Chapter 13 — भू-आकृतिक प्रक्रियाएँ तथा पृथ्वी के प्रमुख स्थलरूप

The surface of the earth is never still. Mountains rise, rivers cut valleys, deltas grow, coasts retreat. The word geomorphology means the study of the form of the earth, and a geomorphic process is any physical or chemical action that changes the shape of the earth's surface. All such processes belong to one of two great groups, distinguished by where their energy comes from.

Endogenic processes (from the Greek for born within) are driven by forces inside the earth. Their energy is the heat of the interior, left over from the earth's formation and constantly generated by radioactive decay in the mantle and core. This heat makes the material of the mantle circulate very slowly, and the movement drags, bends, breaks and lifts the crust. Endogenic processes are of two kinds: slow movements, called diastrophism, which take millions of years and build continents, plateaus and fold mountains; and sudden movements, earthquakes and volcanic eruptions, which act in minutes or days. Because they build up the relief of the surface — raising mountains and plateaus, opening rift valleys — endogenic processes are called the constructive forces or the forces of land-building.

Exogenic processes (born outside) are driven by forces acting on the surface from outside. Their energy comes ultimately from the sun, which drives the winds, the rain, the rivers and the waves, and from gravity, which pulls everything downhill. The agents are the atmosphere, running water, moving ice, wind, the sea and underground water, and the processes are weathering (breaking rock in place), erosion (wearing away and picking up), transportation (carrying) and deposition (laying down). Exogenic processes wear high places down and fill low places up; taken together they are called gradation, the levelling of the land, and they are the destructive or land-wearing forces.

The balance between them. The landscape at any moment is the result of the contest between the two groups. Where endogenic forces are winning, as in the young Himalaya which still rises about 5 mm a year, the relief is high and rugged; where exogenic forces have had a long time and the interior has been quiet, as on the ancient Deccan and Chota Nagpur plateaus, the relief is low and rounded. If the internal forces ever stopped, rivers and weather would in time reduce every continent to a flat plain near sea level; if the external forces stopped, mountains would rise without limit. Neither happens, and the earth's surface is the changing product of both.

Time scale. Some processes are visible within a human lifetime — a landslide, a flood that shifts a river, a volcanic cone that grows in a year. Most are slow: it took about 50 million years to raise the Himalaya, and the Ganga delta has been building for perhaps 10 million. The chapter deals with landforms of all these scales, but its three great landforms, mountains, plateaus and plains, are the work of the slow forces over geological time.

Diastrophism is the name for all the slow, large-scale movements of the earth's crust caused by internal forces — the bending, breaking, tilting, raising and sinking of the rocks over millions of years. The word comes from the Greek for twisting. Because the movements are so slow, they are not felt, but their results are the largest features of the earth: continents, ocean basins, plateaus and mountain ranges. Diastrophic movements are divided into two kinds by their direction and result.

Epeirogenic movements (continent-building; Greek epeiros, continent) are vertical movements that raise or lower a very large area of the crust as a whole, without folding it much. The movement is like a lift going up or down: the land rises (upliftment or emergence) or sinks (subsidence or submergence) over an area of thousands of square kilometres, and the rock layers stay nearly horizontal. When land rises, old beaches are found high above the present sea, as along the Kathiawar coast, and rivers cut down into their old flood plains to form terraces; when land sinks, valleys are drowned to make inlets (the rias of the Konkan coast), forests are found below sea level, and coastal plains are flooded. The Peninsular plateau of India has been slowly rising and tilting eastward for a very long time, which is why its rivers flow east; the Sundarbans and the Netherlands are slowly sinking. Epeirogenic movements make plateaus, continental shelves and broad basins and are the reason land and sea have repeatedly changed places in the geological past.

Orogenic movements (mountain-building; Greek oros, mountain) are horizontal or tangential movements in which the crust is squeezed from the sides. The rocks are compressed into folds or broken along faults, and the crumpled belt is raised into a mountain range. Orogeny works over a long narrow belt rather than a broad area, and it produces the greatest relief on earth. Two kinds of force are involved. Compression, pushing the crust together, produces folds — upfolds called anticlines and downfolds called synclines — and, if pressure continues, overfolds, thrust faults and nappes, sheets of rock pushed many kilometres over their neighbours; the Himalaya, the Alps, the Rockies and the Andes were all made by compression. Tension, pulling the crust apart, produces cracks called faults along which blocks slip up or down: a block that sinks between two faults is a rift valley (the Narmada and Tapi valleys, the Rhine valley, the East African rift), and a block that stands up between two faults is a block mountain or horst (the Vindhya and Satpura, the Vosges and Black Forest).

Comparison. Epeirogenic movement is vertical, affects large areas, leaves strata nearly horizontal and makes plateaus and plains; orogenic movement is horizontal, affects long narrow belts, folds and faults the strata and makes mountains. The two often act together: the Himalaya was folded by orogeny and then, with the whole Tibetan region, lifted by epeirogeny. Geologists recognise several great orogenies in the earth's history; the latest, the Alpine-Himalayan orogeny of the last 50 million years, made the young fold mountains of today.

Alongside the slow movements of diastrophism, the internal forces also act suddenly, in the two most violent events of nature. Both are treated in detail elsewhere in the syllabus; here they are placed among the geomorphic processes and their landforms are noted.

