied to rock fragments, gravel, sand, etc. carried by a glacier.
- Depending on its position, the moraine can be ground moraine and end moraine.
Glacial Cycle of Erosion
- The stage is marked by the inward cutting activity of ice in a cirque.
- Aretes and horns are emerging. The hanging valleys are not prominent at this stage.
- Hanging valleys start emerging. The opposite cirques come closer and the glacial trough acquires a stepped profile which is regular and graded.
- Emergence of a ‘U’-shaped valley marks the beginning of old age.
- An outwash plain with features such as eskers, kame terraces, drumlins, kettle holes etc. is a prominent development.
CHAPTER 8 PART 5
Arid Landforms and Cycle of Erosion
- Arid regions are regions with scanty rainfall. Deserts and Semi-arid regions fall under arid landforms.
Erosional Arid Landforms
Water Eroded Arid Landforms
- In hill slope geomorphology, a rill is a narrow and shallow channel cut into soil by the erosive action of flowing water.
- A gully is a landform created by running water. Gullies resemble large ditches or small valleys, but are metres to tens of metres in depth and width.
- A ravine is a landform narrower than a canyon and is often the product of stream cutting erosion. Ravines are typically classified as larger in scale than gullies, although smaller than valleys.
- In arid regions occasional rainstorms produce numerous rills and channels which extensively erode weak sedimentary formations.
- Ravines and gullies are developed by linear fluvial erosion leading to the formation of badland topography.
- Example: Chambal Ravines.
- The intermontane basins in dry regions are generally known as bolsons.
- Three unique landforms viz. pediments, bajadas and playas are typically found in bolsons.
- Small streams flow into bolsons, where water is accumulated. These temporary lakes are called playas.
- After the evaporation of water, salt-covered playas are called salinas.
- In form and function there is no difference between a pediment and an alluvial fan; however, pediment is an erosional landform while a fan is a constructional one.
- A true pediment is a rock cut surface at the foot of mountains.
- Bajadas are moderately sloping depositional plains located between pediments and playa.
- Several alluvial fans coalesce to form a bajada.
Wind Eroded Arid Landforms
- The wind or Aeolian erosion takes place in the following ways, viz. deflation, abrasion, and attrition.
- Deflation == removing, lifting and carrying away dry, unsorted dust particles by winds. It causes depressions known as blow outs.
- Abrasion == When wind loaded with sand grains erodes the rock by grinding against its walls is called abrasion or sandblasting.
- Attrition == Attrition refers to wear and tear of the sand particles while they are being transported.
Following are the major landforms produced by wind erosion.
- Deflation basins, called blowouts, are hollows formed by the removal of particles by wind. Blowouts are generally small, but may be up to several kilometers in diameter.
- A mushroom rock, also called rock pedestal or a pedestal rock, is a naturally occurring rock whose shape, as its name implies, resembles a mushroom.
- The rocks are deformed in a number of different ways: by erosion and weathering, glacial action, or from a sudden disturbance. Mushroom rocks are related to, but different from, yardang.
- A monadnock or inselberg is an isolated hill, knob, ridge, outcrop, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain.
- These are rock pillars which stand as resistant rocks above soft rocks as a result of differential erosion of hard and soft rocks.
- A table-shaped area of rock found in arid and semi-arid areas formed when more resistant rock is reduced at a slower rate than softer rocks around it.
- Ridge of rock, formed by the action of the wind, usually parallel to the prevailing wind direction.
Wind bridges and windows
- Powerful wind continuously abrades stone lattices, creating holes. Sometimes the holes are gradually widened to reach the other end of the rocks to create the effect of a window—thus forming a wind window. Window bridges, are formed when the holes are further widened to form an arch-like feature.
Arid Depositional Landforms
- Landforms are also created by the depositional force of wind. These are as follows.
- These are depositional features on a small scale formed by saltation (he transport of hard particles over an uneven surface in a turbulent flow of air or water).
- Sand dunes are heaps or mounds of sand found in deserts. Generally their heights vary from a few metres to 20 metres but in some cases dunes are several hundred metres high and 5 to 6 km long.
Some of the forms are discussed below:
- Formed parallel to the wind movement. The windward slope of the dune is gentle whereas the leeward side is steep. These dunes are commonly found at the heart of trade-wind deserts like the Sahara, Australian, Libyan, South African and Thar deserts.
