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 mass, largely under the force of gravity, but frequently affected by water and water content as in submarine environments and mudslides.[1] 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 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 between material help maintain the slope's stability and are known collectively as the slope's shear strength. The steepest angle that a cohesion less slope can maintain without losing its stability is known as its angle of repose. When a slope possesses this angle, its shear strength perfectly 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 rock slides 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.The importance of water in mass wastingWater 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 in order 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.

Types of mass movementTypes of mass movement are distinguished based on how the soil, regolith or rock moves downslope as a whole.CreepsSoil 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.LandslidesA 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.FlowsMovement 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.TopplesTopples are instances when blocks of rock pivot and fall away from a slope.SlumpA 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.FallsA 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.

Triggers of mass wastingSoil 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). Factors that reduce the frictional resistance relative to the downslope forces, and thus initiate slope movement, can include:

seismic shaking 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 bioturbation

TYPES OF MASS WASTING MOVEMENTSVery many different types of mass movement are recognized by geologists, some only very subtly different from others. What follows is a very general classification. Your textbook has a more complete classification, but even it includes only a small subset of all the possible categories and examples.The general classification is based on three things: what moves, how it moves, and how rapidly it moves, not necessarily in that order.The first break is made at the rate of movement. There are two extremely slow types of movement.CREEP is an imperceptibly slow downslope movement of material. The uppermost part of the slope moves more than the deeper parts causing generalized bending and tilting of the slope surface and of objects growing, built, or otherwise attached to that surface. Fig. 11-24 in your book gives good examples.SOLIFLUCTION translates literally "soil flowage". In areas of permafrost, where the ground is permanently frozen at depth, the surface ice melts in the spring and summer but cannot escape the soil because of the ice below. The soil acts like a mush and flows. It even does so on practically non-existent slopes.Other types of movement can go at either very fast or rather slower rates, but all move rapidly enough to be directly observed. There are three major categories.Where very steep slopes exist, things can fall with only occasional contact or none at all with the ground until they reach the foot of a slope. Such movements are called (surprise, surprise) FALLS. Slopes this steep are generally only possible if the material on them is hard rock, so ROCKFALLS are the only really common type of falls.Other mass movements involve material moving in contact with the slope. There are two extreme alternatives, though some geologists recognize intermediate categories as well.AVALANCHES are movements in which loose material moves rapidly down a slope in a chaotic or "incoherent" fashion. If you think of any particular particle, its path is not a straight line down the slope. It might be bouncing and ricocheting all over the place. Other particles might move faster or slower, thus passing or being passed by it. This is what "incoherent" means -- the "bits" don't cohere to each other and move in orderly fashion. The best known avalanches are SNOW AVALANCHES, but ROCK AVALANCHES and DEBRIS AVALANCHES also occur.SLIDES are those movements in which the moving material moves as a coherent mass. Again, think of a single bit -- maybe one molecule or one crystal. It will follow a nearly straight-line path down the slope, it will neither outrun nor be outrun by its neighbors. This is what we mean by "coherent". ROCKSLIDES and DEBRIS SLIDES (or SEDIMENT SLIDES) are the most common types of movement along a planar surface. Very steep slopes on soils, sediment, or fill (near road ditches or construction sites, for example) often fail along curved breaks as the picture shows. Such movements are called SLUMPS. They may occur in several steps.

FLOWS are those movements in which unconsolidated material like soil or sediment becomes over-saturated with water and move as a liquid. Usually so much water is present that the grains do not often touch each other during the movement. They make the flow thick and dangerous, but the water is actually doing most of the work. MUDFLOWS and DEBRIS FLOWS are the important types. The latter can include very large items as well as finer sediment and water.

