|GEOMORPHIC PROCESSES AND LANDFORMS|
|We are going to cover|
- Geomorphic processes
- Endogenic process
- Sudden movement
- Diastrophic movement
- Epeirigenic movement
- Orogenic movement
- Crustal deformation processes
- Types of mountains
- Significance of mountains
- Some relevant definitions related to mountains
- Some details of important mountain ranges
- Mass movement
- Slow and Rapid
- Erosion and deposition
- Geographical Landforms
- Karst (Chalkland)
After learning about the earth formation, the evolution of its crust and other inner layers, the rocks and minerals the crust is composed of, the crustal plates movement, it is time to know in detail about the surface of the earth on which we live.
- Force responsible for features on the earth surface comes from within (Endogenetic Forces)and above (Exogenetic Forces) the earth’s surface.
- The endogenic and exogenic forces cause physical stresses and chemical actions on earth materials andbring changes in the configurationof the surface of the earth are known as geomorphic processes.
- In general terms, the endogenic forces are mainly land building forces and the exogenic processes are mainly land wearing forces.
- The actions of exogenic forces result in wearing down (degradation) of relief/elevations and filling up (aggradation) of basins/ depressions, on the earth’s surface.
- The phenomenon of wearing down of relief variations of the surface of the earth through erosion is known as
- The exogenic processes fail to even out the relief variations of the surface of the earth due to the endogenic forces that continuously elevate or build up parts of the earth’s surface.
- The energy emanating from within the earth is the main force behind endogenic geomorphic processes.
- This energy is mostly generated by radioactivity, rotational and tidal frictionand primordial heat from the origin of the earth.
- Due to variations in geothermal gradients and heat flow from within, the action of endogenic forces are not uniform. Hence the tectonically controlled original crustal surface is uneven.
- Endogenetic forces are of 2 types based on direction
- Endogenetic Forces are of 2 types based on intensity
- Sudden forces
- Diastrophic forces
- It is due to sudden forces from deep inside the earth.
- It can cause huge damage both at the surface and below the surface.
- These are extreme events and become disaster when they occur in densely populated areas.
- It is a result of long period preparation deep within the earth; but the effects on the earth surface were quick and sudden.
- Geologically, they are known as ‘Constructive forces’ as they create relief features on the Earth’s surface.
- The two main phenomenon for sudden movement are
- Volcanism = It includes the movement of molten rock (magma) onto or towards the earth’s surface through narrow volcanic vents or fissures.
- Earthquake =It is a sudden motion or trembling in the earth caused by the abrupt release of slowly accumulated energy.
We will read about volcanism and earthquake in detail in the geographical phenomenon topic.
- All processes that move, lift or build up portions of the crust of Earth come under diastrophism.
- Very slow and effects become discernable after thousands and millions of years.
- Constructive forces– Affect larger areas of the globe and produce meso-level reliefs.
- Example: mountains, plateaus and plains
- Diastrophic movements are further divided
- Epeirogenic movements
- Orogenic movements
- Epeirogenic-or-continent formingmovements are radial movements (act along the radius of the earth).
- It causes upliftment and subsidence of continental masses through upward and downward movements.
- Movements are vertical and they affect larger part.
- Vertical movements are mainly associated with the formation of continents and plateaus (continental building process).
- Upward movement can be of two types-
Upward movement or upliftment
- Of whole continent or part there of
- Of coastal land of the continents called as emergence
- Coramandal coast (Tamil Nadu Coast)
- Malabar coast (Kerala Coast)
- Konkan coast (Maharashtra and Goa Coast)
Downward movement or subsidence
- Of whole continent or part there of
- Of coastal land near the coast called submergence
- Features = Ria, fjord, Dalmatian and drowned lowlands
- The Andamans and Nicobars is the visible part of the submerged Arakan range.
- A part of the Rann of Kachchh was submerged as a result of an earthquake in 1819.
- Horizontal movement or tangential force.
- Either move towardseach other or away from each other.
- They move in opposite directions due to the ‘tensional forces’ or ‘divergent forces’ (create rupture, cracks, fracture and faults).
- They move towards each other due to the ‘compressional forces or convergent force’ (create crustal deformation leading to formation of folds or subsidence of crustal parts).
|CRUSTAL DEFORMATION PROCESSES|
The understanding of crustal deformation processes is very vital to effectively grasp the concept of fold and block mountains formation. These are basically of 3 types
- Compression– shortening (Fold mountain creation)
- Tension– Stretching (Block mountain creation)
- Shearing– stress when two pieces slide past each other (Transverse fault)
A fold is an undulating structure (wave-like) that forms due to the application of compressional stress on rocks or a part of the earth’s crust. The folds are made up of multiple strata (rock layers).Some important terminologies related to folds are
- Anticlines and synclines are the most common up-and-down folds that result from compression.
- Anticline = ∩-shape with the oldest rocks in the centre of the fold.
- Syncline = U-shape, with the youngest rocks in the centre of the fold.
- OverturnedFold= A highly inclined axial plane such that the strata on one limb are overturned.
- RecumbentFold = has an essentially horizontal axial plane.
- Nappe = A sheet of rock that has moved sideways over neighbouring strata as a result of an overthrust or folding.
- A surface along which a rock body has broken and been displaced is known as a fault.
- Based on the elasticity of the rocks and the force, fractures are seen.
- Fault plane: The plane along which the rocks are displaced by tensional and compressional forces acting vertically and horizontally is known as a fault plane.
- Fault plane may be vertical, horizontal, inclined, curved or any other form
- A dip-slip fault in which the block above the fault has moved downward relative to the block below.
- This type of faulting occurs in response to extension.
- Occurs when the “hanging wall” moves down relative to the “foot wall”.
- A dip-slip fault in which the upper block, above the fault plane, moves up and over the lower block.
- This type of faulting is common in areas of compression.
- When the dip angle is shallow, a reverse fault is often described as a thrust fault.
- Occurs where the “hanging wall” moves up or is thrust over the “foot wall”.
- A fault on which the two blocks slide past one another. The San Andreas Fault is an example of a right lateral fault.
Left-lateral strike-slip fault
- If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the left block moves toward you and the right block moves away.
Right-lateral strike-slip fault
- If you were to stand on the fault and look along its length, this is a type of strike-slip fault where the right block moves toward you and the left block moves away.
|TYPES OF MOUNTAINS|
- Fold Mountains are created at the converging site of two or more of Earth’s tectonic plates.
- It is the result of Earth’s crustal rocks folding by compressive forces due to the endogenic or internal forces.
- They are considered as the “true mountains”.
- The term Orogenesis or mountain building is commonly used for Fold Mountains.
- These are extensive mountain chain with lofty heightswhereas their width is considerablysmall.
- These mountains formed along unstable part of the earth and hence have recurrent seismicity.
- They also contain rich mineral resources such as tin, copper, gold etc.
- Rockies (North America) – 4,830 km
- Andes (South America)– 7,000 km
- Alps (Europe)– 1,200 km
- Atlas (Africa) – 2,500 km
- Himalayas (Asia) – 2,400 km
Types of Fold Mountains
On the basis of Nature of Fold
- Simple folded mountains – Folds are arranged in waves like pattern with a well-developed system of anticline and synclines.
