Landslide describes the mass movement of earth down a slope.
Flows vary in type of material and the speed they travel. Soil and rock can move by less than 16 millimetres per year (classed as a very slow flow) to over five metres per second (classed as an extremely rapid flow). Slow flows can form small terraces (up to a metre high) on hill slopes. If you see trees or power-poles that have tilted on a terrace, this may be the result of a slow flow. This process occurs naturally, but can be sped up by human processes, including forest clearance.
Flows commonly occur in steeper terrain where there are already landslides or where intensive farming has stripped the land of vegetation. Heavy rainfall loosens soil and rock, creating a flow of sediment. In a typical flow (right) the source area and main pathway fan out as soil and rock collect at the bottom of the slide.
A slide develops where a crack forms at the top of the slope.
There are three types of ‘slide’:
- Rotational rock slump.
- Translational debris slide.
- Earth block slide.
A typical rotational rock slump (right) occurs when the underlying rock fails due to earthquake movement or a build up of water pressure. A large area of hillside drops down and sideways, leaving behind a sheer exposed wall of earth and rock material (‘headscarp’).
Topples involve rock or soil that tilts and/or rotates forward on a pivot point. There is not necessarily much movement, however it may lead to falls or slides of the displaced material.
Rock and/or soil detaches from the slope and moves rapidly to its new resting place. Often associated with undercut cliffs and riverbanks.
These landslides involve sudden horizontal movement on very gentle terrain. Often initiated by earthquakes that liquefy the layer below the moving material.
This is where two or more of these movements are seen together.