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.
Rock movements are the fastest type of mass movement. They happen
when chunks of rock topple over or break away from cliffs and tumble or roll
downhill. Many pieces of falling, tumbling rock make up a rock avalanche.
