Mass movements

Mass movement

The movement of a large mass of sediment or a section of land down a slope

Rock fall

The rapid mass movement of rock down a steep slope or cliff

Rock slump

A slide that takes place along a curved surface

Rock slide

A sudden downslope movement of rock

Rock flow

Pieces of rock break loose from a steep rock face or cliff

Angle of slope

Greater chance of failure if the slope is greater than the angle of repose (32°)

Lithology

Igneous and metamorphic are more competent and stable. Igneous have no beds so has fewer lines of weakness

Weathering

The alope will be weakened if heavily weathered

Load

A heavier load will need more friction to hold together so a gentle slope is needed

Groundwater regime

Little sticks grains together, lots pushes grains apart (increased pore pressure). A high water table means poor exits so the rock is wet so friction is reduced

Rainfall

Heavy rainfall means the rocks are more saturated so there is less friction holding the grains together

Ground vibration

Earthquakes, fracking, volcanoes and mines all weaken and destabilize the slope

Vegetation cover

Roots provide anchorage for soil and absorb groundwater

What happens when rocks become saturated?

Density and pore pressure increases so grains are pushed apart

Why does shrinking and swelling occur?

As a result of changes in the moisture content of clay rich minerals

Why does subsistence occur?

Soluble rocks dissolve forming caves and cavities which can collapse and cause the ground to subside

Why do landslides occur?

The force of gravity acting on the slope exceeds the strength it the material forming the slope (friction)