- Complex folded mountains – Folds are complex in nature due to extreme compressional forces like overfold, recumbent fold and nappe. Himalayas are one such example.
On the basis of the Period of Origin
- Old fold mountains
- Mountains which originated before the Tertiary period.
- These mountains have been so greatly eroded that they have become residual fold mountains.
- Aravalis – 800 km
- Appalachians – 2,414 km
- Young or New fold mountains
- Fold mountains of the Tertiary period.
- They are further subdivided based on their location
- Andean type
- At the ocean-continental convergent boundaries (C-O).
- Prone to both earthquakes and volcanic activities.
- Andean type
- Himalayan type
- At the continental-continental convergent boundaries (C-C).
- No active volcanism here.
- Presence of sedimentary deposits of marine origin because of
- Great Himalayas
- Block Mountains are created where two or more of Earth’s tectonic plates are drifted away.
- Result of rifting of the Earth’s crustal rocks by tensile forces arising from the endogenic or internal forces.
- Also called fault-block Mountains since they are formed due to faulting as a result of tensile and compressive forces.
- The uplifted blocks are termed as horsts, and the lowered blocks are called graben.
- Great African Rift Valley (valley floor is graben)
- Rhine Valley (graben) and Vosges mountain (horst) in Europe
- A mountain formed due to volcanic activity is called Volcanic Mountain.
- As these are formed by the accumulation of volcanic material, they are also known as mountains of accumulation.
- Kilimanjaro (Tanzania)
- Fujiyama (Japan)
- Andaman and Nicobar islands in India is the tip of the volcanic mountains which rise from the ocean floor.
|RESIDUAL OR DISSECTED MOUNTAIN|
- These mountains are evolved by denudation.
- That is why they are also known as relict mountains or mountains of circumdenudation.
- They have been worn down from previously existing elevated regions.
- Nilgiri Hills
- But Nilgiris got their present height as a result of subsequent uplift.
|CLASSIFICATION OF MOUNTAINS ON THE BASIS OF LOCATION|
- Continental mountains
- Oceanic mountains
- Coastal mountains
- the Rockies
- the Appalachians
- the Alpine mountain chains
- the Western Ghats (India)
- the Eastern Ghats (India)
- Inland mountains
- the Vosges and the Black Forest (Europe)
- the Kunlun, Tienshan, Altai mountains of Asia
- the Urals of Russia
- the Aravallis, the Himalayas, the Satpura, and the Maikal of India
- Found on continental shelves and ocean floors.
- If the height of the mountains is considered from the ocean floor,
- Mauna Kea (9140 m) is the highest mountain when the height of the mountains is considered from the ocean floor.
- A dormant volcanic mountain in the Hawaii hotspot volcanic chain.
|SIGNIFICANCE OF MOUNTAINS|
- The mountains are a storehouse of water.
- Many rivers have their source in the glaciers of mountains.
- Three of the world’s major rivers, the Indus, the Ganges and the Tsangpo-Brahmaputra, all rise near Mount Kailashand cross and encircle the Himalayas.
- Water from the mountains is also used for irrigation and generation of hydro-electricity.
|SOME RELEVANT DEFINITIONS RELATED TO MOUNTAINS|
- Mountain Ridge
- Mountains originated as a result of local folding and faulting.
- Generally, the slope of one side of the ridge is steep and other side is moderate (In case of Himalayas, the southern slope is steeper compared to the northern slope).
- In some cases a ridge may have a symmetrical slope on both sides.
- Mountain Range
- It refers to a series of ridges which originated in the same age and underwent the same processes.
- The most prominent or characteristic feature of mountain ranges is their long and narrow extension.
- Himalayas are a mountain range with Himadri ridge, Himachal ridge, and Shiwalik ridge.
- Mountain System
- A group of mountain ranges formed in a single period, similar in their form, structure and extension, is termed a mountain system.
- Examples are the Basin Range of Nevada (USA), the Rocky mountain system of North America and the Appalachian.
- Mountain Chain
- It consists of mountain ranges which differ in size and periods of formation.
- It refers to highlands composed of different types of mountains, fold, block, or volcanic mountains although there is a proper arrangement of the mountains.
- Cordillera refers to several mountain groups and systems.
- Example = the Western Cordillera in the western part of the USA and in British Columbia of Canada.
|SOME DETAILS OF IMPORTANT MOUNTAIN RANGES|
- Longest continental mountain range in the world.
- World’s highest mountain range outside of Asia with an average height of 4000 meters.
- highest peak is Mount Aconcagua (6,962 m)
- volcanic origin, but now it’s dormant
- World’s highest volcanoes are in the Andes.
- Ojos del Salado (6,893 m),an active volcano, on the Chile-Argentina border is the highest volcano on earth.
- Mountain range forms a part of the American Cordillera.
- Formed due to Ocean-Continent collision.
- The rocks making up the mountains were formed before the mountains were raised.
- The Rocky Mountains took shape during an intense period of plate tectonic activity.
The Ural Mountains
- Their eastern side is usually considered the natural boundary between Europe and Asia.
- Since the 18th century, the mountains have been a major mineral base of Russia.
The Atlas Mountains
- Mountain range across the north-western stretch of Africa extending about 2,500 km through Algeria, Morocco and Tunisia.
- The highest peak is Toubkal (4,165 metres) in south-western Morocco.
- These mountains were formed when Africa and Europe collided.
- The Atlas ranges separate the Mediterranean and Atlantic coastlines from the Sahara Desert.
The Himalayan range
- It is home to the planet’s highest peaks, including the highest, Mount Everest.
- The Himalayas are bordered on the northwest by the Karakoram and Hindu Kush ranges, on the north by the Tibetan Plateau, and on the south by the Indo-Gangetic Plain.
- The mountains were formed as the African and Eurasian tectonic plates collided.
- Extreme folding caused by the event resulted in marine sedimentary rocks rising by thrusting and folding into high mountain peaks such as Mont Blanc (4,810 m) at French–Italian border.
- The Alpine region area contains about a hundred peaks higher than 4,000 meters, known as the four-thousanders.
Why world’s highest mountains are at the equator
- Mountain height depends more on ice and glacier coverage than tectonic forces.
- In colder climates, the snowline on mountains starts lower down causing erosion at lower altitudes. At low latitudes, the atmosphere is warm and the snowline is high.
- At cold locations far from the equator, erosion by snow and ice easily matched any growth due to the Earth’s plates crunching together.
- Hence, colder climates are better at eroding peaks.
- Exogenic (Exogenetic) processes are a direct result of stress induced in earth materials due to various forces that come into existence due to sun’s heat.
- Geomorphic Agent = Any exogenic element of nature (like water, ice, wind, etc.) capable of acquiring and transporting earth materials (mobile medium)
- The effects of most of the exogenic geomorphic processes are small and slow but it affect the rocks severely in the long run due to continued fatigue.
