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
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
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.
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.
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.
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.
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.
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.
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.
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
After weather and erosion have affected the landscape, the next step is
deposition, as the eroded and transported material comes to rest.