The forces which act on the earth's exterior are called exogenic forces or external forces. They wear down the rock, so they are considered land wearing forces. Processes on the earth's surface due to these forces are exogenic processes; weathering, mass wasting and denudation are the major ones. The elements of nature doing them – wind, river, glacier, waves and ground water – are gradational agents.
1. Weathering
Weathering is the disintegration and decomposition of rocks due to climate, plants, animals and other organisms. Three types: physical, chemical and biological.
Physical Weathering
Disintegration of rock induced by elements of weather, producing smaller, angular fragments of the same rock. Categories: thermal weathering, frost wedging and exfoliation.
- Thermal weathering (arid and semi-arid areas): alternate expansion by day and contraction at night cracks rocks. Granular disintegration – rocks break up grain by grain; block disintegration – rocks such as granite break along joints into large rectangular blocks.
- Frost wedging: water becomes larger when it freezes; ice in cracks wedges the walls apart and deepens the crack.
- Exfoliation: outer layers peel off in concentric layers like an onion skin, leaving dome shaped monoliths; also called 'onion weathering'; common in arid areas.