- All the exogenic geomorphic processes (Weathering, mass wasting/movements, erosion and transportation) are included in denudation.
- ‘Denude’ means to strip off or to uncover.
- Internal resisting force applied per unit area is called stress. The basic reason that leads to weathering, erosion and deposition is development of stresses in the body of the earth materials.
- Forces acting along the faces of earth materials are shear stresses (separating forces). It is this stress that breaks rocks and other earth materials.
- Temperature and precipitation are the two important climatic elements that control various processes by inducing stress in earth materials.
- Earth materials become subjected to molecular stresses caused due to temperature changes.
- Loosening of bonds between grains caused due to chemical processes.
- Mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
- An in-situ or on-site process (very little or no motion of materials takes place).
|3 MAJOR GROUPS OF WEATHERING PROCESSES|
- A group of weathering process acts on the rocks to decompose, dissolve or reduce them to a fine state.
- A group of weathering process acts on the rocks to chemically decompose, dissolve or reduce them to a fine state.
- Something is dissolved in water or acids.
- This phenomenon is depends upon the solubility of a mineral in water or weak acids.
- Soluble rock forming minerals like nitrates, sulfates, and potassium etc. are affected by this process.
- Easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
- Reaction of carbonate and bicarbonate with minerals.
- Carbonic acid form by absorbing Carbon Dioxide from the atmosphere that acts as a weak acid on various minerals (like limestone).
- Chemical addition of water.
- Minerals take up water and expand causing an increase in the volume of the material itself or rock.
- This process is reversible and long. Continued repetition causes fatigue leading to disintegration of rocks.
- The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration.
- Oxidation and reduction
- Oxidation is gain of oxygen. Reduction is loss of oxygen.
- Oxidation occurs where there is ready access to the atmosphere and water.
- A combination of a mineral with oxygen forms oxides (rusting in case of iron) or hydroxides.
- Reduction takes place on placement of oxidized minerals in an oxygen deficient environment.
- Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground.
- Red color of iron upon reduction turns to greenish or bluish grey.
- Water and air (oxygen and carbon dioxide) along with heat speed up all chemical reactions.
- These weathering processes are interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.
- Biological weathering is physical changes (removal of minerals and ions) due to growth or movement of organisms.
- New surfaces for chemical attack is exposed by
- Burrowing and wedging by organisms like earthworms, rodents etc.
- Human being by disturbing vegetation, ploughing and cultivating soils.
|PHYSICAL OR MECHANICAL|
- Depend on some applied forces like
- Gravitational forces.
- Expansion forces due to temperature changes, crystal growth or animal activity.
- Water pressures controlled by wetting and drying cycles.
|PHYSICAL OR MECHANICAL WEATHERING PROCESSES|
Unloading and Expansion
- Removal of overlying rock load because of continued erosion releases vertical pressure. It causes expansion of upper layers of the rock, resulting in disintegration of rock masses.
- In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock.
- Large, smooth rounded domes called exfoliation domes forms due to this process.
Exfoliation is the term used to describe the removal of sheets of rock from a rock’s surface due a range of physical and chemical processes during weathering.
- A form of weathering where the grains of a rock become loosened and fall out.
Exfoliation due to the Temperature Changes and Expansion
- Due to the diurnal changes in the temperatures, the surface layers of the rocks tend to expand more than the rock at depth that leads formation of stress within the rock.
- Due to this, surface layers undergo from exfoliation that result in smooth rounded surfaces in rocks known as exfoliated tors.
- This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic.
- This type of disintegration takes place in rocks with numerous joints acquired by shrinkage due to cooling or mountain-making pressures.
- This type of disintegration in rocks can be achieved by comparatively weaker forces.
- A huge rock may undergo disintegration along weak zones. This produces highly angular pieces with sharp corners and edges. The process is known as shattering.
Freezing, Thawing and Frost Wedging
- Water penetrates the pore spaces or fractures in rocks during the warm season. This water freezes into ice during the cold season and its volume expands as a result.
- This exerts tremendous pressure on rock walls to tear apart even where the rocks are massive.
- Frost weathering occurs due to growth of ice within pores and cracks of rocks during repeated cycles of freezing and melting.
- Salts in rocks expand due to thermal action, hydration and crystallisation.
- Many salts like calcium, sodium, magnesium, potassium and barium have a tendency to expand.
- High temperature ranges in deserts favor such salt expansion.
- Salt crystals in near-surface pores split the individual grains within rocks which eventually fall off.
- This process of falling off of individual grains may result in granular disintegration or granular foliation.
|EFFECTS OF WEATHERING|
- Weathering and erosion tend to level down the irregularities of landforms and create a peneplane (a more or less level land surface produced by erosion over a long period, undisturbed by crustal movement).
- The strong wind erosion leaves behind whale-back shaped rocks in arid landscape. These are calledinselberg(an isolated hill or mountain rising abruptly from a plain)or ruware.
- Sometimes, differential weathering of soft strata exposes the domelike hard rock masses, called tors. Tors are a common feature of South Indian landscape.
|WEATHERING AND EROSION|
- Weathering is a static process (disintegrated material do not involve any motion except the falling down under force of gravity).
- Erosion is a mobile process (involves motion)
|SIGNIFICANCE OF WEATHERING|
- The initial step of soils formation.
- Weathering helps in soil enrichment (enhancing certain valuable ores of iron, manganese, aluminium, copper etc.) making extraction of the same valuable material sufficient and economically viable.
- Mass movement, also called Mass Wasting, is a bulk movement of soil and rock debris down slopes under the influence of gravity.
- Mass of rock debris moves down the slope under the direct influence of gravity rather than air, water, or ice (but mass may carry with it air, water, ice).
- Weathering is not pre-requisite for mass movement though it aid mass movement (mass movement is more active on weathered slope).
- Mass movement does not come under erosion (no geomorphic agents participate in the mass movement process).
- Several activating causes precede mass movements
- Removal of support from below.
- Increase in gradient and height of slopes.
- Overloading through addition of materials.
- Heavy rainfall, saturation and lubrication of slope materials.
- Removal of material or load from above.
- Occurrence of earthquakes, explosions or machinery.
- Excessive natural seepage.
- Heavy drawdown of water from lakes, reservoirs and rivers leading to slow outflow of water from under the slopes or river banks.
- Indiscriminate removal of natural vegetation.
|TYPES OF MASS MOVEMENT PROCESSES|
We can use a classification that divides mass movement processes into two broad categories.
- Slow mass movements
- Rapid mass movements
- Earth flow
- Mud flow
- Debris avalanche
- Rock slide
- Debris Slide
These mass movements occur mainly due to two reasons
- Slope Failures – a sudden failure of the slope resulting in transport of debris downhill by sliding, rolling, falling, or slumping.
- Sediment Flows – debris flows down hill mixed with water or air.
- Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope.
- Debris falls are similar, except they involve a mixture of soil, regolith, vegetation, and rocks.
- The accumulation of rock debris at the base of a steep slope is called talus. Slumps (also called Rotational Slides)
- Types of slides wherein downward rotation of rock or regolith occurs along a concave-upward curved surface (rotational slides).
