Erosion is the wearing down and the carrying away of the Earth’s rock by the action of wind and moving water. Erosion happens fastest on steep hillsides after heavy rain, when LANDSLIDES sometimes strike.
Rocks are gradually broken down into smaller pieces by the wind, rain, snow, and frost. For example, when water freezes in rock cracks, the water expands, slowly widening the cracks and fracturing the rock. The fragments of weathered rock are then blown away by the wind or carried away by water in the form of streams and rivers, ice in glaciers, or waves pounding coasts.
Weathering and erosion usually wear away landscapes fairly slowly. A landslide occurs when a huge mass of rock and soil suddenly breaks off a hillside, engulfing everything in its path. Landslides can cause great destruction and can even wipe out whole towns.
Landslides often strike after heavy rain or snow has fallen. Loose soil and rocks begin to trickle downhill; then gravity takes over and the whole hillside slips away. Landslides are common on slopes where the vegetation has been removed by tree felling or farming. Without tree or plant roots to anchor the soil, heavy rain easily washes it away. Landslides, mudflows, and avalanches are called mass wasting. Mass wasting is sometimes triggered by a volcanic eruption or by an earthquake.