Changing Face of the Earth | ISC Class 11 Geography Notes

This note covers the changing face of the earth: endogenetic forces and the theories of continental drift, sea floor spreading, plate tectonics and isostasy; the three great landforms of mountains, plateaus and plains and their types; weathering and gradation; and the work of rivers, wind, glaciers, groundwater and the sea, with their erosional and depositional landforms, diagrams and Indian examples.

What are endogenetic forces, and how do slow and sudden forces differ?

The earth's crust is dynamic. Forces that operate from within the earth are called endogenetic forces, and the energy behind them comes mostly from radioactivity, rotational and tidal friction and primordial heat left from the earth's origin.

These forces build up parts of the crust, so they are mainly land-building forces. Their action is not uniform, because geothermal gradients, heat flow, crustal thickness and crustal strength all vary from place to place, and so the original crustal surface is uneven.

Endogenetic forces are grouped in two ways. By speed, they are either slow or sudden.

Slow forces act over millions of years and include the folding and warping of crustal rocks and the slow uplift or sinking of whole continents; their effects are vast but almost imperceptible in a human lifetime.

Sudden forces act in seconds or days and include earthquakes and volcanic eruptions; their effects are local but violent and immediately visible. By direction, they are either vertical or horizontal.

Vertical forces move the crust up or down, producing uplift, subsidence and broad warping, and are called epeirogenic. Horizontal forces push the crust sideways, producing compression, tension, folding and faulting, and are called orogenic when they build mountains.

Definition: Endogenetic forces are the internal forces of the earth, driven by energy from within, which elevate or build up parts of the earth's surface and so create relief.

The two great endogenetic processes are diastrophism and volcanism. Diastrophism covers all processes that move, elevate or build up portions of the crust: orogenic processes (mountain building through severe folding along long, narrow belts), epeirogenic processes (uplift or warping of large parts of the crust), earthquakes (local, relatively minor movements) and plate tectonics (horizontal movement of crustal plates).

Orogeny is a mountain-building process, whereas epeirogeny is a continental-building process. Volcanism includes the movement of molten rock, or magma, onto or toward the surface and the formation of intrusive and extrusive volcanic forms.

Folding and its types

When horizontal compressional forces act on layers of sedimentary rock that are still plastic, the layers bend into wave-like folds. The upfolded arch is an anticline, the downfolded trough is a syncline, and the sloping sides of a fold are its limbs.

• Symmetrical fold: both limbs dip at about the same angle, formed where the compression is gentle and equal from both sides.
• Asymmetrical fold: one limb is steeper and shorter than the other.
• Overturned fold: one limb is pushed over the other, so that both limbs dip in the same direction.
• Recumbent fold: the fold is pushed over so far that it lies almost horizontal.
• Overthrust fold or nappe: under very strong pressure a recumbent fold breaks and its upper limb is pushed forward over other rocks, sometimes for many kilometres, as in the Alps and the Himalayas.

Faulting and its types

When rocks are too brittle to bend, tension or compression makes them crack, and the blocks on either side move along the crack. Such a fracture with displacement is a fault, and the surface along which the blocks move is the fault plane.

• Normal fault: tension pulls the crust apart and one block slips down along the fault plane.
• Reverse fault: compression pushes one block up and over the other.
• Thrust fault: a reverse fault whose fault plane has a low angle, so one block rides far over the other.
• Tear or strike-slip fault: the blocks move horizontally past each other, as along the San Andreas Fault in California.

Two parallel normal faults produce two contrasting landforms. Where the block between them sinks, a rift valley forms, such as the East African Rift Valley, the Rhine rift valley, and the Narmada and Tapi valleys of India.

Where the block between them stands up, or the land on both sides sinks, a block mountain or horst forms, such as the Vosges and the Black Forest on either side of the Rhine rift valley.

Folds and faults (to draw). Draw five small sections showing a symmetrical, an asymmetrical, an overturned and a recumbent fold and a nappe, and three blocks showing a normal, a reverse and a tear fault, with arrows for the direction of force and movement. Add a rift valley between two normal faults.

What is the theory of continental drift?

The coastlines on either side of the Atlantic Ocean show a striking symmetry. Abraham Ortelius, a Dutch map maker, first proposed as early as 1596 that the two Americas, Europe and Africa may once have been joined.

Antonio Pellegrini drew a map showing the three continents together. But it was Alfred Wegener, a German meteorologist, who put forth a comprehensive argument in 1912 as the continental drift theory.

According to Wegener, all the continents once formed a single continental mass surrounded by a mega ocean. The supercontinent was named Pangaea, meaning all earth, and the mega ocean was called Panthalassa, meaning all water.