Earthquakes. An earthquake is the shaking of the ground caused by the sudden release of strain stored in rocks along a fault. As the plates move, rocks on either side of a fault are bent like a spring until they snap and slip into a new position; the stored energy travels outward as seismic waves from the focus, and the surface point above it, the epicentre, shakes most. Earthquakes are geomorphic agents in several ways. The ground may be raised or lowered by metres in a moment: the Alaska earthquake of 1964 lifted the coast 10 m in places, and the 2004 Sumatra earthquake dropped parts of the Andaman coast by a metre so that the sea moved inland. Fault scarps, fresh cliffs along the fault line, appear, as in the Rann of Kutch after the 1819 earthquake, when a 6 m ridge 80 km long, the Allah Bund, rose across the Indus channel and dammed it. Landslides are set off on slopes, rivers change course — the Brahmaputra shifted after the 1950 Assam earthquake and its bed rose — lakes are formed when landslides dam valleys, and undersea earthquakes throw up tsunamis that reshape coasts. Over geological time repeated earthquakes along a fault build the great fault-block mountains and rift valleys.

Volcanoes. A volcano is a vent through which molten rock (magma), gas and ash from below the crust reach the surface, usually where plates meet or over a hot spot. Volcanoes are the most direct builders of landform, because they add new material to the surface. Volcanic cones are built of lava and ash around the vent: steep cinder cones of ash, gently sloping shield volcanoes of fluid basalt lava (Mauna Loa in Hawaii, 9 km high from the sea floor), and the classic composite or strato-volcanoes of alternate lava and ash (Fuji, Vesuvius, Mayon). Craters at the summit and calderas, huge basins formed when a cone collapses into its emptied chamber, may fill with water to form crater lakes. Where lava is very fluid it does not build cones but spreads in sheets over vast areas as lava plateaus: the Deccan plateau of India, 5 lakh square kilometres of basalt in layers up to 2 km thick, poured out about 65 million years ago from fissures and is the largest example, giving the flat-topped, step-like (trap) hills of Maharashtra and the black cotton soil. Volcanic islands rise from the sea floor (Hawaii, Iceland, Barren Island in the Andamans), geysers and hot springs mark dying volcanic areas, and volcanic plugs and dykes of hardened magma stand out as hills when the softer cone around them is worn away. Volcanic ash also builds the most fertile soils, which is why crowded farmland surrounds Vesuvius and the volcanoes of Java.

Where they occur. Earthquakes and volcanoes cluster along the same belts, because both are caused by the movement of the plates: the Circum-Pacific belt or Ring of Fire around the Pacific, with about two-thirds of the world's earthquakes and most of its active volcanoes; the Alpine-Himalayan belt from the Mediterranean through Iran and the Himalaya to Indonesia; and the mid-ocean ridges. The theory that explains this pattern is the subject of the next topic.

For a long time the endogenic forces were a mystery: why should the crust fold here and sink there, and why should earthquakes and volcanoes follow the same narrow belts? Since the 1960s the answer has been the theory of plate tectonics, which has become the framework of all geology.

The plates. The outermost rigid shell of the earth, the lithosphere — the crust and the uppermost mantle, about 100 km thick — is not one piece but is broken into about seven large and a dozen smaller plates, like the cracked shell of a boiled egg. The large plates are the Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic; smaller ones include the Arabian, Philippine, Nazca and Burma plates. The plates float on the asthenosphere, a hot, soft, slowly flowing layer of the upper mantle, and they move over it at 2 to 10 cm a year — about the speed at which fingernails grow — carried by slow convection currents in the mantle, in which hot rock rises, spreads sideways under the plates, cools and sinks again. The Indian plate is moving north-east at about 5 cm a year.

The evidence. Alfred Wegener proposed in 1912 that the continents had drifted apart from one supercontinent, Pangaea, and pointed to the jigsaw fit of South America and Africa, the matching rocks, fossils and glacial deposits on continents now far apart (the Gondwana rocks of India, Africa, Australia, South America and Antarctica carry the same Glossopteris fossil), and the same ancient mountain belts on both sides of the Atlantic. His idea was accepted only when ocean-floor surveys in the 1950s and 60s found the mid-ocean ridges, the youth of the ocean floor, the symmetrical magnetic stripes on either side of the ridges recording sea-floor spreading, and the exact fit of earthquake belts with plate edges.

Three kinds of plate boundary.

• Divergent (constructive) boundaries, where plates move apart: magma rises to fill the gap and forms new crust, building the mid-ocean ridges (the Mid-Atlantic Ridge, on which Iceland sits) and, on land, rift valleys such as the East African rift, which will one day become a new ocean. Shallow earthquakes and quiet basaltic volcanoes occur here.
• Convergent (destructive) boundaries, where plates collide. If an oceanic plate meets a continental one, the denser oceanic plate slides beneath in a subduction zone, forming a deep ocean trench, a chain of explosive volcanoes and a fold mountain range on the continent (the Andes, with the Peru-Chile trench); if two oceanic plates meet, an island arc forms (Japan, the Philippines, the Andaman-Nicobar chain above the Sunda trench); if two continental plates meet, neither sinks, the sediments between them are crumpled and thrust up into the highest fold mountains (the Himalaya, from the collision of India with Eurasia beginning about 50 million years ago, and the Alps from Africa and Europe). The greatest earthquakes occur at convergent boundaries.
• Transform (conservative) boundaries, where plates slide past each other sideways, neither creating nor destroying crust; the friction produces frequent earthquakes but no volcanoes. The San Andreas fault of California is the best-known example.