- The upper surface of each slump block remains relatively undisturbed, as do the individual blocks.
- Slumps leave arcuate scars or depressions on the hill slope.
- Heavy rains and earthquakes can also trigger slumps.
Slides (also called Translational Slides)
- Rock slides and debris slides result when rocks or debris slide down a pre-existing surface.
- Piles of talus are common at the base of a rock slide or debris slide.
- Slides differ from slumps in that there is no rotation of the sliding rock mass along a curved surface.
- A sediment flow is a mixture of rock, and/or regolith with some water or air.
- It occurs when sufficient force is applied to rocks and regolith that they begin to flow down slope.
- They can be broken into two types depending on the amount of water present.
- Slurry Flows (considered water-saturated flows)
- Solifluction – Fntaining water (slow process). These occur in areas where the soil remains saturated with water for long periods of time.
- Debris Flows – These occur at higher velocities than solifluction and often result from heavy rains causing saturation of the soil and regolith with water.
- Mudflows – These are a highly fluid, high velocity mixture of sediment and water that has a consistency ranging between soup-like and wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Volcanic mudflows are often referred to as lahars.
- Granular Flows (not saturated with water)
- Creep – The very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles.
- Earthflows – Usually associated with heavy rains and move at velocities between several cm/yr and 100s of m/day. They usually remain active for long periods of time.
- Grain Flows – Usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope.
- Debris Avalanches – These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions.
- Snow Avalanches are similar to debris avalanches, but involve only snow, and are much more common than debris avalanches.
|EROSION AND DEPOSION|
- Erosion involves acquisition and transportation of rock debris.
- Denudational processes like erosion and transportation are controlled by kinetic energy.
- The erosion and transportation of earth materials is brought about by wind, running water, glaciers, waves and ground water.
- The first three agents are controlled by climatic conditions and represent three states of matter
- Solid (glacier)
- Liquid (running water)
- Gaseous (wind)
- The work of the other two agents of erosion — waves and ground water — is not controlled by climate.
- Waves = Location along the interface of litho and hydro sphere (coastal region) determine the work of waves
- Ground water = Determined by the lithological character of the region (karst topography develops in thepermeable and soluble rocks)
- The landforms produced by each of these agents of erosion.
- Deposition is a consequence of erosion.
- erosional agents loose their velocity The and hence energy on gentler slopes.
- Due to this, the materials carried by them start to settle themselves.
- In other words, deposition is not actually the work of any agent.
- The same erosional agents viz., running water, glaciers, wind, waves and groundwater act as aggradational or depositional agents also.
This erosion of the materials is carried in following ways
- Hydraulic Action: This is the mechanical loosening that sweeps away the materials by the sheer force or river water itself. No load or material is involved in this process.
- Corrosion or Solution: This is the chemical or solvent action of water on soluble or partly soluble rocks while coming in contact with them.
- Attrition:A form of fluvial erosion in which the bed load is eroded by itself when the transported material rolls and collides into one another. The coarser boulders are broken down into smaller stones and pebbles.
- Corrasion or Abration: The rock particles break off additional rock fragments when they bounce, scrape and drag along the bottom and sides of the river. They are two types:
- Vertical corrosion which acts downward
- Lateral corrosion which acts on both sides
This transportation of eroded materials is carried in four ways:
- Traction: The heavier and larger rock fragments like gravels, pebbles etc are forced by the flow of the river to roll along its bed. This process is called as traction and the load transported in this way are called traction load.
- Saltation: Some of the fragments of the rocks move along the bed of a stream by jumping or bouncing continuously. This process is called as saltation.
- Suspension: The holding up of small particles of sand, silt and mud by the water as the stream flows is called suspension.
- Solution: Some parts of the rock fragments dissolved in the river water and transported. This type of transportation is called solution transportation.
- A glacier is a large mass of ice that is persistently moving under its own weight over the land or as linear flows down the slopes of mountains in broad trough-like valleys.
- Glaciers move under the influence of the force of gravity.
- It generally gives rise to erosional features in the highlands and depositional features on lowlands.
- The movement of glaciers is slow, unlike water flow. Glaciers flow like very slow rivers.
- It erodes its valley by two processes viz. plucking & abrasion.
- Plucking→Occurs when rocks and stones become frozen to the base or sides of the glacier and are plucked from the ground or rock face as the glacier moves. This leaves behind a jagged landscape.
- Abrasion→Occurs when rocks and stones become embedded in the base and sides of the glacier. These are then rubbed against the bedrock (at the bottom of the glacier) and rock faces (at the sides of the glacier) as the glacier moves.
- Moraines are accumulations of dirt and rocks that have fallen onto the glacier surface or have been pushed along by the glacier as it moves.
- The dirt and rocks composing moraines can range in size from powdery silt to large rocks and boulders.
- A receding glacier can leave behind moraines that are visible long after the glacier retreats.
- Ground moraine = Deposited beneath glaciers, widespread beneath continental glaciers, very poorly drained area with many closed depressions.
- End or terminal moraine = Deposited as a ridge along the edge of a stationary glacier.
- Lateral moraine = Valley-side debris accumulated along the sides of a mountain valley glacier.
- Medial moraine = Formed by the confluence of two lateral moraines as tributary mountain glaciers merge.
- A crevasse is simply a deep crack in a glacier or ice sheet caused by changing stresses as ice moves.
- An arête is a thin, crest of rock left after two adjacent glaciers have worn a steep ridge into the rock.
- A horn(pyramidal peak)results when glaciers erode three or more arêtes, usually forming a sharp-edged peak.
- Example = The Matterhorn (highest peak in the Alps) and the Everest (highest peak in the Himalayas)
- Cirques (or corrie or cwm) are concave, circular basins carved by the base of a glacier as it erodes the landscape.
- A lake of water can be seen frequently inside the cirques after the glacier vanishes. Such lakes are calledCirque or Tarn lakes.
- At the head of a glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a Bergschrund or Rimaye.
- This happens in summer when although the ice continues to move out of the corrie, there is no new snow to replace it.
U-shape glacier trough
- U-shaped valleys, trough valleys or glacial troughs, are formed by the process of glaciation.
- They are characteristic of mountain glaciation in particular.
- They have a characteristic U shape, with steep, straight sides and a flat or rounded bottom.
- A fjord is formed when a glacier retreats, after carving its typical U-shaped valley, and the sea fills the resulting valley floor.
- This forms a narrow, steep sided inlet connected to the sea.
- A small mountain glacier may join a larger valley glacier, just as a stream may join a larger river.
- The smaller glacier, however, may not be as deep as the main one, and its base may be higher in elevation than the main glacier’s base.
- A Roche moutonnée called as
- It is a rock formation created by the passing of a glacier over underlying bedrock.
- Results in asymmetric erosional
Crag and tail
- When a more resistant rock persists in the passage of glaciers, the upstream side is smoothened by abrasion and it’s downstream side is roughened & steepened by plucking called Roche moutonnee.
- A tadpole-shaped landform developed by glacial erosion of rocks on unequal resistance.
- It is a larger rock mass than a Roche moutonnee.
