Geomorphic Processes: Weathering & Erosion – NDA Geography Notes

The Earth’s surface is never still. Mountains are being worn down. Valleys are being carved deeper. Cliffs are collapsing into the sea. Sand is being blown across deserts. Rivers are cutting into bedrock. All of this is happening around us, slowly enough that we do not notice it in a single lifetime, but fast enough that it has completely reshaped the Earth’s surface over millions of years.

This chapter is about the processes that break down, move, and deposit Earth’s material. These processes work from the outside, driven by the Sun, water, wind, gravity, and living organisms. NDA tests this chapter through process identification questions: is a given example physical, chemical, or biological weathering? Is a boulder moving downhill mass wasting or erosion?

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Endogenic and Exogenic Processes

Endogenic (internal) processes are driven by heat from inside the Earth. They build up the surface, creating mountains, volcanoes, and plateaus through plate tectonics.

Exogenic (external) processes are driven by energy from outside, mainly from the Sun, which drives water and wind. They break down and level the surface, wearing away mountains and filling valleys with sediment.

Denudation is the overall process of lowering and levelling the Earth’s surface through the combined action of weathering, erosion, mass wasting, and transportation. [NDA 2022-I]

Rocks do not remain in their original form for long. They undergo transformation caused by weathering, erosion, and metamorphic action. [NDA 2022-I]

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Weathering

Weathering is the process of breaking down rocks in place: without moving them. The key distinction: in weathering, the material does not move. It just breaks down where it is. In erosion, the broken material is carried away by a moving agent such as water, wind, or ice.

There are three types of weathering: physical, chemical, and biological.

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Physical Weathering

Physical weathering breaks rocks into smaller pieces without changing their chemical composition. The minerals in the rock remain the same. The rock just becomes smaller fragments.

Physical weathering is most effective in hot deserts (extreme temperature variation between day and night) and cold climates (freeze-thaw action).

The most prevalent form of weathering in hot tropical desert areas is mechanical (physical) weathering. [NDA 2022-II]

Forces applied in mechanical weathering: Gravitational force, expansion force, and force due to water pressure: all three are forces involved in mechanical weathering. [NDA 2025-I]

Processes of Physical Weathering

Thermal Expansion: In deserts, rocks heat up to very high temperatures during the day and cool sharply at night. This repeated expansion and contraction creates stress that eventually cracks and breaks rocks apart.

Freeze-Thaw Action (Frost Action): Water seeps into cracks in rocks. When temperature drops below 0°C, the water freezes and expands by about 9%. This expansion widens the crack. When ice melts, water seeps deeper; the next freeze widens it further. Frost action is synonymous with freeze-thaw action. [NDA 2014-I]

Salt Crystal Growth (Haloclasty): In coastal areas and arid regions, water containing dissolved salts seeps into pores and cracks. When the water evaporates, salt crystals form and grow, pushing on surrounding rock and widening cracks. Salt crystal growth is an example of physical weathering: NOT chemical. [NDA 2019-I]

Salt crystallisation is the most effective of all salt weathering processes. It is more effective over sedimentary rocks than igneous rocks because sedimentary rocks are more porous.

Exfoliation: The outer layers of a rock peel off like the layers of an onion. The outer surface heats and expands while the inner rock stays cooler. Exfoliation is physical weathering. [NDA 2018-II]

Crystallisation: When minerals crystallise inside rock pores, the growing crystals exert pressure on the surrounding rock. Crystallisation is a form of mechanical weathering. [NDA 2014-I]

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Chemical Weathering

Chemical weathering changes the chemical composition of rocks, converting minerals into new, weaker minerals through chemical reactions. It is most effective in hot and humid climates, because high temperature speeds up chemical reactions and water is essential for most processes. [NDA 2018-I]

Chemical weathering processes are more active in hot and humid environments because high temperature and rainfall help decomposition of rocks. [NDA 2014-I]

Exfoliation is NOT a process of chemical weathering: it is physical. [NDA 2018-II]

Processes of Chemical Weathering

Solution (Dissolution): Rainwater absorbs carbon dioxide, forming weak carbonic acid. This acid reacts with rocks, particularly limestone, dissolving them. The dissolved minerals are carried away in solution.

Carbonation: Carbonic acid reacts with calcium carbonate (limestone), breaking it into soluble calcium bicarbonate which washes away. This is especially important in karst (limestone) landscapes.

