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Weathering, mass movement and the work of rivers

Geomorphology: plates, earthquakes, volcanoes and landforms · Part 3 / 4 · Fact-checked on 8 November 2026 · how we check

Written with AI from the sources listed at the bottom of this page, then fact-checked by a separate AI.

Physical, chemical and biological weathering, mass movement and India's landslide regions, how rivers erode, transport and deposit, the stages of a river, and the valleys, gorges, waterfalls, meanders, ox-bow lakes, floodplains, levees, alluvial fans, deltas and estuaries they create.

The first two parts dealt with forces from inside the Earth that raise mountains, open oceans and shake the ground. This part turns to the exogenic forces that work on the surface, powered by the Sun and by gravity, and that slowly wear the raised land down. The work proceeds in a sequence. Rock first breaks down where it lies; loosened material then slides or creeps downhill under its own weight; and running water, the most important agent of all, carries it away and lays it down elsewhere. This part follows that sequence, ending with the valleys, waterfalls, meanders and deltas that rivers create.

weathering is the breaking down of rock at or near the surface, in place, without the broken material being carried away. That last point separates it from erosion, which involves removal by water, ice, wind or waves. Weathering prepares the ground for erosion: it loosens rock so that the agents of erosion can move it, and it supplies the mineral particles from which soil forms over time. Geographers group it into physical (mechanical) weathering, in which rock is broken into smaller pieces without any change in its minerals; chemical weathering, in which the minerals themselves are altered or dissolved; and biological weathering, the work of plants, animals and microbes. Climate largely decides which kind dominates: chemical weathering is fastest in hot, wet regions, while physical weathering dominates in deserts and cold mountains.

Physical weathering works by stress. In frost wedging, water seeps into cracks, freezes and expands by about nine per cent, prising the crack wider; repeated freezing and thawing shatters rock on cold mountain slopes. In hot deserts, rock surfaces heat strongly by day and cool sharply at night, and the repeated expansion and contraction loosens grains and flakes. Exfoliation is the peeling away of curved sheets from the surface of a rock mass, like the layers of an onion, often helped by the release of pressure as overlying rock is removed; it shapes rounded domes of granite. Salt crystallisation is important in deserts and on coasts: salty water enters pores, and the growing crystals force the grains apart.

Chemical weathering changes the minerals of a rock. In solution, minerals such as rock salt simply dissolve in water. In carbonation, rainwater absorbs carbon dioxide and becomes a weak carbonic acid that dissolves limestone; this is the process behind karst caves, taught in the next part. In oxidation, iron in rocks combines with oxygen, much as an iron nail rusts, giving many weathered rocks and soils their red and yellow colours. In hydration, minerals take up water and swell, which strains the rock. In hydrolysis, water reacts with minerals such as feldspar in granite and turns them into clay. Biological weathering adds to both kinds: roots widen cracks as they grow, burrowing animals bring fresh rock to the surface, and lichens and decaying plants release acids that attack minerals.

mass movement is the movement of weathered rock and soil down a slope under the direct pull of gravity, without being carried by water, ice or wind, although water often helps by adding weight and reducing friction. The slowest form is soil creep, a gradual downhill shift of soil that is noticed only when fence posts, walls and tree trunks begin to tilt. In cold regions, solifluction is the slow flow of waterlogged soil over ground that stays frozen below. Faster forms include mudflows, in which soil soaked by heavy rain pours down like a thick liquid; slumping, a block slipping and rotating backwards on a curved surface; landslides, the rapid sliding of large masses of rock and earth; and rockfalls, the free fall of rock from cliffs. Steep slopes, heavy rain, earthquakes, the clearing of forests, road cutting and the undercutting of slopes by rivers all make such movements more likely.

