Shaping of the Earth's Surface: Class 9 Social Science · Chapter 2

The ground under your school feels perfectly still. Yet the Himalaya is still rising by a few millimetres every year. The Brahmaputra carries mountains of silt to the Bay of Bengal, and the sea keeps nibbling at India's coastline. The Earth's surface is never finished. Forces from deep inside push it up and crack it, while rain, rivers, ice, wind and waves wear it down and rebuild it elsewhere.

The Western Ghats, the Narmada valley, the Chota Nagpur plateau, the Ganga plain, the dunes of Jaisalmer and the beach at Puri are each a landform (स्थलरूप). Two teams of forces make them. Internal forces, powered by the Earth's inner heat, lift, fold and break the crust. External forces, such as running water and wind, slowly grind the high places down and spread the waste over the low places. This chapter looks at both teams, and then at the disasters that strike when the land changes suddenly.

2.1 Plate tectonics

Look at a world map. The bulge of South America would fit neatly into the bend of West Africa, like two torn pieces of a page. Puzzles like this led scientists to ask whether continents move. In 1967–68 the American geophysicist W. Jason Morgan described the Earth's outer shell as a set of rigid blocks moving against one another. Dan McKenzie and Robert Parker reached the same idea at about the same time. Their idea grew into the theory of plate tectonics, which today explains mountains, ocean basins, earthquakes and volcanoes with one simple picture.

2.1.1 Inside the Earth

No one has drilled more than about 12 km into the Earth. Scientists learn about its interior from earthquake waves, which bend and change speed as they pass through different materials. These clues show three main layers, like the skin, flesh and stone of a mango (Fig. 2.1).

• Crust: the thin, solid outer skin we live on. It is only about 5–10 km thick under the oceans but 30–70 km thick under the continents, and thickest beneath high ranges such as the Himalaya.
• Mantle: a very thick layer of hot rock reaching down to about 2,900 km. It is mostly solid, but over millions of years it can flow very slowly, like toffee left in the sun.
• Core: the centre, made mainly of iron and nickel. The outer core is liquid metal; the inner core, under enormous pressure, is a solid ball and the hottest and densest part of the planet.

Geographers also divide the outer part of the Earth by how stiff it is. The crust together with the top, rigid part of the mantle forms the lithosphere (स्थलमंडल), roughly 100 km thick. Below it lies the asthenosphere, a hot layer that is soft and partly molten. Think of the lithosphere as the shell of a cracked boiled egg. It is split into pieces, and each piece can slide over the soft layer beneath.

2.1.2 Tectonic plates and why they move

Each piece of the lithosphere is a tectonic plate (विवर्तनिक प्लेट): a gigantic slab of solid rock, often thousands of kilometres across. Plates creep along at a few centimetres a year, roughly as fast as your fingernails grow. The Indian Plate is one of the faster ones, moving north-east at about 4–5 cm a year. That sounds tiny, but over 10 million years it adds up to 400–500 km.

Geographers also sort plates by what rides on top of them. Some bear mainly land (continental plates), some mainly sea floor (oceanic plates), and many bear a mix of the two (mixed plates). Oceanic crust is thinner but heavier than continental crust, which matters when two plates collide.

• Seven major plates: Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic.
• Minor plates fill the gaps between them, for example the Nazca, Arabian, Philippine and Caribbean plates.
• Many recent maps split the Indo-Australian Plate into an Indian Plate and an Australian Plate, because the two parts move slightly differently.

What moves such enormous slabs? The answer lies in the mantle. Heat flows out from the core and from radioactive elements in the rocks. Mantle rock near the bottom warms up, becomes a little lighter and rises very slowly. Near the top it spreads sideways, cools, grows heavier and sinks again. These slow loops are convection currents (संवहन धारा). Like a slow conveyor belt under a carpet, they carry the plates along with them (Fig. 2.2).

2.1.3 Plate boundaries

The most dramatic changes happen where plates meet, along a plate boundary. Plates can meet in three ways, and each way leaves its own signature on the land (Fig. 2.3).