What the theory explains. Plate tectonics accounts for the distribution of earthquakes and volcanoes along plate edges, for the building of the fold mountains and the rift valleys, for the shape and age of the ocean basins, for the drift of the continents through time — India was once joined to Africa and Antarctica and has travelled 6,000 km north — and for the slow, continuing rise of the Himalaya. It is the unifying idea behind every endogenic landform in this chapter.

The external forces begin their work with weathering: the breaking down and decay of rocks in place by the action of the weather — temperature, water, air and living things — without any movement of the broken material. Weathering prepares the rock for erosion; the loose products, called regolith, are what rivers, wind and ice then carry away, and the finest of them, mixed with humus, become soil. Weathering is of three kinds.

1. Physical or mechanical weathering breaks the rock into smaller pieces without changing its chemical nature.

• Temperature change (insolation weathering). In deserts and on bare mountain rock the surface heats to 60–70 °C by day and cools sharply at night; the outer layer expands and contracts more than the inside, and cracks and peels off in sheets (exfoliation, giving rounded domes like those of Bankura's Susunia and the Deccan granite tors) or, in rocks of several minerals, splits into grains (granular disintegration).
• Frost action. Water in cracks freezes at night, expands by about 9 per cent and wedges the rock apart; repeated freezing and thawing shatters it into angular fragments that pile up as scree at the foot of Himalayan cliffs.
• Pressure release. When overlying rock is removed by erosion, the rock beneath expands and splits in curved sheets.
• Salt crystallisation. In dry coasts and deserts, salt water in cracks evaporates and the growing crystals prise the rock apart.

2. Chemical weathering changes the minerals of the rock into new, weaker substances by reaction with water, oxygen and carbon dioxide; it is strongest in hot, wet climates like that of Bengal.

• Oxidation: iron in the rock combines with oxygen to form rust-coloured oxides that crumble; this reddens the laterite of Purulia and Bankura.
• Carbonation: rain dissolves carbon dioxide to form weak carbonic acid, which dissolves limestone and marble, opening joints into caves and sinkholes (the karst of the Meghalaya and Borra caves).
• Hydration: minerals take up water, swell and soften, as feldspar in granite turns to clay.
• Solution: rock salt and gypsum dissolve directly in water.
• Hydrolysis: water itself reacts with minerals, the main process turning granite into kaolin clay.

3. Biological weathering is the work of living things: tree roots grow into cracks and force them open; burrowing animals, earthworms and termites loosen and mix the rock waste; lichens and mosses secrete acids that etch the rock; and humans quarry, plough and blast. Decaying vegetation adds humic acids that speed chemical decay.

Factors controlling weathering. Climate is the chief: hot wet climates favour chemical weathering, cold climates frost action, dry climates insolation and salt. Rock type: limestone weathers by carbonation, granite by hydrolysis, and rocks with many joints weather fastest. Slope: on steep slopes the products are removed quickly and fresh rock is exposed. Vegetation both protects and attacks. Time: the old plateaus of the peninsula have been weathered for hundreds of millions of years.

Importance. Weathering forms soil, without which there is no agriculture; it lowers mountains and prepares material for rivers to build plains; it concentrates ores such as bauxite and laterite iron; and it shapes the rounded hills, tors, caves and scree slopes of the landscape. The mass movement of weathered material downhill under gravity — soil creep, landslides, mudflows — is the link between weathering and erosion.

Once weathering has loosened the rock, the moving agents of the surface take over. Erosion is the wearing away of the land and the removal of the loosened material by a moving agent; transportation is the carrying of that material by the agent; deposition is its laying down when the agent loses energy. The three are stages of one continuous process, and each agent has its own name for the load it carries and the landforms it makes.

The agents of erosion.

• Running water — rain wash and rivers — is the most important agent over most of the earth. A river erodes by hydraulic action (the force of the water), abrasion or corrasion (grinding with the sand and pebbles it carries), attrition (the load wearing itself down) and solution. It carries its load in solution, in suspension (silt and clay), by saltation (sand bouncing) and by traction (rolling boulders). It erodes V-shaped valleys, gorges (the Teesta gorge), waterfalls and pot-holes in its upper course; it deposits alluvial fans and cones at the foot of hills (the Terai), meanders, flood plains, levees and ox-bow lakes in its middle course (the Ganga in Murshidabad), and deltas at its mouth (the Ganga-Brahmaputra delta, the largest in the world).
• Moving ice (glaciers) in high mountains and polar lands erodes by plucking (tearing out rock frozen to it) and abrasion (scraping with the rock it carries), making U-shaped valleys, cirques, arêtes, horns and hanging valleys in the Himalaya; it deposits moraines, drumlins and outwash plains, as in the northern plains of Europe and North America.
• Wind in deserts erodes by deflation (blowing away loose sand to leave stony floors and hollows) and abrasion (sand-blasting rocks into mushroom rocks and yardangs); it deposits sand dunes (the barchans of the Thar) and, far away, loess, the fertile wind-blown silt of northern China.
• Sea waves erode coasts by hydraulic action, abrasion and solution into cliffs, caves, arches, stacks and wave-cut platforms (the Konkan and Kanyakumari coasts); they deposit beaches, spits, bars and lagoons (Digha's beach and dunes, the Chilika lagoon).
• Groundwater dissolves limestone into sinkholes, caverns and underground rivers, and deposits stalactites and stalagmites in caves (Meghalaya, the Borra caves of Andhra Pradesh).

