Synoptic Description of Mass Movements

Mass movement or mass wasting is movements of masses of bodies of soil, bed rock, rock debris, soil, or mud which usually occur along steep-sided hills and mountains because of the pull of gravity. This slipping of large amounts of rock and soil is seen in landslides, mud slides, and avalanches.
  • 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.

Causes of Mass Movements

Mass movements are caused by various conditions:
  • 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.
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Factors that affect mass movements are the steepness and instability of slopes, the nature of the slope materials, and the amount of water in material.

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.

Occurrences of Mass Movements

Landslides occur in every state and U.S. territory. California, West Virginia, Utah, Kentucky, Tennessee, Puerto Rico, Ohio, Washington, Alaska, Hawaii, have the most severe landslide problem. Basically it's areas around the Appalachian Mountains, the Rocky Mountains, the Pacific Coastal Ranges or any area composed of very weak or fractured materials resting on a steep slope can and will experience landslides.

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.

For example, in California mass movements are accelerated by the constant movement of plates and earthquakes in this area. In Miami, there are various example of creep, where there is damage to buildings and houses.

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).

Outside the United States, at least one form of mass movement can be detected in all areas of the world.

Social and Economical Impact of Mass Movements

Mass movements produce a variety of effects. For example, a landslide may fall across a river, damming the water and causing it to form a lake. However, landslides can also suddenly rush down a steep slope can cause great destruction across a wide area of habitable land and sometimes cause floods by damming up bodies of water. In Alberta, Canada, in 1903, a mass of about 30.6 million cubic m slipped from the top of Turtle Mountain and fell to the floor of the adjacent valley. The mass of earth and rock spread across a 3.2-km-wide (2-mi-wide) valley, annihilating the town of Frank and killing 70 people. Similar landslides have occurred in other high mountains, such as the Rocky Mountains, the Alps, and the Himalaya. In Portuguese Bend about 156 houses have been torn apart or moved to another area because of the movement of the land. A slurry can flow rapidly down slopes or through channels, and can strike with little or no warning at avalanche speeds. It can travel several miles from its source, growing in size as it picks up trees, cars, and other materials along the way. In Colombia, the eruption of a volcano near the town of Almero erupted sending a mud flow slurry that wiped the town from the face of the earth; from 25,000 people only 3,000 survived.

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

Expenses related to landslides include actual damages to structures or property, as well as loss of tax revenues on devalued properties, reduced real estate values in landslide prone areas, loss of productivity of agricultural lands affected by landslides, and loss of industrial productivity because of interruption of transportation systems by landslides. All type of mass movements are harmful. The slow movement of creep does more long term economic damage to railroads, building structure and underground pipes.

Natural Hazards Data Resources Directory
Mass Movement
FEMA Landsilde Information