Landslides are among the most common types of geologic hazards. A landslide is an event in which a mass of rock, soil and/or debris moves down a slope.
The velocity at which the landslide travels is dependent on the cohesion between the grains or particles that make up the landslide (cohesion is how well grains or particles stick to each other), the angle of the slope, and other factors.
Landslides can travel quickly (sometimes even at several miles per hour) or slowly (taking advantage of gravity over many hours or even days).
Depending upon the type of movement of materials several types are identified in this category.
Slump is slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place
Rapid rolling or sliding of earth debris without backward rotation of mass is known as debris slide. Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope.
Causes include factors that increase the effects of down-slope forces and factors that contribute to low or reduced strength.
Landslides can be initiated in slopes already on the verge of movement by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.
