A landslide is a massive downward movement of soil, rock, debris, or other earth material along a slope or hill due to the force of gravity. The movement takes place in several ways such as flowing, falling, spreading, toppling, sliding, etc., or a combination of movements. There can be dry debris movement or water-saturated mud movement. Another kind of landslide is a mudslide, which is a fast-moving landslide usually along a channel or canal. When either occurs, they may cover a small area, uphill to downhill, or travel over several miles from its source.
Landslides and mudslides can be extremely powerful and depending on the volume of debris and its speed, it can wipe out houses, move cars, trees, electricity poles, and other structures in its path. Landslides can occur just about anywhere with a hill or slope, although there are some places where the risk may be greater.
There are several causes of landslides and depending on the area of the world, one cause may occur more often than another cause. Saturated slopes include intense rainfall, snowmelt, and seepage which may cause saturated land surfaces. The result is flooding and water overflowing channels, and land surfaces get wiped out and mudflows occur. Earthquakes, tremors, and other vibrations cause the ground to shake and rumble weakening the bond of rock leading to landslides, which is the second cause, seismic activity.
Volcanic activity leads to lava from eruptions melting snow at high rates, and the combination of the melted snow, debris, volcanic ash, and soils may flow quickly along a hill or slope resulting is devastating landslides. Cohesive rock often holds itself in place and are less prone to weathering and movement, but weathered rock and sheared or jointed rock material leads to a geologic cause of landslides. The rocks are no longer cohesive and prone to weathering/erosion.
The different forms of erosion such as water, wave, glacial and others along a slope may cause landslides, mudslides, or other flows. The deposits from erosion build-up over time and eventually gives way in the form of a morphological cause of a landslide. Of course, human activities can also cause landslides such as mining, farming, construction, deforestation, irrigation, dams, and reservoirs. In mining areas, many tunnels and deep wells are dug into the ground and then if left unfilled, air and water get in and may result in a landslide.
Many landslides are classified by the nature of their movements and the earth material involved. There are five main types of landslides: slide, topple, fall, flow, and torrent. There are two kinds of slide landslides, transitional and rotational. During a transitional slide the earth remains in place after it slides downhill, and during a rotational slide the movement of the earth rotates. The second type is called topple when the mass of earth rotates forward on a pivot. There is often a tilt without collapse and is usually caused by cracks or the fracture in the bedrock.
A fall slide usually takes place along steep slopes or cliffs. The earth material is influenced by gravity after large blocks and boulders become detached. A torrent slide is a sporadic discharge of debris and water found along low-lying channels between highlands.
Finally, the last type of landslide is called a flow. There are several types including debris flow, which involves a rapid downhill movement of loose earth material often with water. A debris avalanche is simply much faster, and a mudflow is made up of fine silt, sand and clay material saturated with water and flowing very rapidly. A creep is a very slow flow but can bend electric poles and roads slightly. An earth flow's material is finer and if it washed away it leaves a depression bowl.
The effects of landslides can range from minor to devastating. In the United States each year, about 25 to 50 people die and worldwide between 2004 and 2010 there were over 32,000 landslides killing over 2,600 people, but it is believed the number is underestimated. There is also property destruction, such as the 1980 landslide caused by the volcanic eruption of Mt. St. Helens in the state of Washington. The landslide traveled 14 miles and wiped away bridges, rods, and buildings.
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