Although gravity is the driving force of mass wasting, it is impacted mainly by the slope material’s strength and cohesiveness as well as the amount of friction acting on the material. If friction, cohesion and strength (collectively known as the resisting forces) are high in a given area, mass wasting is less likely to occur because the gravitational force does not exceed the resisting force.
The angle of repose also plays a role in whether a slope will fail or not. This is the maximum angle at which loose material becomes stable, usually 25°-40°, and is caused by a balance between gravity and the resisting force. If, for example, a slope is extremely steep and the gravitational force is greater than that of the resisting force, the angle of repose has not been met and the slope is likely to fail. The point at which mass movement does occur is called the shear-failure point.
Types of Mass WastingOnce the force of gravity on a mass of rock or soil reaches the shear-failure point, it can fall, slide, flow or creep down a slope. These are the four types of mass wasting and are determined by the speed of the material’s movement downslope as well as the amount of moisture found in the material.
Falls and Avalanches
The first type of mass wasting is a rockfall or avalanche. A rockfall is a large amount of rock that falls independently from a slope or cliff and forms an irregular pile of rock, called a talus slope, at the base of the slope. Rockfalls are fast moving, dry types of mass movements. An avalanche, also called a debris avalanche, is a mass of falling rock, but also includes soil and other debris. Like a rockfall, an avalanche moves quickly but because of the presence of soil and debris, they are sometimes moister than a rockfall.
Landslides are another type of mass wasting. They are sudden, fast movements of a cohesive mass of soil, rock or regolith. Landslides occur in two types- the first of which is a translational slide. These involve movement along a flat surface parallel to the angle of the slope in a stepped-liked pattern, with no rotation. The second type of landslide is called a rotational slide and is the movement of surface material along a concave surface. Both types of landslides can be moist, but they are not normally saturated with water.
Flows, like rockfalls and landslides, are fast moving types of mass wasting. They are different however because the material within them is normally saturated with moisture. Mudflows for example are a type of flow that can occur quickly after heavy precipitation saturates a surface. Earthflows are another type of flow that occur in this category, but unlike mudflows, they are not usually saturated with moisture and move somewhat slower.
The final and slowest moving type of mass wasting is called soil creep. These are gradual but persistent movements of dry surface soil. In this type of movement, soil particles are lifted and moved by cycles of moistness and dryness, temperature variations and grazing livestock. Freeze and thaw cycles in soil moisture also contribute to creep through frost heaving. When soil moisture freezes, it causes soil particles to expand out. When it melts though, the soil particles move back down vertically, causing the slope to become unstable.
Mass Wasting and PermafrostIn addition to falls, landslides, flows and creep, mass wasting processes also contribute to the erosion of landscapes in areas prone to permafrost. Because drainage is often poor in these areas, moisture collects in soil. During the winter, this moisture freezes, causing ground ice to develop. In the summer, the ground ice thaws and saturates the soil. Once saturated, the layer of soil then flows as a mass from higher elevations to lower elevations, through a mass wasting process called solifluction.
Humans and Mass WastingAlthough most mass wasting processes occur via natural phenomena like earthquakes, human activities like surface mining or the building of a highway or shopping malls can also contribute to mass wasting. Human induced mass wasting is called scarification and can have the same impacts on a landscape as natural occurrences.
Whether human induced or natural though, mass wasting plays a significant role on the erosion landscapes all over the world and different mass wasting events have caused damage in cities as well. On March 27, 1964, for example an earthquake measuring a magnitude of 9.2 near Anchorage, Alaska caused almost 100 mass wasting events like landslides and debris avalanches throughout the state that impacted cities as well as more remote, rural regions.
Today, scientists use their knowledge of local geology and provide extensive monitoring of ground movement to better plan cities and aid in reducing the impacts of mass wasting in populated areas.
To learn more about mass wasting view Mass Wasting, a presentation by the United States Geological Survey.