Understanding landslides
Discovering Geology – Earth hazards
In this section:
- What is a landslide?
- Why do landslides happen?
- What can increase the chance of a landslide?
- Landslides in the UK and around the world
- How to classify a landslide
What is a landslide?
A landslide is a mass movement of material, such as rock, earth or debris, down a slope. They can happen suddenly or more slowly over long periods of time. If the force of gravity acting on a slope exceeds the resisting forces of a slope then the slope will fail and a landslide occurs. External factors, such as heavy rainfall leading to saturation of the ground, erosion of the base of a slope or changes to the materials strength through weathering can lead to landslides happening.
Rock fall landslide at Pennington Point, near Sidmouth on the south-east Devon coast, UK. © Eve Mathews. All rights reserved.
Landslides are classified by their type of movement. The four main types of movement are falls, topples, slides (rotational and translational), and flows.
Landslides can be classified as just one of these movements or, more commonly, can be a mixture of several.
At BGS we study the landslides that happen in the UK, but here we also include some examples of large landslides that have occurred elsewhere around the world and also in the sea.
Why do landslides happen?
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.
On 24 April 2016, a landslide at East Cliff, Bournemouth caused damage to a funicular railway, a toilet block and an array of fences and benches that were at the top of the cliff. BGS ©UKRI. All rights reserved.
What can increase the chance of a landslide?
Several factors can increase a slope’s susceptibility to a landslide event:
- water — adding water to the material on a slope, makes a landslide more likely to happen. This is because water adds weight, lowers the strength of the material and reduces friction, making it easier for material to move downslope
- erosion processes — such as coastal erosion and river erosion — if the bottom of a slope is continually eroded by the sea or a river, the slope will eventually become too steep to hold itself up
- steepness of slope — the slope angle is a key factor as far as landslides are concerned. Any change to this that makes it steeper (such as coastal erosion) increases the likelihood of a landslide
- type of ‘rocks’ — soft rock such as mudstone or hard rock such as limestone — the type of rocks in the slope, and their combination
- 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
- human activity — mining, traffic vibrations or urbanisation which changes surface water drainage patterns
Following a period of heavy rain, on 1 August 2012 a debris flow landslide occurred along the A83 Rest and Be Thankful pass (Argyll and Bute, Scotland). It was reported that between 50 to 100 tonnes of material blocked the road that was subsequently closed in both directions resulting in a long diversion. BGS ©UKRI. All rights reserved.
Further information on landslides:
Landslides in the UK and around the world
BGS maintains the National Landslide Database, with over 16 500 records, and is the definitive source of landslide information in Great Britain.
How to classify a landslide
Landslides are classified by their type of movement. The four main types of movement are falls, topples, slides, and flows.
You may also be interested in:
Landslide case studies
The landslides team at the BGS has studied numerous landslides. This work informs our geological maps, memoirs and sheet explanations and provides data for our National Landslide Database, which underpins much of our research.
Landslides
The BGS landslides team is involved in many aspects of landslide research, with the primary objective of building resilience both in the UK and internationally.