• Landslides occur when masses of rock, earth, or debris move down a slope. They may be very small or very large, and can move at slow to very high speeds. However slow movement is also seen in the gradual downhill creep of soil on gently sloping land.
• Mudflows (or debris flows) are rivers of rock, earth, and other debris saturated with water.
• An avalanche is a sudden flow of a large mass of snow or ice down a slope or cliff, sometimes at speeds exceeding 160 km/hr (100 mph). The two most important processes in mass movement is the angle of repose and the surface tension.

• Volcanic activity many times causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption.
• Mudslides can also develop when water rapidly accumulates in the ground, such as during heavy rainfall or rapid snow melt, changing the earth into a flowing river of mud or "slurry.".
• Earthquake shocks cause sections of mountains and hills to break off and slide down.
• Human modification of the land or weathering and erosion help loosen large chunks of earth and start them sliding downhill.
• Vibrations from machinery, traffic, weight loading from accumulation of snow; stockpiling of rock or ore; from waste piles and from buildings and other structures.
• However, the trigger mechanism for mass movement is the gravitational pull of the earth on soil, rocks, and mud.

Types of Mass Movements, Slump and Earthflow.
Pictures from: http://www.geo.duke.edu/geo41/mmo2.htm

Types of mass wasting:

• Rate of Movement - rapid or slow
• Type of Movement - falling, sliding or flowing
• Type of Material involved - rock, soil or debris

Kinds of Material Moved:

Bedrock: Most common are slides or falls of small blocks of bedrock that break off by enlargement of joints during weathering. Massive rockslides or rockfalls involving much greatervolumes of material are rare but are spectacular.

Soil (Regolith): Mud and earth imply materials that consist mostly of clay and silt (mineralparticles finer than sand). In a mudflow there is enough water to allow the mixture to flow easily, as a viscous stream. An earthflow is slower moving than a mudflow and involves a mass of material that retains rather distinct boundaries as it moves. Debris is a term used generally for soil and associated materials such as plants and pieces of bedrock. The term implies a heterogeneous mixture of materials including a considerable fraction of particles that are coarser than the particles in mud.

Water: Movements of snow and ice (snow avalanches and glacial flow) under the influence of gravity are not usually considered to be mass wasting. But sometimes snow is involved in mass-wasting events (as in the case of the event involving Nevado Huascarán in Peru in 1970). Soil water, the water between soil particles in the upper part of ground (the part that is usually not completely saturated with water), moves with the soil during mass wasting. Of more importance is the fact that an excess of water in the soil often "triggers" mass wasting events. Also, the more or less contiunous process of creep occurs more rapidly in soils that are usually wet than in soils that are relatively dry most of the time.

This landslide took place in the Siletz River Basin just outside of Lincoln City. During a heavy rain in the winter of 1994/95, water collected on the uphill side of logging road USFS 1980, super-saturating the road grade and eventually causing the road to slide.

On the left is an example of creep(Creep, Marathon, TX (Photo from NOAA)). On the right is a displaced Curb, Photo by DH, 3/4/84).

Yungay, Peru's Main Plaza before(left) and after(right) pictures of landslide's destruction.