• CONTINENTAL DRIFT THEORY
• CONVECTIONAL CURRENT THEORY
• SEA FLOOR SPREADING THEORY
• THE THEORY OF PLATE TECTONICS

EARTH’S DYNAMIC SURFACE – THEORIES

1.1 Continental Drift – Introduction

• Continental drift is the theory that the Earth’s continents have moved over geologic time relative to each other, thus appearing to have “drifted” across the ocean bed.
• The speculation that continents might have ‘drifted’ was first put forward by Abraham Ortelius in 1596.
• The concept was independently and more fully developed by Alfred Wegener in 1912, but his theory was rejected by many for lack of any motive mechanism.
• A sea called Tethys divided the Pangaea into two huge landmasses: Laurentia (Laurasia) to the north and Gondwanaland to the south of Tethys.
• Drift started around 200 million years ago (Mesozoic Era), and the continents began to break up and drift away from one another.

1.2 Force for Continental Drift

Continents were drifted equator wards and westwards.

• Equator wards due to the interaction of forces of gravity, pole-fleeing force and buoyancy (ship floats in water due to buoyant force offered by water), and
• westwards due to tidal currents because of the earth’s motion (earth rotates from west to east, so tidal currents act from east to west.).

1.3 Evidences in support of Continental Drift

The Matching of Continents (Jig-Saw-Fit) He found the continental margins of the subequatorial portions of Africa and South America fit together with jigsaw- puzzle-like precision.

Rocks of Same Age across the Oceans He also determined that the petrologic (rock) records on both sides of the Atlantic show many distributions—such as ancient coal deposits—that would be continuous if the ocean did not intervene.

Tillite Glacial deposits that indicated that large portions of the southern continents and India were extensively glaciated about 300 million years ago.

Placer Deposits The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it is obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.

Distribution of Fossils Supporting evidence came from paleontology: the fossils of some dinosaur and other reptile species, such as the freshwater swimming reptile the Mesosaurus, are found on both sides of the southern Atlantic Ocean, but nowhere else in the world . Fossilized plants, such as the fernlike Glossopteris, are found in similar-aged rocks in South America, South Africa, Australia, India, and Antarctica—its seeds too large and heavy to have been carried across the expanse of the present-day oceans by wind.

Rejection of Continental Drift Despite the vast amount of evidence Wegener presented, most scientists felt that two difficulties made the theory improbable if not impossible:

(1) Earth’s crust was believed to be too rigid to permit such large-scale motions—after all, how could solid rock plow through solid rock?

(2) Further, Wegener did not offer a suitable mechanism that could displace such large masses for a long journey. For these reasons, most Earth scientists ignored or even debunked the idea of continental drift for the better part of half a century after Wegener’s theory was presented.

2. CONVECTIONAL CURRENT THEORY Arthur Holmes in 1930s discussed the possibility of convection currents operating in the mantle portion. These currents are generated due to radioactive elements causing thermal differences in the mantle portion. Holmes argued that there exists a system of such currents in the entire mantle portion. This was an attempt to provide an explanation to the issue of force, on the basis of which contemporary scientists discarded the continental drift theory.

3. SEA FLOOR SPREADING THEORY

(i) Along the mid oceanic ridges, volcanic eruptions are common. (ii) The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the mid-oceanic ridges have normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest. (iii) The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old. Some of the continental rock formations are as old as 3,200 million years. (iv) The sediments on the ocean floor are unexpectedly very thin.

The post-drift studies provided considerable information that was not available at the time Wegener put forth his concept of continental drift. Particularly, the mapping of the ocean floor and palaeomagnetic studies of rocks from oceanic regions revealed the following facts :

These facts and a detailed analysis of magnetic properties of the rocks on either sides of the mid-oceanic ridge led Hess (1961) to propose his hypothesis, known as the “sea floor spreading”. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads. The younger age of the oceanic crust as well as the fact that the spreading of one ocean does not cause the shrinking of the other, made Hess think about the consumption of the oceanic crust. He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed. The basic concept of sea floor spreading has been depicted in Figure.

4. THE THEORY OF PLATE TECTONICS

Since the advent of the concept of sea floor spreading, the interest in the problem of distribution of oceans and continents was revived. It was in 1967, McKenzie and Parker and also Morgan, independently collected the available ideas and came out with another concept termed Plate Tectonics. A tectonic plate (also called lithospheric plate) is a massive, irregularly-shaped slab of solid rock, generally composed of both continental and oceanic lithosphere.

Plate tectonics is the theory that explains the global distribution of geological phenomena. Principally it refers to the movement and interaction of the earth’s lithosphere. This includes the formation, movement, collision and destruction of plates and the resulting geological events such as seismicity, volcanism, continental drift, and mountain building.

Plates move horizontally over the asthenosphere as rigid units. The lithosphere includes the crust and top mantle with its thickness range varying between 5 and 100 km in oceanic parts and about 200 km in the continental areas. A plate may be referred to as the continental plate or oceanic plate depending on which of the two occupy a larger portion of the plate.

Depending on how they are defined, there are usually seven or eight “major” plates: African, Antarctic, Eurasian, North American, South American, Pacific, and Indo-Australian. The latter is sometimes subdivided into the Indian and Australian plates. Some important minor plates are listed below: (i) Cocos plate : Between Central America and Pacific plate (ii) Nazca plate : Between South America and Pacific plate (iii) Arabian plate : Mostly the Saudi Arabian landmass (iv) Philippine plate : Between the Asiatic and Pacific plate.

(v) Caroline plate : Between the Philippine and Indian plate (North of New Guinea) (vi) Fuji plate : North-east of Australia.

PLATES

Rates of Plate Movement

The current motion of the tectonic plates is today determined by remote sensing satellite data sets, calibrated with ground station measurements.

The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement. These rates vary considerably. The Arctic Ridge has the slowest rate (less than 2.5 cm/yr), and the East Pacific Rise near Easter Island, in the South Pacific about 3,400 km west of Chile, has the fastest rate (more than 15 cm/yr).

PLATES

Types of plate boundaries

Three types of plate boundaries exist, with a fourth, mixed type, characterized by the way the plates move relative to each other. They are associated with different types of surface phenomena. The different types of plate boundaries are:

• Transform boundaries (Conservative) occur where two lithospheric plates slide, or perhaps more accurately, grind past each other along transform faults, where plates are neither created nor destroyed. Transform faults occur across a spreading center. Strong earthquakes can occur along a fault. The San Andreas Fault in California is an example of a transform boundary exhibiting dextral motion.
• Divergent boundaries (Constructive) occur where two plates slide apart from each other. At zones of ocean-to-ocean rifting, divergent boundaries form by seafloor spreading, allowing for the formation of new ocean basin. As the ocean plate splits, the ridge forms at the spreading center, the ocean basin expands, and finally, the plate area increases causing many small volcanoes and/or shallow earthquakes. Active zones of mid-ocean ridges (e.g., the Mid-Atlantic Ridge and East Pacific Rise), and continent-to-continent rifting (such as Africa’s East African Rift and Valley and the Red Sea), are examples of divergent boundaries.

3. Convergent boundaries (Destructive) (or active margins) occur where two plates slide toward each other to form either a subduction zone

• continental collision – Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together. Continental collision is known only to occur on Earth. Continental collision is not an instantaneous event, but may take several tens of millions of years before the faulting and folding caused by collisions stops. The collision between India and Asia has been ongoing for about 50 million years already and shows no signs of abating. Collision between East and West Gondwana to form the East African Orogen took about 100 million years from beginning (610 Ma) to end (510 Ma). Collision between Gondwana and Laurasia to form Pangea occurred in a relatively brief interval, about 50 million years long.
• ocean-to-continent subduction (e.g. the Andes mountain range in South America, and the Cascade Mountains in Western United States), the dense oceanic lithosphere plunges beneath the less dense continent. Earthquakes trace the path of the downward-moving plate as it descends into asthenosphere, a trench forms, and as the subducted plate is heated it releases volatiles, mostly water from hydrous minerals, into the surrounding mantle. The addition of water lowers the melting point of the mantle material above the subducting slab, causing it to melt. The magma that results typically leads to volcanism.

3. ocean-to-ocean subduction (e.g. Aleutian islands, Mariana Islands, and the Japanese island arc), older, cooler, denser crust slips beneath less dense crust. This motion causes earthquakes and a deep trench to form in an arc shape. The upper mantle of the subducted plate then heats and magma rises to form curving chains of volcanic islands. Deep marine trenches are typically associated with subduction zones, and the basins that develop along the active boundary are often called “foreland basins”. Closure of ocean basins can occur at continent-to-continent boundaries (e.g., Himalayas and Alps): collision between masses of granitic continental lithosphere; neither mass is subducted; plate edges are compressed, folded, uplifted.

4. Plate boundary zones occur where the effects of the interactions are unclear, and the boundaries, usually occurring along a broad belt, are not well defined and may show various types of movements in different episodes.

Ocean floor spreading

The most compelling evidence is ocean floor spreading found at divergent (constructive) plate boundaries. At divergent plate boundaries a constant supply of magma is beng supplied. This supply of magma is the result of large convection cells within the mantle. Ageing Rocks

The map to the left shows the age of crustal rock away from the Mid Atlantic Ridge. As you can see a clear age gradient can be observed away from the ridge with the youngest crustal rock found on the ridge itself and the oldest crustal rock found close to the land continents.

Polar Reversal

The Schematic diagram shows the polar movement from 1900 to 1996. Scientists now understand that over millenia the Earth’s magnetic field has switched its polarity many times. Over the last 20 million years Earth has followed a distinct and regular pattern of pole reversal, with a reversal occuring approximately every 200,000 to 300,000 years. Our last reversal was however more than 700 000 years ago.