He argued that around 200 million years ago Pangaea began to split. It first broke into two large masses, Laurasia in the north and Gondwanaland in the south, and these later broke into the smaller continents that exist today.

Evidence for drift

Wegener offered several kinds of evidence:

1. Jig-saw fit: the shorelines of Africa and South America facing each other have a remarkable and unmistakable match. Bullard presented a computer-fitted map of the Atlantic margin in 1964, tried at the 1,000-fathom line instead of the present shoreline, and it proved to be quite perfect.
2. Rocks of the same age: radiometric dating shows that the belt of ancient rocks of 2,000 million years from the Brazil coast matches those of western Africa. The earliest marine deposits along the South American and African coasts are of Jurassic age, suggesting the ocean did not exist before that time.
3. Tillite: tillite is the sedimentary rock formed out of deposits of glaciers. The Gondwana system of sediments from India has thick tillite at its base, indicating extensive and prolonged glaciation, and its counterparts are found in Africa, the Falkland Island, Madagascar, Antarctica and Australia.
4. Placer deposits: rich placer deposits of gold occur on the Ghana coast with an absolute absence of source rock, while the gold-bearing veins are in Brazil. The gold of Ghana was evidently derived from the Brazil plateau when the two continents lay side by side.
5. Distribution of fossils: identical species of plants and animals adapted to land or fresh water are found on either side of marine barriers. Lemurs occur in India, Madagascar and Africa, which led some to consider a contiguous landmass called Lemuria. Skeletons of Mesosaurus, a small reptile adapted to shallow brackish water, are found only in the Southern Cape province of South Africa and the Iraver formations of Brazil, two localities now 4,800 km apart with an ocean between them.

Forces for drifting

For the force of drifting, Wegener suggested the pole-fleeing force and the tidal force. The pole-fleeing force relates to the rotation of the earth, which gives the earth a bulge at the equator.

The tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed these forces would become effective when applied over many million years, but most scholars considered them totally inadequate.

What is the theory of isostasy?

Isostasy is the state of balance, or equilibrium, in which the earth's crustal blocks stand with respect to one another, so that a block of lighter crust floats in the denser material beneath it much as a block of wood floats in water. High land must therefore be made either of thicker crust or of lighter crust, so that every column of crust exerts the same pressure at some depth below. The theory explains why the great continental blocks and the ocean floors stand at different levels and yet remain in balance.

Two classic explanations were put forward in 1855, both prompted by survey work in India's Himalayan region. J. H.

Pratt, a British mathematician who was Archdeacon of Calcutta, held that the crust is made of columns of different density that all end at the same depth, the level of compensation: a mountain stands high because its column is made of lighter rock, while lowlands and ocean floors are made of denser rock.

G. B. Airy held instead that the crust has the same density everywhere but different thickness, so a high mountain has a deep root that sinks into the denser layer below, as a large block of wood floats both higher and deeper than a small one.

The British geologist Arthur Holmes added that the layer below the crust is not rigid.

In the 1930s he proposed that slow convection currents in the mantle, kept going by heat from radioactive decay, can move the crust, and that the crust keeps adjusting isostatically as it is thickened in some places and thinned in others.

Note: Isostasy is a state of balance, not a force. When erosion wears a mountain down, the crust slowly rises to restore the balance, and when ice sheets or sediments load a region, it slowly sinks.

How did sea floor spreading and plate tectonics replace the drift theory?

After the Second World War, mapping of the ocean floor gave new information. Expeditions revealed that the ocean floor is not a vast plain but is full of relief, with submerged mountain ranges and deep trenches mostly near the continent margins.

The mid-oceanic ridges were found to be most active in volcanic eruptions, and rocks on either side of a ridge crest, at equal distances from it, showed remarkable similarities in their constituents and their age.

Palaeomagnetic evidence

Palaeomagnetic studies of rocks from oceanic regions revealed five key facts:

1. All along the mid-oceanic ridges, volcanic eruptions are common and bring huge amounts of lava to the surface.
2. Rocks equidistant on either side of the crest show remarkable similarities in period of formation, chemical composition and magnetic properties. Rocks closer to the ridge have normal polarity and are the youngest; age increases as one moves away from the crest.
3. Ocean crust rocks are much younger than continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old, while some continental rock formations are as old as 3,200 million years.
4. The sediments on the ocean floor are unexpectedly very thin, and nowhere is the sediment column older than 200 million years.
5. Deep trenches have deep-seated earthquakes, while in mid-oceanic ridge areas the earthquake foci are shallow.