- Dunes deposited perpendicular (transverse) to the prevailing wind direction.
- Crescent shaped dunes. The windward side is convex whereas the leeward side is concave and steep.
- They are U-shaped and are much longer and narrower than barchans.
- Have a high central peak, radically extending three or more arms.
- In some parts of the world, windblown dust and silt blanket the land. This layer of fine, mineral-rich material is called loess.
- Extensive loess deposits are found in northern China, the Great Plains of North America, central Europe, and parts of Russia and Kazakhstan.
- The thickest loess deposits are near the Missouri River in the U.S. state of Iowa and along the Yellow River in China.
- Loess accumulates, or builds up, at the edges of deserts. For example, as wind blows across the Gobi, a desert in Asia, it picks up and carries fine particles. These particles include sand crystals made of quartz or mica. It may also contain organic material, such as the dusty remains of skeletons from desert animals.
- Loess often develops into extremely fertile agricultural soil. It is full of minerals and drains water very well. It is easily tilled, or broken up, for planting seeds.
- Loess usually erodes very slowly – Chinese farmers have been working the loess around the Yellow River for more than a thousand years.
CHAPTER 9 Lakes
- A lake is a body of water of considerable size, localized in a basin, that is surrounded by land apart from a river or other outlet that serves to feed or drain the lake.
- Lakes lie on land and are not part of the ocean, and therefore are distinct from lagoons, and are also larger and deeper than ponds.
- Natural lakes are generally found in mountainous areas, rift zones, and areas with ongoing glaciation.
- Most lakes have at least one natural outflow in the form of a river or stream, which maintain a lake’s average level by allowing the drainage of excess water
- Other lakes are found in endorheic basins. Some lakes do not have a natural outflow and lose water solely by evaporation or underground seepage or both. They are termed endorheic lakes.
- The majority of lakes on Earth are fresh water, and most lie in the Northern Hemisphere at higher latitudes. Canada, Finland and Siberia contain most of the fresh water lakes.
Classification of Lakes
- Lakes may exist temporarily filling up the small depressions of undulating ground after a heavy shower.
- In this kind of lakes, Evaporation > Precipitation.
- Example: Small lakes of deserts.
- In this kind of lakes, Evaporation < Precipitation.
- These lakes are deep and carry more water than could ever be evaporated.
- Example: Great Lakes of North America, East African Rift Lakes.
Fresh water lakes
- Most of the lakes in the world are fresh-water lakes fed by rivers and with out-flowing streams e.g. Great Lakes of North America.
- Salt lakes (also called saline lakes) can form where there is no natural outlet or where the water evaporates rapidly and the drainage surface of the water table has a higher-than-normal salt content.
- Because of the intense evaporation (negative freshwater balance == more water is lost in evaporation than gained from rivers) these lakes are saline.
- Examples of salt lakes include Great Salt Lake, the Aral Sea and the Dead Sea.
- For example the Dead Sea has a salinity (salt content) of 250 parts per thousand, and the Great Salt Lake of Utah, U.S.A. has a salinity of 220 parts per thousand.
- Playas or salt lakes, are a common feature of deserts (recall desert landforms).
Lakes Formed by Earth Movement
- Due to the warping (simple deformation), subsidence (sliding downwards), bending and fracturing (splitting) of the earth’s crust, tectonic depressions occur.
- Such depressions give rise to lakes of immense sizes and depths.
- They include Lake Titicaca, and the Caspian Sea.
Rift valley lakes
- A rift valley is formed when two blocks of earth move apart letting the ‘in between’ block slide downwards. Or, it’s a sunken land between two parallel faults.
- Rift valleys are deep, narrow and elongated. Hence the lakes formed along rift valleys are also deep, narrow and very long.
- Water collects in troughs (Valley in the rift) and their floors are often below sea level.
- The best known example is the East African Rift Valley which runs through Zambia, Malawi, Tanzania, Kenya and Ethiopia, and extends along the Red Sea to Israel and Jordan over a total distance of 3,000 miles.
- It includes such lakes as Lakes Tanganyika, Malawi, Rudolf, Edward, Albert, as well as the Dead Sea 1,286 feet below mean sea level, the world’s lowest lake.