India's two most landslide-prone regions show these causes at work. The Himalaya are young mountains of folded, faulted and fractured rock, still rising and shaken by frequent earthquakes; when monsoon rain falls on these unstable slopes, especially where roads and settlements have been cut into them, landslides block highways and dam rivers. The Western Ghats have steep slopes facing the south-west monsoon and receive very heavy rain within a few days; thick weathered soil over hard rock becomes saturated and can give way suddenly. The landslides of 30 July 2024 in Wayanad district of Kerala, are a recent example. The Geological Survey of India is the nodal agency for landslide studies, and the Landslide Atlas of India, published by ISRO's National Remote Sensing Centre in 2023, maps the areas most at risk.

Running water does more to shape the land than any other agent. A river carries out erosion in four ways: hydraulic action, the sheer force of moving water; abrasion, in which pebbles and sand carried by the current scrape the bed and banks; attrition, in which the carried fragments knock against each other and become smaller and rounder; and solution, in which soluble rocks are dissolved. It transports its load in solution, in suspension (fine silt and clay held up in the water), by saltation (sand grains bouncing along) and by traction (boulders rolled along the bed). When the current slows, at a bend, on entering a plain or where it meets the sea, the river lays down its load, coarsest first; this is deposition. A river cannot cut below its base level, the lowest level to which it can erode; for most rivers this is sea level.

A river's work changes along its course, and geographers describe this as stages of youth, maturity and old age. In its youthful upper course, near the source, the slope is steep, the channel narrow and the water fast; the river cuts mainly downward, carving deep valleys and leaving waterfalls and rapids. In maturity the slope eases, the river begins to cut sideways as well as down, the valley widens, and the river starts to swing in bends. In old age, on a nearly level plain, the river flows slowly in wide loops across a broad floodplain and deposits more than it erodes. Given enough time without uplift, a whole region could be worn down to a low, gently rolling surface called a peneplain, close to base level. If the land is uplifted, the river gains energy and begins cutting down again, which is called rejuvenation.

Erosion by rivers produces a distinctive set of landforms. Where a river cuts down faster than weathering can widen the sides, it carves a V-shaped valley. A very deep, narrow valley with almost vertical sides is a gorge; a canyon is a gorge on a vast scale, typical of dry regions where the walls are not worn back by rain, such as the Grand Canyon in the United States. Waterfalls form where a river drops over a sudden step in its bed, often where a band of hard rock lies over softer rock that wears away faster; rapids are stretches of fast, broken water over a rocky bed. Jog Falls on the Sharavathi in Karnataka is a well-known Indian waterfall. Pebbles whirled round by eddies drill circular potholes into the bed, and a deep plunge pool forms at the foot of a waterfall. River terraces are step-like benches on a valley side, the remains of an older, higher floodplain left behind when the river cut down again.

On gentle slopes a river swings from side to side in loops called meanders. Water flows fastest on the outside of each bend, so it erodes that bank into a steep river cliff, while slower water on the inside drops sand and gravel to form a point bar. When two bends come close, the river may break through the narrow neck during a flood and take the shorter path; the cut-off loop, sealed by sediment, becomes a crescent-shaped ox-bow lake. When the river overflows, it spreads silt over the land beside it and builds a flat, fertile floodplain. The coarsest material is dropped first, right at the edge of the channel, so repeated floods raise low ridges called natural levees along the banks. Where a river carries a heavy load and its flow varies greatly between seasons, it may split into many channels separated by sandbars, forming a braided channel, as the Brahmaputra does in Assam.

Two more landforms mark places where a river slows abruptly. Where a stream rushes out of a mountain valley onto a plain, the sudden drop in slope makes it shed its load in a fan-shaped spread of gravel and sand called an alluvial fan; the Kosi has built a very large one in north Bihar and Nepal. At the river's mouth, if the sea or lake cannot carry sediment away as fast as it arrives, the river builds a delta, a low, often triangular plain crossed by branching distributaries. The Ganga and Brahmaputra together have built the largest delta in the world, in West Bengal and Bangladesh, and the Mahanadi, Godavari, Krishna and Kaveri build deltas on India's east coast. Where strong tides scour the mouth and sweep sediment out to sea, no delta forms; the river instead ends in a funnel-shaped estuary. The west-flowing Narmada and Tapi, which run through rift valleys to the Arabian Sea, end in estuaries.