These facts led Hess in 1961 to propose the hypothesis of sea floor spreading. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust and new lava wedges into it, pushing the crust on either side, so the ocean floor spreads.

The younger age of the oceanic crust, and the fact that the spreading of one ocean does not cause the shrinking of the other, made him think about the consumption of oceanic crust: the floor pushed by eruptions at the crest sinks down at the oceanic trenches and is consumed.

Draw a cutaway block of the crust and upper mantle and label a divergent plate boundary with an oceanic spreading ridge, a convergent plate boundary with a trench and a subducting plate, a transform plate boundary, a continental rift zone, a strato volcano, a shield volcano, a hotspot, the lithosphere, the asthenosphere, oceanic crust and continental crust.

Plate tectonics

In 1967, McKenzie and Parker, and also Morgan, independently collected the available ideas and came out with the concept of plate tectonics.

A tectonic plate, also called a lithospheric plate, is a massive, irregularly shaped slab of solid rock, generally composed of both continental and oceanic lithosphere. Plates move horizontally over the asthenosphere as rigid units.

The lithosphere includes the crust and top mantle, with thickness varying between 5 and 100 km in oceanic parts and about 200 km in continental areas.

A plate may be called a continental plate or an oceanic plate depending on which occupies the larger portion: the Pacific plate is largely oceanic, while the Eurasian plate may be called continental.

The theory proposes that the lithosphere is divided into seven major and some minor plates.

The major plates are the Antarctic plate with the surrounding oceanic plate; the North American plate; the South American plate; the Pacific plate; the India-Australia-New Zealand plate; the African plate with the eastern Atlantic floor; and the Eurasian plate with the adjacent oceanic plate.

Important minor plates are the Cocos plate (between Central America and the Pacific plate), the Nazca plate (between South America and the Pacific plate), the Arabian plate (mostly the Saudi Arabian landmass), the Philippine plate (between the Asiatic and Pacific plates) and the Caroline plate (between the Philippine and Indian plates, north of New Guinea).

Mark the seven major plates and the five minor plates named above, and draw the Mid-Atlantic Ridge, the East Pacific Rise, the San Andreas Fault, the Aleutian Trench and the Java Trench, using different lines for divergent, convergent and transform boundaries.

Types of plate boundary

There are three types of plate boundary. At divergent boundaries new crust is generated as plates pull away from each other; these are spreading sites, and the best-known example is the Mid-Atlantic Ridge, where the American plate or plates are separated from the Eurasian and African plates.

At convergent boundaries crust is destroyed as one plate dives under another; the location where sinking occurs is called a subduction zone, and convergence can occur between an oceanic and a continental plate, between two oceanic plates, or between two continental plates.

At transform boundaries crust is neither produced nor destroyed as plates slide horizontally past each other; transform faults are planes of separation generally perpendicular to the mid-oceanic ridges.

The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement.

The Arctic Ridge has the slowest rate, less than 2.5 cm/yr, and the East Pacific Rise near Easter Island, about 3,400 km west of Chile, has the fastest rate, more than 15 cm/yr.

The driving force is the slow movement of hot, softened mantle below the rigid plates. Heated material rises, spreads, cools and sinks back, and the cycle repeats to generate a convection cell.

Heat within the earth comes from two main sources, radioactive decay and residual heat, and Arthur Holmes first considered this idea in the 1930s.

How did the Indian plate move, and what did it create?

The Indian plate includes Peninsular India and the Australian continental portions. Its northern boundary is the subduction zone along the Himalayas, in the form of continent-to-continent convergence.

In the east it extends through the Arakan Yoma mountains of Myanmar towards the island arc along the Java Trench, and its eastern margin is a spreading site east of Australia in the form of an oceanic ridge in the south-west Pacific.

The western margin follows the Kirthar Mountain of Pakistan and extends along the Makran coast, joining the spreading site from the Red Sea rift south-eastward along the Chagos Archipelago.

The boundary with the Antarctic plate is also marked by an oceanic ridge running roughly west to east.

India was a large island situated off the Australian coast in a vast ocean. The Tethys Sea separated it from the Asian continent till about 225 million years ago.

India is supposed to have started her northward journey about 200 million years ago, when Pangaea broke, and collided with Asia about 40 to 50 million years ago, causing rapid uplift of the Himalayas.

About 140 million years before the present, the subcontinent was located as far south as 50°S latitude.

During the movement towards the Eurasian plate, a major event was the outpouring of lava and the formation of the Deccan Traps, which started somewhere around 60 million years ago and continued for a long period; the subcontinent was still close to the equator at that time.