- Crag is a mass of hard rock with a steep slope on the upward side.
- The tail is formed in softer rocks.
- Drumlins are elongated, teardrop-shaped hills of rock, sand, and gravel that formed under moving glacier ice.
- Steeper slope at upstream & gentle slope along downstream.
- One end of the drumlins facing the glacier called the stoss end is blunter and steeper than the other end called tail.
- Drumlins give an indication of the direction of glacier movement.
- The stoss end gets blunted due to pushing by moving ice.
DRUMLINS VS ROCHE MOUNTONNE
- Type of landform
- “Drumlins” are glacial depositional landforms, whereas “Roche moutonnee”are glacial erosional landform.
- Place of origin
- “Drumlins” are formed in outwash plains at the foothills of the mountainous area, whereas “Roche moutonnee” are formed at a high altitude compared to drumlins.
- Type of surface
- “Drumlins” have smooth surface at both upstream and downstream side, whereas “Roche moutonnee” have smooth surface at upstream side and rough surface at downstream side.
Boulder clay and glacial till
- Boulder clay containing many large stones and boulders, formed by deposition from melting glaciers and ice sheets.
- Glacial erratics are stones and rocks that were transported by a glacier, and then left behind after the glacier melted.
- Erratics can be carried for hundreds of kilometers, and can range in size from pebbles to large boulders.
- They are called erratics bucause they are composed of materials entirely different from those of the region in which they are found.
- The large quantities of water that flowed from the melting ice deposited various kinds of materials, the most important of which is called glacial outwash.
- Outwash plains made up of outwash deposits are characteristically flat and consist of layers of sand and other fine sediments.
- It is also known as called a sandur.
- Such plains with their sandy soils are often used for specialized kinds of agriculture.
- Glaciers melting in summer –> Water flow on the surface of the ice or seeps down along the margins or even moves through holes in the ice.
- These waters accumulate beneath the glacier and flow like streams in a channel beneath the ice.
- Such streams flow over the ground (not in a valley cut in the ground) with ice forming its banks.
- Very coarse materials like boulders and blocks along with some minor fractions of rock debris carried into this stream settle in the valley of ice beneath the glacier.
- After the ice melts, these subglacial river deposit can be found as a sinuous ridge called esker. .
- The landforms created as a result of degradational action (erosion) or aggradational work (deposition) of running water is called fluvial landforms.
- The fluvial processes may be divided into three physical phases – erosion, transportation and deposition.
- Most of the erosional landforms made by running water are associated with vigorous and youthful rivers flowing over steep gradients. With time, The stream channels over steep gradients turn gentler due to continued erosion with time. As a consequence, it loses the velocity, facilitating active deposition.
Running water effects of landforms: Erosion, Transportation, and Deposition
- Erosion occurs when overland flow moves soil particles and rock materials downslope.
- These particles and materials carried by erosion is the load of the river.
- This load acts as a grinding tool. It hels in cutting the bottom and sides of the river bed. This results in deepening and widening of the river channel.
- A valleyis a low area between hills or mountains typically with a river running through it.
- Evolution = Rills –> Gully –> Valley
- A rillis a shallow channel in some soil, created by the erosion of flowing water.
- Rillscan generally be easily removed by tilling the soil. When rills get large enough that they cannot easily be removed, they’re known as gullies.
- There are many types of valleys like V-shaped valley, gorge, canyon, etc. depending upon dimensions and shape.
- Valley types depend upon the type and structure of rocks in which they form. For example, canyons commonly form in horizontal bedded sedimentary rocks and gorges form in hard rocks.
- A gorge is almost equal in width at its top as well as its bottom
- A canyon is wider at its top than at its bottom (a variant of gorge).
- Potholes are more or less circular depressions over the rocky beds of hills streams.
- The pebbles collected in the small and shallow depression rotates due to the flowing water causing growth of a depressions to form potholes.
- Plunge pools are nothing but large, deep potholes commonly found at the foot of a waterfall.
- They are formed because of the sheer impact of water and rotation of boulders.
- When a river runs over alternating layers of hard and soft rock (creating unequal resistance), rapids and waterfalls may form.
- The outcrop of a band of hard rocks may cause a river to jump or fall down stream.
- Waterfalls often form in the upper stages of a river where it flows over different bands of rock. It erodes soft rock more quickly than hard rock and this may lead to the creation of a waterfall.
- Formation of a waterfall:
- The soft rock erodes more quickly, undercutting the hard rock.
- The hard rock is left overhanging. It eventually collapses due to the absence of any support.
- The fallen rocks crash into the plunge pool. They swirl around, causing more erosion.
- Over time, this process is repeated and the waterfall moves upstream.
A steep-sided gorge is formed as the waterfall retreats.
- Incised meanders are meanders which are particularly well developed and occur when a river’s base level has fallen giving the river a large amount of vertical erosion power, allowing it to downcut.
- There are two types of incised meanders, entrenched meanders and ingrown
- Entrenched meanders are symmetrical and form when the river downcuts particularly quickly. Due to the speed which the river downcuts, there is little opportunity for lateral erosion to occur giving them their symmetrical shape.
- Ingrown meanders are asymmetrical. They form when the river downcuts at a less rapid pace, giving the river opportunity to erode laterally as well as vertically.
- A rejuvenating river can erode vertically into the former flood plain to produce features called river terraces.
- If vertical erosion is rapid then paired terraces are formed either side of the channel.
- If vertical erosion is slower though, unpaired terraces form as the river is given opportunity to meander.
- River terraces are particularly useful for settlements as they provide flat areas above the present floodplain.
- An alluvial fan is a wide fan shaped deposit by a river.
- An alluvial fan is formed when a river flows out of a mountain valley.
- The river becomes wider and more shallow making it be slower and the sediment is deposited in a fan shape.
- Deltas are like alluvial fans (i.e. a depositional feature) but develop at a different location .
- Unlike in alluvial fans, the deposits making up deltas are very well sorted with clear stratification.
- The coarsest materials settle out first and the finer fractions like silts and clays are carried out into the sea.
- Deltas are found at the mouth of large rivers – for example, the Ganges delta.
- The load carried by the rivers is dumped and spread into the sea.
- As the delta grows, the river distributaries continue to increase in length and delta continues to build up into the sea.
- There are three main types of delta, named after the shape they create.
- Arcuate or fan-shaped – the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect.
- Examples: Nile, Ganga, Indus
- Cuspate – the land around the mouth of the river juts out arrow-like into the sea.
- Example: Tiber river on west coast of Italy.
- Bird’s foot – the river splits on the way to the sea, each part of the river juts out into the sea, rather like a bird’s foot.
- Arcuate or fan-shaped – the land around the river mouth arches out into the sea and the river splits many times on the way to the sea, creating a fan effect.
- A floodplain is the area around a river that is covered in times of flood.
- The rivers have a wide floodplain in the lower course.
- It is a depositional feature where large sized materials are deposited first when the river enters the gentle slope.
- Active (near to the river) and Inactive Flood plains (receded part-away from the river) are seen.
