Landslides often have devastating effects on humans. The wall of mud wipes out buildings, covers roads and changes the landscape. The economic cost of landslides in the United States ranges from $1 billion to $2 billion annually. NBC News reports a death toll of 41 individuals with two missing from the March 22, 2014, landslide in Oso, Washington.
Many landslides are the aftereffects of volcanoes or earthquakes. The damage they cause is often more significant than the damage caused by the triggering event.
Both natural and human forces increase the likelihood of a landslide. Activity at the base of the incline, such as the cutting action of a river or excavation by humans, removes some of the supporting material, giving gravity a greater advantage at moving the sediment from the higher layer.
According to HowStuffWorks.com, geologists classify landslides as "mass movement," a term describing any kind of "gravity induced movement of sediment down a slope." While some mass movements occur slowly over several years, landslides often occur in a matter of minutes. Mass movements are sometimes small, as when a deer kicks a few rocks down an incline. Others are large, such as the 1980 landslide due to Mount St. Helens' eruption. From a technical point of view, a landslide is a specific type of mass movement in which sediment loosens from its underlying bedrock along a distinct line of weakness, separates and moves down a slope in a rapid fashion.