Mass Wasting
Mass Wasting
Mass wasting processes
Mass wasting in loose aggregates
Mass wasting in rocks and soils
What can we do about mass wasting?
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.
Mass wasting processes
The four processes of mass wasting are distinguished by 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. A mudflow, 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 move at 32 feet/second2 (9.8 m/sec2 —the fastest rate possible in a natural system. 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.
Moving mountains to the sea
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 that 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 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 movement is an example of mass wasting, known as a rockfall. As talus pile adjusts to the weight of the overlying rocks, its base extends outward;
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 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 material in its flowing water. This erosion would result in deep canyons with vertical walls if the erosion by the river were the only factor. But very high vertical walls 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.
Mass wasting in loose aggregates
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 you dump dry sand from a bucket it forms a conical hill. The more sand you dump the larger the hill gets, but the slope of the hill stays the same. If you stop dumping sand and start to dig into the bottom of the hill, sand will avalanche down into the hole you are trying to make. Loose, dry sand flows easily and will quickly reestablish its preferred slope whenever you do anything to steepen it. The flow of sand is a simple example of mass wasting.
If sand is moist, the slope can be higher. A sand castle can have vertical walls when it is built of moist sand in the morning, but as the afternoon wears on and the sand dries out, it 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 quicksand), then the sand will flow in a process known as lateral spreading. Water-saturated sand flows because its weight is supported (at least temporarily) by the water, and the grains are not continuously in contact. This means that the slope stability depends on water content; either too little or too much lowers the stable slope.
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 a 34° angle of repose. This is the slope that you can see inside a sand-filled hour glass. 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 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. This permits you to run up the windward side where the sand has been tightly packed by the wind, then jump off the crest, land on the leeward slope where you cause (and become part of) a sand avalanche down that slope.
So far we’ve discussed loose deposits of particles on land, but similar conditions exist 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.
Mass wasting in rocks and soils
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 materials. 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 when a portion of the slope breaks off and slides down the hill. We usually call these events 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.
When people build on slopes they add to the gravitational forces by constructing very heavy things, such as houses and swimming pools. A period of heavy rain can add even more weight to the slope, and can also change the strength of the material making up the slope. As we have seen, saturated soil with no surface tension is much weaker than moist soil. But there is another effect that occurs even in solid rock.
Even very solid rocks contain pores, and many of these are interconnected. It is through such pores that water and oil move toward wells. Below the water table, all pores are filled with water with no surface tension to eliminate. So it might seem that rock 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 southern California gets heavy rains, television news cameras record beautiful homes moving down slope in a landslide or mudflow to become rubble at the bottom. Such dramatic examples of mass wasting are impressive, but represent only a fraction of what is all around us everyday.
Soil creep
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
KEY TERMS
Angle of repose —The slope made by a pile of loose material, such as the sand in an hour glass.
Avalanche —A mass movement in which a chaotic mass of rock and/or soil very rapidly flows down slope along a discrete surface; an avalanche is a type of flow.
Creep —A mass movement that involves gradual down-slope movement (flow), too slow to be observed directly, but apparent in many long term observations.
Erosion —Movement of material caused by the flow of ice, water, or air, and the modification of the surface of Earth (by forming or deepening valleys, for example) produced by such transport.
Flow —A mass movement process in which a chaotic mass of Earth material moves down slope; rates of flow are a function of fluid content of the mass and the grain size the higher the fluid content and finer the particles, the faster the flow.
Landslide —A mass movement in which a fairly coherent mass of rock and/or soil rapidly moves down slope along a discrete plane; sometimes used to include all types of moderately rapid mass movements involving flows, falls, or slides.
Lateral spreading —Mass wasting of water-saturated sediments; lateral spreading involves the flow of finegrained sediments (clay, silt, or sand).
Pore pressure —The pressure of fluids contained within the pores of a rock, which influences the strength of the rock and is often a factor in dramatic landslides or mudslides.
Rock creep —Very slow mass movement of large rock fragments (pebbles, cobbles, and boulders) in response to the weight of overlying rocks.
