Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, sand, regolith, and rock move downslope typically as a solid, continuous or discontinuous mass, largely under the force of gravity, frequently with characteristics of a flow as in debris flows and mudflows. 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. Mass wasting occurs on both terrestrial and submarine slopes, and has been observed on Earth, Mars, Venus, and Jupiter’s moon Io.
When the gravitational force acting on a slope exceeds its resisting force, slope failure (mass wasting) occurs. The slope material’s strength and cohesion and the amount of internal friction within the material help maintain the slope’s stability and are known collectively as the slope’s shear strength. The steepest angle that a cohesionless slope can maintain without losing its stability is known as its angle of repose. When a slope made of loose material possesses this angle, its shear strength counterbalances the force of gravity acting upon it.
Mass wasting may occur at a very slow rate, particularly in areas that are very dry or those areas that receive sufficient rainfall such that vegetation has stabilized the surface. It may also occur at very high speed, such as in rockslides or landslides, with disastrous consequences, both immediate and delayed, e.g., resulting from the formation of landslide dams. Factors that change the potential of mass wasting include: change in slope angle, weakening of material by weathering, increased water content; changes in vegetation cover, and overloading.
Volcano flanks can become over-steep resulting in instability and mass wasting. This is now a recognised part of the growth of all active volcanoes. It is seen on submarine volcanoes as well as surface volcanoes: Loihi in the Hawaiian volcanic chain and Kick ’em Jenny in the Caribbean volcanic arc are two submarine volcanoes that are known to undergo mass wasting. The failure of the northern flank of Mount St Helens in 1980 showed how rapidly volcanic flanks can deform and fail.
Role of water
Water can increase or decrease the stability of a slope depending on the amount present. Small amounts of water can strengthen soils because the surface tension of water increases soil cohesion. This allows the soil to resist erosion better than if it were dry. If too much water is present the water may act to increase the pore pressure, reducing friction, and accelerating the erosion process and resulting in different types of mass wasting (i.e. mudflows, landslides, etc.). A good example of this is to think of a sand castle. Water must be mixed with sand for the castle to keep its shape. If too much water is added the sand washes away, if not enough water is added the sand falls and cannot keep its shape. Water also increases the mass of the soil, this is important because an increase in mass means that there will be an increase in velocity if mass wasting is triggered. Saturated water, however, eases the process of mass wasting in that the rock and soil debris are easily washed down-slope.
Based on how the soil, regolith or rock moves downslope as a whole, mass movements can be broadly classified as creeps and landslides.
Soil creep is a slow and long term mass movement. The combination of small movements of soil or rock in different directions over time is 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. Landslides are often preceded by soil creep accompanied with soil sloughing — loose soil that falls and accumulates at the base of the steepest creep sections.
A landslide, also called a landslip, is a slow or 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.
In particular, if the main feature of the movement is a slide along a planar or curved surface, the landslide is termed slump, earth slide, debris slide or rock slide, depending on the prevailing material.
Movement of soil and regolith that more resembles fluid behaviour is called a flow. These include avalanches, mudflows, debris flows, earth flow, lahars and sturzstroms. Water, air and ice are often involved in enabling fluid-like motion of the material.
A fall, including rockfall and debris fall occurs 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 characterized by the presence of vertical cracks. Falls can also result from undercutting by running water as well as by 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 and 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.
Soil and regolith remain on a hillslope only while the gravitational forces are unable to overcome the frictional forces keeping the material in place (see slope stability). Some factors that reduce the frictional resistance relative to the downslope forces, and thus can trigger slope movement, can include:
- increased overburden from structures
- increased soil moisture
- reduction of roots holding the soil to bedrock
- undercutting of the slope by excavation or erosion
- weathering by frost heave or chemical dissolution
- Terracing steps on slopes or, more generally, re-modeling its shape
- Slope stabilization