- Sometimes in inactive floodplains, cut off streams can be seen.
- The flood plains in a delta are called delta plains.
- A floodplain is a very fertile area due to the rich alluvium deposited by floodwaters.
- This makes floodplains a good place for agriculture.
- A build up of alluvium on the banks of a river can create levees, which raise the river bank.
- Levees are natural embankments which are formed when a river floods.
- When a river floods friction with the floodplain leads to a rapid decrease in the velocity of the river (causing decrease in the capacity to transport material), larger material is deposited closest to the river bank. This often leads to large,raised mounds being formed.
- Smaller material is deposited further away and leads to the formation of gently sloping sides of the levees.
- These act as a natural protection againstfloods.A breach in a levee causessudden floods in adjoining areaslike in the case of the Hwang-He or Yellow riverof China (China’s sorrow).
Point bars or meander bars
- They are sediments deposited in linear fashion by the flowing rivers.
- If there are more than one ridge, narrow and elongated depressions are found in between the point bars.
- As the rivers build point bars on the convex side, the banks on the concave side will erode actively.
- These are thread-like streamsof water rejoin and subdivide repeatedly to give a typical braided pattern.
- A braided channel is one that is divided into smaller channels by temporary islands called eyots.
- Braided channels tend to form in rivers that have a significant amount of sedimentary load, a steep profile and where discharge regularly fluctuates.
- When the river’s carrying capacity is exceeded the river deposits, its load into the channel and eyots
Meanders (Lateral erosion)
- As the river erodes laterally, to the right side then the left side, it forms large bends, and then horseshoe-like loops called meanders.
- In large flood and delta plains, rivers rarely flow in straight courses. Loop-like channel patterns called meanders develop over flood and delta plains.
- The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream.
- Outside of the bend where water flow has most energy due to decreased friction, the force of the water erodes and undercuts the river bank.
- Inside of the bend, river flow is slower due to the more friction leading deposition of material.
- An oxbow lake is a meander that is no longer attached to the river.
- The water has to find a straighter route downstream during the floods, so the water flows over the ends of the meander.
- As the flood starts to go down, the water deposits sediment and covers up the ends of the meander making an oxbow lake.
|UPPER COURSE RIVER FEATURES (YOUTH STAGE = EROSION DOMINATES)|
- It starts from the source of the river in hilly or mountainous areas.
- As the river moves through the upper course, it cuts downwards.
- The gradient (slope) here is steep and the river channel is narrow.(image)
- Vertical erosion in this highland part of the river helps to create steep-sided V-shaped valleys, interlocking spurs, rapids, waterfalls and gorges
|MIDDLE COURSE (MATURE STAGE = TRANSPORTATION DOMINATES)|
- In the middle course the river has more energy and a high volume of water.
- In this stage, vertical erosion slowly starts to replace with lateral erosion (sideways). This causes widening of the river channel.
- The river channel has also deepened.
- A larger river channel means there is less friction, so the water flows faster.
- Landforms like alluvial fans, piedmont alluvial plains, meanders etc. can be seen at this stage.
|LOWER COURSE FEATURES (OLD STAGE = DEPOSITION DOMINATES)|
- In the lower course, the river has a high volume and a large discharge.
- Heavy debris brought down from upper and middle courses.
- Vertical erosion has almost stopped but lateral erosion still goes on.
- The work of the river is mainly deposition. This builds up its bed and form an extensive flood plain.
- Landforms like braided channels, floodplains, levees, meanders, oxbow lakes, deltas etc. can be seen at this stage
- A karst landform is a geological feature created on the earth’s surface by the drainage of water into the ground resulting chemical erosion of bedrock.
- For this reason, the development of karst landforms is limited to areas where comparatively soluble rocks — principally limestone — exist.
- Approximately 8% of the earth’s land surface is karst terrain.
- A landform consisting of a flat, incised surface of exposed limestone that resembles an artificial pavement.
Features of limestone pavements
Clint: Section of a limestone pavement separated from adjacent sections by grikes.
- Grike: Vertical crack that develops along a joint in limestone.
- Karren: small hollow that forms on the surface of a limestone clint.
- Limestone caverns and caves are large sub-surface voids where the rocks have been dissolved by carbonation.
- Calcium carbonate precipitates out of the saturated carbonate solution and accumulates as deposits.
- Stalactites: Deposits that grow from the ceiling downward.
- Stalagmites:Deposits that grow from the ground up.
- Column: Forms when the stalactite and stalagmites get attached forming a pillar.
- Collapsed/depressed limestone features that develop in karst landscapes.
- The ground water slowly dissolves the limestone rock below the surface until it eventually becomes unstable and collapses creating local depressional features.
- Uvala: Collection of multiple smaller individual sinkholes that coalesce into a compound sinkhole.
- The swallow holes are cylindrical tunnel-like holes lying underneath the sinkhole at some depth.
- The surface streams which sink disappear underground through swallow holes because these are linked with underground caves through vertical shafts.
- The open, broad area formation due to thecollapse of adjoining multiple sink holes is called a karst window.
- A number of uvalas may coalesce to create a valley called polje which is actually a flat-floored depression.
- If the streams lose themselves in these valleys, then these are called blind valleys. These valleys may have surface streams and may be used for agriculture.
- 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.
|DRY VALLEY/BOURNE/HANGING VALLEY|
- Sometimes, a stream cuts through an impermeable layer to reach a limestone bed. It erodes so much that it goes very deep. The water table is also lowered. Now the tributaries start serving the subterranean drainage and get dried up. These are dry valleys or bournes.
- Lack of adequate quantities of water and reduced erosion leaves them hanging at a height from the main valley. Thus, they are also referred to as hanging valleys.
- These are curved relicts of limestone rocks after erosion.
- The Coastal Landforms are formed by the constant action of the waves, tides, and currents.
- The coastline changes the coastal landforms under the influence of these denudational agents and gives shapes to various types of marine landform features.
|AGENT OF EROSION|
- Most of the changes along the coasts are accomplished by waves.
- Constant impact of breaking waves drastically affects the coasts.
- When waves break, the water is thrown with great force onto the shore along with a great churning of sediments on the sea bottom.
- Storm waves and tsunami waves can cause far-reaching changes in a short period of time than normal breaking waves.
- Other than the action of waves, the coastal landforms depend upon
- the configuration of land and sea floor
- whether the coast is advancing (emerging) seaward or retreating (submerging) landward
- Assuming sea level to be constant, two types of coasts are considered to explain the concept of evolution of coastal landforms:
- High, rocky coasts (submerged coasts)
- The sea will be very close to the land with a narrow coast or no coast.
- Erosional features are dominant here.
- The shores of these high rocky coasts do not show any depositional landforms.
- Wave-cut platforms, cliffs, sea caves etc. are common here.
- Most of the west coasts of the Indian Peninsula belong to this category.
- Low, smooth and gently sloping sedimentary coasts (emerged coasts).
- The length of these coasts is extended by the rivers by building coastal plains and deltas.
- Depositional features are dominant here.
- Bars, Barriers, spits, lagoons etc are common on these coasts.