From 40 million years ago and thereafter the formation of the Himalayas took place, and scientists believe the process is still continuing, so the height of the Himalayas is rising even today.

Draw the Eurasian plate and the Indian Ocean, mark the equator, and plot the position of India at 71, 55, 38 and 10 million years ago and India today, showing the northward journey and the closing of the Tethys Sea.

What are mountains, plateaus and plains, and how are they classified?

The three main landforms of the earth differ in height, slope and the way their surface is made. A mountain is a high, elevated landform with steep slopes and a small summit area, standing well above the surrounding land.

A plateau is an elevated tableland with a comparatively flat top and steep sides, standing above the surrounding plain.

A plain is a broad, low, almost level area of land, usually below 200 m and built of sediments brought down by rivers or other agents.

Types of mountains

Mountains are classified on the basis of their origin or mode of formation:

• Fold mountains are formed by the folding of sedimentary rocks along converging plate margins. Examples are the Himalayas, the Alps and the Andes.
• Block mountains are formed when a block of crust is raised between two faults, or when the land on either side sinks. Examples are the Vosges and the Black Forest in Europe and the Sierra Nevada in North America.
• Volcanic mountains are built by the eruption and piling up of lava and ash. Examples are Mount Fuji in Japan and Mount Kilimanjaro in Africa.
• Residual mountains are old, worn-down mountains left standing as remnants after long erosion. Examples are the Aravalli and the Vindhya ranges in India.

Types of plateaus

Plateaus are classified on the basis of their situation:

• Intermontane plateaus lie enclosed between mountain ranges. The Tibetan Plateau, between the Himalayas and the Kunlun, is the highest in the world.
• Piedmont plateaus lie at the foot of a mountain range, on its plainward side. The Appalachian plateau and the Patagonian plateau are examples.
• Continental plateaus are vast plateaus standing in the interior of a continent, away from mountains. The Deccan Plateau of India and the East African Plateau are examples.

Plains are classified on the basis of formation, and the depositional plains are the most widespread. Depositional plains are built of sediments laid down by rivers, wind, glaciers or the sea.

The Ganga plain is a river-built plain, the Northern European plain is largely a glacial and river plain, and the delta plains of the Ganga, the Nile and the Mississippi are built by river deposition at the coast.

What is the difference between weathering and gradation?

Weathering is the 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. Gradation is the phenomenon of wearing down of relief variations of the surface of the earth through erosion, together with the filling up of basins and depressions. Weathering breaks the rock in place; gradation moves the broken material and lowers the land.

The exogenic processes derive their energy from the atmosphere, ultimately from the sun, and from the gradients created by tectonic factors.

All of them are covered under the general term denudation, which means to strip off or uncover, and which includes weathering, mass wasting, erosion and transportation.

Weathering prepares the rock for gradation: erosion cannot be significant if the rocks are not weathered, and weathering aids mass wasting, erosion and the reduction of relief. Weathering also prepares the way for the formation of regolith and soils.

Forests depend on the depth of the weathering mantle, and biomes and biodiversity depend on forests.

Draw a flow chart in which denudational processes branch into weathering, mass movements and erosion and transportation, and against each write its driving force: gravitational, molecular stresses and chemical actions for weathering, gravitational force for mass movements, and kinetic energy for erosion and transportation.

Types of weathering

There are three major groups of weathering processes, and very rarely does any one operate completely by itself, though a dominance of one can often be seen.

• Chemical weathering includes solution, carbonation, hydration, oxidation and reduction, which act on rocks to decompose, dissolve or reduce them to a fine clastic state through chemical reactions by oxygen, surface or soil water and other acids. Water and air, along with heat, must be present to speed up the reactions.
• Physical or mechanical weathering depends on applied forces: gravitational forces such as overburden pressure, load and shearing stress; expansion forces due to temperature changes, crystal growth or animal activity; and water pressures controlled by wetting and drying cycles. Most physical weathering is caused by thermal expansion and pressure release.
• Biological weathering is the contribution to or removal of minerals and ions from the weathering environment, and physical changes due to the growth or movement of organisms. Burrowing and wedging by earthworms, termites and rodents expose new surfaces, and plant roots exert tremendous pressure, mechanically breaking materials apart.

Exfoliation is the flaking off of more or less curved sheets or shells from over rocks or bedrock, leaving smooth and rounded surfaces. It is a result, not a process, and can occur due to expansion and contraction induced by temperature changes. Exfoliation domes and tors result from unloading and thermal expansion respectively.

How do rivers shape the land?

A river's work is controlled by its base level, the lowest level to which it can erode, usually sea level. A river erodes in three ways. Headward erosion extends the valley upstream into the source region.

