Rocks may appear to be solid and unchanging, but they are being slowly worn away. They are always under “attack” by the weather. Rainwater, frost or even just changes in the temperature can all break up rocks. This is called weathering. The weathering of rocks makes them liable to be eroded. Erosion is the removal of rock fragments by water, ice or wind.

Weathering

How does the weather break down rocks? Rocks heat up and expand in the sunshine during the day. At night, they cool down and contract (get smaller). This expansion and contraction, repeated day after day, causes the rock’s surface to crack. Pieces flake off and are blown or washed away (see Erosion, below). Over many years, the rock disappears. Hard rocks take longer to break down than softer ones.

Weathering wears away the bare rocks found in mountainous areas. Ice, wind and water carry the rocky fragments away and deposit them (lay them down) in lower regions such as plains, rivers and lakes. These fragments—gravel, sand and mud (silt and clay)—are called sediments. The process of laying them down is called deposition.

Erosion

Erosion is the carrying away of fragments of rock by the action of running water, glaciers or wind. Rivers, especially if they are fast-flowing or are in flood, can carry quite large pieces of rock. Waves crashing on to cliffs, sometimes hurling pebbles or boulders at the rock, are also powerful forces of erosion.