Oxidation: Iron-bearing minerals react with oxygen and water, forming iron oxides (rust). This is why many rocks and soils appear red or reddish-brown.

Hydration: Some minerals absorb water molecules and expand. This absorption changes their chemical structure and weakens the rock. The process whereby minerals absorb water, expand and change is called hydration. [NDA 2021-I]

Hydration causes chemical change in minerals. It does NOT cause granular disintegration. Granular disintegration is a physical process. [NDA 2014-I]

Hydrolysis: Water chemically reacts with minerals, breaking them down into new compounds. Feldspars break down through hydrolysis into clay minerals. Hydrolysis is effective in humid tropics because of high temperature and high humidity, NOT because of high diurnal range in temperature. The tropics have low diurnal range. [NDA 2014-I]

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Biological Weathering

Biological weathering is caused by living organisms: plants, animals, bacteria, and fungi.

Plant roots grow into cracks in rocks and push them wider over time. This is physical biological weathering. Lichens secrete acids that dissolve the rock surface. This is chemical biological weathering. Bacteria break down minerals in rock through chemical processes.

Type	Changes Chemistry?	Most Effective Climate	Key Processes	NDA Tag
Physical (Mechanical)	No	Hot deserts; Cold climates	Freeze-thaw, Salt crystals, Exfoliation, Thermal expansion	[NDA 2014-I
Chemical	Yes	Hot and humid	Solution, Carbonation, Oxidation, Hydration, Hydrolysis	[NDA 2014-I
Biological	Both	Varies	Plant roots (physical); Lichens, Bacteria (chemical)	—

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Soil Formation

Weathered rock material, mixed with organic matter, forms soil, a process taking hundreds to thousands of years to form just a few centimetres.

Classes of soil forming processes: Translocation, enrichment, removal, and transformation are all classes of soil forming processes. [NDA 2025-II]

In translocation, fine particles are transported downward by eluviation and accumulate in lower horizons by illuviation. [NDA 2025-II]

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Erosion

Erosion is the wearing away of the Earth’s surface and the transportation of the loosened material by agents like water, wind, ice, and waves.

The key difference from weathering: in erosion, the material is carried away. In weathering, the material stays in place.

Main agents of erosion: Running water (rivers): the most powerful overall | Wind: important in deserts | Glaciers (ice): powerful in polar and high mountain regions | Waves: in coastal areas | Underground water: in limestone regions.

The specific landforms created by each agent are covered in Chapter 11.

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Mass Wasting

Mass wasting is the downward movement of rock, soil, and debris under the direct influence of gravity alone: without a fluid agent (water or wind) carrying the material.

The key distinction: in mass wasting, gravity does the work directly. In erosion, a medium (water, wind, ice) carries the material.

A boulder loosened by heavy rains and moving downhill is an example of mass wasting: not erosion. [NDA 2022-I]

Weathering, mass wasting, erosion, and transportation together constitute denudation. [NDA 2022-I]

Types of Mass Wasting

Creep: the slowest form; soil and rock move downhill extremely slowly (millimetres per year). Evidence includes fence posts leaning downhill. Characterised by slow speed with soil.

Earthflow: water-saturated soil and soft rock flowing slowly downslope. Faster than creep.

Mudflow: fast-moving flow of water-saturated soil and rock after heavy rainfall on unstable slopes.

Slump: rotational movement where a block of earth slides along a curved slip surface.

Rockslide: a mass of rock fragments sliding rapidly down a steep slope.

Rockfall: individual rocks falling freely from steep cliffs. The fastest type.

Landslide: a general term for rapid downslope movement of rock, soil, or debris.

Landslides generally occur in clay-rich soil and are triggered by earthquakes and heavy rainfall. They occur on steep slopes, NOT gentle slopes. [NDA 2022-I]

Avalanche: a sudden, rapid movement of snow and ice down a steep mountain slope. It involves falling, rolling, sliding, and flowing of materials and is hazardous to skiers and mountaineers. [NDA 2022-I]

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Karst Topography

In areas underlain by limestone, slightly acidic rainwater dissolves the rock over time, creating a distinctive landscape called karst topography: characterised by sinkholes, caves, and underground streams.

The most common topographic form in karst terrain is the sinkhole: a circular or funnel-shaped depression formed when the roof of an underground limestone cavern collapses. A sinkhole is a depression, not formed by upliftment of land.

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