Lakes Formed by Glaciation
Cirque lakes or tarns
- Cirque is a hollow basin cut into a mountain ridge. It has steep sided slope on three sides, an open end on one side and a flat bottom.
- When the ice melts, the cirque may develop into a tarn lake.
- The advance and retreat of glaciers can scrape depressions in the surface where water accumulates; such lakes are common in Scandinavia, Patagonia, Siberia and Canada.
- These are formed by ice-scouring (eroding) when ice sheets scoop out (dig) hollows on the surface.
- Such lakes of glacial origin are abundant in Finland – Land of Lakes. It is said that there are over 35,000 glacial lakes in Finland.
Lakes due to morainic damming of valleys
- Valley glaciers often deposit morainic debris across a valley so that lakes are formed when water accumulates behind the barrier.
Lakes Formed by Volcanic Activity
Crater and caldera lakes
- During a volcanic explosion the top of the cone may be blown off leaving behind a natural hollow called a crater.
- This may be enlarged by subsidence into a caldera.
- In dormant or extinct volcanoes, rain falls straight into the crater or caldera which has no superficial outlet and forms a crater or caldera lake.
- Examples: Lonar in Maharashtra and Krakatao in Indonesia.
Others are Lava-blocked lakes and Lakes due to subsidence of a volcanic land surface.
Lakes Formed by Erosion
- The solvent action of rain-water on limestone carves out solution hollows. When these become clogged with debris lakes may form in them.
- The collapse of limestone roofs of underground caverns may result in the exposure of long, narrow- lakes that were once underground.
- The winds in deserts creates hollows. These may reach ground water which seeps out forming small, shallow lakes. Excessive evaporation causes these to become salt lakes and playas. Example: Great Basin of Utah, U.S.A.
Lakes Formed by Deposition
Lakes due to river deposits
- Ox-bow lake, e.g. those that occur on the flood-plains of Lower Mississippi, Lower Ganges etc..
Lakes due to Marine deposits
- Also called Lagoons.
- Example: Lake Chilka
Lakes due to damming of water
- Lakes formed by these processes are also known as barrier lakes. Landslides, avalanches may block valleys so that rivers are dammed. Such lakes are short-lived.
- Example: Lakes that are formed in Shiwaliks (Outer Himalayas). Dehradun (all Duns) were lakes few centuries ago.
- Besides the natural lakes, man has now created artificial lakes by erecting a concrete dam across a river valley so that the river water can be kept back to form reservoirs.
- Example: Lake Mead above the Hoover Dam on the Colorado River, U.S.A.
- Man’s mining activities, e.g. tin mining in West Malaysia, have created numerous lakes. Inland fish culture has necessitated the creation of many fishing-lakes.
Lakes and Man
- In countries where they are found in abundance, such as Finland, Canada, U.S.A., Sweden and the East African states, lakes are used as inland waterways.
Means of communication
- Large lakes like the Great Lakes of North America provide a cheap and convenient form of transport for heavy and bulky goods such as coal, iron, machinery, grains and timber.
- The Great Lakes-St. Lawrence waterways penetrate more than 1,700 miles into the interior. They are thus used as the chief arteries of commerce.
Economic and industrial development
- The Great Lakes-St. Lawrence waterways were responsible for the development of the interior wheat farms and lakeside industries.
- Example: Kolleru lake in Andhra Pradesh.
Hydro-electric power generation
- Artificial lakes like Hirakud.
- Many dams are built across artificial lakes.
- Bhakra Nangal Dam. Its reservoir, known as the “Gobind Sagar Lake” and Hirakud Dam (Madhya Pradesh) on the Mahanadi in India.
Regulating river flows
- Hoover Dam on the River Colorado and the Bhakra and Nangal Dams on the Sutlej in India.
- The Hirakud dam was originally conceived as a flood control measure. But the project is criticized for doing more damage than good.
Moderation of climate
Source of food
- Many large lakes have important supplies of protein food in the form of freshwater fish. Sturgeon is commercially caught in the Caspian Sea, salmon and sea trout in the Great Lakes.
Source of minerals
- Salt lakes provide valuable rock salts. In the Dead Sea, the highly saline water is being evaporated and produces common salt. Borax is mined in the salt lakes of the Mojave Desert.
