Geography Reference
In-Depth Information
Erosion
The removal of material in dissolved or particulate form can occur
through several processes. Dissolved removal occurs in water.
Water carrying the dissolved material removes it from the slope
through the ground or over the surface. The concentration of
solutes is generally highest in dry climates, but the total amounts of
the dissolved material removed is less than in wetter areas. Once
dissolved material is removed it generally travels far downstream.
For some limestones, 90 per cent of their original volume can be
dissolved, often producing dramatic landforms and caves known as
karst
landforms. Rapid karst development in wet tropical areas can
result in amazing rock towers. Hot desert areas have the slowest
karst development due to a lack of water for solutional weathering.
Around 15 per cent of the Earth's surface has some karst landforms.
Soil material being transported downslope can do so as a large
mass or as independent particles. During mass movements (which
can be very fast, or very slow), a section of rock or soil moves
together. Where large flows of water generate mass movement of
sediment this tends to be faster than drier mass movements of sedi-
ment. The net effect of all forces operating on a mass of material
controls when the material will move. Forces that encourage
movement include gravity, water and wind. Flowing water can
detach fragments of rock or soil if it passes over them rapidly by
picking up material from the surface, or by detachment of soil
grains by raindrop impact. Friction and
cohesion
resist movement.
Material begins to move when the forces promoting its movement
become larger than the resistance forces. The
safety factor
is the
ratio of these forces. In simple terms, moving material will slow
down and stop when it meets lower gradients or where water car-
rying the material spreads out and moves more slowly, or seeps
into the ground.
There are many types of mass movements. In rapid mass move-
ments, there are slides where the mass moves as a block, and flows
within which different parts of the material move over each other
at different speeds. Fractures in rock provide weaknesses which fail
and slabs of rock can slide off downslope. Toppling can also occur
when columns of rock become overhanging. Flows occur when
there is more water mixed into the moving mass, in proportion to