Vertical erosion deepens the channel, and dominates in the upper course where the gradient is steep. Lateral erosion widens the valley, and dominates in the lower course where the gradient is gentle.

The load is transported in solution, in suspension, by saltation (bouncing along the bed) and by traction (rolling and dragging).

Deposition occurs when the velocity and energy of the river fall on gentler slopes; the coarser material is deposited first and the finer material later.

1. The river cuts a deep, narrow channel in its upper course, where vertical erosion dominates and the valley sides are steep.
2. Headward erosion lengthens the valley upstream, and tributaries cut back into the uplands, so the drainage basin grows.
3. In the middle course lateral erosion widens the valley and the river begins to meander across its floodplain.
4. In the lower course the gradient is gentle, deposition exceeds erosion, and the river builds levees, floodplains and finally a delta at its mouth.

Landforms made by rivers

The landforms made by rivers include the following. A V-shaped valley is a narrow, steep-sided valley cut by vertical erosion in the upper course.

A gorge is a very narrow, deep valley with almost vertical sides, cut where a river crosses hard rock.

A waterfall forms where a river drops over a band of hard rock that lies above softer rock, which is worn back faster. Levees are natural embankments of coarse sediment built along the banks when a river overflows.

A floodplain is the flat land on either side of a river in its lower course, built of fine sediment laid down by repeated floods.

A meander is a curve or bend in a river's course, formed where the current swings from side to side across a flat plain.

A braided channel is a river channel divided into many shallow channels by bars of sediment, common where the load is heavy and the flow variable.

An oxbow lake is a crescent-shaped lake left when a meander loop is cut off from the main channel.

A delta is the triangular plain of sediment built where a river enters a standing body of water and drops its load; the delta plain is the low, flat land of the delta itself.

Draw the course of a river from source to sea and label a V-shaped valley and a waterfall in the upper course, meanders and an oxbow lake in the middle course, and levees, a floodplain, a braided channel and a delta in the lower course.

Case study: the Ganga delta and the Sundarbans

The Ganga and the Brahmaputra together carry an enormous load of silt from the Himalayas and drop it where they meet the Bay of Bengal, building the world's largest delta.

The delta plain is a maze of distributaries, tidal creeks and low islands, and its seaward part is the Sundarbans, the mangrove forest of India and Bangladesh.

The heavy silt load and the gentle gradient make the lower Ganga a classic example of deposition at work, and the shifting distributaries show how a delta plain is built and rebuilt over time.

How does wind shape the land?

Wind erodes in three ways. Abrasion is the wearing away of rock by sand and dust carried by the wind. Attrition is the wearing down of the carried grains themselves as they collide with one another.

Deflation is the lifting and removal of loose, dry material from the surface by the wind.

The ideal conditions for wind erosion in hot deserts are a dry surface with little or no vegetation, strong and frequent winds, abundant loose sand and dust, and a large daily range of temperature that breaks up the rock.

The landforms resulting from wind erosion are:

• Deflation hollows, shallow depressions scooped out where the wind has removed loose material, such as the Qattara Depression in Egypt.
• Pedestal rocks, or mushroom rocks, formed where abrasion near the ground wears away the lower part of a rock faster than the top, leaving a narrow stem and a broad cap.
• Yardangs, long, narrow ridges aligned with the prevailing wind, carved from rock of uneven hardness.
• Desert pavement, a surface of closely packed pebbles left behind after the wind has blown away the finer material.

Depositional landforms of wind

The landforms resulting from wind deposition are sand dunes and loess. A sand dune is a mound or ridge of sand built by the wind. Barchans are crescent-shaped dunes with horns pointing downwind, formed where sand supply is limited.

Seif dunes are long, narrow ridges parallel to the wind. Transverse dunes are ridges at right angles to the wind, and star dunes are pyramidal mounds with arms radiating out, formed where the wind blows from several directions.

Loess is a deposit of fine, wind-blown silt and clay, often forming fertile soils far from its desert source.

Draw a desert scene and label a deflation hollow, a pedestal rock, a yardang and a desert pavement on the eroded surface, and a barchan, a seif dune and a sheet of loess on the depositional side, with arrows for the prevailing wind.

Case study: the Thar Desert

The Thar Desert of Rajasthan and adjoining Pakistan is a hot desert of sand, bare rock and strong winds.

Strong south-westerly winds in summer shift its barchan and seif dunes, and dust storms carry its fine dust far to the east over the northern plains.

Bare, wind-scoured rocky surfaces between the dunes show deflation and abrasion at work, and the shifting dunes are a constant problem for roads, canals and settlements.