THE EVIDENCE FOR PLATE TECTONICS

View larger map

Evolution of land forms due to Internal Forces

EVOLUTION OF LANDFORMS DUE TO INTERNAL FORCES

ENDOGENETIC FORCES

Sudden

Diastrophic/Slow

Orogenic/Horizontal

Epeirogenic/Vertical

Earthquakes

Volcanos

Tention

Compression

Upward

Downward

ENDOGENETIC FORCES

Sudden

Diastrophic/

Slow

Orogenic/Horizontal

Epeirogenic/Vertical

Earthquakes

Volcanos

Tention

Compression

Upward

Downward

Endogenetic Movements The interaction of matter and temperature generates these forces or movements inside the earth’s crust. The earth movements are mainly of two types: diastrophism and the sudden movements. 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 friction and primordial heat from the origin of the earth. This energy due to geothermal gradients and heat flow from within induces diastrophism and volcanism in the lithosphere.

Diastrophism Diastrophism is the general term applied to slow bending, folding, warping and fracturing. All processes that move, elevate or build up portions of the earth’s crust come under diastrophism. They include: Orogenic processes/Horizontal Movements –involving mountain building through severe folding and affecting long and narrow belts of the earth’s crust; In the process of orogeny, the crust is severely deformed into folds.Orogeny is a mountain building process whereas epeirogeny is continental building process. Epeirogenic processe/Vertical Movements involving uplift or warping of large parts of the earth’s crust; Due to epeirogeny, there may be simple deformation. plate tectonics involving horizontal movements of crustal plates. Through the processes of orogeny, epeirogeny, earthquakes and plate tectonics, there can be faulting and fracturing of the crust. All these processes cause pressure, volume and temperature (PVT) changes which in turn induce metamorphism of rocks.

Orogenic or the mountain-forming movements

Orogenic or the mountain-forming movements act tangentially to the earth surface, as in plate tectonics.

• Tensions produces Faults
• Compression produces Folds

Faults

A fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement.

Fault types

Strike-slip faults – the fault surface (plane) is usually near vertical and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known as sinistral faults. Those with right-lateral motion are also known as dextral faults.

Dip-slip faults

Dip-slip faults

Dip-slip faults can be either “normal” or “reverse”. In a normal fault, the hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is a graben. An upthrown block between two normal faults dipping away from each other is a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults. A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust. The dip of a reverse fault is relatively steep, greater than 45°. The terminology of “normal” and “reverse” comes from coal-mining in England, where normal faults are the most common.

• A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.
• Flat segments of thrust fault planes are known as flats, and inclined sections of the thrust are known as ramps.
• Thrust faults form nappes and klippen in the large thrust belts. Subduction zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.

Oblique-slip faults

A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault.

Listric fault

Listric faults are similar to normal faults but the fault plane curves, the dip being steeper near the surface, then shallower with increased depth.

Ring fault

Ring faults, also known as caldera faults, are faults that occur within collapsed volcanic calderas

Synthetic and antithetic faults

Synthetic and antithetic faults are terms used to describe minor faults associated with a major fault.

Folds

In structural geology, geological fold occurs when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified.

Types of Folds

Types of Folds

• Monoclinal folds: These are those in which one limb inclines moderately with regular slope while the other limb inclines steeply at right angle and the slope is almost vertical.
• Isoclinal folds: When the compressive forces are so strong that both the limbs of the fold become parallel but not horizontal.
• Recumbent folds: These are formed when the compressive forces are so strong that both the limbs of the fold become parallel as well as horizontal.
• Overturned folds: These are those folds in which one limb of the fold is thrust upon another fold due to intense compressive forces. Limbs are seldom horizontal.
• Plunge folds: These are found when the axis of the fold, instead of being parallel to the horizontal plane, becomes tilted and forms plunge angle which is the angle between the axis and the horizontal plane.
• Fan folds: These with anticlinorium or obtuse angle.
• Open folds: These are those in which the angle between the two limbs of the fold is more than but less than
• Closed folds: These are those folds in which the angle between the two limbs of a fold is acute angle. Such folds are formed because of intense compressive force.

Epeirogenic or continent forming movements In geology, Epeirogenic movement refers to upheavals or depressions of land exhibiting long wavelengths [undulations] and little folding. The broad central parts of continents are called cratons, and are subject to epeirogeny. The movement is caused by a set of forces acting along an Earth radius, such as those contributing to Isostasy and Faulting in the lithosphere. Epeirogenic or continent forming movements act along the radius of the earth; therefore, they are also called radial movements. Their direction may be away (uplift) or towards (subsidence) from the center. The results of such movements may be clearly defined in the relief.

Uplift Raised beaches, elevated wave-cut terraces, sea caves and fossiliferous beds above sea level are evidences of uplift. Example of uplift

• Raised beaches, some of them elevated as much as 15 m to 30 m above the present sea level, occur at several places along the Kathiawar, Nellore, and Thirunelveli coasts.
• Several places which were on the sea some centuries ago are now a few miles inland. For example, Coringa near the mouth of the Godavari, Kaveripattinam in the Kaveri delta and Korkai on the coast of Thirunelveli, were all flourishing sea ports about 1,000 to 2,000 years ago.

Subsidence Submerged forests and valleys as well as buildings are evidences of subsidence. In 1819, a part of the Rann of Kachchh was submerged as a result of an earthquake.

Example of subsidence

• Presence of peat and lignite beds below the sea level in Thirunelveli and the Sunderbans is an The Andamans and Nicobars have been isolated from the Arakan coast by submergence of the intervening land.
• On the east side of Bombay island, trees have been found embedded in mud about 4 m below low water mark. A similar submerged forest has also been noticed on the Thirunelveli coast in Tamil Nadu.
• A large part of the Gulf of Mannar and Palk Strait is very shallow and has been submerged in geologically recent times. A part of the former town of Mahabalipuram near Chennai (Madras) is submerged in the sea.

Sudden Movements These movements cause considerable deformation over a short span of time, and may be of two types.

• Earthquake
• Volcano

Earthquake An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earth’s lithosphere that creates seismic waves.

About 50,000 earthquakes large enough to be noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centers are near areas of habitation.

Terms associated with earthquakes

• Focus – The place of origin of an earthquake inside the earth.
• Epicenter – Point on the earth’s surface vertically above the focus. Maximum damage is caused at the epicenter.
• Wave Velocity – 5 to 8 km per second through the outer part of the crust but travel faster with depth.
• Isoseismic Line – A line connecting all points on the surface of the earth where the intensity is the same.

Every tremor produces different types of seismic waves, which travel through rock with different velocities:

• Longitudinal P-waves (shock- or pressure waves)
• Transverse S-waves (both body waves)
• Surface waves – (Rayleigh and Love waves)

2. Seismic waves

Primary Waves (P waves)

• Primary waves are the fastest body waves (twice the speed of s-waves) and are the first to reach during an earthquake.
• They are similar to sound waves, i.e, they are longitudinal waves, in which particle movement is in the same direction of wave propagation.
• They travel through solid, liquid and gaseous materials.
• They create density differences in the earth material leading to stretching and squeezing.

Secondary Waves (S waves)

• These are called secondary waves.
• S-waves arrive at the surface with some time lag.
• They can travel only through a solid medium.
• S-waves vibrate perpendicular to the direction of the wave in the vertical plane.
• Hence, they create troughs and crests in the material through which they pass.

Surface Waves (L waves)

• Also called as long period waves.
• They are low frequency, long wavelength, and transverse vibration.
• Generally affect the surface of the Earth only and die out at smaller depth.
• Develop in the immediate neighborhood of the epicenter.
• They cause displacement of rocks, so, the collapse of structures occurs.
• These waves are the most destructive.
• Recoded last on the seismograph.

3. Richter magnitude of an earthquake

Causes of Earthquakes

Earthquakes are caused by disturbances in the interior of the earth and other causes.

Tectonic Movements: The disturbances inside the earth are called tectonic movements. These forces bring about changes on the earth surface and physical features like mountains, plateaus and rift valleys are formed. Most disastrous earthquakes are caused by tectonic forces. Tectonic forces create tension and pressure and the stress begins to build up inside the earth. When the stress tends to be more than what the rocks of the earth can bear, the rocks are broken and displaced from their state of equilibrium. It is known as faulting. The energy accumulated during faulting is released. This release of energy gives rise to mighty waves. These waves originate from a point called Focus in the interior of the earth and then spread out in all directions. On the surface whatever comes into their contact begins to vibrate. The chief cause of earthquakes felt often in California in the USA is often the San Andreas Fault found there.

Volcanic Eruptions: The volcanic eruptions are often very violent and cause vibrations in the earth crust. Sometimes the vent of a volcano is blocked temporarily and explosive eruption takes place suddenly causing tremors in the earth crust. The Krakatoa that erupted in 1883 became the cause of a violent earthquake there.

Other Reasons: The roofs of underground caves sometimes give way and release great force to cause minor tremors in the earth crust. Nuclear explosions also release massive energy to cause tremors in the earth crust.

Geographical Distribution of Earthquakes

It is true that the earthquakes can happen in any part of the world. But in the areas of faulting and folding or of crustal weakness, the frequency of earthquakes is more than anywhere else. The earthquakes are concentrated in two main belts.

• Circum Pacific Belt accounts for 66% of total earthquakes. Circum Pacific belt runs through the west coasts of North and South America, Aleutian Islands and the island group along the eastern coast of Asia.
• Mid-Atlantic Belt causes 11% of total earthquakes. This range causes mid-atlantic Ridge and several islands nearer the ridge. Earthquake even here are of moderate to shallow magnitude.
• Mid Continental Belt causes 21% of total earthquakes. Mid-continental mountain belt runs through the middle of Asia from east to west and goes beyond the Mediterranean Sea. Its axis lies along the mountain belt of the Himalayas, Caucasus, and the Alps.

Major Earthquakes in the World

The most deadly earthquake in history was in Shaanxi, China in 1556. It’s estimated to have killed 830,000 people. This is more than twice that of the second most fatal: the recent Port-au-Prince earthquake in Haiti in 2010. It’s reported that 316,000 people died as a result.