Gradation. All these agents work towards one end: they lower the high places and fill the low ones, tending to bring the whole land surface to a gentle, even slope towards the sea, called the base level. This levelling is called gradation, and it has two halves. Degradation (or denudation) is the wearing down of the high land by weathering, mass movement and erosion; aggradation is the building up of the low land by deposition. The material removed from the Himalaya by degradation is the material that has built the Ganga plain and the Bengal delta by aggradation: the two are the same process seen from its two ends. Gradation is never complete, because the internal forces keep raising new land, but its tendency is everywhere visible in the rounded old hills of the peninsula and the vast flat plains of the north.

The cycle of erosion. The geographer W. M. Davis described the life of a landscape as a cycle: a newly uplifted land in its youth has high relief, steep slopes and deep narrow valleys; in maturity the valleys widen, the slopes are graded and the relief is greatest in variety; in old age the land is worn to a nearly flat plain, a peneplain, with a few resistant hills, monadnocks, standing above it — as in the Chota Nagpur plateau of Purulia with its scattered Ayodhya and Panchet hills. Renewed uplift starts the cycle again.

The landforms produced by all these processes are grouped into three major types by their height and shape: mountains, plateaus and plains. Together they make the first-order relief of the continents, and every country's geography begins with them.

What a mountain is. A mountain is a part of the earth's surface that rises steeply and considerably above the surrounding land, usually to more than 600–900 m, with a small summit area, steep slopes and a large difference between its top and its foot. A lower rise is called a hill, though the distinction is not exact — the Nilgiris, called hills, rise to 2,637 m, and the Ayodhya hills of Purulia are hills at 677 m. A line of mountains is a range (the Pir Panjal), a group of ranges of the same age and origin is a system (the Himalayan system), a group of systems is a chain (the Alpine-Himalayan chain), and a broad belt of ranges is a cordillera (the Western Cordillera of North America). The mountains of the world are arranged in two great belts that follow the plate boundaries: the Circum-Pacific belt round the Pacific and the Alpine-Himalayan belt across Eurasia.

Characteristics. Mountains have great height and great differences of height within a short distance; steep slopes that make them hard to farm, build on and cross; a cold climate that grows colder with height, so that vegetation changes in belts from forest to alpine meadow to permanent snow; heavy rain or snow on the windward side and dryness in the rain shadow; swift rivers that rise in them and cut deep valleys; sparse population concentrated in valleys and on terraces; and rich resources of forest, water power, minerals and pasture.

Classification by age. Young mountains were formed in the last 50 million years, in the Tertiary era; they are high, sharp-peaked, deeply cut, still rising and prone to earthquakes — the Himalaya, Alps, Rockies, Andes. Old mountains were formed hundreds of millions of years ago, in the Palaeozoic or earlier; erosion has worn them to low, rounded ranges with gentle slopes — the Aravalli, the oldest fold mountains in the world, the Urals, the Appalachians, the Scottish Highlands. The Aravalli, once perhaps as high as the Himalaya, now rise to only 1,722 m at Guru Shikhar.

Classification by origin is the one the chapter follows. Mountains are of four main kinds:

• Fold mountains, made by the compression and folding of the crust — the highest and longest ranges.
• Block mountains, made by faulting, where a block of the crust is raised or the land on either side sinks.
• Volcanic mountains, built by the piling up of lava and ash.
• Residual or relict mountains, left standing when erosion has removed the surrounding land.

The first three are made by endogenic forces and the fourth by exogenic; the next topics take them one by one. A fifth type sometimes named is the dome mountain, where the crust is arched up without breaking by magma pushing from below, as in the Black Hills of Dakota.

Fold mountains are the highest, longest and youngest mountains of the earth, and they are formed by the folding of the crust under horizontal compression. The Himalaya, the Alps, the Rockies, the Andes, the Atlas, the Caucasus, the Zagros and the Ural are all fold mountains; the Himalaya-Alps chain stretches 10,000 km across Eurasia, and the Andes-Rockies belt runs 15,000 km down the Americas.

Formation. The story of a fold mountain has four stages. (1) A geosyncline. Between two continental masses lies a long, shallow, slowly sinking trough of the sea called a geosyncline; the Tethys sea between the Angaraland of Asia and the Gondwanaland of India was one. Rivers from the two lands pour sediment into it for tens of millions of years, and as the floor sinks under the weight the sediments accumulate to a thickness of many kilometres — sandstone, shale, limestone full of marine shells. (2) Compression. The two continental plates, driven by convection currents in the mantle, move towards each other and the geosyncline is squeezed between them like a carpet pushed from both ends. (3) Folding. The soft, layered sediments buckle into a series of wave-like folds, upfolds or anticlines and downfolds or synclines; as pressure continues the folds tilt over into asymmetrical folds, overturned folds and recumbent folds, and finally break along thrust faults so that great sheets of rock, nappes, ride forward over the rocks in front. (4) Uplift. The crumpled mass, thickened and lightened, rises like a cork to form the mountain range, and the process may be repeated in several pulses; the Himalaya rose in three main phases that produced its three parallel ranges, the Himadri, the Himachal and the Shiwalik, from north to south.

