Mass Wasting

Earth and Space


Soil creep is a long term process. The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope. The steeper the slope, the faster the creep. The creep makes trees and shrubs curve to maintain their perpendicularity, and they can trigger landslides if they lose their root footing. The surface soil can migrate under the influence of cycles of freezing and thawing, or hot and cold temperatures, inching its way towards the bottom of the slope forming terracettes. This happens at a rate that is not noticeable to the naked eye.


A landslide, also called a landslip, is a rapid movement of a large mass of earth and rocks down a hill or a mountainside. Little or no flowage of the materials occurs on a given slope until heavy rain and resultant lubrication by the same rainwater facilitate the movement of the materials, causing a landslide to occur. The common forms of landslides are slump, debris slide, rock slide, rock fall, debris fall and avalanche.


Movement of soil and regolith that more resembles fluid behavior is called a flow. These include avalanches, mudflows, debris flows, earth flow, lahars and sturzstroms. Water, air and ice are often involved in enabling fluidlike motion of the material.


A slipping of coherent rock material along the curved surface of a decline. Slumps involve a mass of soil or other material sliding along a curved surface (shaped like a spoon). It forms a small, crescent-shaped cliff, or abrupt scarp at the top end of the slope. There can be more than one scarp down the slope.


A fall, including rockfall, is where regolith cascades down a slope, but is not of sufficient volume or viscosity to behave as a flow. Falls are promoted in rocks which are characterised by presence of vertical cracks. Falls are a result of undercutting of water as well as undercutting of waves. They usually occur at very steep slopes such as a cliff face. The rock material may be loosened by earthquakes, rain, plant-root wedging, expanding ice, among other things. The accumulation of rock material that has fallen and resides at the base of the structure is known as talus.

Earth Flow

Typically develop at the low end of a large slump, where the slump block breaks apart and material continues moving down slope. This down-slope movement can be rapid and short-lived, as a debris flow (example: the La Conchita event of 2005), or the movement can be slow and variable, and prolonged over a long period of time (example: the Portuguese Bend earthflow). The speed of an earth flow can be controlled by several factors, the most important being the amount of water introduced into the earth flow - the more water, the faster it will move.

Mud Flow

originates in drainage basin in arid and semiarid country when a heavy rain following a long dry spell produces a cascading runoff too voluminous to absorbed in the soil. Fine debris is picked up from the hillsides by the runoff and concentrating the valley bottoms, where it flows down-valley with the consistency of wet concrete. The leading edge of the mudflow continues to accumulate load, becoming increasingly stiff and regarding the flow of the more liquid upstream portions, so that the entire mudflow moves halting down the valley. Mudflows often pick up large rocks and carry them along as part of the their load