Two very recent earthquakes — the Sumatra earthquake and tsunami of 2004, and 2010 Port-au-Prince earthquake — feature amongst the most deadly in human history. But equally, some of the most fatal occurred in the very distant past. Making the top three was the earthquake in Antakya (Turkey) in the year 115. Both old and very recent feature near the top the list. The deadly nature earthquakes has been a persistent threat throughout our history.

Earthquakes in India

Effects of Earthquakes

Earthquakes are less advantageous and more harmful to man. Damage done is chiefly in following respects:

• Loss of Property: Severe earthquakes reduce to rubble human structures ranging from huts to palaces and single storey to multi storey buildings. Even pipelines laid under the ground and railway lines are damaged or displaced. The best example of this type of damage is Koyana earthquake in 1970.
• Loss of Life: Earthquake tremors of a few seconds takes the lives of thousands of people. Many people have been rendered homeless or suffered injuries in various ways.
• Changes in the course of rivers: On account of the impact of earthquakes, sometimes rivers also change their course. Consequently, when floods come they play havoc with people’s lives.
• Tsunamis: The earthquakes in the sea generate massive waves called Tsunami in Japanese language. It sometimes rises to the height of 20-25 metres. It causes great damage to life and property of people living in coastal areas as well as to tourists. Tsunami caused by an earthquake in the sea near Sumatra on 26th Dec, 2004 hit southeast Asian countries including India and Sri Lanka. There was heavy damage in these countries. More than 3 lakh people died.
• Mud Fountains: On account of earthquakes of high intensity, warm water and mud fountains also burst.
• Cracks in Earth Crust: Earthquake cause cracks in earth’s crust anywhere in fields, roads, parks and even hills. They are thus rendered useless. The San Andreas fault in California, U.S.A. was created in a similar manner.

Prediction of earthquake

Earthquake can occur at any time of the year, day or night. Its impact is very sudden. There are no warning signs of earthquakes. Extensive and sincere research has been conducted in the forecast or prediction of an earthquake.

For the first time in India, a system to detect earthquakes and disseminate warnings was installed in Uttarakhand, in 2015. It issues warnings 1-40 seconds before earthquakes of magnitude 5 occur. All sensors under this system that warn of earthquakes are based on the detection of P and S waves generated during an earthquake. The P wave, which is harmless and travels faster than the S wave, is detected by the sensors for advance warning.

IIT Roorkee is conducting research to develop first of its kind sensors to be deployed in all seismic prone major cities in North India.

Volcanoes A volcano is a vent or fissure in Earth’s crust through which lava, ash, rocks, and gases erupt.

An active volcano is a volcano that has erupted in the recent past.

The mantle contains a weaker zone known as asthenosphere.

Magma is the material present in the asthenosphere.

Material that flows to or reaches the ground comprise of lava flows, volcanic bombs, pyroclastic debris, dust, ash and gases. The gases maybe sulphur compounds, nitrogen compounds, and trace amounts of argon, hydrogen and chlorine.

Causes of Volcanism

• In the interior of the earth, the radioactive substances undergo chemical reactions which generate a large amount of heat. Apart from this, some amount of residual heat which was captured at the centre of the earth during its formation is also present. This leads to a creation of large temperature difference between the inner and Outer layers of earth.
• This huge temperature difference leads to the formation of convection currents in the outer Core and the Mantle. Due to this, the molten magma along with the gaseous materials comes out to the earth s surface at the first available opportunity. This mainly occurs in the weak zones of earth surface such as divergent plate boundaries, and convergent plate boundaries etc.
• Sometimes, the earthquakes may expose the fault zones in the rock strata through which the Magma can escape to the earth’s surface leading to volcanic eruptions.

3. Types of Volcanoes

Active Volcano

• • Keeps on ejecting volcanic material at frequent intervals
  • Ex – Etna (Italy), Stromboli (Sicily – largest island in the Mediterranean Sea, near Italy)
  • Mt Stromboli → Lighthouse of the Mediterranean

Dormant Volcano

• • One in which eruption has not occurred for a long time but can occur any time in future
  • Barren Island (Andaman), Versuris (Italy)

Extinct Volcano

• No eruption has occurred in historic times & possibility of future eruption is also remote
• Mt. Popa (Myanmar). But we can never be thoroughly sure about them.
• Vesuvius (Bay of Naples near Italy) & Mt. Krakatau (Sunda straits b/w Java & Sumatra) were thought to be extinct & yet both erupted violently

Types of Lava in volcanoes

Acidic or Andesitic or composite Lava

• The acidic or composite Lava is highly viscous and has a high melting point. It has a high percentage of silica content, low density and light colour.
• The acidic lava flows slowly and they rarely travel far before solidification. This leads to the formation of the cone-like structure having steep sides.
• Due to the rapid solidification of this acidic Lava, the openings obstruct the flow of new Lava, which results in loud explosions and pyroclasts (volcanic bombs).

Basic or Shield or Basaltic Lava

• The basaltic or basic lava is highly fluid, and their temperature is about 1,000 C. Basaltic lava is poor in silica, but are rich in Iron and manganese.
• They have a dark colour and high fluidity. Due to their high fluidity, the basaltic Lava is not very explosive, and they spread over great distances as thin sheets of Lava.
• The volcano formed by Basic Lava is gently sloping and they form a flattened shield or dome with a wide diameter.

Landform due to Volcano

Intrusive landforms Sills – When an intrusion of molten magma is made horizontally along the bedding planes of sedimentary rocks, the resultant intrusion is called a Sill. Dikes – Similar intrusion when injected vertically as narrow walls of igneous rocks within the sedimentary layers are termed as Dikes.

Laccoliths – An igneous mound with a dome shaped upper surface & a level base, fed by a pipe like conduit from below Lopolith – An igneous intrusion with a saucer shape Phacolith – A lens shaped mass of igneous rock occupying the crest of an anticline or the bottom of a syncline & being fed by a conduit from beneath Batholith – A large emplacement of igneous intrusive rock, mainly granite, that forms from cooled magma deep in the Earth’s crust

Extrusive Landforms

• Lava or molten magma ejects at a very high pressure through a pipe known as Volcano’s neck or vent.
• Top portion of volcano is known as crater and a crater lake is formed when rain water gets accumulated in Some volcanoes may have greatly enlarged depressions like cauldron known as Calderas.
• Volcanic dust or ash (finer particles) that emerges out of volcano travels round the world & falls as black snow, which can bury house & people.
• The coarser fragmental rocks are collectively called as Pyroclasts which include cinders, pumice & volcanic bombs.

Distribution of Volcanoes across the World

Till now, around 480 major active volcanoes have been found out of which around 400 are found in the areas around the Pacific ocean. While the others are in the Alpine Himalayan belt, Atlantic Ocean, Indian ocean etc. The Himalayas do not have an active volcano. The converging plate margins and the mid-oceanic ridges are the areas of high volcanic activity and earthquakes. The volcanic zones and earthquake zones are more prominent around the converging plate boundaries. Pacific Ring of Fire The circum Pacific region or Pacific Ring of Fire has the largest concentration of active volcanoes. It has almost two-thirds of active volcanoes. The Aleutian islands of Kamchatka, Japan, the areas of Philippines, Indonesia, Islands of Solomon, Tonga and North Island, New Zealand, the Andes to Central America and up to Alaska are the part of Pacific rim of fire.

volcanoes along the Atlantic coast The Atlantic coast has a comparatively fewer number of active volcanoes. But it has many dormant volcanoes such as Saint Helena, Cape Verde islands etc. The volcanoes of Iceland and Azores along the Atlantic coast are active volcanoes. Volcanoes in the Mediterranean region The Alpine folds, such as Vesuvius, stromboli (also known as the Lighthouse of Mediterranean) and the Aegean Islands are the areas of the Mediterranean region where active volcanoes are found. Volcanoes in the great rift region Mount Kilimanjaro and Mount Kenya of the East African Rift Valley have some extinct volcanoes. Mount Cameroon is the only volcano active in West Africa. Volcanoes in other parts of the world Other regions such as West Indian Islands have experienced some volcanic activity in the recent past. Mount Pelee of the Lesser Antilles is a volcanic Island where the last eruption took place in 1929.

Volcanos in India

There are no volcanoes in the Himalayan region or in the Indian peninsula. Barren Island, lying 135 km north-east of Port Blair became active again in 1991 and 1995. The other volcanic island in Indian territory is Narcondam, about 150 km north-east of Barren Island; it is probably extinct. Its crater wall has been completely destroyed.

Before a volcano becomes extinct, it passes through a waning stage during which steam and other hot gases and vapours are exhaled. These are known as fumaroles or solfataras. Krakatao volcano became active in 1883, killing 36,000 people in West Java. Today, Krakatao is no more than a low island with a caldera lake inside its crater.

Geysers and Hot Springs

When the water comes in contact with the intense heat of these rocks, it gets heated and rises in the form of capillaries and narrow roots through the porous rocks. When this heated water comes to the surface it undergoes expansion and gets converted into steam leading to the formation of geysers and Springs.

Geysers: when the heated water at high pressure comes out of the surface and bursts into steam, it is known as Geysers. In most of the cases, a carter like structure is formed at its mouth. Springs: When the hot water comes out to the surface in a smooth manner it is known as a spring. Most of the world s geysers are found in the areas of Iceland, New Zealand and the Yellowstone National Park of USA. The hot springs and geysers of Japan and Hawaii are great tourist attractions. Geysers are found in very few regions, while the hot water springs are found all over the world.