Rock fall —Very rapid mass wasting of large rock fragments that have fallen from an exposed cliff.
Slide —A mass movement process in which a fairly coherent mass of rock and/or soil rapidly moves down slope over a discrete surface.
Slumps —A mass movement process in which a fairly coherent mass of rock and/or soil moves down slope over a discrete surface, but curved (concave-upward) surface. Slumps occur much faster than creep but slower than slides.
Talus —Large rock fragments that accumulate at the base of a steep slope, transported there by mass wasting.
Weathering —Biological, chemical, and mechanical attack on rock which breaks it up and alters it at or near the surface of Earth.
after they are emplaced. 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 Earth it is responsible for most mass wasting.
Influence of climate
Rivers in the desert regions of the southwestern states form canyons with fairly steep walls, and the buttes and mesas in those regions also have very steep walls. The topography of the eastern states is much more subdued, dominated by rolling hills and gentle slopes. How does rainfall control this contrast in landscape?
Weathering needs water for freezing and thawing, chemical reactions, and growth of roots. Less water means the rate of weathering will be reduced. Also, rainfall affects erosion by water. Less water means the rate at which rivers cut valleys will be reduced. If these were the only factors, however, only the rates would vary; arid regions would weather and erode more slowly than humid ones.
Much of the difference in landscape results from variations in mass wasting. Very steep slopes on dry rock with little or no pore pressure are much more stable than similar slopes on wet rock with high pore pressures. Water adds to the weight trying to break the rock and increases the pore pressure which weakens the rock. Hence the height of a vertical slope which can exist in dry rock is much greater than that in wet rock. Arid climates also have less vegetation to stabilize sediment at the base of a slope, so it is likely to be washed away during the infrequent storms.
What can we do about mass wasting?
In a natural setting, mass wasting presents little threat. Most slopes are relatively stable most of the time. However, when people modify slopes—their gravitational loads, or their water content—they may become unstable and fail. Engineers can study the stresses acting on a slope, test the material of which it is made, make some assumptions about behavior with higher pore pressure, etc., and predict how likely that slope is to fail if the additional load of a house were added to it. This study might conclude that the slope would still be stable, and so the house might be constructed. Later, after it has changed hands a few times, a new owner might decide to put in a swimming pool. A neighbor in the property just downhill, might decide to cut into the slope in order to widen a driveway, or a neighbor just uphill might inadvertently introduce large amounts of water into the ground while trying to maintain a gracious green lawn. Any one of these actions could bring the slope close to where it is unstable. A period of heavy rain could provide the final impetus, and a sudden mass wasting event could occur. To avoid such problems, property owners on such slopes must have their freedoms restricted. One way to accomplish this is to prohibit certain activities through zoning, deed restrictions, or insurance requirements. A less satisfactory means is by threat of litigation.
Resources
BOOKS
Cooke, Ronald U. Geomorphological Hazards in Los Angeles: A Study of Slope and Sediment Problems in a Metropolitan County. London; Boston: Allen & Unwin, 1984.
Costa, John E., and Gerald F. Wieczorek, eds. Debris Flows/Avalanches: Process, Recognition, and Mitigation. Boulder, CO: Geological Society of America, 1987.
Haneberg, William C. and Scott A. Anderson, eds. Clay And Shale Slope Instability. Boulder, CO: Geological Society of America, 1995.
Press, Frank, and Raymond Siever. Understanding Earth. New York: W.H. Freeman and Co., 1994.
Small, R.J., and M.J. Clark Slopes and Weathering. Cambridge; New York: Cambridge University Press, 1982.
OTHER
Tulane University. “Mass Wasting” <http://www.tulane.edu/~sanelson/geol111/masswasting.htm> (accessed December 3, 2006).
University of Wisconsin–Madison; Department of Geology and Geophysics. “Mountains and Mass Wasting” <http://www.geology.wisc.edu/courses/g112/mass_wasting.html> (accessed December 2, 2006).
Otto H. Muller