LO 1.2 - Physical factors in soil formation

Weathering: Is a process by which rock is broken into smaller particles by physical, chemical or biological agents. Nutrients such as phosphorus, potassium and sulphur are released which become available to plants.

Erosion: Is the movement of rock particles and soil, due to action of wind, water, ice and gravity.

• Heating/cooling • Freezing/thawing • Wetting/drying • Expansion of plant roots • Abrasion

• Acids • Dissolution • Oxidation • Plants/fungi/lichens

• Different minerals expand at different rates when heated. • When rocks composed of different minerals are heated, the different minerals undergo different rates of expansion. • ‘Onion-skin weathering’ happens when outer parts of a rock are at different temperature from inner parts.

• Water penetrates small cracks in rocks. • Water can then freeze and expand. • This is known as ‘frost shattering’. • Some minerals swell when exposed to water and shrink on drying

• Roots grow into small cracks and crevices in rocks. • They grow and expand and widen the cracks which exposes the rock to surface elements.

Water or air/wind carrying rock fragments ‘scours’ or ‘sand-blasts’ the surface of rocks. Fast-flowing streams/rivers and seas carry stones/pebbles that are tumbled against each other. Glaciers carry particles of varying sizes that scour the rock below, wind carries sand. Gravity can contribute to abrasion too – rocks falling or sliding down a slope scrape against each other and against bedrock. The physical force of water alone is also a powerful agent of weathering, including rivers/streams and waves along shorelines.

• Abrasion also happens when animals (including humans) repeatedly pass over an area of rock. • Burrowing animals can have a similar effect below ground.