landslide, also called landslip , 

landslide, also called landslip ,  the movement downslope of a mass ofrock, debris, earth, or soil (soil being a mixture of earth and debris). Landslides occur when gravitational and other types of shear stresseswithin a slope exceed the shear strength (resistance to shearing) of the materials that form the slope.Shear stresses can be built up within a slope by a number of processes. These include oversteepening of the base of the slope, such as by naturalerosion or excavation, and loading of the slope, such as by an inflow of water, a rise in the groundwater table, or the accumulation of debris on the slope’s surface. Short-term stresses, such as those imposed byearthquakes and rainstorms, can likewise contribute to the activation of landslides. Landslides can also be activated by processes that weaken the shear strength of a slope’s material. Shear strength is dependent mainly on two factors: frictional strength, which is the resistance to movement between the slope material’s interacting constituent particles, and cohesive strength, which is the bonding between the particles. Coarse particles such as sand grains have high frictional strength but low cohesive strength, whereas the opposite is true for clays, which are composed of fine particles. Another factor that affects the shear strength of a slope-forming material is the spatial disposition of its constituent particles, referred to as the sediment fabric. Some materials with a loose, open sediment fabric will weaken if they are mechanically disturbed or flooded withwater. An increase in water content, resulting from either natural causes or human activity, typically weakens sandy materials through the reduction of interparticle friction and weakens clays through the dissolution of interparticle cements, the hydration ofclay minerals, and the elimination of interparticle (capillary) tension.