The earth’s land moves at least a few centimetres a year. But there are times when it can move metres, even kilometres at a time. This is when there is a landslide, avalanche or mudslide.
Landslides are the movement of large amounts of earth such as rock, soil, sand, mud or the combination of any of these.
The earth’s land is dynamic and is always moving. It moves very slowly and is calculated to be moving about a few centimetres each year. This is one reason why we have landslides. Some other factors in causing landslides are:
- Other vibrations in the earth due to machinery, blasting, construction and mining.
- Erosion as a result of loss of vegetation, rivers, floods or the ocean’s waves.
- Avalanches that can be caused by significant changes to the amount of moisture that seeps into the land.
- Moisture levels can change in the earth from agriculture, the farming and irrigation of fields, or the melting of snow and ice.
All these elements are contributing factors in destabilising the earth.
When land is destabilised, particularly on sloping, mountainous or land with even a slight gradient – landslides can occur.
Landslides vary in size
Landslides can vary in size. They can be as small as the movement of a single boulder in a minor rock fall or as big as thousands of tonnes of earth and debris that fall to the bottom of a slope or a cliff.
How a landslide occurs
As the land slowly moves, by a few millimeters a day, it causes tension cracks in the earth and in the bedrock. As this tension increases and significant changes in moisture occur, from things like heavy rainfall or the fast melting of snow, the land begins to move more and more.
The image on the right shows how this movement in the land can cause a landslide.
Effects of landslides
Landslides can happen in different places and have different effects on people, buildings, animals and landscape.
Landslides can affect and endanger homes and lives.
Did you know?
- Landslides are quick. In Canada, one landslide caused a riverbank to move 680 m in less than an hour.
- In Australia, the worst recorded landslide was in Tasmania, when 35 houses were destroyed in two adjacent landslides in the 1960s.
- Melting ice in the northern hemisphere can cause ice dams that block rivers and force water to burst through shorelines. When the pressure of the water is too strong, sudden outburst floods surge downstream with blocks of ice five to seven metres tall.
- In the USA, between 25 and 50 people are killed in landslides each year.
- The May 1980 eruption of the Mount St. Helens, USA volcano caused the largest landslide in history – a rock slide debris avalanche large enough to fill 250 million trucks.