A landslide is the movement of soil, rock, or other earth materials, downhill in response to gravity. Landslides include 'rock falls' and 'topples', 'debris flows' and 'debris avalanches', 'earthflows', 'mudflows, creep', and 'lateral spread' of rock or soil. Frequently landslides occur in areas where the soil is saturated from heavy rains or snowmelt. They can also be started by 'earthquakes', 'volcanic activity', changes in groundwater, disturbance or change of a slope by man-made construction activities, or any combination of these factors. A variety of other natural causes may also result in landslides, and they may trigger additional hazards, such as 'tsunamis' caused by submarine landslides. A landslide occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. Rock falls or topples are usually sudden and occur on steep slopes. In a rock fall, rocks may fall, bounce, or roll down the slope. A topple occurs when part of a steep slope breaks loose and rotates forward. A debris flow is a combination of water-saturated loose soil, rock, organic matter, and air, with material varying in size from grains of clay to large boulders. Such flows are formed when loose masses of unconsolidated wet debris become unstable. A 'lahar' is a special type of debris flow that originates from the slopes of a volcano. Water for a debris flow may be supplied by rainfall, by melting of snow or ice, or by overflow of a lake, and the flow may be either hot or cold, depending on how it starts and the temperature of the constituent debris. When moving, a debris flow resembles a mass of wet concrete and tends to flow along channels or stream valleys. It can travel down a hillside at speeds up to 200 miles per hour (more commonly, 30 to 50 miles per hour), depending on the slope angle, the water content, and the type of earth and debris in the flow. Burned areas are particularly susceptible to debris flows. Very rapidly moving debris flows are known as debris avalanches. Earthflows usually occur on moderate slopes, and consist of saturated soil or fine-grained rock deposits that flow downhill. Dry earthflows are also possible. A mudflow is an earthflow consisting of material that is wet enough to flow rapidly. Creep is the imperceptibly slow, steady, downward movement of slope-forming soil or rock. Creep can occur seasonally, where movement is within the depth of soil affected by seasonal changes in soil moisture and soil temperature, or can be continuous or progressive. Creep is indicated by curved tree trunks, bent fences or retaining walls, tilted poles or fences, and small soil ripples or ridges. Most landslides happen on steep or moderate slopes, but lateral spreads usually occur on very gentle slopes or in flat terrain. These spreads are caused by liquefaction, the process whereby saturated, loose sediments that will not stick together (usually sands and silts) are transformed from a solid into a liquefied state. Lateral spread is usually triggered by rapid ground motion, such as that experienced during an earthquake, but can also be artificially induced. The combination of two or more types of landslides is known as a complex landslide. Landslides constitute a major geologic hazard because they are widespread, occurring in all 50 States, and because they cause more than $2 billion in damages and more than 25 fatalities on average each year. Casualties in the United States are primarily caused by rockfalls, rock slides, and debris flows. Worldwide, landslides cause thousands of casualties and billions in monetary losses annually. One of the largest landslides in the world the 20th century occurred at Mount St. Helens, Washington, in 1980. A moderate earthquake caused roughly 1.7 cubic miles of rocks and mud to break free and slide down the side of the volcano, releasing pent-up pressure to produce the major eruption of May 18. Although this was the largest landslide recorded in historic time, fewer than 60 people were killed because most residents and visitors had been evacuated. The most costly landslide in U.S. history was a relatively slow-moving event in Thistle, Utah, in the spring of 1983. The landslide, caused by the wet El Nino winter of 1982-83, dammed the Spanish Fork River and buried U.S. Highway 6 and the main line of the Denver and Rio Grande Western Railroad. The town of Thistle was inundated under the floodwaters rising behind the landslide dam. Total losses were estimated at more than $400 million in 1983 dollars