You may have seen rock walls or high
fences between a highway and a steep
slope. The barriers are there to prevent
loose rocks and soil from falling onto the
road. Mass movement is a from of erosion
in which soil,rock,or sediments move downslope
due to gravity.
First, weathering breaks up large rocks
high on the hill. Gravity causes loose rocks
and soi; to move down the hill Deposition
takes place when the sediments lose their
energy of motion and stop. This usually
occurs when the slope becomes flat or
when there is a depression in the hill.
Some types of mass movement, such
as a landslide, happen very quickly. A
landslide occurs when heavy rains or an
earthquake cause large rocks on a steep
slope to break loose. the loose rocks
tumble down the slope, striking other rocks
and breaking them free. As long as the
slope remains steep. the landslide continues
to gather more material and gain speed. A
landslide can have enough energy to knock
over trees or houses, carry them away, or
bury them.
Sediments are deposited when theyreach the bottom of the slope. The loose mixture of rocks that form at the base of very steep rock slopes is called talus. During winter, heavy snows often accumulate on mountain slopes. when the weight of the snow is greater than the forces holding the snow to the mountain,an avalanche may occur. An avalancha is a landlide that carries snow, rocks, and soil.
On very steep slopes, a large chunk of sediments may quickly slip downslope a short way when the material beneath it weakens. This mass movement, called slump, leaves a curved scar where the material was originally located.
A mudflow is thick downward movement of soil or sediments mixed with water. Mudflows sometimes occur in fairly dry areas. Over time, dry sediments build up in deep layers. If heavy rains fall in the areas, they turn the sediment into a thick paste. The water adds to the mass of the sediments and increases the effect of the force of gravity until the mass flows downhill.
Like a landslide, a mudflow has enough energy to move huge rocks, cars, or houses. At the bottom of the slope, the mudslide deposits sediments and debris in a cone-shaped mass that may cover roads or communities. During a mudflow, the land that supports houses near the top of the slope can slide away, causing the house to fall.
Trees or utility poles on a hillside sometimes lean to one side. This is caused by a slow mass movement called creep. Creep occurs where the ground often freezes and thaws. Each time the ground freezes, water in the soil at right angles to the slope. When the ground thaws, the soil settles slightly downslop. Over time, the soil supporting trees and fence posts shifts, and they begin to lean downhill
Mass movements tend to happen on steep slopes. Mass movements occur more often after a heavy rain. Not only dose the water add to the mass of the sediments, but it also acts as a lubricant that makes the sediments slippery and easier to move.
Many areas of the country are hilly. Many houses are built on or near steep slopes. Careful planning and management are necessary to prevent mass movements from affecting the people who live there..