How do glaciers shape the land?

A glacier is a mass of ice that moves slowly under its own weight. A continental glacier is a vast ice sheet covering a whole landmass, such as the ice sheet of Antarctica or the one that once covered northern Europe.

A mountain or valley glacier is a long tongue of ice confined between valley walls, such as the Siachen Glacier in the Karakoram.

Glacial erosion works in three ways. Plucking is the lifting and tearing away of blocks of rock frozen into the base and sides of the ice.

Abrasion is the wearing away of the bed and sides by rock fragments carried in the ice. Attrition is the wearing down of those fragments as they knock against one another.

The main erosional feature is the cirque, a steep-sided, armchair-shaped hollow cut at the head of a glacier; when filled with water it becomes a tarn.

The main depositional features are moraines, the ridges of unsorted rock debris left by a glacier.

A lateral moraine lies along the sides of the glacier, a medial moraine runs down its middle where two glaciers join, a terminal moraine marks the farthest point reached by the ice, and a ground moraine is a sheet of debris left beneath the ice.

Draw a valley glacier from its cirque to its snout and label the cirque and tarn, the lateral, medial and terminal moraines, and the ground moraine, with arrows showing the direction of ice flow.

Case study: the Siachen Glacier

The Siachen Glacier in the eastern Karakoram is one of the longest valley glaciers outside the polar regions. It flows between steep rock walls, plucking and abrading its bed, and its snout feeds the Nubra river with meltwater.

Dark stripes of medial moraine run down its surface where tributary glaciers join it, and the glacier is a working example of a mountain glacier in the Karakoram.

How does groundwater shape the land?

Groundwater is the water held in the pores and cracks of rock below the surface. It erodes in two ways. Solution is the dissolving of soluble rock, especially limestone, by rainwater made slightly acidic by carbon dioxide. Corrasion is the wearing away of rock by the load carried by the moving water. The landscape formed by the solution of limestone is called karst topography, after the Kras region of Slovenia. It develops only where the rocks are permeable and soluble and water is available.

The features of karst topography include sinkholes, swallow holes, caverns and underground streams. The depositional features are formed when water dripping in a cave loses carbon dioxide and deposits calcium carbonate.

A stalactite is the icicle-like deposit hanging from the roof of a cave. A stalagmite is the pillar-like deposit growing up from the floor. Where the two meet, a pillar is formed.

Draw a limestone region in section and label a sinkhole and swallow hole on the surface, a cavern and an underground stream below, and stalactites hanging from the roof and stalagmites rising from the floor of the cave.

Case study: the Borra Caves

The Borra Caves in the Ananthagiri hills of Andhra Pradesh are cut in limestone of the Eastern Ghats.

The Gosthani river has dissolved the rock along joints and bedding planes to open a large cavern, and the roof and floor are hung and studded with stalactites and stalagmites of calcium carbonate.

The caves show both the solution and the deposition work of groundwater in one place.

How does the sea shape the coast?

Waves, tides and currents erode the coast by hydraulic action, corrasion, attrition and solution. The erosional features of the coast are the following.

A sea cliff is a steep rock face at the edge of the sea, cut back by wave action. A sea cave is a hollow worn into a cliff along a line of weakness.

A sea arch is formed when a cave is worn right through a headland and the sea passes through it.

A headland and inlet pattern is produced where hard rock stands out as a headland and soft rock is worn back into a bay.

A stack is an isolated pillar of rock left standing in the sea after the arch above it has collapsed.

Depositional features of the coast

The depositional features are the following. A bay is a broad, curved inlet of the sea, often lined with a beach.

A bar is a ridge of sand and shingle built across a bay or the mouth of a river by waves and currents. A lagoon is the shallow stretch of water cut off behind a bar.

Draw a headland and a bay and label a sea cliff, a sea cave, a sea arch and a stack on the headland, and a beach, a bar and a lagoon in the bay.

Case study: the Kerala coast and Chilika Lake

The Malabar coast of Kerala is a low, sandy coast where waves and currents have built long sand bars parallel to the shore, cutting off stretches of backwater behind them.

The Vembanad backwaters, behind a bar, are a lagoon-like stretch of water used for fishing and inland transport.

On the east coast, Chilika Lake in Odisha is the largest brackish lagoon in India, separated from the Bay of Bengal by a long sand bar, and it shows the same process of bar building and lagoon formation.