The Himalaya. About 200 million years ago India broke away from Gondwanaland and drifted north across the Tethys. About 50 million years ago it collided with the Eurasian plate; the Tethys sediments were folded and thrust up, and the collision continues — India still pushes north at about 5 cm a year, the mountains still rise about 5 mm a year, and the great earthquakes of Kangra (1905), Bihar-Nepal (1934), Assam (1950) and Nepal (2015) mark the strain. Marine fossils, ammonites and shells, are found in the limestone of the Spiti valley at 5,000 m and on Everest's summit rock, proving that these peaks were once sea floor.

Characteristics of fold mountains. They are arc-shaped, convex towards the side from which they were pushed (the Himalaya bows south); they occur in long parallel ranges with valleys between; they are made mainly of sedimentary rocks (with metamorphosed cores) containing marine fossils; they have the highest peaks (Everest 8,849 m, Aconcagua 6,961 m, Mont Blanc 4,808 m) and the sharpest relief; they are young, mostly of the Tertiary era, and so are still rising and shaken by earthquakes; they carry glaciers and give rise to the largest rivers; and they are rich in minerals, water power, forests and scenery. Old fold mountains such as the Aravalli, Appalachians and Urals, folded in far earlier orogenies, are now worn low and quiet.

Importance. The Himalaya shields India from the cold winds of Central Asia, forces the monsoon to give up its rain, feeds the Ganga, Indus and Brahmaputra with snowmelt, stores hydroelectric power, holds forests, pastures, tea and orchards, and has been India's northern rampart and its place of pilgrimage. The Darjeeling Himalaya of West Bengal is its eastern part, with Sandakphu (3,636 m) as the state's highest point.

When the crust is stretched or squeezed but is too rigid to fold, it breaks along faults, and the blocks between the faults move up or down. A block that stands high between faults is a block mountain; a block that sinks between faults is a rift valley. The two are made by the same process and usually occur together.

Faulting. A fault is a fracture in the rocks along which movement has taken place. Under tension (pulling apart) a block slides down along an inclined fault — a normal fault; under compression a block is pushed up over the other — a reverse or thrust fault; where blocks slide sideways past each other it is a strike-slip fault (the San Andreas). The cliff produced at the surface where one side has moved relative to the other is a fault scarp.

Formation of block mountains. There are two ways. (a) Two roughly parallel faults form, and the block between them is raised by internal forces while the land on either side stays where it is; the raised block, called a horst, stands up with steep fault-scarp sides and a flat or gently rolling top. (b) Two parallel faults form and the land on either side sinks while the central block stays where it is; the central block is again left standing as a block mountain. In both cases the mountain has a flat summit, since it is a piece of the old surface, and steep straight sides, since they are fault scarps. Where a single fault forms and one side is raised, the result is a tilted block with one steep fault-scarp face and one gentle back slope, as in the Sierra Nevada of California.

Formation of rift valleys. When the block between two parallel faults sinks, or the blocks on either side rise, a long, narrow, steep-sided, flat-floored trough is left — a rift valley or graben. Rift valleys are often very long, often hold lakes or rivers, and are often the first stage of an ocean opening. The greatest is the East African rift, 6,000 km from Syria through the Red Sea, Ethiopia, Kenya and Tanzania to Mozambique, holding the Dead Sea, the Red Sea and lakes Tanganyika and Malawi; the Rhine rift valley lies between the Vosges and the Black Forest; and in India the Narmada and Tapi valleys are rifts between the Vindhya and Satpura blocks, which is why these two rivers, unlike all others of the peninsula, flow west in straight troughs to the Arabian Sea.

Examples of block mountains. In India the Vindhya and the Satpura are the classic examples: flat-topped, steep-sided blocks left standing when the Narmada trough sank between them. The Vosges in France and the Black Forest in Germany are horsts on either side of the Rhine rift. The Sierra Nevada is a tilted block 600 km long; the Harz in Germany, the Salt Range in Pakistan and the block mountains of the Basin and Range province of Nevada are others. In West Bengal the small plateau blocks of Purulia, bounded by faults, belong to this family.

Characteristics. Block mountains have flat or table-like summits (they are old surfaces lifted bodily), steep, straight, cliff-like sides (fault scarps), are usually lower than fold mountains, are made of any kind of rock, are often associated with rift valleys, and are found in regions of old, rigid crust that could not fold. The relative movement along the faults may continue, so earthquakes occur along the scarps.

Volcanic mountains are built by the accumulation of lava, ash, cinders and rock fragments thrown out of a volcano around its vent. Unlike fold and block mountains, which are made by moving pieces of crust that already exist, volcanic mountains are made of new material added to the surface, and they can grow visibly: Paricutin in Mexico rose from a farmer's field to 424 m in a year (1943), and Anak Krakatoa has grown from the sea since 1927. They are also called mountains of accumulation.