- Most of the east coasts of the India Peninsula are of this category.
- High, rocky coasts (submerged coasts)
|MARINE EROSIONAL LANDFORMS|
- These are narrow, deep indentations (a deep recess or notch on the edge or surface of something).
- It is carved by wave actionthrough headward erosion (downcutting) of weak vertical planes in the rocks.
- With time, further headward erosion is hindered by lateral erosion of chasm mouth, which itself keeps widening till a bay is formed.
Cliff erosion and wave-cut platforms
- A very steep rock face adjoining the coast forms a cliff.
- Their steep nature is the result of wave-induced erosion near sea level and the subsequent collapse of rocks at a higher elevation.
- At the base of the cliff, the sea cuts a notch, which gradually undermines the cliff so that it collapses.
- The best-known cliffs are the Chalk cliffs of the English channel and the White Cliffs of Dover
Stages in cliff retreat
- Waves attack the bottom of the cliff, particularly during storms and at high tide, forming a wave-cut notch.
- At the same time weathering attacks and weakens the top of the cliff.
- The weakened cliff is left unsupported and eventually collapses.
- Once the sea has removed the fallen rocks it can start the process again.
- The cliff will move back and leave a rocky platform at the base called a wave-cut platform.
- Prolonged attack of waves against the base of the cliff and the rock debris that gets smashed against the cliff along with lashing waves create holes in regions of weakness.
- These holes get further widened and deepened to form sea caves.
- Example Flamborough head, England
- When two caves approach one another from either side of a headland and unite, they form a bridge like structure, known as arch.
- Sea cave –> || <– Sea cave
- These archways may have an arcuate or rectangular shape, with the opening extending below water level.
- Example the Neddle Eye near Wick, Scotland
- Continued erosion, under the attack of the wave, can result in the total collapse of an arch.
- The seaward portion of headland will remain as an isolated pillar of rock known as stack.
- Like all other features, sea stacks is also temporary and eventually, the stack will also disappear
- The stack is gradually eroded, leaving behind only the stump.
- Stumps are only just visible above the sea level.
- When sea caves grow towards the land and upwards creating a vertical shaft that exposed on the surface, it results in a blowhole.
- Water often gushes out at the top part of the landform when waves move to the sea cave with significant force.
- Example Holborn Head, Scotland
|MARINE DEPOSITIONAL LANDFORMS|
- Beaches are temporary features covering the rock debris on or along a wave-cut platform.
- These are characteristic of shorelines that are dominated by deposition but may occur as patches along even the rugged shores.
- Sands and gravels loosened from the land are moved by waves to be deposited along the shore as beaches.
- Most of the sediment making up the beaches comes from land carried by the streams and rivers or from wave erosion.
- Most of the beaches are made up of sand sized materials. Beaches called shingle beaches contain excessively small pebbles and even cobbles.
- Just behind the beach, the coastal sands lifted and winnowed from over the beach surfaces will be deposited as sand dunes.
- On shore, winds play a major part in the formation of these dunes
- Sand dunes forming long ridges parallel to the coastline are very common along low sedimentary coasts.
- Sand dunes are common in the coasts of Belgium, Denmark and the Netherlands
- When a ridge of sand and shingle formed in the sea in the off-shore zone (from the position of low tide waterline to seaward), it is called a bar.
- The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay.
- Bars are submerged features.
- When bars show up above water, they are called barrier bars.
- Generally, bars are approximately parallel to the coast.
- An off-shore bar which is exposed due to further addition of sand is termed a barrier bar.
- The off-shore bars and barriers commonly form across the mouth of a river or at the entrance of a bay.
- They usually occur in chains.
- They are subject to change during storms and other action, but absorb energy and protect the coastlines and create areas of protected waters where wetlands may flourish.
Spit and Hook
- Barrier bar which gets keyed up to the headland of a bay is called a spit.
- Spits are projected depositional landforms with one end attached to the land and the other end projecting into the sea.
- Spits may also develop attached to headlands/hills.
- The mode of formation of spit is similar to a bar or barrier.
- A shorter spit with one end curved towards the land is called a hook.
- When barrier bars and spits form at the mouth of a bay and block it, a lagoon
- The lagoons would gradually get filled up by sediments from the land giving rise to a coastal plain.
- Tombolo joins two landmasses by a connecting bar.
- The tombolo is a deposition landform in which an island is attached to the mainland by a narrow piece of lands such as a spit or bar.
- A tombolo is a sandy isthmus.
- Example = Tombolo can be found in Chesil beach in England which links the Isle of Portland with mainland.
|ARID EROSIONAL LANDFORMS|
Mechanism of Arid Erosion
- Lifting and blowing away of loose materials from the ground.
- It results in the lowering of the land surface to form large depressions called deflation hollows.
- The sand-blasting of rock surfaces by winds when they hurl sand particles against them is called abrasion.
- Abrasion is most effective at or near the base of rocks, where the amount of maternal the wind is able to carry is greatest.
- Wind borne particles wear each other away when they roll against one another in collision.
- This process is known as attrition. The sizes of particles reduce by this and the grains are rounded into millet seed sand.
- Slightly larger particles move by rolling or bouncing on the ground.
|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.
- Rillscan generally be easily removed by tilling the soil. When rills get large enough that they cannot easily be removed, they’re known as gullies.
- 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.
- Badlands are a type of dry terrain where softer sedimentary rocks and clay-rich soils have been extensively eroded by wind and water.
- They are characterized by steep slopes, minimal vegetation, lack of a substantial regolith, and high drainage density.
- The intermontane basins in arid or semiarid areas are generally called bolsons.
- Such basins are characterized by 3 unique landforms which from the mountain front downward are pediments, bajadas and playas.
- Numerous ephemeral streams after originating from the surrounding mountain fronts drain into the bolsons. These temporary lakes are called playas.
- After the evaporation of water, salt-covered playas are called salinas.
- The playa plain covered up by salts is called alkali flats.
- Pediments situated between mountain front and bajada in intermontane basin.
- These are broad, extensive, and gently sloping areas of rockcut surfaces which spread as aprons around the bases of mountains.
- There is no difference between a pediment and an alluvial fan in form and function but pediment is an erosional landform while a fan is a constructional one.
- Gently sloping depositionalplain between pediments and playa is called bajada.
- Bajada is formed due to coalescence of several alluvial fans. Thus, bajada is a wholly depositional feature.
- A pediplain is an extensive flat terrain formed by the coalescence of pediments.
- The steep wash slope and free face of pediments retreat backward after its formation.
- Through parallel retreat of slopes, the pediments extend backwards at the expense of mountain front.
- Gradually, the mountain gets reduced leaving an inselberg (a remnant of the mountain).
- That’s how the high relief in desert areas is reduced to low featureless plains called pediplains.
|WIND ERODED AND DEPOSITIONAL ARID LANDFORMS|
- The wind or Aeolian erosion takes place in the following ways, viz. deflation, abrasion, and attrition.
- 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.
- Formation of rock pillars shaped like a mushroom with narrow pillars and broad top surfaces.