Tourist attraction and health resorts
- Lake Chilka, Leh, Dead Sea etc..
No lake is permanent over geologic time
- Lakes are only temporary features of the earth’s crust; they will eventually be eliminated by the double process of draining and silting up.
- The process of lake elimination may not be completed within our span of life, it takes place relatively quickly in terms of geological time.
Important Lakes on Earth
Note 1:Black Sea is not a lake since Bosporus and Dardanelles Straits connect it to the Mediterranean Sea. Many big rivers fall into the Black Sea, making the salinity of its surface water half that of the ocean: 17‰.
Note 2:Caspian Sea and Dead Sea are lakes. The surface and shores of the Dead Sea are 423 metres below sea level, making it Earth’s lowest elevation on land.
Note 3:While writing facts about lakes, people ignore Caspian Sea because for them it is too big to be considered a lake. But it is still a lake.
Lake Baikal [Deepest]
- Located in Siberia, Russia.
- The deepest lake in the world [1,637 metres deep]
- It is the world’s largest lake by volume.
- It is the second longest.
Lake Tanganyika [Longest]
- The longest lake in the world. [660 kilometres long]
- It is also the second largest by volume.
- It is the second deepest lake in the world, after lake Baikal.
World’s Highest and Lowest Lakes
- The world’s highest lake, if size is not a criterion, may be the crater lake of Ojos del Salado, at 6,390 metres. It is in Andes.
- The highest large lake in the world is the Pumoyong Tso (Pumuoyong Tso), in the Tibet Autonomous Region of China. [5,018 metres above sea level]
- The world’s highest commercially navigable lake is Lake Titicaca in Peru and Bolivia border at 3,812 m. It is also the largest lake in South America.
- The world’s lowest lake is the Dead Sea, bordering Israel and Jordan at 418 metres below sea level. It is also one of the lakes with highest salt concentration.
The largest lakes (surface area) by continent
- Australia – Lake Eyre (salt lake)
- Africa – Lake Victoria, also the third-largest freshwater lake on Earth. It is one of the Great Lakes of Africa.
- Antarctica – Lake Vostok (subglacial)
- Asia – Lake Baikal (if the Caspian Sea is considered a lake, it is the largest in Eurasia, but is divided between the two geographic continents)
- Europe – Lake Ladoga, followed by Lake Onega, both located in northwestern Russia.
- North America – Lake Superior.
- South America – Lake Titicaca, which is also the highest navigable body of water on Earth at 3,812 metres above sea level. The much larger Lake Maracaibo is a contiguous body of water with the sea, so it is ignored. ,
- Great Lakes of North America are a series of interconnected freshwater lakes which connect to the Atlantic Ocean through the Saint Lawrence Seaway.
- Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario[in the order of west to east]. Superior, Huron, Michigan, Erie, and Ontario [In the order of largest to smallest].
- Lake Superior is the largest continental lake in the world by area, and Lake Michigan is the largest lake that is entirely within one country.
- The Great Lakes are today used as a major water transport corridor for bulk goods.
- The Great Lakes Waterway connects all the lakes; the smaller Saint Lawrence Seaway connects the lakes to the Atlantic oceans.
- Also called the Salt Sea.
- Lake bordering Jordan to the east, and Palestine and Israel to the west.
- It Earth’s lowest elevation on land.
- It was a lake lying between Kazakhstan in the north and Uzbekistan, in the south.
- Aral Sea has been steadily shrinking since the 1960s after the rivers that fed it were diverted by Soviet irrigation projects.
The Aral Sea in 1989 (left) and 2008 (right)
African Great Lakes
Already given under “Divergent Boundary”
- Series of lakes constituting the part of the Rift Valley lakes in and around the East African Rift.
- They include Lake Victoria, the second largest fresh water lake in the world, and Lake Tanganyika, the world’s second largest in volume as well as the second deepest.
Largest Lakes by Surface Area
- Caspian Sea – Asia
- Lake Superior – North America
- Lake Victoria – Africa
- Lake Huron – North America
- Lake Michigan – North America
Largest Lakes by Volume
- Baikal – Asia
- Tanganyika – Africa
- Superior – North America
Deepest Lakes in the World
- Lake Baikal – Asia
- Lake Tanganyika
- Caspian Sea
CHAPTER 10 Plateau
- A plateau is a flat-topped table land.