Formation and shape. Magma from the mantle rises through a vent and erupts. If the lava is thick and gas-rich, the eruption is explosive, throws out ash and cinders, and builds a steep-sided cinder cone or, with alternating layers of ash and lava over many eruptions, a tall, symmetrical composite cone or strato-volcano — Fujiyama in Japan, Vesuvius and Etna in Italy, Mayon in the Philippines, Cotopaxi in Ecuador, Kilimanjaro in Africa (5,895 m, the highest free-standing mountain on earth), Mount Rainier and Mount St Helens in the USA. If the lava is fluid basalt, it flows far before hardening and builds a broad, gently sloping shield volcano — Mauna Loa and Mauna Kea in Hawaii, which rise 9 km from the ocean floor, higher than Everest from base to summit. A crater at the summit may hold a lake, and if the top collapses into the emptied magma chamber a huge caldera forms — Crater Lake in Oregon, Lake Toba in Sumatra. Volcanic mountains are typically conical, isolated rather than in ranges (though they may stand in lines along a plate boundary or a hot-spot trail, like the Hawaiian chain), and are found along the Ring of Fire, the mid-ocean ridges and the rift valleys. India has few: Barren Island (active) and Narcondam (dormant) in the Andaman Sea, and the ancient volcanic hills of the Deccan.

Residual or relict mountains are the only mountains made not by internal forces but by the external forces of erosion. They are the remnants left standing when weathering and erosion have removed a plateau or an old mountain mass around them, because their rock was harder or they lay away from the main rivers. They are also called mountains of circumdenudation or dissected mountains. In Davis's cycle they are the monadnocks of an old-age landscape.

Formation. A high plateau or an old fold mountain is attacked by rivers, which cut it into blocks; the softer rocks are worn away faster, and the harder ones — quartzite, granite, basalt cappings — are left as isolated hills and ridges above the lowered surface. The Nilgiris, Palni and Cardamom hills of the south, the Parasnath hill of Jharkhand (1,365 m), the Rajmahal hills, the Girnar in Gujarat, the Mahadeo hills and the hills of the Eastern Ghats are all residual mountains, cut out of the Peninsular plateau; so, on a small scale, are the Ayodhya, Biharinath and Susunia hills of Purulia and Bankura, left above the peneplain of the Chota Nagpur fringe. The Aravalli, though folded in origin, has been so worn that it is often counted as residual; and the Highlands of Scotland, the Catskills of New York and Monadnock itself in New Hampshire are world examples.

Characteristics of residual mountains. They are low (usually under 1,500 m), rounded or flat-topped, isolated or in short ridges, made of hard resistant rock, occur on old plateaus in regions long free of internal disturbance, and are rich in minerals exposed by the erosion around them. Their existence is the proof that erosion has removed hundreds of metres of rock from the plateaus of the peninsula.

A plateau is an extensive area of high, flat or gently rolling land that rises sharply above the surrounding country on at least one side. It is a tableland: high like a mountain but flat like a plain on top, with a large summit area, and it is usually bounded by steep edges called escarpments. Plateaus range from a few hundred metres to over 4,000 m in height — the Tibetan plateau, the roof of the world, averages 4,500 m — and they cover about a third of the earth's land, more than mountains.

Characteristics. A plateau has a flat or undulating summit of great extent; steep sides or escarpments on one or more edges; a considerable height above the neighbouring lowland; rivers that have cut deep valleys and gorges into it, so that an old plateau is dissected into a rolling surface with steep-sided valleys; usually thin, poor soils on hard old rock except where lava has weathered to black soil; and, because plateaus are typically made of ancient crystalline or volcanic rock, great mineral wealth — coal, iron, manganese, bauxite, gold, copper.

Classification by mode of origin.

• Tectonic plateaus are raised by epeirogenic uplift of a large block of the crust, with the strata left nearly horizontal; the Deccan (in part), the plateau of Arabia, the Meseta of Spain and most of the old continental shields are of this kind.
• Volcanic or lava plateaus are built by sheet after sheet of fluid basalt pouring from fissures; the Deccan Trap of Maharashtra and neighbouring states, the Columbia-Snake plateau of the north-west USA, the plateau of Ethiopia and the Antrim plateau of Northern Ireland are examples.
• Dissected or erosional plateaus are old uplands so deeply cut by rivers that they have become a maze of flat-topped hills and valleys; the Chota Nagpur plateau, the Colorado plateau with its Grand Canyon, and the Scottish Highlands are examples.
• Glacial plateaus are formed where ice has planed the surface flat, as in Greenland and parts of Canada.

Classification by location.