• Convergent boundary (plates move towards each other). When two continental plates collide, neither sinks easily, so the crust crumples and is pushed up into fold mountains. The Himalaya rose this way as the Indian Plate pushed into the Eurasian Plate. When an oceanic plate meets a continental plate, the heavier oceanic plate bends down and slides underneath (this is called subduction). A deep trench forms on the sea floor, and melting rock rises to feed volcanoes on land, as in the Andes. Strong earthquakes are common at both kinds of collision.
• Divergent boundary (plates move apart). Hot mantle rock wells up into the gap, partly melts as the pressure on it drops, and hardens into new crust. Under the oceans this builds long underwater mountain chains called mid-ocean ridges; the Mid-Atlantic Ridge runs down the middle of the Atlantic, and Iceland sits on top of it.
• Transform boundary (plates slide past each other). Crust is neither made nor destroyed, but the plates lock, strain and then jerk free, causing earthquakes. The San Andreas Fault in California, USA, is the best-known example.

2.1.4 Earthquakes, volcanoes and the Ring of Fire

Because plates jostle mainly at their edges, earthquakes and volcanoes are not scattered at random. On a world map they trace narrow belts that match the plate boundaries almost line for line. The busiest belt loops around the Pacific Ocean. It runs from New Zealand through Indonesia, the Philippines and Japan to Alaska, and then down the west coast of the Americas to Chile. This horseshoe is called the Ring of Fire. According to the US Geological Survey, about 90 per cent of the world's earthquakes happen along it, and it holds most of the world's active volcanoes.

India lies away from the Pacific, but it is not safe from earthquakes. The Indian Plate is still pushing into Asia, and the strain builds up along the Himalaya and the North-East. On India's national map of seismic zones, the highest-risk zone covers parts of the Himalaya, the North-East, the Rann of Kachchh and the Andaman and Nicobar Islands.

• Drop, cover and hold on: get under a strong table and hold it until the shaking stops; stay away from windows and tall cupboards.
• If you are outdoors, move to an open space away from buildings, trees, poles and wires.
• Do not use lifts. After the shaking stops, leave carefully and expect aftershocks.
• Prevention matters most: buildings designed for earthquakes, and a family plan with an emergency kit.

A volcano is an opening in the crust through which molten rock escapes, along with gases, ash and rock fragments. Molten rock is called magma underground and lava once it reaches the surface. Repeated eruptions pile these materials into a mountain. Most volcanoes stand where plates pull apart or where one plate sinks beneath another.

Plate tectonics, then, explains why the continents and oceans are where they are, why mountains rise, and why some places shake far more often than others. That last point is very practical. By mapping plate boundaries and past quakes, planners can mark hazard zones, set stricter building rules there and prepare people in advance.

2.2 Weathering and erosion

Internal forces build the land up. External forces take it apart, grain by grain, and they never stop. Two processes do most of this work. Weathering loosens and breaks rock, and erosion carries the broken pieces away. Working together for thousands to millions of years, they lower mountains, cut valleys, spread plains, and carve caves, cliffs and deltas.

2.2.1 Weathering

Old stone steps at a temple tank are rounded and crumbly; a new granite slab is sharp-edged. The difference is weathering (अपक्षय): rock breaking down where it lies, with no transport involved. There are three kinds (Fig. 2.4).

• Physical weathering breaks rock into smaller pieces without changing what it is made of. In the Thar desert, rock heats up sharply by day and cools at night; the repeated swelling and shrinking cracks its outer layers. In Ladakh, water seeps into cracks, freezes, expands and wedges the rock apart.
• Chemical weathering changes the minerals themselves. Rainwater, slightly acidic because it dissolves carbon dioxide from the air, eats into limestone and marble. Oxygen reacts with iron in rocks and stains them reddish-brown, much as an iron gate rusts.
• Biological weathering is done by living things. A peepal sapling sprouting from an old fort wall sends roots into a crack and slowly splits the stones. Burrowing animals, and lichens that release weak acids, also loosen rock.

Weathering is not only destructive. The fine particles it produces, mixed over time with rotting leaves and living organisms, become soil. Every field of wheat or rice grows on rock that weathering broke down long ago.

2.2.2 Erosion

Erosion (अपरदन) begins where weathering stops: the loosened soil and rock are picked up and carried away by a moving agent. Watch a heavy monsoon shower on a bare slope and you can see it happen, as muddy water streams downhill. Erosion has many agents.

• Water erosion: raindrops, sheets of runoff, gullies and rivers carry soil away. The deep ravines along the Chambal river were cut this way.
• Wind erosion: where rain is scarce and the ground bare, as in western Rajasthan, gusts strip off the loose top layer of soil.
• Glacial erosion: a glacier freezes onto rock fragments and grinds them along its bed like a giant file.
• Coastal erosion: waves batter the shore and wear the land back.

