An earth slide consists of rock, gravel, sand and soil that start to move. Earth slides are commonly triggered in periods of significant precipitation, and occur on slopes with uncompacted material covering the bedrock.
Three conditions must be in place in order for an earth slide to occur:
- The terrain must be steep enough, normally an incline of 30° or more. In areas without vegetation, earth slides can occur in slopes of as little as 25°.
- There must be uncompacted material on top of bedrock that can slide out.
- The uncompacted material must be affected in a way that causes it to become unstable, which could cause it to start moving.
Norwegian mountainsides are often covered in uncompacted material, which means that this type of slide can occur many places in Norway. People live in many of these areas.
Why do earth slides occur?
The addition of water is usually the direct triggering cause of an earth slide. The water either comes as precipitation or melt water from glaciers, or from snow at higher elevations.
The combination of melt water and precipitation increases the risk of slides. Areas are particularly vulnerable in periods where they have both melt water and intense precipitation, while the ground is already saturated.
Unstable uncompacted material is an important precondition for the occurrence of slides. Humans can contribute to this instability through activities such as construction and forestry. Natural processes can also cause areas to become unstable, such as weathering, forest fires and the timberline moving lower. Over time, weathering breaks down minerals and rock in the upper layer of soil, making the soil looser. Fewer plants and trees in an area can cause this uncompacted material to become unstable, as the roots help keep the soil in place.