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Mass wasting, or mass movement, is the process that moves Earth materials down a slope, under the influence of gravity. Mass wasting processes range from violent landslides to imperceptibly slow creep. Mass wasting decreases the steepness of slopes, leaving them more stable. While ice formation or water infiltration in sediments or rocks may aid mass wasting, the driving force is gravity. All mass wasting is a product of one or more of the following mass wasting processes: flow, fall, slide, or slump.

The four processes of mass wasting are distinguished based on the nature of the movement that they produce. Flow involves the rapid downslope movement of a chaotic mass of material. Varying amounts of water may be involved. Amud flow, for example, contains a large amount of water and involves the movement of very fine-grained Earth materials. Fall involves very rapid downslope movement of Earth materials as they descend (free fall) from a cliff. Ignoring wind resistance, falling materials accelerate at 32 ft/sec² (9.8 m/sec²)—the average gravitational force of the earth. Slides result when a mass of material moves downslope, as a fairly coherent mass, along a planar surface. Slumps are similar to slides, but occur along a curved (concave-upward) surface and move somewhat more slowly.

Consider a chunk of rock currently attached to a jagged outcrop high on a mountain. It will move to the sea as a result of three processes: weathering, mass wasting, and erosion.

On warm days, water from melting snow trickles into a crack which has begun to form between this chunk and the rest of the mountain. Frigid nights make this water freeze again, and its expansion will widen and extend the crack. This and other mechanical, biological, and chemical processes (such as the growth of roots, and the dissolution of the more soluble components of rock) break apart bedrock into transportable fragments. This is called weathering.

Once the crack extends through it and the chunk has been completely separated from the rest of the mountain, it will fall and join the pile of rocks, called talus, beneath it that broke off the mountain previously. This pile of rocks is called a talus pile. This movement is an example of mass wasting, known as a rockfall. As the rocks in the talus pile slip and slide, adjusting to the weight of the overlying rocks, the base of the talus pile extends outward and eventually all the rocks making up the pile will move down slope a little bit to replace those below that also moved downslope. This type of mass movement is known as rock creep, and a talus pile that is experiencing rock creep is called a rock glacier.

In the valley at the bottom of this mountain, there may be a river or a glacier removing material from the base of the talus slope and transporting it away. Removal and transport by a flowing medium (rivers, glaciers, wind) is termed erosion.

These processes occur in many other situations. A river erodes by cutting a valley through layers of rock, transporting that material using flowing water. This erosion would result in deep canyons with vertical walls if the erosion by the river were the only factor. Very high, vertical walls, however, leave huge masses of rock unsupported except by the cohesive strength of the material of which they are made. At some point, the stresses produced by gravity will exceed the strength of the rock and an avalanche (another type of mass movement) will result. This will move some of the material down the slope into the river where erosion will carry it away.

Erosion and mass wasting work together by transporting material away. Erosion produces and steepens slopes, which are then reduced by mass wasting. The steepness of a natural slope depends on the size and shape of the material making up the slope and environmental factors, principally water content. Most people learn about this early in life, playing in a sandbox or on the beach. If dry sand is dumped from a bucket, it forms a conical hill. The more sand dumped, the larger the hill becomes, but the slope of the hill stays the same. Digging into the bottom of the hill causes sand to avalanche down into the hole you are trying to make. Loose, dry sand flows easily, and will quickly re-establish its preferred slope whenever anything is done to steepen it. The flow of sand is a simple example of mass wasting.

If sand is moist, the slope of a sand pile can be higher. A sandcastle can have vertical walls of moist sand when it is built in the morning, but, as the afternoon wears on and the sand dries out, it eventually crumbles and collapses (mass wastes) until a stable slope forms. This is because the water makes the sand more cohesive. With the proper moisture content, there will be both water and air between most of the grains of sand. The boundary between the water and the air has surface tension—the same surface tension that supports water striders or pulls liquids up a capillary tube. In moist sand, surface tension holds the grains together like a weak cement.

However, if sand becomes saturated with water (that is, its pores become completely water-filled as they are in quick sand), then the sand will flow in a process known as lateral spreading. Water-saturated sand flows because the weight of the sand is supported (at least temporarily) by the water, and so the grains are not continuously in contact. The slope of a pile of sand is dependent on water content, and either too little or too much water lowers the stable slope. This illustrates how slope stability is a function of water content.

The steepest slope that a material can have is called the angle of repose. Any loose pile of sediment grains has an angle of repose. As grain size increases, the angle of repose also increases. Talus slopes high on mountain sides may consist of large, angular boulders and can have slopes of up to 45°, whereas fine sand has an angle of repose of 34°. This is the slope that you can see inside a sand-filled hourglass. In nature, however, slopes less than the angle of repose are common because of wind activity and similar environmental processes.

A typical sand dune has a gentle slope on the windward side where erosion by the wind is responsible for the slope. On the leeward side, where sand falls freely, it usually maintains a slope close to the angle of repose. As with loose deposits of particles on land, similar conditions exist if they are under water, although stable slopes are much gentler. When sudden mass wasting events occur under water, large quantities of material may end up being suspended in the water producing turbidity currents that complicate the picture. Such currents occur because a mass of water with sediment suspended in it is denser than the clear water surrounding it, so it sinks, moving down the slope, eroding as it goes. Still, the initial adjustment of the slope was not the result of these currents, so the mechanism that produces turbidity currents is an example of mass wasting.

Most slopes in nature are on materials that are not loose collections of grains. They occur on bedrock or on soils that are bound together by organic or other material. Yet, many of the principles used to explain mass wasting in aggregates still apply. Instead of mass wasting taking place as an avalanche, however, it results from a portion of the slope breaking off and sliding down the hill. These events are usually called landslides, or avalanches—if they are large and damaging—or slumps if they are smaller.

If the gravitational forces acting on a mass of material are greater than its strength, a fracture will develop, separating the mass from the rest of the slope. Usually this fracture will be nearly vertical near the top of the break, curving to a much lower angle near the bottom of the break. Such events can be triggered by an increase in the driving forces (for example, the weight of the slope), a decrease in the strength of the material, or both.

Even solid rocks contain pores, and many of these pores are interconnected. It is through such pores that water and oil move toward wells. Below the water table, all the pores are filled with water with no surface tension to eliminate. It might seem that rocks down there would not be affected by rainfall at the surface. As the rains come, however, the water table rises, and the additional water increases the pressure in the fluids in the pores below. This increase in pore pressure pushes adjacent rock surfaces apart, reducing the friction between them, which lowers the strength of the rock and makes it easier for fractures to develop. Elevated pore pressures are implicated in many dramatic mass wasting events.

When mass wasting by flow occurs so slowly that it cannot be observed, it is called creep. Most vegetated slopes in humid climates are subject to soil creep, and there are many indicators that it occurs. Poles and fence posts often tip away from a slope a few years after they are placed. Trees growing on a slope usually have trunks with sharp curves at their bases. Older trees are bent more than younger ones. All this occurs because the upper layers of soil and weathered rock move gradually down the slope while deeper layers remain relatively fixed. This tips inanimate objects such as power poles. It would tip trees, too, except that they grow toward the Sun, keeping the trunk growing vertically, and so a bend develops.

This gradual downslope movement requires years to result in significant transport, but because it occurs over a great portion of the surface of the earth it is responsible for most mass wasting.

See also Catastrophic mass movements