Seen together, the processes in this part form a chain. weathering loosens rock in place, mass movement brings it down the slopes to the valley floor, and rivers carry it onward, cutting valleys, gorges and waterfalls where they are young and steep, and building floodplains, levees and deltas where they are old and slow. The same chain explains why landslides strike the Himalaya and the Western Ghats in the monsoon, why the plains of north India are deep in fertile alluvium, and why the east-flowing rivers of the peninsula end in deltas while the Narmada and Tapi end in estuaries. The next part completes the picture with glaciers, wind, groundwater and waves.

Exam traps

• Weathering breaks rock down in place; erosion removes and carries it. Weathering involves no transport.
• Carbonation dissolves limestone; oxidation gives rocks and soils red and yellow colours; hydrolysis turns feldspar into clay.
• Exfoliation (onion-skin peeling) and frost wedging are physical weathering; carbonation and hydrolysis are chemical. Chemical weathering is fastest in hot, wet climates.
• Mass movement is driven directly by gravity; soil creep is the slowest form and solifluction is typical of cold regions with frozen subsoil.
• Waterfalls, rapids, gorges and V-shaped valleys belong to a river's youthful upper course; ox-bow lakes, levees and deltas to its old-age lower course.
• An ox-bow lake forms when a meander neck is cut through; the river cliff is on the outer bank and the point bar on the inner bank.
• The Narmada and Tapi end in estuaries, not deltas; the Mahanadi, Godavari, Krishna and Kaveri build deltas on the east coast.
• The Ganga-Brahmaputra delta is the world's largest; the Brahmaputra in Assam is a classic braided river; the Kosi has built a huge alluvial fan.

Recap

• Exogenic forces (Sun and gravity) wear land down: weathering, then mass movement, then erosion, transport and deposition.
• Weathering is breakdown in place: physical (frost wedging, heating and cooling, exfoliation, salt), chemical (solution, carbonation, oxidation, hydration, hydrolysis) and biological.
• Climate decides the dominant kind: chemical in hot, wet regions; physical in deserts and cold mountains; weathering supplies the particles for soil.
• Mass movement under gravity: soil creep, solifluction, mudflows, slumping, landslides, rockfalls; most common in the Himalaya and Western Ghats (Wayanad, 30 July 2024).
• River erosion: hydraulic action, abrasion, attrition, solution; transport in solution, suspension, saltation, traction; base level usually sea level.
• Stages: youth (downcutting, V-valleys, gorges, waterfalls), maturity (lateral erosion, wider valleys), old age (deposition, floodplains); peneplain; rejuvenation after uplift.
• Meanders: river cliff outside, point bar inside; neck cut-off makes an ox-bow lake; floods build floodplains and natural levees; heavy loads make braided channels.
• Alluvial fans at the mountain foot (Kosi); deltas where sediment outpaces the sea (Ganga-Brahmaputra, east-coast rivers); estuaries where tides scour (Narmada, Tapi).

Practice questions

Sources

• United States Geological Survey: educational material on weathering, landslides and the work of running water (US government) — https://www.usgs.gov/programs/landslide-hazards
• US National Park Service, Geologic Resources: fluvial landforms (canyons, meanders, floodplains, levees, alluvial fans, deltas) (US government) — https://www.nps.gov/subjects/geology/fluvial-landforms.htm
• USGS publications on rock weathering and soil formation (US government) — https://pubs.usgs.gov
• Geological Survey of India, Ministry of Mines: landslide studies (nodal agency), landslide susceptibility mapping in the Himalaya and Western Ghats — https://www.gsi.gov.in
• ISRO National Remote Sensing Centre: Landslide Atlas of India (2023); India-WRIS river basin information on deltas and estuaries — https://www.nrsc.gov.in