How Landslides Work

Two homes in the Hollywood Hills of Los Angeles sit perilously on the edge of a cliff in January 2017 after a mudslide caused by heavy rains washed away the land beneath. ROBYN BECK/AFP/Getty Images

When it comes to natural disasters, the tornadoes and tsunamis of the world tend to get all of the attention. Rarely do landslides seize as many headlines as the volcanoes and earthquakes that can cause them. But when the ground literally rips downhill, the effect is often more damaging than the trigger. The force of landslides can cave houses, dam rivers and annihilate entire towns. Worldwide, landslides were responsible for more than 30,000 deaths in 2005 [source: Petley]. They inflict damage that costs the United States alone at least $1 billion to $2 billion each year, making them more damaging than all other natural disasters combined [source: United States Search and Rescue Task Force].

Landslides are a form of mass movement, a term used to describe any sort of gravity-induced movement of sediment down a slope. Mass movements can occur slowly over a period of years, or they can happen in a matter of minutes. A mass movement can be as small as some rocks and debris you kick down a small incline or as big as the 1980 landslide set off by the eruption of Mount St. Helens.

­There are many different kinds of mass movements categorized by the type of material involved, the way it is moved and how fast it moves­. However, with any mass movement, a soil layer is separated to some degree from the underlying bedrock. Soil is the relatively loose mixture of worn-down rock, minerals, air, water and decayed organic matter that covers the ground. Bedrock is the more stable, solid layer of rock underneath.

Although the word landslide often is used (incorrectly) to encompass many types of mass movements, a landslide is actually something more specific. A slide refers to a mass movement where rocks and sediment are loosened from the stable, underlying bedrock along a distinct zone of weakness. The rocks and sediment separate and move down the slope rapidly. You could think of it as a poster fastened to a wall with tape. The poster will remain on the wall barring any outside force acting on it. But if extra weight is attached to the poster, or if the tape is moistened, the connection will be weakened and the poster will fall.

In this article you'll learn what happens if a landslide happens underwater, why deforestation and water don't mix and just how powerful (and hot!) volcanic landslides can be.

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