As soon as any rock is exposed on the Earth's surface, it is assaulted by the processes of weathering and the agents of erosion.

Geologists estimate that, on average, about 3.5 centimeters of land are worn away from land surfaces every 1,000 years.

Weathering starts and finishes in one place.

However, the agents of erosion - rivers, glaciers, the wind and the ocean - transport rock fragments from one place to another. The rock debris then used erodes even more material. For example, waves hurl loose rocks against cliffs and undercut them.

Denudation - the cycle of weathering, transportation and erosion -  is both destructive and constructive. It constantly changes the face of the Earth by breaking rocks down and redistributing the debris to form new rocks and landscapes.

Landscape and climate affect this cycle.

Weathering is quickest in mountainous areas. As soon as a rock is weakened and dislodged from its neighbors, gravity causes it to fall.

A new surface is then exposed to the elements.

Climate dictates water supply, temperatures and wind. The forces at work in arid regions differ from those in moist regions.

The speed at which weathering and erosion occur depends largely on the type of rock under attack. Massive igneous rocks can usually resist weathering for long periods. However, over millions of years, even granite will decompose into soft kaolin (china clay).

On the other hand, rainwater will dissolve a relatively soft limestone quite rapidly.


Mechanical Weathering

Exfoliation is a type of mechanical weathering in which the outer layers of exposed rocks are peeled away like the skins of an onion. This process occurs in deserts, because of the rapid heating during the day followed by fast cooling at night.

Water is important in mechanical weathering, especially when it fills cracks in rocks or is absorbed by minerals. When water turns into ice, it expands, occupying a larger volume than the original water. The force of this expansion loosens and eventually shatters rocks. This process creates large piles of loose rock called scree, which often accumulate at the bottoms of mountain slopes.

The roots of trees and other plants also exert pressure and widen cracks in rocks. Burrowing animals, such as worms, ants and moles, also loosen rocks and bring material to the surface.

Human beings contribute to mechanical weathering through such activities as road-building, farming, mining and quarrying.

Chemical Weathering

In chemical weathering, rocks are decomposed and destroyed by chemical reactions.

New minerals may be created elsewhere from the same material.

While pure water dissolves minerals such as salt, rainwater is even more powerful. It absorbs materials such as carbon dioxide, from the air and soil, and acid from rotting plant and animal material.

This transforms it into carbonic acid. As it percolates through carbonic acid attacks minerals, dissolving them and carry them away, perhaps to redeposit them elsewhere.

This process is particularly marked in limestone.

Another example of chemical weathering occurs when oxygen in water combines with, or oxidizes, minerals in rocks to form compounds called oxides. For example, when iron in rocks is oxidized, a weak, flaky red rust called iron oxide is formed.


The chief agents of erosion are running water, moving bodies of ice, the ocean (along coasts) and the wind.

In moist, temperate regions, rivers are the most important of these agents. From their sources, rivers are divided into three main stages: youth, maturity and old age. Each stage has its own features and characteristic forms of erosion and transportation.