Some significant Volcanic Eruptions

In the history of mankind perhaps the most disastrous eruptions were those of Vesuvius, Mt. Krakatau and Mt. Pelee. Mt. Vesuvius

• Vesuvius is a Stratovolcano (composite volcano) in Italy.
• Vesuvius, standing 4,000 feet above the Bay of Naples, erupted violently in A.D. 79.The city of Pompeii, located to the south-west, was buried beneath twenty feet of volcanic ashes cemented by the torrential downpours of heavy rain.
• Fertility of the solidified Volcanic ashes tempted many farmers to begin anew on the slopes of Vesuvius.
• Then came the catastrophic eruption of December 1631, ruined fifteen towns and killed inhabitants.

Mt. Krakatau

• The greatest volcanic explosion known to men is perhaps that of Mt. Krakatau in August 1883.
• Krakatau is a small volcanic island in the Sunda Straits, between Java and Sumatra.
• The explosion could be heard in Australia, almost 3,000 miles away.
• Though Krakatau itself was not inhabited and nobody was killed by the lava flows, the vibration set up enormous waves over 100 feet high which drowned 36,000 people in the coastal districts of Indonesia.

Mt. Pelee

• The eruption of Mt. Pelee of the West Indies in May 1902 was the most catastrophic of modem times.
• Pierre, the capital of Martinique, lying on the path of the lava, was completely destroyed within minutes.
• Its entire population of 30,000 was killed almost instantly.

View larger map

Evolution of land forms due to External Forces

EVOLUTION OF LANDFORMS DUE

TO

EXTERNAL FORCES

Exogenic Process
Deposition
Weathering
Mass Movements

Erosion

• Physical
• Chemical
• Biological

• Slow
• Rapid

• Running W.
• Ground W
• Glaciar
• Waves
• Winds

• Running W.
• Ground W
• Glaciar
• Waves
• Winds

Exogenic Process
Deposition
Weathering
Mass Movements
Erosion

• Physical
• Chemical
• Biological

• Slow
• Rapid

• Running W.
• Ground W
• Glaciar
• Waves
• Winds

• Running W.
• Ground W
• Glaciar
• Waves
• Winds

1 Weathering
1.1 Weathering – Introduction
1.2 Weathering – Classification
1.2.1 Chemical Weathering Processes 1.2.1.1 Solution 1.2.1.2 Carbonation 1.2.1.3 Hydration 1.2.1.4 Oxidation and Reduction 1.2.2 Biological activity and weathering 1.2.3 Physical Weathering Processes 1.2.3.1 Unloading and Expansion 1.2.3.2 Granular Disintegration 1.2.3.3 Exfoliation – Temperature Changes and Expansion 1.2.3.4 Block Separation 1.2.3.5 Shattering 1.2.3.6 Freezing, Thawing and Frost Wedging 1.2.3.7 Salt Weathering 1.2.3.8 Mass Wasting 1.2.4 Effects of Weathering 1.2.5 Weathering and Erosion 1.2.6 Significance of weathering
1.1 Weathering – Introduction
Weathering is defined as mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate. As very little or no motion of materials takes place in weathering, it is an in-situ or on-site process.
Factors Influencing Weathering Processes Types and rates of weathering vary, at the regional and local spatial scales. Climate : Different climatic conditions are associated with different weathering processes. Example, chemical weathering, is particularly effective and rapid in humid climates. Chemical weathering is much more restricted in arid climates. Arid regions typically receive sufficient moisture for physical weathering by salt crystal growth and the hydration of salts. Abundant salts, high humidity, and contact with seawater make salt weathering processes very effective in marine coastal location. Rock Type : Weathering is greatly influenced by the character of the bedrock: hard or soft, soluble or insoluble, broken or unbroken. Areas of diverse rock types undergo differential weathering and erosion; easily eroded rocks exhibit more extensive effects of weathering and erosion than the resistant rocks. Slope orientation : The geographic orientation of a shop- whether it faces north, south east, or west- controls the slope’s exposure to sun, wind, and precipitation. Vegetation : Although vegetative cover can protect rock by shielding if from raindrop impact and providing roots to stabilize soil, it also produces organic acids from the partial decay of organic matter; these acids contribute to chemical weathering. Plant roots can enter crevices and break up a rock, exerting enough pressure to force rock segments apart, thereby exposing greater surface area to other weathering processes. In the complexity of nature, physical and chemical weathering processes usually operate together.
1.2 Weathering – Classification
There are three major groups of weathering processes: (i) chemical; (ii) physical or mechanical; (iii) biological weathering processes.
1.2.1 Chemical Weathering Processes
A group of weathering processes viz; solution, carbonation, hydration, oxidation and reduction act on the rocks to decompose, dissolve or reduce them to a fine state. Water and air (oxygen and carbon dioxide) along with heat speed up all chemical reactions.
Solution Carbonation
Hydration
Oxidation and Reduction
Solution When something is dissolved in water or acids, the water or acid with dissolved contents is called solution. On coming in contact with water many solids disintegrate. Soluble rock forming minerals like nitrates, sulphates, and potassium etc. are affected by this process. So, these minerals are easily leached out without leaving any residue in rainy climates and accumulate in dry regions.
Carbonation Carbonation is the reaction of carbonate and bicarbonate with minerals. Carbon dioxide from the atmosphere and soil air is absorbed by water, to form carbonic acid that acts as a weak acid on various minerals.
Hydration Hydration is the chemical addition of water. Minerals take up water and expand; this expansion causes an increase in the volume of the material itself or rock. This process is reversible and long, continued repetition of this process causes fatigue in the rocks and may lead to their disintegration. The volume changes in minerals due to hydration will also help in physical weathering through exfoliation and granular disintegration. Oxidation and Reduction In weathering, oxidation means a combination of a mineral with oxygen to form oxides (rusting in case of iron) or hydroxides. Red soils appear red due to the presence of iron oxides. Oxidation occurs where there is ready access to the atmosphere and water. The minerals most commonly involved in this process are iron, manganese, sulphur etc. When oxidized minerals are placed in an environment where oxygen is absent, reduction takes place. Such conditions exist usually below the water table, in areas of stagnant water and waterlogged ground. Red colour of iron upon reduction turns to greenish or bluish grey. These weathering processes are interrelated. Hydration, carbonation and oxidation go hand in hand and hasten the weathering process.
1.2.2 Biological activity and weathering
Biological weathering is removal of minerals and ions from the weathering environment and physical changes due to growth or movement of organisms. Burrowing and wedging by organisms like earthworms, rodents etc., help in exposing the new surfaces to chemical attack and assists in the penetration of moisture and air. Human beings by disturbing vegetation, ploughing and cultivating soils, also help in mixing and creating new contacts between air, water and minerals in the earth materials. Decaying plant and animal matter help in the production of humic, carbonic and other acids which enhance decay and solubility of some elements. Algae utilise mineral nutrients for growth and help in concentration of iron and manganese oxides. Plant roots exert a tremendous pressure on the earth materials mechanically breaking them apart.
1.2.3 Physical Weathering Processes Physical or mechanical weathering processes depend on some applied forces like (i) gravitational forces (ii) expansion forces due to temperature changes, crystal growth or animal activity; (iii) water pressures controlled by wetting and drying cycles. Unloading and Expansion Removal of overlying rock load because of continued erosion causes vertical pressure release with the result that the upper layers of the rock expand producing disintegration of rock masses. In areas of curved ground surface, arched fractures tend to produce massive sheets or exfoliation slabs of rock. Granular Disintegration Rocks composed of coarse mineral grains commonly fall apart grain by grain or undergo granular disintegration.
Exfoliation – Temperature Changes and Expansion With rise in temperature, every mineral expands and pushes against its neighbor and as temperature falls, a corresponding contraction takes place. Because of diurnal changes in the temperatures, this internal movement among the mineral grains takes place regularly. This process is most effective in dry climates and high elevations where diurnal temperature changes are drastic. The surface layers of the rocks tend to expand more than the rock at depth and this leads to the formation of stress within the rock resulting in heaving and fracturing parallel to the surface. Exfoliation results in smooth rounded surfaces in rocks.
Block Separation This type of disintegration takes place in rocks with numerous joints acquired by mountain-making pressures or by shrinkage due to cooling. This type of disintegration in rocks can be achieved by comparatively weaker forces.
Shattering A huge rock may undergo disintegration along weak zones to produce highly angular pieces with sharp corners and edges through the process of shattering.
Freezing, Thawing and Frost Wedging During the warm season, the water penetrates the pore spaces or fractures in rocks. During the cold season, the water freezes into ice 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. Salt Weathering 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 favour such salt expansion. Salt crystals in near-surface pores cause splitting of individual grains within rocks, which eventually fall off. This process of falling off of individual grains may result in granular disintegration or granular foliation. Mass Wasting Since gravity exerts its force on all matter, both bedrock and the products of weathering tend to slide, roll, flow or creep down all slopes in different types of earth and rock movements grouped under the term ‘mass wasting’.
1.2.4 Effects of Weathering Weathering and erosion tend to level down the irregularities of landforms and create a The strong wind erosion leaves behind whale-back shaped rocks in arid landscape. These are called inselberg or ruware.
Sometimes a solid layer of chemical residue covers a soft rock. Sometimes, differential weathering of soft strata exposes the dome like hard rock masses, called tors. Tors are a common feature of South Indian landscape.
1.2.5 Weathering and Erosion Lead to simultaneous process of ‘degradation’ and ‘aggradation’. Erosion is a mobile process while weathering is a static process [disintegrated material do not involve any motion except the falling down under force of gravity].
1.2.6 Significance of weathering Weathering is the first step in formation of soils. Weathering of rocks and deposits helps in the enrichment and concentrations of certain valuable ores of iron, manganese, aluminium, copper etc. Weathering helps in soil enrichment. Without weathering, the concentration of the same valuable material may not be sufficient and economically viable to exploit, process and refine. This is what is called enrichment. Another important process in Exogenetic movements is erosion. We will study about erosion in Indian Geography.
Mass Movements

• These movements transfer the mass of rock debris down the slope under the direct influence of gravity.
• Mass movements are very active over weathered slopes rather than over unweathered slopes.
• Usual geographic agents like running water, glaciers, wind, waves etc do not have much role to play in mass movements, and it is the gravity, which is the main driving force.
• Mass movements are classified into slow movements and rapid movements.

