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Weathering and soils
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Rocks do not last for ever. Weathering breaks them down, and over a very long time the pieces help make soil.
Types of weathering
Mechanical (physical): freeze-thaw, when water in a crack freezes, expands and widens the crack.
Chemical: rainwater is a weak acid and slowly dissolves rocks such as limestone and chalk.
Biological: plant roots grow into cracks, and animals burrow.
What is soil?
It takes a very long time to form, so losing soil is a serious problem.
Soil layers and soil loss
Topsoil: dark, rich in humus, full of roots and living things.
Subsoil: lighter, with fewer living things.
Weathered rock, then solid bedrock at the bottom.
Soil can be eroded by wind and rain, especially when trees and plants are removed. Planting trees and hedges and ploughing across slopes help protect it.
Explain how freeze-thaw weathering breaks a rock.
- Rainwater seeps into a crack in the rock.
- At night it freezes; ice takes up more space than water, so it pushes the crack wider.
- This repeats many times until pieces break off.
Answer: Water in cracks freezes and expands, widening the crack again and again until the rock breaks.
Weathering breaks rock in place: mechanical (freeze-thaw), chemical (acid rain dissolves limestone) and biological (roots, burrowing). Soil = minerals + humus + water + air. Layers: topsoil, subsoil, weathered rock, bedrock. Removing plants increases soil erosion.
The interactive lesson includes the diagrams for this topic.
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