MASS WASTING PROCESSES1 The downslope movement of rock, mud, or other material under the influence of gravity is known as mass wasting. While the angle of the slope is a major factor in the potential for mass wasting, the slope is not the sole determiner of mass wasting events. Water plays a significant role, especially where it is plentiful during the rainy season. Earthquakes may cause rockslides, mudflows, and other mass movements. Factors such as the presence or absence of vegetation and human activities can also influence the potential for mass wasting.2 One way to classify mass wasting processes is on the basis of the material involved, such as rock, debris, earth, or mud. The manner in which the material moves is also important and is generally described as a fall, a slide, or a flow. A fall occurs when weathering loosens boulders from cliffs or rock faces, causing the boulders to break away and fall. A slide takes place whenever material remains fairly coherent and moves along a well-defined surface. A flow involves the movement of debris containing a large amount of water.3 Many mass wasting processes are described as slides. Rockslides occur when a coherent mass of rock breaks loose and slides down a slope as a unit. If the material involved is mostly separate pieces, it is called a debris slide. Slides are among the fastest and most destructive mass movements. Usually rockslides occur in a geologic setting where the rock layers are inclined, or where there are joints and fractures in the rock that are parallel to the slope. When such a rock unit is undercut at the base of the slope, it loses support and the rock eventually collapses. Rain or snowmelt can trigger a rockslide by wetting the underlying surface to the point that friction can no longer hold the rock in place. The fastest type of slide is a rock avalanche, in which a mass of rock literally floats on air as it moves downslope. The high speed of a rock avalanche is the result of air becoming trapped and compressed beneath the falling mass of debris, allowing it to move down the slope as a buoyant sheet.4 Mudflows are relatively rapid mass wasting events that involve soil and a large amount of water. Because of their fluid properties, mudflows follow canyons and stream channels. Mudflows often take place in semiarid mountainous regions and on the slopes of some volcanoes. Although rainstorms in semiarid regions are infrequent, they are typically heavy when they occur. When a rainstorm or rapidly melting snow creates a sudden flood, large quantities of soil and loose rock are washed into nearby stream channels because there is usually little or no vegetation to anchor the surface material. The result is a flowing mass of well-mixed mud, soil, rock, and water. The consistency of the mudflow may be similar to that of wet concrete, or it may be a soupy mixture not much thicker than muddy water. The water content influences the rate of flow across the surface. When a mudflow is dense, it moves more slowly, but it can easily carry or push large boulders, trees, and even houses along with it.5 In dry mountainous areas such as southern California, mudflows are a serious hazard to development on and near canyon hillsides. The removal of native vegetation by brush fires has increased the probability of these destructive events. Past mudflows have contributed to the buildup of fan-shaped deposits at canyon mouths. Such fans are relatively easy to build on and often have scenic views, so many have become desirable sites for residential development. However, because mudflows occur infrequently, homeowners are often unaware of the potential danger of building on the site of a previous mudflow.6 Highly fluid, fast-flowing mudflows incorporate fine-grained sediment and are common after volcanic eruptions that produce large volumes of volcanic ash. Mudflows containing volcanic debris are called lahars, a word originating in Indonesia, a region that experiences many volcanic eruptions. Lahars occur when highly unstable layers of ash and debris become saturated with water and flow down steep volcanic slopes along stream channels. In the northwestern United States, the eruption of Mount St. Helens in 1980 created several lahars that raced down the valley of the Toutle River, altering the landscape in a relatively short period. Why does a rock avalanche move faster than other types of rockslides

A. The rock moves over a layer of compressed air.
B. The rainfall is heavy where avalanches occur.
C. The rock breaks apart as it moves downslope.
D. The angle of the slope is almost vertical.