(1) Slow movements:
1. Soil Creep: This is a slow, gradual but more or less continuous movement of soil down the hill slopes. It is shown on the figure to the right. 2. Soil Flow ( Solifluction ) : When the soil is completely saturated with water the individual particles are almost suspended in the water and move easily over one another and over the underlying rock. The soils acts like a liquid and a soil-flow or mud-flow occurs. In temperature and tundra regions soil flow occur when the surface layers of frozen ground than in spring. In Ireland such flows are known as ‘bog-bursts’.
(2) Rapid movements
LANDSLIDES
There are very rapid kinds of movement and occur when a large mass of soil or rock falls suddenly.
Slump: It is a type of landslide in which slipping of several units of rock debris occurs with a backward rotation with respect to the slope over which the movement takes place. Debris slide: In this type of landslide, there is no backward rotation. The fall is almost vertical. Rock slide: It is nothing but the slide of individual rock masses.
EARTHFLOW: Movement of water-saturated clayey or silty earth materials down low angle terraces or hillsides is called earthflow MUDFLOW: In the absence of vegetation and cover and with heavy rainfall, thick layers of weathered materials get saturated with water and either slow or rapidly flow down along definite channels is called as mudflow. DEBRIS AVALANCHE: It is more in humid regions with or without vegetation. It occurs in narrow tracks on sleep slopes and is similar to snow avalanche.
Erosion and Deposition
Erosion
Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth’s crust, and then transports it to another location.(not to be confused with weathering which involves no movement).
Deposition
Deposition is the geological process in which sediments, soil and rocks are added to a landform or land mass. Wind, ice, water, and gravity transport previously weathered surface material, which, at the loss of enough kinetic energy in the fluid, is deposited, building up layers of sediment.
Action of Wind
The wind is the main geomorphic agent in the hot deserts. Winds in hot deserts have greater speed which causes erosional and depositional activities in the desert. The landforms which are created by erosional and depositional activities of wind are called as Aeolian Landforms. This process is not unique to the Earth, and it has been observed and studied on other planets, including Mars. An erg (also known as sand sea / dune sea / sand sheet if it lacks dunes) is a broad, flat area of desert covered with wind-swept sand with little or no vegetative cover. It is defined as a desert area that contains more than 125 square kilometres of aeolian or wind-blown sand and where sand covers more than 20% of the surface. Smaller areas are known as “dune fields”. The largest hot desert in the world, the Sahara, contains several ergs.
Erosional Landforms due to Wind 1. Pediplains When the high relief structures in deserts are reduced to low featureless plains by the activities of wind, they are called as Pediplains. 2. Deflation Hollows Deflation is the removal of loose particles from the ground by the action of wind. When deflation causes a shallow depression by persistent movements of wind, they are called as deflation hollows. 3. Mushroom Tables Ventifacts are rocks that have been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently strong winds, and where there is a steady but not overwhelming supply of sand. Mushroom Tables / Mushroom rocks are Ventifacts in the shape of a mushroom. 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. This result in the formation of rock pillars shaped like a mushroom with narrow pillars with broad top surfaces.
Depositional Landforms of Wind 1. Sand dunes Dry hot deserts are good places for sand dune formation. According to the shape of a sand dune, there are varieties of sand dune forms like Barchans, Seifs etc. The crescent-shaped dunes are called as Barchans and they are the most common one. Seif is similar to Barchans but has only one wing or point.
2. Loess

• In several large areas of the world, the surface is covered by deposits of wind-transported silt that has settled out from dust storms over many thousands of years. These depositions are called as Loess.

Action of Waves: Coastal processes are the most dynamic and hence most destructive. Some of the changes along the coast take place very fast. Storm waves and tsunami waves can cause far-reaching changes in short period of time than normal breaking waves. The coastal landforms in the world can be classified into two categories:
1. High Rocky Coast/ Submerged Coast/ Retreating Coast In these type of coasts, the sea will be very close to the land without any coast or sometimes a narrow coast. The shores of these high rocky coasts do not show any depositional landforms. Erosional feature dominates here. Wave-cut platforms, cliffs, sea caves etc are common here. Most of the west coasts of the Indian Peninsula belong to this category.
2. Low Sedimentary Coast/ Emerging Coast/ Advancing Coast
The rivers in these coasts extend the length of the coast by building coastal plains and deltas. Thus, 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.
Erosional Landforms due to Waves 1. Cliffs, Terraces, Caves, Stacks and Stumps Cliffs are common on the high rocky coasts. At the foot of such cliffs, there may be flat or gently sloping platform covered by rock debris derived from the sea cliff behind. Such platforms occurring at an elevation above the average height of waves is called as a wave-cut terrace. When the upper part of a coastal rock is hard and the lower part is soft, the erosion will not be uniform. The lower part erodes easily which results in the formation of a hollow part. This hollow part, by frequent wave action, gradually develops into a sea-cave. Sea arches are also formed in the same manner. Sea stacks are nothing but the isolated standing rocks in the sea which were once a part of the cliff. These stacks look will like small islands in the sea. Small underwater stacks are known as stumps.
Depositional Landforms due to Waves 1. Beaches and dunes Beaches are characteristics of shorelines that are dominated by deposition. Beaches are temporary features which are made up of sand-sized materials. Beaches which contain excessively small pebbles and even cobbles are called as Shingle Beaches. Sand dunes are formed just behind the beaches as long ridges parallel to the coastline. 2. Bars, spits, and Lagoons Bars are deposits of sand and gravel laid down by waves and currents which separate the shoreline from the sea. They act as a barrier between the mainland and the sea. When one end of such bar is attached to the coast and other extends into the sea, it is called as a spit. Tombolo is a deposition landform in which an island is attached to the mainland by a narrow piece of land such as a spit or bar. Sometimes due to deposition of waves and currents, both ends of the bar join to enclose a part of sea water between the coast and the bar. This enclosed part of the sea forms a lake of saline water called as Lagoon. A lagoon is generally connected with the sea through a narrow passage. Chilika and Pulicat lakes are examples of Lagoon Lake.
Action of Glaciers
Glaciers are a mass of ice moving under its own weight. They are commonly found in the snow-fields. We know that the landmass on the earth is not entirely the same as we see around. Some areas are covered by thick green forests, some with dry hot deserts, some with permanent ice covers etc. Among these varied landmasses, the permanently ice-covered regions on the earth surface are called as snow-fields. The lowest limit of permanent snow or snow-field is called as the snowline. A Glacier forms in areas where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries.
Action of Glaciers
They form features like crevasses, seracs etc. A crevasse is a deep crack, or fracture, found in an ice sheet or glacier, as opposed to a crevice that forms in rock. A serac is a block or column of glacial ice, often formed by intersecting crevasses on a glacier. Ogives are alternating wave crests and valleys (troughs) that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals the annual movement of the glacier. Glaciers cover about 10 percent of Earth’s land surface and they are the largest freshwater reservoirs on earth. On the basis of the location of the glacier, they can be classified as: Continental Glacier/Piedmont Glacier: they move outward in all directions Valley/Mountain Glaciers: Move from higher elevation to lower
Erosional landforms due to Glaciers 1. Cirque or Corris They are deep, long and wide troughs or basins with very steep concave to vertically dropping high walls at its head as well as sides. They are simply a bowl-shaped depression formed due to the erosional activity of glaciers. When these depressions are filled with water, they are called as Cirque lake or Corrie Lake or Tarn Lakes.
2. Hanging Valleys or U-shaped Valleys, Fjords/fiords The Glacier doesn’t create a new valley like a river does but deepens and widens a pre-existing valley by smoothening away the irregularities.
These valleys, which are formed by the glacial erosions assume the shape of letter ‘U’ and hence are called as U-shaped Valleys or Hanging Valleys.
A fjord is a very deep glacial trough filled with sea water and making up shorelines. A fjord is formed when a glacier cuts a U-shaped valley by ice segregation and abrasion of the surrounding bedrock and this valley gradually gets filled with the seawater (formed in mountains nearby sea).
3. Horns and Aretes

• Horns are sharp pointed and steep-sided peaks.
• They are formed by headward erosion of cirque wall.
• When the divide between two cirque walls gets narrow because of progressive erosions, it results in the formation of a saw-toothed ridge called Arete.