• Intermontane plateaus lie between mountain ranges, enclosed on all sides, and are the highest and largest: the Tibetan plateau between the Himalaya and the Kunlun (4,500 m, 25 lakh sq km), the plateau of Iran between the Zagros and the Elburz, the Bolivian plateau (Altiplano) between the two ranges of the Andes, the Mexican plateau, and in India the small plateau of Ladakh.
• Piedmont plateaus (foot-of-the-mountain) lie at the base of a mountain range with the mountain on one side and the plain or sea on the other; the Piedmont plateau of the eastern USA between the Appalachians and the Atlantic coast, the plateau of Patagonia between the Andes and the Atlantic, and the Malwa plateau at the foot of the Vindhya are examples.
• Continental plateaus rise directly from the plains or the sea and form the heart of a continent, usually on ancient shield rock; the plateau of Peninsular India as a whole, the plateaus of Africa, Australia, Arabia, Greenland and Antarctica are the largest. They are also called plateaus of accumulation when built of lava, and shield plateaus when of old crystalline rock.
• Coastal plateaus border the sea, like the Coromandel coast plateau fragments.

These classes overlap: the Deccan is at once continental (by location), partly volcanic and partly tectonic (by origin) and, at its edges, dissected. The next topic looks at India's plateau in detail, including the part of it that lies in West Bengal.

The Peninsular plateau is the oldest and most stable part of India, a triangular tableland of about 16 lakh square kilometres south of the Ganga plain, made of the ancient crystalline rocks of Gondwanaland — granite, gneiss and schist over 2,500 million years old — and, in the north-west, of the basalt of the Deccan Trap. It has never been under the sea since the Cambrian and has been worn by erosion for so long that its surface is a series of rolling uplands 600 to 900 m high with residual hills, tilted gently towards the east so that all its major rivers except the Narmada and Tapi flow to the Bay of Bengal. The plateau is bounded by the Aravalli in the north-west, the Vindhya-Satpura and the Ganga plain in the north, the Western Ghats in the west and the Eastern Ghats in the east, and it is divided into two parts.

The Central Highlands north of the Narmada: the Malwa plateau of lava and black soil, drained by the Chambal and Betwa into the Yamuna; the Bundelkhand and Baghelkhand uplands of granite; and, in the east, the Chota Nagpur plateau, described below.

The Deccan plateau south of the Narmada: the Deccan Trap region of Maharashtra, where fissure eruptions 65 million years ago poured out layer upon layer of basalt that weathered into the black cotton soil; the crystalline plateaus of Karnataka and Telangana; and the Western and Eastern Ghats along its edges. The Western Ghats are the true escarpment of the plateau, 1,000–1,600 m high and continuous; the Eastern Ghats are lower, broken residual hills. The two meet in the Nilgiris. The plateau holds most of India's minerals — iron, manganese, coal, bauxite, mica, gold — and nearly all its hydroelectric sites on the Ghats.

The Chota Nagpur plateau. This north-eastern lobe of the Peninsular plateau covers most of Jharkhand and extends into the western districts of West Bengal, northern Odisha and Chhattisgarh. It is a dissected plateau of Archaean granite and gneiss, with Gondwana sandstones and coal seams preserved in the faulted Damodar valley, and it rises in steps: the Pat lands of the west above 1,000 m, the Ranchi plateau at about 700 m, the Hazaribagh plateau at 600 m and, lowest, the Rajmahal hills and the eastern fringe that slopes down at 150–300 m into Purulia, Bankura, Bardhaman and Birbhum. The rivers that drain it — Damodar, Subarnarekha, Barakar, Ajay, Kangsabati — have cut deep valleys and gorges and fall over its edges in waterfalls such as Hundru and Dassam; residual hills of hard rock, Parasnath (1,365 m) and the Ayodhya hills (Gorgaburu 677 m), stand above its surface, and the whole is a peneplain in Davis's old age. Its lateritic soil is poor, its rainfall runs off the hard rock, and drought is common; but beneath it lies the richest mineral belt of India — the Jharia and Raniganj coal, the iron of Singhbhum, the copper of Ghatsila, the mica of Koderma, the bauxite of Lohardaga, the uranium of Jaduguda — so that this poor plateau carries the industrial cities of Jamshedpur, Bokaro, Dhanbad, Asansol and Durgapur and is called the Ruhr of India.

The West Bengal plateau fringe. Purulia, western Bankura, Paschim Bardhaman, western Birbhum and Jhargram form the eastern edge of the Chota Nagpur plateau: a rolling laterite upland with the Ayodhya, Baghmundi, Panchet, Biharinath and Susunia hills as residual mountains, the Raniganj Gondwana basin with its coal, and the Damodar, Ajay and Kangsabati flowing east off it to the delta. Here the student can see all the plateau's features — the flat summit, the steep edge, the dissected valleys, the residual hills, the thin soil and the buried wealth — within a day's journey of Kolkata.

A plain is an extensive area of flat or gently undulating land of low elevation, usually less than 200 m above sea level, with very little difference of height within it. Plains cover about 55 per cent of the earth's land surface, and they are the home of most of its people: the great civilisations of the Nile, the Tigris-Euphrates, the Indus, the Ganga and the Huang He all rose on plains.

Characteristics. Plains have a flat or gently rolling surface with a gradient often of only a few centimetres per kilometre; low altitude; usually deep, fertile soil where they are built of river or glacial deposits; slow, meandering rivers that flood and shift; easy transport by road, rail and water; and dense population and intensive agriculture. Not all plains are fertile — the Sahara is a plain — but the alluvial plains are the world's granaries.

Classification by origin. Plains are of three kinds by how they were made.

