External Landform Processes
Senior 2 · Geography and Environment
External Landform Processes
Lesson 1 of 3
20 min
Every handful of soil on a Rwandan farm began as solid rock. The deep red soils of the central plateau, the dark soils around the volcanoes and the sand in the eastern valleys were all made by the slow breaking-down of rock in place. That process is weathering. Internal processes (the last unit) build the land up; external processes such as weathering, erosion and mass wasting wear it down. Weathering comes first, because it prepares the loose material that the other processes carry away.
Weathering is the breaking down and decay of rocks in place, at or near the Earth's surface, by the weather, water, chemicals and living things. The key words are "in place": weathered material has not been moved. Once it is carried away, that is erosion.
Two observations from a hillside in Huye:
Ask: has the material moved? In (1) the crumbly layer is still on the boulder, so this is weathering. In (2) the material has been carried downhill by running water, so this is erosion. The weathering in (1) supplied the material that was eroded in (2).
Physical (mechanical) weathering breaks rock into smaller pieces without changing its minerals. Chemical weathering changes or dissolves the minerals. Biological weathering is done by living things, and can be physical or chemical.
| Process | Type | How it works |
|---|---|---|
| Exfoliation (onion weathering) | Physical | Strong daytime heating and night-time cooling make the outer layers expand and contract more than the inside, so they peel off in sheets. Common in hot, dry areas with a large daily temperature range. |
| Block disintegration | Physical | Repeated expansion and contraction along joints splits rock into blocks. |
| Frost action (freeze–thaw) | Physical | Water in cracks freezes, expands by about 9%, and forces the crack wider. Found on high, cold mountains. |
| Salt crystallisation | Physical | Salty water evaporates in cracks; the growing crystals prise the rock apart. |
| Pressure release | Physical | When overlying rock is removed, the rock below expands and cracks in layers. |
| Hydrolysis | Chemical | Water reacts with minerals such as feldspar in granite, turning them into soft clay. |
| Oxidation | Chemical | Oxygen reacts with iron in rocks, forming iron oxides (rust). This gives many Rwandan soils their red colour. |
| Carbonation | Chemical | Rainwater containing carbon dioxide is a weak acid that dissolves limestone. |
| Solution | Chemical | Some minerals, such as rock salt, simply dissolve in water. |
| Hydration | Chemical | Minerals absorb water, swell and weaken. |
| Roots, animals and people | Biological | Tree roots widen cracks; termites and moles open the ground; people quarry and dig; decaying plants release acids that attack rock. |
Weathering forms soil, the basis of farming; produces clay for bricks and pottery; forms rounded boulders and bare rock domes (tors and inselbergs) where hard rock is left behind; and weakens slopes, preparing them for landslides.
Tip
Memory hook. Weathering stays, erosion goes. For the types, think "break, change, living": physical breaks, chemical changes, biological is done by the living.
Solution: the breaking down and decay of rocks in place at or near the Earth's surface.
Solution: oxidation of iron in the rock, forming iron oxide.
Solution: the climate is warm and wet: warmth speeds up reactions and plentiful rain supplies water for them.
Solution: biological weathering (roots widening cracks).
Solution: physical weathering breaks rock into pieces without changing its minerals; chemical weathering changes or dissolves the minerals.
Summary
Sources and credits
The worked example and practice items are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
