A landslide is a mass movement of material, such as rock, earth or debris, down the slope of a hill or cliff. They can happen suddenly or move slowly over long periods of time.

Landslides are classified by their type of movement. The four main types of movement are:

  1. falls
  2. topples
  3. slides
  4. flows

Landslides can be classified as just one of these movements or, more commonly, can be a mixture of several.

At the BGS we study the landslides that happen in the UK. Large landslides occur around the world and also in the sea.

Why do landslides happen? 

Mountain builders

A landslide may occur because the strength of the material is weakened. This reduces the power of the 'glue' that cements the rock or soil grains together. Located on a slope, the rock is then no longer strong enough to resist the forces of gravity acting upon it.

What can increase the chance of a landslide?

Several factors can increase a slopes susceptibility to a landslide event:

  • water (rainfall or the movement of the sea) — this acts as a grease to the material increasing the likelihood that it will slip and also adds extra weight to the rock
  • erosion processes — such as coastal erosion and river erosion
  • steepness of slope
  • type of 'rocks' — soft rock such as mudstone or hard rock such as limestone
  • shape of the rock 'grains'
  • jointing and orientation of bedding planes
  • arrangement of the rock layers
  • weathering processes — for example freeze-thaw reduces the stickiness (cohesion) between the rock grains.
  • lack of vegetation which would help bind material together
  • flooding
  • volcanoes and earthquake activity nearby
  • man's activity — mining, traffic vibrations or urbanisation which changes surface water drainage patterns