Depositional Landforms due to Glaciers Glacial deposits are of two types: (i) Glacial Till – unassorted coarse and fine debris; (ii) Outwash – assorted roughly stratified deposits. 1. Moraines Moraines are long ridges of deposits of glacial till. When these deposits are at the end of a glacier, they are called as Terminal moraines and when they are deposited on both sides, they are called as Lateral moraines. When lateral moraines of two glaciers join together, they form Medial moraines. When the lateral moraines of both sides of a glacier join together, it forms a horse-shoe shape. Ground moraines are deposits left behind in areas once covered by glaciers.
2. Eskers When glaciers melt in summer, the water which formed as a result of melting accumulates beneath the glacier and flows like streams in channels beneath that ice. Very coarse material like boulders, blocks and some minor fractions of rock debris are carried away by these streams. They later get deposited in the valleys itself and once the ice melts completely, they are visible to the surface as sinuous ridges. These ridges are called as Eskers.
3. Drumlins They are smooth oval-shaped ridge-like structures composed mainly of glacial till. It shapes like an inverted spoon with the highest part is called as Stoss End and the lowest narrow part is called as Tail End. They are formed as a result of glacial movement over some minor obstruction like small surface rocks. The glacial till gets deposited in those obstructions and the movement of glacier shapes these deposits like an inverted spoon.
Action of Running Water
Running water, which doesn’t need any further explanation, has two components: one is overland flow on the general land surface as a sheet and the other is linear flow as streams and rivers in valleys. The overland flow causes sheet erosion and depending upon the irregularities of the land surface, the overland flow may concentrate into narrow to wide paths. During the sheet erosion, minor or major quantities of materials from the surface of the land are removed in the direction of flow and gradual small and narrow rills will form. These rills will gradually develop into long and wide gullies, the gullies will further deepen, widen and lengthen and unite to give rise to a network of valleys. (Note: A valley can be formed in various ways like faulting, but here we are dealing only with the formation by means of exogenic geomorphic agent). Once a valley is formed, it later develops into a stream or river.
Courses of a river A river, which is the best example of the linear flow of running water through a valley, can be divided into three, on the basis of its course – upper course, middle course and lower course. Upper Course / Stage of Youth (Erosion dominates): It starts from the source of the river in hilly or mountainous areas. The river flows down the steep slope and, as a result, its velocity and eroding power are at their maximum. Streams are few, with poor integration. As the river flows down with high velocity, vertical erosion or downward cutting will be high which results in the formation of V-Shaped Valleys. Waterfalls, rapids, and gorges exist where the local hard rock bodies are exposed.
Middle Course/ Stage of Maturity (Transportation dominates): In this stage, vertical erosion slowly starts to replace with lateral erosion or erosion from both sides of the channel. Thus, the river channel causes the gradual disappearance of its V-shaped valley (not completely). Streams are plenty at this stage with good integration. Wider flood plains start to visible in this course and the volume of water increases with the confluence of many tributaries. The work of river predominantly becomes transportation of the eroded materials from the upper course (little deposition too). Landforms like alluvial fans, piedmont alluvial plains, meanders etc. can be seen at this stage. Lower Course/ Stage of Old (Deposition dominates): The river starts to flow through a broad, level plain with heavy debris brought down from upper and middle courses. Vertical erosion has almost stopped and lateral erosion still goes on. The work of the river is mainly deposition, building up its bed and forming an extensive flood plain. Landforms like braided channels, floodplains, levees, meanders, oxbow lakes, deltas etc. can be seen at this stage.
Running water: erosion, transportation, and deposition Erosion occurs when overland flow moves soil particles downslope. The rock materials carried by erosion is the load of the river. This load acts as a grinding tool helping in cutting the bottom and sides of the river bed, resulting in deepening and widening of the river channel.
Erosion Types The work of river erosion is accomplished in different ways, all of which may operate together. They are corrasion, corrosion, hydraulic action etc. Corrasion or Abration: As the rock particles bounce, scrape and drag along the bottom and sides of the river, they break off additional rock fragments. This form of erosion is called corrasion or abration. They are two types: vertical corrosion which acts downward and lateral corrosion which acts on both sides. Corrosion or Solution: This is the chemical or solvent action of water on soluble or partly soluble rocks with which the river water comes in contact. Hydraulic Action: This is the mechanical loosening and sweeping away of material by the sheer force or river water itself. No load or material is involved in this process.
Transportation types After erosion, the eroded materials get transported with the running water. 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. These fragments can be seen rolling, slipping, bumping and being dragged. 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.
When the stream comes down from the hills to plain areas with the eroded and transported materials, the absence of slope/gradient causes the river to lose it energy to further carry those transported materials. As a result, the load of the river starts to settle down which is termed as deposition.
Erosion, transportation, and deposition continue until the slopes are almost completely flattened leaving finally a lowland of faint relief called peneplains with some low resistant remnants called monadnocks.
Erosional Landforms due to Running Water 1. Valleys, Gorges, Canyon

• • As we discussed above, valleys are formed as a result of running water.
  • The rills which are formed by the overland flow of water later develop into gullies.
  • These gullies gradually deepen and widen to form valleys.
  • A gorge is a deep valley with very steep to straight sides.
  • A canyon is characterized by steep step-like side slopes and may be as deep as a gorge.
  • A gorge is almost equal in width at its top as well as bottom and is formed in hard rocks while a canyon is wider at its top than at its bottom and is formed in horizontal bedded sedimentary rocks.

2. Potholes, Plunge pools

• Potholes are more or less circular depressions over the rocky beds of hills streams.
• Once a small and shallow depression forms, pebbles and boulders get collected in those depressions and get rotated by flowing water. Consequently, the depressions grow in dimensions 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.

3. Incised or Entrenched Meanders

• • They are very deep wide meanders (loop-like channels) found cut in hard rocks.
  • In the course of time, they deepen and widen to form gorges or canyons in hard rock.
  • The difference between a normal meander and an incised/entrenched meander is that the latter found on hard rocks.

4. River Terraces

• They are surfaces marking old valley floor or flood plains.
• They are basically the result of vertical erosion by the stream.
• When the terraces are of the same elevation on either side of the river, they are called as paired terraces.
• When the terraces are seen only on one side with none on the other or one at quite a different elevation on the other side, they are called as unpaired terraces.

Depositional Landforms due to Running Water
1. Alluvial Fans

• They are found in the middle course of a river at the foot of slope/ mountains.
• When the stream moves from the higher level break into foot slope plain of low gradient, it loses its energy needed to transport much of its load.
• Thus, they get dumped and spread as a broad low to the high cone-shaped deposits called an alluvial fan.
• The deposits are not roughly very well sorted.

2. Deltas

• Deltas are like an alluvial fan but developat a different location.
• They are found in the mouth of the river, which is the final location of depositional activity of a river.
• Unlike alluvial fans, the deposits making up deltas are very well sorted with clear stratification.
• The coarser material settle out first and the finer materials like silt and clay are carried out into the sea.

3. Flood Plains, Natural Levees

• Deposition develops a flood plain just as erosion makes valleys.
• A riverbed made of river deposits is the active flood plain and the flood plain above the bank of the river is the inactive flood plain.
• Natural levees are found along the banks of large rivers. They are low, linear and parallel ridges of coarse deposits along the banks of a river.
• The levee deposits are coarser than the deposits spread by flood water away from the river.

4. Meanders and oxbow lakes

• Meanders are loop-like channel patterns develop over the flood and delta plains.
• They are actually not a landform but only a type of channel pattern formed as a result of deposition.
• They are formed basically because of three reasons: (i) propensity of water flowing over very gentle gradient to work laterally on the banks; (ii) unconsolidated nature of alluvial deposits making up the bank with many irregularities; (iii) Coriolis force acting on fluid water deflecting it like deflecting the wind.
• The concave bank of a meander is known as cut-off bank and the convex bank is known as a slip-off
• As meanders grow into deep loops, the same may get cut-off due to erosion at the inflection point and are left as oxbow lakes.
• For large rivers, the sediments deposited in a linear fashion at the depositional side of a meander are called as Point Bars or Meander Bars.

5. Braided Channels

• When selective deposition of coarser materials causes the formation of a central bar, it diverts the flow of river towards the banks, which increases lateral erosion.
• Similarly, when more and more such central bars are formed, braided channels are formed.
• Riverine Islands are the result of braided channels

What does Groundwater do?

• The part of rain or snow-melt water which accumulates in the rocks after seeping through the surface is called underground water or simply groundwater.
• The rocks through which water can pass easily are called as permeable rocks while the rocks which do not allow water to pass are called as impermeable rocks.
• After vertically going down to some depth, the water under the ground flows horizontally through the bedding planes, joints or through the materials themselves.
• Although the amount of groundwater varies from place to place, its role in shaping the surface features of the earth is quite important.
• The works of groundwater are mainly seen in rocks like limestone, gypsum or dolomite which are rich in calcium carbonate.
• Any limestone, dolomite or gypsum region showing typical landforms produced by the action of groundwater through the process of solution and deposition is called as Karst Topography (Karst region in the Balkans)
• The zones or horizons of permeable and porous rocks which are fully filled with water are called as the Zones of Saturation.
• The marks which show the upper surface of these saturated zones of the groundwater are called as the Water Tables.
• And these rocks, which are filled with underground water, are called as aquifers.
• The water table is generally higher in the areas of high precipitation and also in areas bordering rivers and lakes.
• They also vary according to seasons. On the basis of variability, water tables are of two types: (i) Permanent water table, in which the water will never fall below a certain level and wells dug up to this depth provide water in all seasons; (ii) Temporary water tables, which are seasonal water tables.
• Springs: They are the surface outflow of groundwater through an opening in a rock under hydraulic pressure.
• When such springs emit hot water, they are called as Hot Springs. They generally occur in areas of active or recent volcanism.
• When a spring emits hot water and steam in the form of fountains or jets at regular intervals, they are called as geysers.
• In a geyser, the period between two emissions is sometimes regular (Yellowstone National Park of USA is the best example).

Erosional Landforms due to Groundwater
Sinkholes and caves are erosional landforms formed due to the action of ground water.
1. Sinkholes

• Small to medium sized rounded to sub-rounded shallow depressions called swallow holes forms on the surface of rocks like limestone by the action of the solution.
• A sinkhole is an opening more or less circular at the top and funnel-shaped towards the bottom.
• When as sinkhole is formed solely through the process of solution, it is called as a solution sink.
• Some sinkhole starts its formation through the solution process but later collapse due to the presence of some caves or hollow beneath it and becomes a bigger sinkhole. These types are called as collapse sinks.
• The term Doline is sometimes used to refer collapse sinks.
• Solution sinks are more common than collapse sinks.
• When several sink holes join together to form valley of sinks, they are called as valley sinks or Uvalas.
• Lapies are the irregular grooves and ridges formed when most of the surfaces of limestone are eaten by solution process.

2. Caves

• In the areas where there are alternative beds of rocks (non-soluble) with limestone or dolomite in between or in areas where limestone are dense, massive and occurring as thick beds, cave formation is prominent.
• Caves normally have an opening through which cave streams are discharged
• Caves having an opening at both the ends are called tunnels.

