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 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 re-establish 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 of a sand pile 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 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. Apparently then, 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 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 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. 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 discussed loose deposits of particles on land, but 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 which 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.

An avalanche. Photograph by Ned Gillette. The Stock Market. Reproduced with permission.