Landslides occur when masses of rock, earth, or debris move down a slope. Mudslides develop when water rapidly accumulates in the ground and results in a surge of water-saturated rock, earth, and debris. Mudslides usually start on steep slopes and can be activated by natural disasters.

This massive movement of rock, ash, water and downed trees swept into Spirit Lake and down the north fork of the Toutle River Valley at speeds in excess of 175 miles per hour,” according to a website on the history of the eruption.

Mudslides can be slow- or fast- moving, though they tend to grow in size and momentum as they pick up trees, boulders, cars and other materials. Mudslides can occur at any time of the year, regardless of weather conditions, according to the Federal Emergency Management Agency (FEMA).

Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place over timescales from seconds to hundreds of years.

Creep. A slow, gradual movement of soil or regolith downhill over time is called creep. Velocities are typically less than a centimeter per year. Freezing and thawing contribute to soil creep by progressively moving soil particles down the hill.

Mass wasting – is movement in which bed rock, rock debris, or soil moves downslope in bulk, or as a mass, because of the pull of gravity. Flow – The debris is moving downslope as a viscous fluid. A mudflow is a flowing mixture of debris and water, usually moving down a channel.

Short-term prediction of mass-wasting events is somewhat more problematical. For earthquake triggered events, the same problems that are inherent in earthquake prediction are present. Slope destabilization and undercutting triggered events require constant monitoring.

Humans can contribute to mass wasting in a few different ways: Excavation of slope or its toe. Loading of slope or its crest. Drawdown (of reservoirs)