Depositional Landforms of Groundwater
1. Stalactites and stalagmites

• They are formed when the calcium carbonates dissolved in groundwater get deposited once the water evaporates.
• These structures are commonly found in limestone caves.
• Stalactites are calcium carbonate deposits hanging as icicles while Stalagmites are calcium carbonate deposits which rise up from the floor.
• When a stalactite and stalagmite happened to join together, it gives rise to pillars or columns of different diameters.

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Evolution of land forms due to External Forces

Rocks

Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a common rock, is a combination of the minerals quartz, feldspar and biotite. The Earth’s outer solid layer, the lithosphere, is made of rock.

Feldspar and quartz are the most common minerals found in rocks. Petrology is science of rocks.

1 ROCKS

Rock Types

• Igneous Rocks — solidified from magma and lava.
• Sedimentary Rocks — the result of deposition of fragments of rocks.
• Metamorphic Rocks — formed out of existing rocks undergoing recrystallization.

Igneous Rocks

• • Formed out of magma and lava and are known as primary rocks.
  • If molten material is cooled slowly at great depths, mineral grains may be very large.
  • Sudden cooling (at the surface) results in small and smooth grains.
  • Granite, gabbro, pegmatite, basalt, etc.are some of the examples of igneous rocks.

There are two types of igneous rocks: intrusive rocks (Granite) and extrusive rocks (Basalt-Deccan Traps).

• Having their origin under conditions of high temperatures, the igneous rocks are
• Acid igneous rocks, such as granite, are less dense and are lighter in colour than basic rocks.

Plutonic Rocks or intrusive rocks Sometimes, the molten matter is not able to reach the surface and instead cools down very slowly at great depths. Slow cooling allows big-sized crystals (large grains) to be formed. Granite is a typical example. These rocks appear on the surface only after being uplifted and denuded.

Lava or Volcanic Rocks or Extrusive rocks These are formed by rapid cooling of the lava thrown out during volcanic eruptions. Rapid cooling prevents crystallization, as a result such rocks are fine-grained. Basalt is a typical example. The Deccan traps in the peninsular region is of basaltic origin. Basic rocks contain a greater proportion of basic oxides, e.g. of iron, aluminium or magnesium, and are thus denser and darker in colour.

Hypabyssal or Dyke Rocks or Intermediate rocks Hypabyssal rocks are volcanic magma which cools and solidifies in cracks, pores and hollows beneath the earth’s surface, pores, batholiths, lopoliths, phacoliths, laccoliths, sills and dykes are its examples.

Based on place and time taken in cooling of the molten matter, igneous rocks can be divided into Plutonic and Volcanic rocks.

Based on the presence of acid forming radical, silicon, igneous rocks are divided into Acid Rocks and Basic Rocks.

Sedimentary Rocks

• • Sedimentary or detrital rocks are formed as a result of denudation (weathering and erosion).
  • These deposits through compaction turn into rocks. This process is called lithification.
  • Cover 75 percent of the earth’s crust but volumetrically occupy only 5 per cent.
  • They are layered or stratified of varying thickness. Example: sandstone, shale etc.

Till or Tillite == Ice deposited sedimentary rocks. Loess == Wind deposited sediments.

• Depending upon the mode of formation, they are classified into Mechanically formed — sandstone, conglomerate, limestone, shale, loess etc. Organically formed — geyserite, chalk, limestone, coal etc. Chemically formed — chert, limestone, halite, potash etc..

Chief Characteristics of Sedimentary Rocks These rocks consist of a number of layers or strata. These rocks are characterized by marks left behind by water currents and waves etc.. These rocks have fossils of plants and animals. These rocks are generally porous and allow water to percolate through them.

Chief Characteristics of Sedimentary Rocks These rocks consist of a number of layers or strata. These rocks are characterized by marks left behind by water currents and waves etc.. These rocks have fossils of plants and animals. These rocks are generally porous and allow water to percolate through them.

Spread of Sedimentary Rocks in India Alluvial deposits in the Indo-Gangetic plain and coastal plains is of sedimentary accumulation. These deposits contain loam and clay. Different varieties of sandstone are spread over Madhya Pradesh, eastern Rajasthan, parts of Himalayas, Andhra Pradesh, Bihar and Orissa. The great Vindhyan highland in central India consists of sandstones, shales, limestones. Coal deposits occur in river basins of the Damodar, Mahanadi, Godavari in the Gondwana sedimentary deposits.

Economic Significance of Sedimentary Rocks Sedimentary rocks are not as rich in minerals of economic value as the igneous rocks. But important minerals such as hematite iron ore, phosphates, building stones, coals, petroleum and material used in cement industry are found. The decay of tiny marine organisms yields petroleum. Petroleum occurs in suitable structures only. Important minerals like bauxite, manganese, tin are derived from other rocks but are found in gravels and sands carried by water. Sedimentary rocks also yield some of the richest soils.

Metamorphic Rocks

• The word metamorphic means ‘change of form’.
• Form under the action of pressure, volume and temperature (PVT) changes.
• Metamorphism occurs when rocks are forced down to lower levels by tectonic processes or when molten magma rising through the crust comes in contact with the crustal rocks.
• Metamorphism is a process by which already consolidated rocks undergo recrystallization and reorganization of materials within original rocks.
• In the process of metamorphism in some rocks grains or minerals get arranged in layers or lines. Such an arrangement is called foliation or lineation.
• Sometimes minerals or materials of different groups are arranged into alternating thin to thick layers. Such a structure in is called banding.
• Gneissoid, slate, schist, marble, quartzite etc. are some examples of metamorphic rocks.

Causes of Metamorphism

Orogenic (Mountain Building) Movements: Such movements often take place with an interplay of folding, warping and high temperatures. These processes give existing rocks a new appearance.

Lava Inflow: The molten magmatic material inside the earth’s crust brings the surrounding rocks under the influence of intense temperature pressure and causes changes in them.

Geodynamic Forces: The omnipresent geodynamic forces such as plate tectonics also play an important role in metamorphism.

Examples of Metamorphosis

Metamorphic Rocks in India The gneisses and schists are commonly found in the Himalayas, Assam, West Bengal, Bihar, Orissa, Madhya Pradesh and Rajasthan. Quartzite is a hard rock found over Rajasthan, Bihar, Madhya Pradesh, Tamil Nadu and areas surrounding Delhi. Marble occurs near Alwar, Ajmer, Jaipur, Jodhpur in Rajasthan and parts of Narmada Valley in Madhya Pradesh. Slate, which is used as a roofing material and for writing in schools, is found over Rewari (Haryana), Kangra (Himachal Pradesh) and parts of Bihar. Graphite is found in Orissa and Andhra Pradesh.

Some Rock-Forming Minerals

Feldspar: Half the crust is composed of feldspar. It has a light colour and its main constituents are silicon, oxygen, sodium, potassium, calcium, aluminium. Quartz: It has two elements, silicon and oxygen. It has a hexagonal crystalline structure. It is uncleavaged, white or colorless. It cracks like glass and is present in sand and granite. It is used in manufacture of radio and radar. Bauxite: A hydrous oxide of aluminium, it is the Ore of aluminium. It is non-crystalline and occurs in small pellets. Cinnabar: It is mercury sulphide and mercury is derived from it. It has a brownish colour. Dolomite: A double carbonate of calcium and magnesium. It is used in cement and iron and steel industries. It is white in colour. Gypsum: It is hydrous calcium sulphate and is used in cement, fertilizer and chemical industries. Haematite: It is a red ore of iron. Magnetite: It is the black ore (or iron oxide) of iron.

THANKS

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Evolution of land forms due to External Forces

Major Landforms Mountains Plateaus Plains Desert Island Delta

Mountains

A mountain is a large landform that rises above the surrounding land in a limited area, usually in the form of a peak. A mountain is generally steeper than a hill. Mountains are formed through tectonic forces or volcanism.

• • Nearly 27% of the world’s land surface is covered by mountains.
  • It is from the mountains that up to 80% of the planet’s fresh surface water come from.
  • 12% of the world’s population lives in the mountains, but over 50% are directly or indirectly dependent on mountain resources.

A mountain may have several forms.

Mountain Ridge — a system of long, narrow and high hills. Shimla Ridge

Mountain Range — a system of mountain and hills with several ridges.Himalayas

Mountain Chain — Which have several paralleL, tong and narrow mountains belonging to different periods. Alps

Mountain Group — Consists of several unsystematic patterns of different mountain system.

Mountain System — Consists of different mountain ranges belonging to the same period. Appalachian

CLASSIFICATION OF MOUNTAINS

The mountains, on the basis of their mode of formation, can be classified as:

• • Structural or Tectonic Mountain

Fold Mountains

Block Mountains

Dome Mountains

• Volcanic Mountains/ Accumulated Mountains
• Residual Mountains/ Relict Mountains

Fold Mountains

Fold mountains form when two tectonic plates move towards each other at convergent plate boundary.

These mountains have three types as follows:

• Young Fold mountain: Some young fold mountain are Alps in Europe, the Rockies of North America, the Andes of South America, the Himalayas of Asia and Atlas of North Africa. These young fold mountains are still rising under the influence of the earth’s tectonic forces.
• ​Matured Fold mountain: The Urals, the Appalachians, the Tien Shan and the Nan Shan were formed during an earlier mountain-building period.
• Old Fold mountain: Folded mountains which have originated before tertiary period are called old fold mountain e.g. – Caledonian and Hercynian, Vindhyachal, Aravalis. These are also called relict fold mountain because of denudation.

Block Mountains

• • Block mountains are caused by faults in the crust: a plane where rocks have moved past each other.
  • The down-lifting or uplifting of land in between two parallel faults results in the formation of Block Mountains.
  • A block mountain is also called as Horst and the rift valley formed as a result of faulting is called as Graben.
  • Examples: The Sierra Nevada in North America, Black Forest Mountains in Germany etc are typical examples of Block Mountains.