Types of Mass WastingReference: Dept of Conservation, State of CaliforniaDMG Note 50 Definition. Translational/Rotational Landslide This kind of slide is characterized by large blocks of material and a failure plane that is relatively deep. The movement is forms straight or almost straight lines downhill. It occurs in relatively cohesive, homogeneous soils and rock. The soil mantle may be greater than 5 feet thick, but sliding is not restricted to the zone of weathering. Failure commonly occurs along bedrock bedding planes that are deep-seated and dip in the same direction as the slope surface. In saturated conditions, incompetent clayey bedrock material may fail under overburden weight and high pore pressures, resulting in a deep-seated rotational-type failure. Translational slides commonly are controlled structurally by surfaces of weakness such as faults, joints, bedding planes, and contacts between bedrock and overlying deposits. Translational/rotational slides generally form a concentric (like) near vertical scarp in the head region and, occasionally, along the lateral margins of the slide. Slide materials are characterized by hummocky topography consisting of rolling, bumpy ground, frequent benches, and depressions. The toe of the slide may be steep where slide material has accumulated. Steep crownscarps and margins of the translational/rotational slide and toe areas of large slides may be subject to debris sliding. Diagrammatic sketch of a translational/rotational landslide by Janet Appleby, Richard Kilbourne, and Thomas Spittler; modified from Varnes (1978) Earthflow An earthflow is a landslide resulting from slow to rapid movement of saturated soil and debris in a liquidy state. After initial failure, the earthflow may move, or creep, seasonally in response to destabilizing forces. Earthflows are composed of clay-rich materials that swell when wet, causing a reduction in friction between the soil particles. When saturated, the fine-grained, clay-rich matrix may carry larger, more resistant boulders with them in slow, creeping movements. Slide materials erode easily, resulting in gullying and irregular drainage patterns. The irregular, hummocky ground characteristic of earthflows is generally bare of trees. Failures commonly occur on slopes that are gentle to moderate, although they may also occur on steeper slopes where vegetation has been removed. Undercutting of the toe of an earthflow is likely to reactivate downslope movement.

Diagrammatic sketch of an earthflow by Janet Appleby and Richard Kilbourne; modified from Varnes (1978)Debris Slide A debris slide is characterized by unconsolidated rock and soil that has moved downslope along a relatively shallow failure plane. Debris slides form steep, unvegetated scars (depressions) in the head region and irregular, hummocky deposits (when present) in the toe region. Debris slide scars are likely to ravel and remain unvegetated for many years. These scars can be recognized by the nature of the slope, steepness of the slope, and the light bulb-shaped form left by many mid- and upper-slope failures. Debris slides are most likely to occur on slopes greater than 65 percent where unconsolidated colluvium overlie a shallow soil/bedrock. The shallow slide surface is usually less than 15 feet deep. The probability of sliding is low where bedrock is exposed, except, where weak bedding planes and extensive bedrock joints and fractures parallel the slope.

Diagrammatic sketch of a debris slide by Janet Appleby and Richard Kilbourne; modified from Varnes (1978). Debris Flow/Torrent Track Debris flow and debris torrent tracks are characterized by long stretches of bare, generally unstable stream channel banks that have been scoured and eroded by the extremely rapid movement of water-laden debris. They commonly are caused by debris sliding or the failure of fill materials along stream crossings in the upper part of a drainage during high intensity storms. Debris flow/torrent tracks are formed by the failure of water-charged soil and organic material down steep stream channels. They are often triggered by debris slide movement on adjacent hill slopes and by the mobilization of debris accumulated in the stream channels themselves. Debris flows and torrents commonly entrain large quantities of inorganic and organic material from the stream bed and banks. Occasionally, the channel may be scoured to bedrock. When momentum is lost, scoured debris may be deposited as a tangled mass of large organic debris in a matrix of sediment and finer organic material. Such debris may be reactivated or washed away during subsequent events. The erosion of steep debris slide-prone streambanks below the initial failure may caused further failure downstream.

Diagrammatic sketch of a debris flow/torrent track.by Janet Appleby and Richard Kilbourne.