Erosion shapes landforms, but it also shapes people's lives and work.

• Farmers lose the thin, fertile topsoil that took centuries to form, so harvests shrink and more fertiliser is needed.
• Families by rivers and coasts can see fields, houses and village roads swallowed by the water. Majuli, the river island in the Brahmaputra, has lost a large part of its area to bank erosion.
• Builders and miners face unstable slopes, sinking foundations and collapsing pits.
• Tourism and fishing suffer when beaches wash away and rivers fill with silt.

2.2.3 Protecting land from erosion

On a slope, the faster water runs, the more soil it carries. So almost every method of saving soil on hills tries to slow the water down and let it soak in (Fig. 2.5).

• Contour trenches (continuous contour trenches, or CCTs): shallow trenches dug along a contour line, joining points at the same height, so that each one catches runoff and lets it sink into the ground.
• Contour bunding: a low ridge of earth or stones laid across the slope at one level. Runoff is stopped behind it, so its silt settles there and the water has time to seep in.
• Terracing: cutting a hillside into a staircase of level fields. Water stays on each flat step instead of rushing downhill. Terraced fields are a familiar sight in Uttarakhand, Himachal Pradesh and the North-East.
• Check dams: low walls of stone or earth across a seasonal stream. Water pools behind them, its silt settles, and more of it seeps into the ground instead of rushing away.

2.3 Agents of gradation

If nothing pushed the land up again, external forces would in the end grind every continent down to a low, flat surface. The natural forces that do this levelling are the agents of gradation. Each of them erodes (wears away and removes material), transports it, and deposits it (drops it) somewhere lower. So every agent makes two families of landforms: some carved by erosion and some built by deposition (Table 2.3).

2.3.1 Running water

Rivers are the busiest agents on land. A river's journey from the mountains to the sea is usually divided into three stretches, and each has its own landforms (Fig. 2.6).

• Upper course: the slope is steep and the water fast, so the river cuts mostly downward, making deep V-shaped valleys, gorges, waterfalls and rapids.
• Middle course: the slope eases, the river cuts sideways more than downward, and it begins to swing in loops called meanders. Cut-off loops become oxbow lakes, and floods spread silt over a wide floodplain.
• Lower course: the land is almost flat and the river slow, so it deposits most of its load, building raised banks called levees and, at its mouth, a delta. Where a stream rushes out of hills onto a plain, it drops its load in a fan shape called an alluvial fan.

Waterfall

A waterfall forms where a river drops over a steep step in its bed. The step often appears where a band of hard rock lies over softer rock. The river wears away the soft rock faster, so the hard layer is left hanging as a ledge. The falling water churns a deep plunge pool at the foot and undercuts the cliff until the ledge breaks off. Then the whole process starts again a little further upstream, leaving a gorge behind (Fig. 2.7).

Waterfalls matter to people too. Jog Falls on the Sharavati river in Karnataka drops 253 m, and power stations nearby use the same river to make hydroelectricity. Falls such as Dudhsagar in Goa, Athirappilly in Kerala and Chitrakote in Chhattisgarh draw tourists and support local guides, shops and homestays. Many are also linked with local festivals and beliefs.

Meander and oxbow lake

On gentle ground a river rarely flows straight. Around any bend, the water on the outside moves faster and scours the bank, while the slower water on the inside drops sand and silt. So the outer bank keeps retreating and the inner bank keeps growing, and the bend swells into a loop called a meander (विसर्प). In time the neck of the loop becomes so narrow that a flood cuts straight through it. The abandoned loop is left behind as a curved oxbow lake (Fig. 2.8).

Meandering rivers deposit fertile silt on their floodplains, so villages and farms crowd along them, and the rivers supply water for irrigation and, in places, routes for boats. Oxbow lakes become rich wetlands: Kabartal (Kanwar Lake) in Begusarai, Bihar's first Ramsar site, is an old loop of the Burhi Gandak that was cut off long ago. People have long tapped such rivers. The Kallanai, or Grand Anicut, was built across the Kaveri near Tiruchirappalli about two thousand years ago and is credited to the Chola king Karikāla. It still diverts water into canals that irrigate the Kaveri delta.