Dome Mountains

• Originated by magmatic intrusions and upwraping of the crustal surface e.g. Normal domes, Lava domes, Laccolithic domes, Salt domes etc.

Volcanic Mountains or Accumulated Mountains

The mountains formed by the accumulation of volcanic materials are called as Volcanic Mountains or Mountains of accumulation. Examples: Mount Mauna Loa in Hawaii Island, Mount Popa in Myanmar, Fuji Yama in Japan etc are some examples.

Residual Mountains or Relict Mountains

Residual mountains are those mountains which have been eroded by the agents of degradation such as winds, rain, frost and running water. The hard rocks that are left behind are called residual mountains. Examples: Hills like Nilgiri, Palkonda, Parasnath and Rajmahal and Mountains like the Aravalli, the Vindhya, and the Satpura are some of the examples of Relict Mountains in India.

Economic Significance of Mountains

• Storehouse of resources: Mountains are the storehouse of natural resources. Large resources of minerals like petroleum, coal, limestone are found in mountains. The mountains are the main source of timber, lac, medical herbs, etc.
• Generation of hydro-electricity: Hydro-electricity is mainly generated from the waters of perennial rivers in the mountains.
• Abundant source of water: Perennial rivers arising in the snow fed or heavily rain-fed mountains are one of the important sources of water. They help in promoting the irrigation and provide water for many other purposes.
• Formation of fertile plains: The rivers that originate from the high mountain ranges bring silt along with water to the lower valleys. This helps in the formation of fertile plains and further the expansion of agriculture and related activities.
• Natural political frontiers: The mountains can also act as natural boundaries between two countries. They have a prominent role in protecting the country from external threats.
• Effects on climate: They serve as a climatic divide between two adjoining regions. The mountains cause orogenic rainfalls, diversion, and blocking of cold winds, etc.
• Tourist centers: The pleasant climate and beautiful sceneries of the mountains have led to their development as centers of tourist attraction.

MOUNTAIN PEAKS

Plateaus

• • Also called a high plain or a tableland, is an area of a highland, usually consisting of relatively flat terrain, that is raised significantly above the surrounding area, often with one or more sides with steep slopes. Plateaus can be formed by a number of processes, including upwelling of volcanic magma, extrusion of lava, and erosion by water and glaciers.
  • The plateaus cover about 18% of the earth’s land surface.

CLASSIFICATION OF PLATEAUS

1. Plateau formed by exogenetic processes

(I) Glacial Plateau. e.g. Garhwal plateau, Greenland. Antarctica.

(ii) Fluvial Plateau e.g. Kaimur plateau. Rhander plateau, Rewa Plateau, Rohtas Plateau.

(iii) Aeollan Plateau e.g. Potwar Plateau (Pak), Loess plateau (China)

1. Plateau formed by endogenetic processes

(i) Interniontane Plateau: The plateaus which are partly or fully enclosed by mountains are known as intermonl ane plateaus. These are the results of the mountain-building process which was accompanied by a vertical uplift of the adjoining enclosed lands. e.g. Tibetan plateau. Bolivian plateau. Péruvien plateau, Columbian plateau and Mexican plateau.

(ii) Piedmont Plateau : Situated at the foot of a mountain, they are bounded on the opposite side by a plain or an ocean. These are also called the plateaus of denudation because areas which were formerly high have now been reduced in elevation by various agents of erosion. e.g. Appalachian plateau. Patagonien plateau (Argentina).

(iii) Dome Plateau : Formed when land mass is uplifted e.g. Ozark Plateau (USA). Chotanagpur plateau (Jharkhand)

(iv) Lava Plateau : Formed due to accumulation of thick layers of basaltic lava e.g. Columbia plateau (USA). Mahab alcshwar plateau. Panchgani tableland,

(v) Continental Plateau: They rise abruptly from the lowlands or from the sea. e.g. Deccan plateau of India, Ranchi plateau, Shillong plateau, Columbia plateau (USA), Mexican plateau etc.

(vi) Coastal Plateau : e.g. Coromandel coastal upland of India.

(vii) Desert Plateau : Arabian Plateau.

(viii) Humid Plateau: e.g. Shillong Plateau, Assam Plateau, Mahabaleshwar Plateau etc.

(ix) loung Plateau: e.g. Idaho Plateau (USA), Colorado Plateau (USA). Mahabaleshwar Plateau. Khandala Upland (Maharashtra).

(x) Mature Plateau: e.g. Ranchi Plateau. Hazaribagh Plateau (Jharkhand), Appalachian Plateau (USA). (xi) Rejuvenated Plateau: e.g. Missouri Plateau (USA).

Economic significance of Plateaus

• Storehouse of minerals: Most of the minerals in the world are found in plateaus. The extraction of minerals in plateaus is relatively easier on plateaus than mountains. The major portions of industrial raw materials are obtained from plateaus. We get gold from the plateau of Western Australia; copper, diamond and gold from the plateaus of Africa; and coal, iron, manganese and mica from the Chottanagpur Plateau in India.
• Generation of hydel-power: The edges of plateaus form waterfalls which provide ideal sites for generating hydel power.
• Cool climate: The higher parts of the plateaus even in tropical and sub-tropical regions have a cool climate.
• Animal rearing and agriculture: plateaus have large grassland areas suitable for animal rearing especially sheep, goat, and cattle. The lava plateaus when compared to other plateaus are richer in minerals and hence used for agriculture as the soil is very fertile.

Plains

• • A plain is a flat, sweeping landmass that generally does not change much in elevation.
  • About 55% of the earth’s land surface is occupied by plains.
  • Most of the plain have been formed by deposition of sediments brought down by rivers.
  • Beside rivers, some plains have also been formed by the action of the wind, moving ice and tectonic activities.

CLASSIFICATION OF PLAINS

On the basis of their mode of formation, plains can be classified as:

• Structural plain
• Erosional plains
• Depositional plains

Structural Plains

Structural plains are relatively undisturbed horizontal surfaces of the Earth. They are structurally depressed areas of the world that make up some of the most extensive natural lowlands on the Earth’s surface.

These plains are uplift of a part of the sea floor usually bordering a continent, that is, the continental shelf.

Such plains include:

• The great plains of Russian platform
• The great plains of USA
• The Central lowlands of Australia
• The coastal plain lying between the Appalachian Piedmont Plateau and the Atlantic coast of south-eastern United States

Erosional Plains (Peneplains)

• Erosional plains have been leveled by various agents of denudation such as running water, rivers, wind and glacier which wear out the rugged surface and smoothens them.
• The surface of such plains is hardly smooth and hence, they are also called as Peneplains, which means almost plain.
• The plain formed by wind action is called as pediplain.
• Northern Canada, northern Europe and west Siberia are examples of such ice-eroded plains.
• Parts of Sahara in Africa are wind-eroded plain surface.
• The rivers by widening their banks and lowering the higher land between them have eroded parts of the Amazon basin into streameroded type of plains.

Depositional Plains

• Depositional plains formed by the deposition of materials brought by various agents of transportation such as glaciers, rivers, waves, and wind. Their fertility and economic relevance depend greatly on the types of sediments that are laid down.
• When plains are formed by the river deposits, they are called as riverine or alluvial plains.The Indo-Gangetic plain in Indian subcontinent, Hwang-ho plain of north China and Povalley in North Italy are some examples of alluvial plain.
• The depositions of sediments in a lake give rise to a Lacustrine Plain or Lake Plains. The Valley of Kashmir is an example of lacustrine plain.
• When plains are formed by glacial deposits, they are called as Glacial or Drift Plains.
• When the wind is the major agent of deposition, those plains are called as Loess Plains. Plains of N. China and Russian Turkistan is formed due to it.

Economic significance of Plains

• Fertile soil: The plains generally have deep and fertile soil. As they have a flat surface, the means of irrigation can be easily developed. That is why plains are called as the ‘Food baskets of the world’.
• The growth of industries: The rich agricultural resources, especially of alluvial plains, have helped in the growth of agro-based industries. Since the plains are thickly populated, plenty of labour is available for the intense cultivation and for supplying the workforce for the industries.
• Expansion of means of transportation: The flat surface of plains favours the building of roads, airports and laying down railway lines.
• Centers of civilizations: Plains are centers of many civilizations.
• Setting up of cities and towns: Easy means of transportation on land and the growth of agriculture and industries in plains have resulted in the setting up and expansion of cities and towns. The most developed trade centers and ports of the world are found in the plains only and as much as 80% of the world’s population lives here.

Delta

•​Depositional feature of triangular shape at the mouth of a river debouching either into a lake or a sea is called a delta.

Types of Delta

(a)​Bird’s foot delta, Ex-Mississippi delta

b)​Arcuate delta, Ex-Nile, Ganga, Mekong.

c)​Estuarine delta : Amazon, Ob, Vistula, and Tapi.

Their fertility and economic importance is dependent on the type of sediments brought by agents of denudation.

Desert

ISLANDS

Islands are broadly divided into four types—

(I) CONTINENTAL ISLANDS are those Islands that raise from the ‘continental shelf’, for example, the British Isles and NewfoundLand. These islands have the same geological structure, as the continents to which they are related.

(ii) OCEANIC ISLANDS are those that rise from the BOSOM of the oceans. Their geological structure will have no geological relation to that of the nearest shores. They are very often the tops of submarine mountains or submarine volcanoes.

(iii) TECTONIC ISLANDS are created by the movements in the Earth’s crust. The outer-most Layer of the earth made of rigid plates are in very slow, but constant, motion. When one plate is pushed under another plate, the top plate may scrape off pieces of the bottom plate. Over millions of years, this material piles up to form an island.

(iv) CORAL ISLANDS are the work of minute sea organisms called CORAL POLYPS. They congragate in large colonies. When the organisms (constituting biocoenosis of the coral. reafs) die, their skeletons, which are made of a substance resembling limestone, form big clusters, some of which rise above the water.

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