- Plateaus occur in every continent and take up a third of the Earths land.
- They are one of the four major landforms, along with mountains, plains, and hills.
- Plateaus, like mountains may be young or old. The Deccan plateau in India is one of the oldest plateaus.
- Valleys form when river water cuts through the plateau. The Columbia Plateau, between the Cascade and Rocky mountains in the northwestern United States, is cut through by the Columbia River.
- Sometimes, a plateau is so eroded that it is broken up into smaller raised sections called Many outlier plateaus are composed of very old, dense rock formations. Iron ore and coal often are found in plateau outliers.
- Plateaus are very useful because they are rich in mineral deposits. As a result, many of the mining areas in the world are located in the plateau areas.
- Tectonic plateaus are formed from processes that create mountain ranges – volcanism (Deccan Plateau), crustal shortening (thrusting of one block of crust over another, and folding occurs. Example: Tibet), and thermal expansion (Ethiopian Highlands).
- Thermal expansion of the lithosphere means the replacement of cold mantle lithosphere by hot asthenosphere).
- Those caused by thermal expansion of the lithosphere are usually associated with hot spots. The Yellowstone Plateau in the United States, the Massif Central in France, and the Ethiopian Plateau in Africa are prominent examples.
- When the lithosphere underlying a broad area is heated rapidly – e.g., by an upwelling of hot material in the underlying asthenosphere – the consequent warming and thermal expansion of the uppermost mantle causes an uplift of the overlying surface. The high plateaus of East Africa and Ethiopia were formed this way.
- The great heights of some plateaus, such as the Plateau of Tibet is due to crustal shortening.
- Crustal shortening, which thickens the crust as described above, has created high mountains along what are now the margins of such plateaus.
- Plateaus that were formed by crustal shortening and internal drainage lie within major mountain belts and generally in arid climates. They can be found in North Africa, Turkey, Iran, and Tibet, where the African, Arabian, and Indian continental masses have collided with the Eurasian continent.
Volcanic Flood Basalts – Traps
- A third type of plateau can form where extensive lava flows (called flood basalts or traps) and volcanic ash bury preexisting terrain, as exemplified by the Columbia Plateau in the northwestern United States, Deccan Traps of peninsular India, Laurentian plateau or The Canadian Shield and the Siberian Traps of Russia.
- Volcanic plateaus are commonly associated with eruptions that occurred during the Cenozoic or Mesozoic.
- Eruptions on the scale needed to produce volcanic plateaus are rare, and none seems to have taken place in recent time.
- The volcanism involved in such situations is commonly associated with hot spots. The lavas and ash are generally carried long distances from their sources, so that the topography is not dominated by volcanoes or volcanic centers.
- The thickness of the volcanic rock can be tens to even hundreds of metres, and the top surface of flood basalts is typically very flat but often with sharply incised canyons and valleys.
- The volcanic eruptions that produce lava plateaus tend to be associated with hot spots. For example, the basalts of the Deccan Traps, which cover the Deccan plateau in India, were erupted 60–65 million years ago when India lay in the Southern Hemisphere, probably over the same hot spot that presently underlies the volcanic island of Reunion.
- In North America the Columbia River basalts may have been ejected over the same hot spot that underlies the Yellowstone area today. Lava plateaus of the scale of those three are not common features on Earth.
- Some plateaus, like the Colorado Plateau, the Ordos Plateau in northern China, or the East African Highlands, do not seem to be related to hot spots or to vigorous upwelling in the asthenosphere but appear to be underlain by unusually hot material. The reason for localized heating beneath such areas is poorly understood, and thus an explanation for the distribution of plateaus of that type is not known.
- There are some plateaus whose origin is not known. Those of the Iberian Peninsula and north-central Mexico exhibit a topography that is largely high and relatively flat.
- There are two kinds of plateaus: dissected plateaus and volcanic plateaus.
- A dissected plateau forms as a result of upward movement in the Earth’s crust.
- The uplift is caused by the slow collision of tectonic plates. The Colorado Plateau, in the western United States, Tibetan plateau etc. are examples.
