Copyright © 2007 Dorling Kindersley
The process that breaks up and carries away the rocks and soils that make up the Earth’s surface is called erosion. It is caused by flowing water, waves, glaciers, and the wind, and it constantly changes the shape of the landscape. Erosion happens more quickly on bare rock, which is unprotected by soil. It often begins with weathering, where rocks are weakened by the weather’s elements, such as sunshine, frost, and rain. Rocks can be eroded by physical weathering through heat, cold and frost, and CHEMICAL WEATHERING. Erosion may lead to the MASS MOVEMENT of rock and soil.
Waves erode the base of cliffs, undermining them and making them collapse. This can create coastal features such as the Twelve Apostles in Victoria, Australia. The stacks (rock towers) are left when headlands are worn away from both sides until they crumble. The broken rocks form shingle and sand beaches. Erosion happens faster when shingle is thrown against the cliffs by the waves.
Mountain ranges contain deep valleys that have been carved out by glaciers. A glacier is like a slow-moving river of ice that flows downhill, carried forwards by its huge weight. The rocks dragged along underneath it gouge deep into the ground, creating U-shaped valleys with steep sides and flat bottoms.
Sand blown by strong winds has sculpted the slender sandstone pillars of Bryce Canyon, Utah, USA. Their rugged outlines are caused by the softer layers of rock are being eroded more quickly than the harder layers. Wind erosion is common in deserts, where sand is blown about because there are few plants to hold the soil in place and there is no rain to bind the soil particles together.
Some rocks are broken down by chemical action, in a process called chemical weathering. The minerals they contain are changed chemically by the effects of sunlight, air, and especially water. The rocks are weakened and wear away more easily. Limestone, for example, is dissolved by rainwater, because the water contains carbon dioxide from the atmosphere, making it slightly acidic.
Erosion normally breaks down the landscape a tiny piece at a time, but sometimes rocks and soil move downhill in large volumes. These movements, which include landslides, mudflows, and rock falls, are called mass movements. They happen when rock, debris, or soil on a slope becomes unstable and can no longer resist the downward force of gravity.
Soil creep is the extremely slow movement of soil down a steep hillside. It is caused by soil expanding and contracting, when it goes from wet to dry or frozen to unfrozen. The top layers of the soil move faster than the layers underneath. The movement is far too slow to see, but bent trees, leaning fence posts and telegraph poles, and small terraces in fields are all evidence of soil creep. Soil may also build up against a wall or at the bottom of the hillside.
A slump is a mass movement that happens when a large section of soil or soft rock breaks away from a slope and slides downwards. Short cliffs called scarps are left at the top of the slope. Slumps often happen where the base of a slope is eroded by a river or by waves, or when soil or soft rock becomes waterlogged.
A lahar is a mudflow of water mixed with volcanic ash. This forms when ash mixes with melting ice during an eruption, or with torrential rain. The mud flows down river valleys and sets hard when it comes to a stop. Lahars can cause destruction on a massive scale.
Debris is made up of broken rock, sometimes mixed with soil. These pieces of debris may collect on a slope and begin to roll or slide downwards. Debris slides often happen where people have cleared hillsides of trees and other vegetation, which causes the soil and rock to be eroded quickly.