Glossary

• Endogenetic force: an internal force of the earth, driven by energy from within, which builds up or elevates parts of the crust.
• Exogenetic force: an external force, driven ultimately by the sun, which wears down the land by weathering, mass wasting, erosion and deposition.
• Pangaea: the single supercontinent, meaning all earth, that Wegener believed held all the continents about 200 million years ago.
• Panthalassa: the mega ocean, meaning all water, that surrounded Pangaea.
• Isostasy: the state of balance in which crustal blocks of different height, thickness or density float in the denser material beneath them.
• Sea floor spreading: the hypothesis that new oceanic crust forms at ridge crests and spreads outward on either side.
• Tectonic plate: a massive, irregularly shaped slab of solid rock, of continental or oceanic lithosphere, moving over the asthenosphere.
• Subduction zone: the place where one plate sinks beneath another at a convergent boundary.
• Weathering: the mechanical disintegration and chemical decomposition of rocks in place by the elements of weather and climate.
• Gradation: the wearing down of relief by erosion and the filling up of basins by deposition.
• Base level: the lowest level to which a river can erode, usually sea level.
• Yardang: a long, narrow ridge of rock carved by wind abrasion and aligned with the prevailing wind.
• Cirque: a steep-sided, armchair-shaped hollow cut by a glacier at the head of its valley.
• Karst topography: the landscape of hollows, caves and underground drainage formed by the solution of limestone.
• Lagoon: a shallow stretch of water cut off from the sea by a bar or spit.

Common errors and misconceptions

• Misconception: Continents plough through the ocean floor. Correct: It is not the continent that moves but the plate; continents are parts of plates, and the plates move over the asthenosphere as rigid units.
• Misconception: Wegener's drift theory was accepted at once. Correct: Most scholars considered his pole-fleeing and tidal forces totally inadequate, and the theory was revived only after sea floor spreading and plate tectonics.
• Misconception: Weathering and erosion are the same. Correct: Weathering is an in-situ process with little or no movement of material; erosion involves the acquisition and transportation of rock debris.
• Misconception: Wind erosion is mainly the wind blowing dust away. Correct: Deflation is only one process; abrasion and attrition also wear the rock and the carried grains.
• Misconception: Stalactites grow up from the cave floor. Correct: Stalactites hang from the roof; stalagmites grow up from the floor.
• Misconception: A stack is the same as a sea arch. Correct: An arch is a passage through a headland; a stack is the isolated pillar left when the arch collapses.
• Misconception: All plateaus are high mountains with flat tops. Correct: Plateaus are classified by situation as intermontane, piedmont and continental, and their height varies greatly.

Exam-style questions with model answers

Q1. State two forces that Wegener suggested for the drifting of continents. [2 marks]

1. Wegener suggested the pole-fleeing force, which relates to the rotation of the earth and its equatorial bulge.
2. He also suggested the tidal force, due to the attraction of the moon and the sun, which develops tides in oceanic waters.

Q2. Distinguish between a transform boundary and a convergent boundary of plates. [3 marks]

1. At a transform boundary the crust is neither produced nor destroyed, as the plates slide horizontally past each other.
2. At a convergent boundary the crust is destroyed as one plate dives under another at a subduction zone.
3. Transform faults are planes of separation generally perpendicular to the mid-oceanic ridges, while convergence may occur between an oceanic and a continental plate, between two oceanic plates, or between two continental plates.

Q3. Explain the evidence for continental drift from tillite and from placer deposits. [4 marks]

1. Tillite is the sedimentary rock formed out of deposits of glaciers.
2. The Gondwana system of sediments from India has thick tillite at its base, showing extensive and prolonged glaciation, and its counterparts are found in Africa, the Falkland Island, Madagascar, Antarctica and Australia, so these landmasses had remarkably similar histories.
3. Rich placer deposits of gold occur on the Ghana coast with an absolute absence of source rock in the region.
4. The gold-bearing veins are in Brazil, so the gold of Ghana was derived from the Brazil plateau when the two continents lay side by side.

Q4. Describe the three types of plate boundary and give one example of each. [5 marks]

1. At a divergent boundary new crust is generated as plates pull away from each other; these are spreading sites, and the best-known example is the Mid-Atlantic Ridge, where the American plate or plates are separated from the Eurasian and African plates.
2. At a convergent boundary crust is destroyed as one plate dives under another, and the sinking location is called a subduction zone; convergence can occur between an oceanic and a continental plate, between two oceanic plates, or between two continental plates, as along the Himalayas, where the Indian plate meets the Eurasian plate in continent-to-continent convergence.
3. At a transform boundary crust is neither produced nor destroyed as plates slide horizontally past each other; transform faults are planes of separation generally perpendicular to the mid-oceanic ridges, and the San Andreas Fault is a well-known example.
4. The strips of normal and reverse magnetic field that parallel the mid-oceanic ridges help scientists determine the rates of plate movement, which vary from less than 2.5 cm/yr at the Arctic Ridge to more than 15 cm/yr at the East Pacific Rise.
5. The driving force is the slow movement of hot, softened mantle below the rigid plates, which rises, spreads, cools and sinks in a convection cell.