1. Structural plains are formed by the uplift of a part of the sea floor or the continental shelf, or by the subsidence of land, with the rock strata left horizontal; they are flat because they were laid down flat under the sea. The Great Plains of North America, the plains of the Russian platform, the south-eastern plain of England and, in India, the coastal plains of the Coromandel and the raised plains of Kathiawar are examples.

2. Erosional plains are formed by the wearing down

3. Depositional plains are formed by the laying down of material carried by rivers, glaciers, wind or the sea; they are the most extensive and the most fertile, and they are divided by the agent that built them.

• Alluvial plains, built by rivers, are of three sub-types along the river's course: the piedmont alluvial plain of coarse gravel and sand at the foot of the mountains, formed of merged alluvial fans (the bhabar and Terai of the Himalayan foot, the Duars of North Bengal); the flood plain of fine silt spread by annual floods in the middle course, with levees and ox-bow lakes (the great plain of the Ganga from Punjab to Bihar, the plains of the Mississippi and the Huang He); and the delta plain at the mouth (the Ganga-Brahmaputra delta of Bengal, the Nile delta, the Mississippi delta).
• Glacial plains: till plains of unsorted boulder clay left by melting ice sheets (the northern plains of Germany, Poland and the American Mid-West) and outwash plains of sorted sand and gravel washed out by meltwater streams beyond the ice front.
• Loess plains, built of fine yellow dust blown by wind from deserts and glacial outwash and laid down as a thick fertile blanket; the loess plateau-plain of northern China along the Huang He, up to 300 m thick, and the loess belts of the Danube and the Mississippi.
• Coastal and marine plains, built by the sea depositing sand and mud along the shore or by the emergence of the shallow sea floor; the Medinipur coastal plain of West Bengal, the coastal plains of the Gulf of Mexico.
• Lacustrine plains, the flat floors of former lakes filled with sediment and drained; the Valley of Kashmir was a lake bed, and so were the flat farmlands round Chicago.

India has all three kinds within its borders, but its greatest plain, the northern plain of the Indus, Ganga and Brahmaputra, is a depositional alluvial plain, and it is the subject of the next topic.

The northern plain of India. Between the Himalaya and the Peninsular plateau lies the largest alluvial plain in the world, about 2,400 km from the Indus delta to the Brahmaputra valley and 150–300 km wide, covering some 7 lakh square kilometres. Its origin is tied to the mountains: when the Himalaya rose, a long trough or foredeep formed between it and the rigid peninsula, and into this the Indus, Ganga, Brahmaputra and their tributaries have poured the debris of the eroding mountains for millions of years, filling it with alluvium 1,000 to 2,000 m deep, in places far deeper. The plain is almost featureless — from Delhi to Kolkata, 1,400 km, the land falls only 200 m — and it is divided into belts by the age and texture of the deposits: the bhabar, a narrow belt of gravel at the mountain foot where the streams sink underground; the Terai, the wet marshy belt below it where they reappear; the bhangar, the older alluvium of the raised interfluves, with beds of lime nodules (kankar); and the khadar, the newer alluvium of the flood plains renewed every year. This plain, the Punjab-Haryana, Ganga and Brahmaputra plains together, holds over 40 per cent of India's population.

The Bengal delta. Where the Ganga and Brahmaputra reach the Bay of Bengal they have built the largest delta on earth, about 1 lakh square kilometres in India and Bangladesh. A delta forms when a river laden with silt enters still water, slows, drops its load and, choked by its own deposits, splits into many distributaries that spread the sediment in a fan. The Ganga delta is an arcuate delta, its seaward edge a broad curve; it has been growing seaward for millions of years and still advances, and it is divided from the Bhagirathi eastward into the moribund delta of Nadia and Murshidabad, where the river has abandoned its channels, the mature delta of Kolkata and the northern 24 Parganas, and the active delta of the Sundarbans, still flooded by the tides and still being built. Its soil is deep, its water table shallow, and it carries the densest rural population in India.

Importance of the three landforms.

• Mountains are the source of rivers and of the water that irrigates the plains; they hold forests, pastures and wildlife; they store hydroelectric power; they contain minerals; they shield the land from cold winds and force the monsoon to rain, as the Himalaya does for India; they attract tourists and pilgrims; and they form natural frontiers. Their steep slopes, cold, isolation and earthquakes limit farming, settlement and transport.
• Plateaus are the storehouses of minerals and the sites of the great industrial regions — the Chota Nagpur plateau carries India's coal and steel; they give hydroelectricity where rivers fall over their edges; their lava soils grow cotton and their grasslands feed cattle; their cool tablelands in the tropics, like the Deccan and the East African plateau, are healthy places to live. Their thin soils, uncertain rain and dissected surfaces make agriculture hard.
• Plains are the cradle of civilisation: their deep alluvial soil and abundant water make them the granaries of the world; their flatness makes roads, railways, canals and cities easy to build; their rivers give transport and fish; and they hold the densest populations on earth. Their dangers are floods, waterlogging and, in the delta, cyclones and salinity.

West Bengal is a model of all three: the Darjeeling Himalaya in the north, the Chota Nagpur plateau fringe in the west and the Ganga delta plain in the centre and south, each with the resources and the problems of its landform. The whole chapter can be read in a journey from Sandakphu through Purulia to the Sundarbans.