If you have read the topic on Weathering you are now ready for erosion and then deposition. Once rocks have been broken down by mechanical and chemical weathering, the resulting particles are now smaller, and able to be picked up and transported.

The process of picking up and moving weathered rock materials is called erosion.

The process of dropping these materials in a new place is called deposition.

This topic investigates the agents of erosion in the formation of common landscape features.

Erosion - bringing down the mountain
Erosion occurs when rock particles are moved by an external force or agent. The agents of erosion are ice, water, wind, waves and gravity. The degree to which erosion occurs depends on the slope of the land, amount of rain or snow, wind and looseness of the rock and soil.

The gently sloping hills of Australia no longer erode quickly, but the steep mountains in the New Zealand Fjord country tend to erode very quickly. On the slopes of Mt. Cook, New Zealand, you can hear the mountains eroding as rocks slip and roll down the steep sides of the glacial valleys.

The five agents of erosion will now be further investigated.

Gravity
Whether a whole mountain side moves slowly or a landslide occurs quickly and dramatically, the mass movement of rocks and soil under the influence of gravity can remove large amounts of material from a mountain side. Often associated with rain, which weakens a slope, gravity is an important agent of erosion in high steep mountains.

In areas which have been cleared of trees, land movement becomes a real threat. The foothills of the Himalayan mountains of India and the Andes in South America both have large populations trying to cultivate the steep hill sides. In recent years, huge mud slides have caused mass destruction of villages and the flooding of low land areas. Authorities are now working to stop the threat of land slides by reforestation and the ban on removal of trees from these areas.

Water erosion
Rain falling on bare soil on a slope can result in the soil breaking free from the underlying rock and flowing downslope, carrying mud and rock within a mud flow. From the peaks of the mountains, running water forms streams in gullies and canyons. Water is the most important agent of erosion in wet highland areas, quickly moving large quantities of material into lower river valleys and plains.

The removal of trees and their replacement with agriculture has lead to a significant increase in the loss of topsoils in Australia via water run off. Planting of trees around water ways and in areas of high risk from water erosion and better farming practices can significantly decrease the amount of soil lost to water erosion.

Ice erosion
In colder regions, where snow is the predominant form of precipitation, glaciers are the most important agent of erosion. The action of ice, although slow, can be dramatic. The heavy, slow moving ice carries rock, pebbles and sand, which is then trapped within the ice. These trapped rocks ground away at the base and sides of the glacial valley like gigantic sand paper.

During the last ice age, glaciers flattened whole mountains in Tasmania, leaving behind the large lakes of the Tasmanian Western Highlands. On the South Island of New Zealand, steep sided valleys have been formed by glaciers with cliffs hundreds of metres high.

Wind erosion
In dry areas, wind becomes the primary agent of erosion. Sand storms and wind are responsible for the movement of finer dust and sand producing the dominant features of desert landscapes; plains, dunes and sculptured rock outcrops. Over many parts of Australia, the agricultural practice over the last 200 years has involved the removal of many trees from arid regions. This exposes the soil to the wind and increases the chance of dust storms occurring, like the one that occurred over Melbourne in 1983.

Wave erosion
A visit to any beach with a cliff will soon attest to the power of waves as agents of erosion. Coastlines are continually being sculptured and shaped by the action of waves. The Twelve Apostles in Western Victoria with their cliffs, stacks, arches and caves are the classic example of a landscape created by wave erosion. These structures are formed as softer rock is eroded away more quickly than harder sections leaving some parts out in the ocean as the cliff lines slowly recede under the pounding of the waves.