- In deserts, a greater amount of sand and rock particles are transported close to the ground by the winds which cause more bottom erosion in overlying rocks than the top.
Monadnock or Inselbergs
- Monadnock or inselberg is an isolated rock hill, knob, ridge, 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.
- Zeugen is a flat-topped rock masses resembling a capped inkpot.
- It stand on softer rock pedestals like mudstone, shale, etc.
- Zeugens are formed in desert areas characterized by a high range of temperature.
- The alternate freeze and thaw of moisture results in expansion and contraction which ultimately disintegrates rocks along the joints.
- Sculpted landforms, called yardangs, are up to tens of meters high and kilometers long and are forms that have been streamlined by desert winds.
- The famous sphinx at Giza in Egypt may be a modified yardang.
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.
- Ripple marks are sedimentary structures and indicate agitation by water (current or waves) or wind.
- These are ridges of sediment that form in response to wind blowing along a layer of sediment.
- Ripple marks are generally depositional features (formed at a fluid/sediment interface)
- Heaps or mounds of sands are generally called sand dunes or simply dunes.
- Though sand dunes are significant depositional features of desert areas but they are also formed in all those areas where sands are available in profusion and wind is capable of transporting and depositing them in suitable areas.
- Barchan dunes have a crescentic shape with two horns.
- The windward side in convex while the leeward is concave and steep.
- Transverse Dunes are dunes deposited transverse to the prevailing wind direction.
- Longitudinal Dunes are formed parallel to the wind movement.
- In other words, these dunes are formed by a modification in the shape of the transverse dunes.
- They look much like Barchan, but accumulation of sand and formation is opposite to Barchan.
- Some shrubs are also found growing on them.
- They surface of all these types of dunes in marked by ripples.
- It is similar to barchan with a small difference.
- Seif has only one wing or point. This happens when there is shift in wind conditions.
- The lone wings of seifs can grow very long and high.
- It has a high central peak, radically extending three or four arms.
- These are formed when winds blow from opposite direction and are balanced in strength and duration.
- When the longitudinal dunes migrate, the coarser sands are left behind to form whaleback dunes.
- Very large whaleback dunes are known as draas.
- The surface covered by deposits of wind-transported silt that has settled out from dust storms over many thousands of years.
- A flat-topped tableland with steep descent to the surrounding lowland.
- It covers about one-third area of the Earth’s 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 north-western USA) is cut through by the Columbia River.
- Sometimes, a plateau is so eroded that it is broken up into smaller raised sections.
- 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. This is why many of the mining areas in the world are located in the plateau areas.
|TYPES OF PLATEAU|
- Forms as a result of upward movement in the Earth’s crust.
- The uplift is caused by the slow collision of tectonic plates.
- Tibetan plateau
- Formed by basaltic lava which comes from the interior of Earth’s crust and spread over its surface to form successive sheets.
- The Columbia Plateau (USA)
- Deccan Traps
- Plateau that are enclosed by fold mountains.
- These plateau some of the highest and most extensive plateau in the world.
- Tibetan plateau (between the Himalayas and Kunlun)
- Mongolian plateau
- These plateaus are evolved by denudation.
- The Colorado Plateau (Western USA)
- These are bordered on all sides by the plains or seas, forming away from mountains.
- Antarctic Plateau in East Antarctica.
- Bordered by ocean or sea on all sides.
- Mid-Pacific Mountain
- These are bordered by a mountain on one side and a plain or ocean on the other side.
- Pantagonian Plateau (South America
|MAJOR PLATEAUS OF THE WORLD|
- It is the world’s largest and highest plateau.
- The Tibetan Plateau lies between the Himalayas to the south and Taklamakan desert to the north.
- Share between China, Nepal, India, Bhutan, Tajikistan and Kyrgyzstan.
- It is sometimes termed the Third Pole given its ice fields contain the largest reserve of freshwater outside the polar regions.
Plateau of Arabia
- Westernmost part of Asia.
- Very high Aridity due to low Rainfall.
- Rich petroleum reserves.
- It is known as Roof of the world.
- Pamirs Plateau is located in West China, extending across Tajikistan, China and Afghanistan.
- At the junction of the Himalayas (to south east) with the Tian Shan (North East), Karakoram (south east), Kunlun (east), Hindu Kush(west) and Suleiman (south).
- Largest intermontane plateau of the continent.
- The Basin forms an area of inland drainage for rivers.
- Lies in United States, covering state of Nevada Utah, California etc.
- Colorado river and its tributaries have deep cuts in the soft rocks of the region.
- Such deep cuts have formed canyons and Grand canyon is the largest of them.
- Mostly lie in USA.
- It lies between Rockies and cascade range.
- It is a Basaltic Lava Plateau.
- It has rich mineral resources.
- It lies in Ethiopia.
- It is the highest Plateau of Africa.
- It is volcanic in nature.
- Highest peak is Ras Dashan (4,620m).
- Cooler despite close to equator.
East African plateau
- It is shared between Uganda, Kenya and Tanzania.
- Multiple rift valleys and lakes.
- Lies in Democratic Republic of Congo.
- Rich in mineral resources like Cobalt, Copper, and Diamond etc.
- Lies in Cameroon, Nigeria and central African Republic.
- Savannah vegetation.
- Bauxite deposits.
- It occupies nearly two-thirds of the continent.
- Most of the plateau is a desert or semi-desert.
- Rich in Minerals such as gold and iron ore.
- Kimberly plateau is an example of western highlands.
- 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.
- 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.
- Plateau in the Indian Ocean.
- It extends between the Seychelles and Mauritius Islands.
- Deccan Plateau is a large plateau which covers the majority of the southern part of the India.
- It is bordered by two mountain ranges, the Western Ghats on the west of the plateau and the Eastern Ghats on the east of the plateau.
- The Plateau is higher in the west and slopes gently eastwards. This is why most Deccan plateau rivers flow from west to east.
- The plateau includes the Deccan Traps which is the largest volcanic feature on Earth.
- It is made of multiple basaltic lava layers containing some unique fossils and minerals.
We will read about the Deccan Plateau in detail in Indian Physical Geography Section.
- A plain is an area of lowland, either level or undulating.
- It rarely raises more than few hundred feet above sea level.
- It possesses high concentration of human habitation.
Classification of plains
Plains can be classified in following types on the basis of their mode of formation
- Structural plain
- Erosional plains
- Depositional plains
- Structurally depressed area of the world.
- Formed by horizontally bedded rocks, relatively undisturbed by the crustal movement of the earth.
- Plains that are carved by the erosional agents.
- Agents of erosion smooth out the irregularities of the Earth’s surface.
- The surface of such plains is hardly smooth. This is why these plains are also called as Peneplains, which means almost plain.
- These plains are formed by the deposition of materials brought by various agents of transportation.
- Plains are formed by the river deposits are calledriverine or alluvial plains.
- The depositions of sediments in a lake give rise to a Lacustrine Plain or Lake Plains.
- Example: The Valley of Kashmir
- Plains are formed by glacial deposits are called Glacial or Drift Plains.
- Loess Plains form when the wind acts as the major depositional agent.