Delta

When a river finally enters a sea or a lake, it slows down suddenly and drops the silt it has carried from far upstream. Layer by layer the deposits build a low, fan-shaped or triangular plain called a delta. The river splits into many smaller channels, called distributaries, that wind across it to the sea (Fig. 2.9). India's east coast is lined with deltas: those of the Ganga–Brahmaputra, the Mahanadi, the Godavari, the Krishna and the Kaveri.

Why do crores of people live on deltas? Food comes first: the Kaveri delta is known as the rice bowl of Tamil Nadu, and the Ganga–Brahmaputra delta grows rice and jute. Where river water mixes with the sea, fish, crabs and prawns thrive. Boats move goods along the channels, so market towns and ports grow. The price is risk: the land is low and flat, so river floods, storm surges and cyclones can sweep across it quickly.

2.3.2 Waves and currents

Waves and currents keep moving water against the coast, day and night. Where they drop sand and pebbles they build beaches; where they attack rock they carve it into striking shapes. (You will study waves and currents themselves in Part 2 of your textbook.)

Where waves lose energy, in bays and along gently sloping shores, they drop the sand and shingle they carry. That deposit is a beach, and every tide and current rearranges it a little. Currents moving along the shore can also stretch sand out into a long sand bar, sometimes enclosing a lagoon. Such a bar separates Chilika lake in Odisha from the Bay of Bengal. Beaches such as Marina in Chennai and those of Puri and Goa draw visitors and support hotels, stalls and fishing communities. They also act as natural shock absorbers that take the force of storm waves before it reaches houses.

Follow one rocky headland (land jutting into the sea) over thousands of years (Fig. 2.10). Waves armed with sand and stones find a weak joint near its tip and hollow it into a sea cave. When hollowing from both sides breaks right through, the headland has a sea arch. One day the arch roof crashes down, and the seaward end survives as a lone sea stack. Meanwhile the main headland is cut back into a steep sea cliff, like the red cliffs at Varkala in Kerala. The flat rocky floor it leaves behind, washed by every tide, is a wave-cut platform.

These features draw tourists: the rock arch known as the Natural Bridge on Shaheed Dweep in the Andaman Islands is one example. At the same time, crumbling cliffs and shrinking beaches threaten coastal roads and villages, so some stretches of coast need sea walls, tetrapods or planted mangroves for protection.

2.3.3 Glaciers

A glacier is a river of ice that slides downhill a few centimetres to a few metres a day. Frozen to its bed, it plucks out blocks of rock and uses them like sandpaper to scrape the land. Glacial erosion makes these landforms (Fig. 2.11):

• U-shaped valley: an old river valley that the ice has gouged into a broad trough, with a flat floor and cliff-like sides.
• Cirque: the armchair-shaped hollow high on a mountainside where a glacier begins; after the ice melts, it often cradles a small lake called a tarn.
• Arête: when two cirques eat into the same mountain from opposite sides, only a thin, jagged wall of rock survives between them.
• Hanging valley: the valley of a smaller side glacier, left perched high above the deepened main valley, often with a waterfall spilling from its mouth.
• Fjord: a glacial valley drowned by the sea after the ice melts, forming a long, steep-walled arm of the sea, as in Norway and New Zealand.

Picture a glacier as a slow conveyor belt loaded with rubble: rock, sand and clay, together called till. Wherever it drops this load, it leaves moraines (हिमोढ़) (Fig. 2.12). Rubble falling from the valley walls collects in ridges along each edge of the ice: these are lateral moraines. When two glaciers merge, their touching edge ridges ride on together as one medial moraine down the centre. At the front, where the ice melts as fast as it advances, the belt dumps everything in a curved terminal moraine. Long after the ice has gone, it shows how far the glacier once reached.

Glaciers are India's frozen water banks: the Gangotri glacier, where the Bhagirathi emerges at Gaumukh, keeps rivers flowing through the dry months before the monsoon. In the hills, villages, orchards and fields spread over the broad floors of old glacial valleys, and lakes trapped behind moraines supply water and sometimes power. Trekkers, climbers and skiers are drawn to cirques and glacial valleys, and in Norway ships shelter in deep, calm fjords.

