What are they?
- Landslides are the downslope movement of rock or debris or soil under the influence of gravity.
- The process may also be called mass movement or mass wasting.
These terms may be better in some ways because sliding is just one way in which mass movements can happen: rock, debris or soil can also move under gravity by falling or flowing.
Mass movement can be classified according to how the movement happens.
- The main types are falls, slides and flows.
- It is also important to think about what happens to the material when it moves: does it remain a coherent block, similar to the state in which it fell off the face, or is it deformed - does it become jumbled up during the movement?.
- If there is water present, falls and slides may disintegrate and develop into avalanches or debris flows..
Why do they happen?
1. Slope
- Landslides occur on slopes, not on flat ground.
- They are more likely to occur on steeper slopes because the gravitational forces acting on the rock or sediment are larger.
- Anything which steepens the slope or removes its lower regions (for example a valley being glaciated) will make the slope less stable.
2. Tectonic activity
- Earthquakes will trigger mass movements.
- Areas of active mountain building have steep slopes and folded rocks, which are susceptible to movement.
This is particularly important for the very largest landslides.
4. Rock type (lithology)
- Some rocks are stronger than others.
- Some will weather more easily, creating a much weaker material.
- What a slope is made of will help to determine how steep it can become before it gets unstable.
- The permeability (how fast water can filter through it) is also important.
- Less permeable rocks are more susceptible to mass movement.
5. Rock structure
- Bedding in rocks (i.e. how the different layers of rock fit together) and any cracks and joints in the rock will help to determine how stable a slope is.
- If beds or joints dip downwards out of the slope, this will produce an inherently unstable situatio
- A permeable rock which sits directly above an impermeable one will create the possibility for instability if water is introduced.
For example, if sandstone overlies clay, then water will be able to filter through the sandstone more rapidly than it can through the clay, so water can begin to build up at the bedding plane (the junction between them), and slippage will occur along this plane.
6. Water
- The amount of water present is often very important and may be the thing which sets off movement.
- Water will lubricate joints and bedding in rock which will reduce the friction.
- Water can reduce the bonds holding the material together.
In this case, falls or slides may develop into flows. - Water within the rock or sediment will increase its weight and so make it more liable to fall or collapse.
- Water can also be part of the movement in debris flows.
These are rather like superconcentrated rivers - but with more sediment than there is water.
7. People
- Human impacts can cause mass movements
- Changing drainage patterns can lead to areas becoming wetter, and so unstable.
Dam construction can cause particular problems here. - Excavation at the toe end of slopes (for example in road building or when channels are dug) may make the slope above unstable.
- Building in unsuitable locations can change the balance of the slope and make it unstable.