Mass movement in detail

What are the causes of mass movement?

- High rainfall / intense rainfall

- Vibrations from earthquakes or building work or traffic

- Deforestation (the removal of vegetation)

- Weathering

- Adding weight through afforestation or building

Soil creep is the slow downward progression of rock and soil down is low grade slope

It is slow (1cm/year) but a continuous process

It has low water content

What are the causes:

- Creep on Milk Hill, Wiltshire, UK has resulted in teracettes down the valley

- Creep occurs as result of repeated expansion and contraction of material

- Cycles of freeze-thaw move particles up due to freezing (expanding) & then particles fall further downslope (under the influence of gravity) when the ice melts. Alternating wet dry periods can have the same effect

- About 75% of soil creep movement is induced by such moisture (and volume) changes

Involve rapid movement of rock & weathered debris (up to 100 km/hour)

Rock and debris are mixed with water resulting in a fluid movement

They are turbulent and structureless mixtures of sediment & water

Mudflows begin as a narrow flow (often in valleys) & spread out as the travel downslope

What are the causes:

- They occur on steep slopes

- They occur after periods of heavy rain

- When there is not enough vegetation to hold the soil in place saturated soil floors over impermeable subsoil causing great devastation and endangering lives

- Areas of deep regolith (e.g. In the tropics where deep chemical weathering occurs) are particularly susceptible to flows

Slides occur when an entire mass of material (either soil or rock) moves downhill

They have a medium speed and water content

What are the causes:

- They occur when there is a combination of weak rocks, steep slopes and active undercutting

- Weak rocks such as clay have little shear strength and are particularly vulnerable to the development of slip planes

- They can be triggered by periods of rockfall which increases the rate of the soil adding shear stress

- If layers of impermeable rock are present they trap water above them leading to failures

Rockfall is the rapid freefall of rock from a steep cliff face

A dry movement

What are the causes:

Slope must be extremely steep for rockfall to occur

Rock fragments fall from the face of the cliff because of gravity

This is made worse by freeze-thaw action loosening the rock. Bare well-joined rock is very vulnerable to rockfall as water enters the joint, freezes and expends cracking the rock.

Can be triggered by undercutting at the base of the cliff (e.g. by a river or the sea)