- A volcanic plateau is formed by numerous small volcanic eruptions that slowly build up over time, forming a plateau from the resulting lava flows.
- The Columbia Plateau in the northwestern United States of America and Deccan Traps are two such plateaus.
- Intermontane plateaus are the highest in the world, bordered by mountains. The Tibetan Plateau is one such plateau.
- Continental plateaus are bordered on all sides by the plains or seas, forming away from mountains.
Major plateaus of the World
- Highest and largest plateau in the world and hence called the ‘roof of the world’.
- Formed due to collision of the Indo-Australian and Eurasian tectonic plates.
- The plateau is sufficiently high enough to reverse the Hadley cell convection cycles and to drive the monsoons of India towards the south.It covers most of the Autonomous Tibetan Region, Qinghai Province of Western China, and a part of Ladakh in Jammu and Kashmir.
- It is surrounded by mountains to the south by the Himalayan Range, to the northeast by the Kunlun Range, and to the west by the Karakoram Range.
Columbia – Snake Plateau
- River Columbia and its tributary Snake meet in this plateau.
- It is bordered by the Cascade Range and Rocky Mountains and divided by the Columbia River.
- This plateau has been formed as the result of volcanic eruptions with a consequent coating of basalt lava (Flood Basalt Plateau).
- It is lying to western part of U.S.A. It is the largest plateau in America.
- It is divided by the Colorado River and the Grand Canyon.
- This plateau is an example of intermontane plateau. Mesas and buttes are found here at many places [Arid Landforms].
- The plateau is known for the groundwater which is under positive pressure and causes the emergence of springs called Artesian wells.
- Deccan Plateau is a large plateau which forms most of the southern part of India.
- It is bordered by two mountain ranges, the Western Ghats and the Eastern Ghats.
- The plateau includes the Deccan Traps which is the largest volcanic feature on Earth.
- Made of multiple basalt layers or lava flows, the Deccan Traps covers 500,000 square kilometers in area.
- The Deccan Traps are known for containing some unique fossils.
- The Deccan is rich in minerals. Primary mineral ores found in this region are mica and iron ore in the Chotanagpur region, and diamonds, gold and other metals in the Golconda region.
- Lies in the northern part of Australia.
- This plateau is made of volcanic eruption.
- Many minerals like iron, gold, lead, zinc, silver and diamond are found here.
- Diamond is also found here.
- It is lying in Congo.
- It is famous for copper production.
- Other minerals like Cobalt, Uranium, Zinc, Silver, Gold and Tin are also mined here.
- Plateaus also form in the ocean, such as the Mascarene Plateau in the Indian Ocean.
- It extends between the Seychelles and Mauritius Islands.
- Lying in the eastern part of Canada, it is a part of Canadian Shield.
- Fine quality of iron-ore is found here.
- It is called as ‘Mineral Store’. Different types of metallic minerals like silver, copper etc. are obtained from here.
- World’s biggest silver mine Chihuahua is situated in the plateau.
- It is a Piedmont plateau (Arid Landforms) lying in southern part of Argentina.
- It is a rain shadow desert plateau.
- It is an important region for sheep rearing.
Altiplano Plateau or Bolivian Plateau
- It is an intermontane plateau which is located between two ranges of Andes Mountain.
- It is a major area of Tin reserves.
- This plateau lies in the central France.
- It is famous for Grapes cultivation.
- Also known as Asia Minor, most of Turkey lies on this plateau.
- It is an intermontane plateau lying between Pontiac and Taurus Mountain ranges.
- Tigris – Euphrates Rivers flow through this plateau.
- Precious wool producing Angora goats are found here.
- Spanish Plateau or Iberian Plateau: It is situated in the middle of Spain. It is a lava plateau. It is rich in minerals like Iron.
- Loess Plateau: It is in China. The soil here is made of fine particles brought by the wind. This fine loamy soil is extremely productive. Crops grown in this soil along the Yellow River give great yields.
- Potwar Plateau: It is situated in northern plateau (Punjab) region of Pakistan. Its average ‘Salt Range’ is located to the south-west of the plateau.
- Bavarian Plateau: Southern part of Germany.
- Ahaggar Plateau: A small plateau located in Algeria, Sahara.