Q5. Explain the difference between weathering and gradation, and state the role of weathering in gradation. [5 marks]

1. Weathering is the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.
2. As very little or no motion of materials takes place in weathering, it is an in-situ, or on-site, process.
3. Gradation is the wearing down of relief variations of the surface of the earth through erosion, together with the filling up of basins and depressions.
4. Weathering prepares the rock for gradation: erosion cannot be significant if the rocks are not weathered, and weathering aids mass wasting, erosion and the reduction of relief.
5. Weathering also prepares the way for the formation of regolith and soils. Forests depend on the depth of the weathering mantle, and biomes and biodiversity depend on forests.

Q6. Describe the erosional landforms made by wind in hot deserts. [4 marks]

1. Deflation hollows are shallow depressions scooped out where the wind has removed loose material.
2. Pedestal rocks, or mushroom rocks, are formed where abrasion near the ground wears away the lower part of a rock faster than the top, leaving a narrow stem and a broad cap.
3. Yardangs are long, narrow ridges aligned with the prevailing wind, carved from rock of uneven hardness.
4. Desert pavement is a surface of closely packed pebbles left behind after the wind has blown away the finer material.

Q7. Explain how a river builds a delta, and name one Indian example. [3 marks]

1. A river carries a heavy load of sediment in its lower course, where the gradient is gentle and deposition exceeds erosion.
2. When the river enters a standing body of water, its velocity and energy fall and it drops its load, building a triangular plain of sediment called a delta.
3. The Ganga and the Brahmaputra together build the world's largest delta at the Bay of Bengal, whose seaward part is the Sundarbans.

Q8. Describe the depositional landforms of a glaciated valley. [4 marks]

1. Moraines are the ridges of unsorted rock debris left by a glacier.
2. A lateral moraine lies along the sides of the glacier, and a medial moraine runs down its middle where two glaciers join.
3. A terminal moraine marks the farthest point reached by the ice.
4. A ground moraine is a sheet of debris left beneath the ice.

Key takeaways

• Endogenetic forces build the land and exogenetic forces wear it down, and relief survives only while the two opposing groups continue to act.
• Wegener's drift theory rested on the jig-saw fit, matching rocks, tillite, placer deposits and fossils, but its forces were judged inadequate.
• Sea floor spreading and palaeomagnetic stripes led to plate tectonics, in which rigid plates move over the asthenosphere.
• Isostasy is the state of balance in which crustal blocks of different height, thickness or density float in the denser material beneath.
• Mountains are classified by origin, plateaus by situation and plains by formation, each with examples from the world.
• Weathering is an in-situ process, while gradation moves material and lowers the land; weathering prepares rock for erosion.
• Rivers erode headward, vertically and laterally, and deposit levees, floodplains, meanders, oxbow lakes and deltas.
• Wind, ice, groundwater and the sea each leave a distinct set of erosional and depositional landforms, seen in India as well.

Test yourself

Who first proposed that Europe, Africa and America may once have lain side by side?

Abraham Ortelius, a Dutch map maker, proposed the possibility as early as 1596, and Antonio Pellegrini later drew a map showing the three continents together.

What are the two sources of heat within the earth that drive convection?

Heat within the earth comes from two main sources, radioactive decay and residual heat, and it drives the slow movement of hot, softened mantle below the rigid plates.

Name the three major groups of weathering processes.

The three major groups are chemical weathering, physical or mechanical weathering, and biological weathering, and very rarely does any one operate completely by itself.

What is the difference between a stalactite and a stalagmite?

A stalactite is the icicle-like deposit hanging from the roof of a cave, while a stalagmite is the pillar-like deposit growing up from the floor; where the two meet, a pillar is formed.

How is a stack formed on a rocky coast?

A sea cave is worn right through a headland to form a sea arch, and when the arch collapses, the isolated pillar of rock left standing in the sea is a stack.

Give the three classifications of plateaus with one example each.

Intermontane plateaus lie between mountains, such as the Tibetan Plateau; piedmont plateaus lie at the foot of a range, such as the Patagonian plateau; continental plateaus stand in the interior, such as the Deccan Plateau.

What is the base level of a river?

The base level is the lowest level to which a river can erode, usually sea level, and it controls how far down the river can cut its valley.