2.3.4 Wind

In deserts, where there is little water or plant cover to hold the ground, wind becomes the main sculptor. It lifts loose dust and sand and hurls the grains against rock like a sandblaster. Wind erosion makes these landforms (Fig. 2.13):

• Yardangs: rock ridges shaped like an upturned boat, blunt at the end that faces the wind and tapering downwind; softer rock around them has been scoured away.
• Ventifacts: pebbles and boulders whose windward sides have been ground smooth and flat by flying sand.
• Deflation hollows (blowouts): dips left in the ground after the wind has lifted away loose sand and dust. If one is deepened down to the water table, water seeps in and an oasis appears.
• Desert pavements: the wind carries off the fine grains, and the pebbles too heavy to move settle into a tight, flat layer.

Where the wind slows, it drops its sand in hills and ridges called sand dunes. Their shape depends on how much sand there is and how steady the wind is (Fig. 2.14).

• Barchan dunes: shaped like a new moon, with the horns pointing downwind. You find them where there is not much sand and the wind keeps to one direction, as in parts of the Thar.
• Longitudinal dunes: narrow sand ridges that run for kilometres in the same direction as the wind.
• Star dunes: tall heaps with arms radiating several ways, because the wind keeps changing direction.
• Parabolic dunes: curved like a U, with the arms trailing upwind and pinned down by grasses or shrubs; common behind sandy beaches.

In a desert, water decides where people live: oases become green islands of farming, rest and trade. A dune covered with grass or shrubs stops moving, and a belt of such dunes slows the advance of the desert onto farmland. Along the coast, dunes break the force of sea winds and storm waves before they reach villages. Tourists ride camels and camp among the dunes of Sam near Jaisalmer, and builders sometimes dig dune sand for their work.

2.3.5 Underground water

Rainwater that sinks into the ground keeps working out of sight. In limestone it is especially powerful, because the slightly acidic water slowly dissolves the rock along its cracks. The result is karst topography, named after the Kras region on the border of Slovenia and Italy (Fig. 2.15).

• Caves: underground rooms and passages, some large enough to hold a building, left where the limestone has dissolved.
• Stalactites: lime columns hanging down from a cave roof, built drop by drop as water seeping through the roof leaves a trace of lime behind.
• Stalagmites: lime columns growing upward from the cave floor where the drops land. A stalactite and a stalagmite that grow until they meet make a pillar.
• Sinkholes (dolines): funnel-shaped dips at the surface, where the rock below has dissolved or a cave roof has given way.
• Underground rivers: streams that vanish into the ground and flow through cave passages, sometimes for many kilometres.

Karst country matters to people. Its springs and underground streams are valuable sources of fresh water. Caves such as the Borra Caves in Andhra Pradesh and the long cave systems of Meghalaya attract tourists, cavers and geologists. Some caves are places of worship. At the Amarnath cave in Jammu and Kashmir, pilgrims revere a column of ice rising from the floor: a stalagmite of frozen water.

2.4 Landforms and disasters

Most landforms change too slowly for anyone to notice. Sometimes, though, the change comes in minutes: a hillside slides, a slope of snow breaks away, a mountain lake bursts, or a wall of dust swallows a town. Each of these hazards belongs to a particular kind of landform, so knowing the land helps us predict where they are likely and how to prepare. A hazard becomes a disaster when it strikes people who are exposed and unprepared.

2.4.1 Landslides

A landslide is the sudden movement of rock, soil and debris down a slope. A slope stays put as long as the friction holding it is stronger than gravity pulling it down. Anything that adds weight or reduces that grip can tip the balance.

• More weight, less grip: days of heavy rain soak into soil and cracked rock, making the slope heavier and slippery inside. Water held in by blocked drains, and the load of buildings, add to the burden.
• Less support: road cuts, quarries and mines, and rivers eating into the foot of a slope remove what holds it up; clearing a forest removes the roots that knit the soil together.
• A sudden jolt: earthquakes, volcanic eruptions and even heavy blasting can shake an already weak slope loose.
• The danger is greatest on steep slopes made of shattered or deeply rotted rock.

• Prone areas: the Himalaya (Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Sikkim, the Darjeeling hills), the hills of the North-East, and the Western Ghats and Nilgiris (Kerala, Karnataka, Goa, Maharashtra, Tamil Nadu). Worldwide: steep, rainy regions such as the Andes, Nepal, Japan and the Philippines.
• Ways to reduce harm: map slopes by risk and avoid building on the most dangerous ones; plant trees and grass; build retaining walls and proper drains; issue warnings when rainfall crosses safe limits.
• Example: in the early hours of 30 July 2024, after extreme rain, landslides buried the villages of Mundakkai and Chooralmala in Wayanad, Kerala; more than 200 people were killed (figures reported by state officials, August 2024).

