Landslide

A landslide is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes, and shallow debris flows, see flow[?]. Although gravity acting on an over steepened slope is the primary reason for a landslide, there are other contributing factors:

• erosion by rivers, glaciers, or ocean waves create oversteepened slopes
• rock and soil slopes are weakened through saturation by snowmelt or heavy rains
• earthquakes create stresses that make weak slopes fail
• volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
• vibrations from machinery, traffic, blasting, and even thunder may trigger failure of weak slopes
• excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures

Slope material that become saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.

Similarly, ice floes can form in rivers that are clogged with ice, but are generally much slower moving. Nonetheless, they can generate forces strong enough to collapse bridges.

An avalanche is similar in mechanism to landslide and it involves a large amount of ice, snow and rock falling quickly down the side of a mountain. Usually the ice builds in cornices or forms over a weaker layer of snow, creating the danger of an avalanche.

A pyroclastic flow is caused by a collapsing cloud of ash, poisonous gas and hot rocks from a volcanic explosion that moves rapidly down an erupting volcano.

First-draft text taken from USGS fact sheet, public domain

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References and external links:

• United States Geological Survey site (http://landslides.usgs.gov/)