Weathering and Erosion

Weathering breaks a rock down into small pieces. The two main processes are exfoliation and freeze-thaw weathering. Freeze-thaw weathering occurs when water gets into cracks in rock and then freezes (below 0 °C) in cold weather. Water expands when it freezes and turns to ice, and the expansion can cause the rock to split and fragment.

In warmer weather (above 0 °C) the ice melts (called thawing), and new cracks are exposed allowing the process to repeat itself.

Erosion is a slow process of wearing away weathered rock by the action of the wind, rivers and waves. All rocks would have been worn down flat to sea level long ago if hills and mountains were not constantly being formed by the movement of plate tectonics. See the rock cycle.

Transport and compaction

Small pieces of weathered rock (called sediments) are transported by rivers. This means that the rock pieces are moved along as the river flows towards the sea.

During transportation, the sediments are sorted according to size. The largest pieces tend to be deposited first. Sand is next to settle out of the water. The smallest particles are mud or clay, and these are moved furthest by the river and deposited last.

Compaction - With the passing of time, the deposits of sediment get thicker and thicker. The sediment underneath becomes increasingly crushed by the weight of the new sediment which is being deposited on it from above. This is called compaction. It is this process of compaction which eventually makes layers of sedimentary rock.

The process may take millions of years. The largest rock pieces will produce conglomerate. Sand will produce sandstone. Mud or clay will produce mudstone or shale

	Created by miminoma almost 10 years ago

Note on Rocks - GCSE Chemistry, created by miminoma on 14/01/2015.