2.4.2 Avalanches

An avalanche is a large mass of snow, sometimes mixed with ice and rock, that suddenly races down a mountain slope. Snow piles up on a slope in layers. Think of the snowpack as a pile of blankets on a tilted bed: it stays in place only while the friction between the layers is stronger than the pull of gravity.

• Causes: the load grows when a big snowfall arrives within a day or two, or when wind dumps deep drifts on one side of a ridge. The grip between layers fails when a weak, sugary layer lies buried in the pack, or when a warm spell turns some snow to slush. Then a small jolt, such as an earthquake, a blast, a skier, a trekking party or construction work, can set the slab sliding.
• Prone areas: high slopes in Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim and Arunachal Pradesh; worldwide, the Alps, the Rockies and the Himalaya.
• Ways to reduce harm: avalanche forecasts and warnings; snow fences and barriers on known avalanche paths; forests on slopes; keeping roads and camps off avalanche paths and avoiding travel just after heavy snowfall.
• Example: on 4 October 2022, an avalanche on Draupadi ka Danda-II peak in Uttarkashi district, Uttarakhand, swept away a mountaineering training team; more than 25 trainees and instructors died.

2.4.3 Glacial lake outburst floods

As glaciers retreat, meltwater often collects in lakes held back by a natural dam of ice or loose moraine. Such a dam is not built to last. If it gives way, the lake can empty within hours, sending a glacial lake outburst flood (हिमनद झील विस्फोट बाढ़) roaring down the valley (Fig. 2.17).

• Causes: in a warming climate, glaciers shrink and their lakes deepen, so loose moraine dams must hold back more and more water. A cloudburst or a fast melt can top a lake up suddenly. Worse, a chunk of glacier, an avalanche or a landslide can plunge into the lake and push out a wave, and an earthquake can crack the dam itself.
• Prone areas: glacial lakes in Sikkim, Himachal Pradesh, Uttarakhand, Ladakh and Arunachal Pradesh; also Nepal, Bhutan, Pakistan and the Andes.
• Ways to reduce harm: watching glacial lakes with satellites and sensors; sirens and warning systems in the valleys below; lowering dangerous lakes by draining some water through pipes or channels; keeping houses, roads and dams out of the flood path.
• Example: on the night of 3–4 October 2023, South Lhonak Lake in north Sikkim burst; the flood down the Teesta destroyed the Chungthang dam of a large hydroelectric project and bridges and towns downstream, and many people were killed or went missing.

2.4.4 Dust storms

In a dust storm, strong winds pick up so much dry soil and sand that the sky turns brown and you may not see beyond a few metres. In north-west India such storms often blow in the hot months before the monsoon, sometimes with thunder and lightning.

• Causes: the wind needs loose, dry material to lift. Months without rain bake the topsoil into dust; fields left bare after harvest, overgrazed pastures and felled trees leave nothing to hold it down; and the gusts that rush ahead of a summer thunderstorm can be fierce. Scientists expect a warmer climate to bring more such extremes.
• Prone areas: Rajasthan, Punjab, Haryana, western Uttar Pradesh and Delhi; worldwide, the Sahara, West Asia, Central Asia and the Gobi.
• Ways to reduce harm: rows of trees as shelterbelts; grasses that hold sand dunes in place; controlling overgrazing; weather warnings, so people stay indoors and away from weak walls, trees and electricity poles.
• Example: on 2–3 May 2018, violent dust storms swept across Rajasthan and Uttar Pradesh; by 4 May the authorities had counted more than 100 deaths, many caused by falling walls, trees and poles.

Step back and the whole chapter becomes a tug of war. On one side, the Earth's inner heat moves plates, folds and breaks rock, and pours out lava: mountains, rift valleys, ridges and trenches are its handiwork. On the other side, the Sun's energy and gravity drive water, ice, wind and waves, which file the heights away and spread the debris as plains, deltas, dunes and beaches. Every farm, town, road and pilgrim route sits on the result. Knowing how the land was made, and how suddenly it can move, is the first step to using it wisely and staying safe on it.