Creep is the slow downward movement of regolith as a result of gravitational force. Before the initiation of the creeping process, the regolith is in what physicists call a condition of unstable equilibrium: it remains in place, yet a relatively small disturbance would be enough to dislodge
Changes in temperature or moisture are among the leading factors that result in the disturbance of regolith. A change in either can cause material to expand or contract, and freezing or thawing may be enough to shake regolith from its position of unstable equilibrium. In fact, some geomorphologists cite a distinct mass-wasting process, known as solifluction, that occurs in the active layer of permafrost, which thaws in the summertime. Water also can provide lubrication or additional weight that assists the material in moving. One of the only causes of creep not associated with changes in temperature or moisture is the burrowing of small animals.
Slump occurs when a mass of regolith slides over or creates a concave surface (one shaped like the inside of a bowl). The result is the formation of a small, crescent-shaped cliff, known as a scarp, at the upper end—rather like the crest of a wave. Soil flow takes place at the bottom end of the slump. One is likely to see slumps in any place where forces, whether man-made or natural, have graded material to a slope too steep for its angle of repose. This may happen along an interstate highway, where a road crew has cut the slope too sharply, or on a riverbank, where natural erosion has done its work.
Often, slump is classified as a variety of slide, in which material moves downhill in a fairly coherent mass (i.e., more or less in a section or group) along a flat or planar surface. These movements sometimes are called rock slides, debris slides, or, in common parlance, landslides. Among the most destructive types of mass wasting, they may be set in motion by earthquakes, which are caused by plate tectonic processes, or by hydrologic agents (i.e., excessive rain or melting snow and ice).
When a less uniform, or more chaotic, mass of material moves rapidly downslope, it is called flow. Flow is divided into categories, depending on the amounts of water involved: granular flow (0-20% water) and slurry flow (20-40% water). Creep and solifluction often are classified as very slow forms of granular and slurry flow, respectively. In order of relative speed, these categories are as follows:
Granular Flow (0-20% Water)
Slurry Flow (20-40% Water)
Earth flow moves at a rate anywhere from 3.3 ft. (1 m) per year to 330 ft. (100 m) per hour. Grain flow can be nearly 60 mi. (100 km) per hour, and debris avalanche may achieve speeds of 250 mi. (400 km) per hour, making it extremely dangerous. Among types of slurry flow, debris flow is roughly analogous to earth flow, falling into a range from about 4 in. (10 cm) per year to 0.6 mi. (1 km) per hour. Mudflow is slightly faster than grain flow. If the water content is more than 40%, a slurry flow is considered a stream.
Earth flows involve fine-grained materials, such as clay or silt, and typically occur in humid areas after heavy rains or the melting of snow. Debris flows usually result from heavy rains as well and may start with slumps before flowing downhill, forming lobes with a surface broken by ridges and furrows. Grain flows can be caused by a small disturbance, which forces the dry, unconsolidated material rapidly downslope. Debris avalanches are commonly the result of earthquakes or volcanic eruptions.
Seismic disturbances or volcanic activity may cause the collapse of a mountain slope, sending debris avalanches moving swiftly even along the gentler slopes of the mountainside. Likewise, mudflows may be the result of volcanic activity, in which case they are known as lahars. In some situations, the material in a lahar is extremely hot. Mudflows tend to be highly fluid mixtures of sediment (material deposited at or near Earth's surface from a number of sources, most notably preexisting rock) and water and typically flow along valley floors.
Most other forms of mass wasting entail movement along slopes that are considerably less than 90°, whereas fall takes place at angles almost perpendicular to the ground. Anyone who has driven through a wide mountain area, with steep cliffs on either side, has seen signs that say "Watch for Falling Rock." These warnings, which appear regularly on the drive through the Rockies in Colorado or on highways across the Blue Ridge and Great Smoky mountains in the southern United States, indicate the threat of rock fall.
The mechanism behind rock fall is simple enough. When a rock at the top of a slope is in unstable equilibrium, it can be dislodged such that it either falls directly downward or bounces and rolls. Usually, the bottom of the slope or cliff contains accumulated talus, or fallen rock material. Freezing and thawing as well as the growth of plant roots may cause fall. The latter is not limited to rock fall: debris fall, which is closely related, includes soil, vegetation, and regolith as well as rocks.
Among the most dramatic and well-known varieties of mass wasting are avalanches, a variety of flow, and landslides, which (as their name suggests) are a type of slide. These can result, and have resulted, in enormous loss of life and property. Some notable modern occurrences of mass wasting, and the type of movement involved, are listed below. With each incident, the approximate number of fatalities is shown in parentheses.
Note how many times an instance of mass wasting was either caused by or "related to" (meaning that geologists could not establish a full causal relationship) volcanic or seismic activity. Both, in turn, are the result of plate movement in most instances, and thus it is not surprising that several of the locales noted here are either at plate margins or in mountainous regions where plate tectonic and other processes are at work. (For more on this subject, see the entries Plate Tectonics and Mountains.)
To set mass wasting into motion, it is necessary to have a steep slope and some type of force to remove material from its position of unstable equilibrium. Plate tectonic processes provide both. Not only does an earthquake, for instance, jar rocks loose from the upper portion of a slope, but the movement of plates also helps create steep slopes, for example, the collision of the Indo-Australian and Eurasian belts that produced the Himalayas.
Some of the most vigorous plate tectonic activity occurs underwater, and, likewise, there are remarkable manifestations of mass wasting beneath the seas. Off Moss Landing, a research facility that serves a consortium of state universities in northern California, is an underwater canyon more than 0.6 mi. (1 km) deep. At one time, Monterey Canyon was thought to be the result of erosion by a river flowing into the ocean; however, today it is believed to be the result of underwater mass wasting.
The dramatic instances of mass wasting discussed here hardly require any effort at detection. Their effect is obvious and, to those unfortunate enough to be nearby, inescapable. Other types of mass wasting occur so slowly that they do not invite immediate detection. This can be unfortunate, because in some cases slow mass wasting is a harbinger of much more rapid movements to follow.
A dwelling atop a hill is subject to enormous gravitational force, and the more massive the dwelling, the greater the pull of gravity. (Weight is, after all, nothing but gravitational force.) If a homeowner adds a swimming pool or other items that contribute to the weight of the dwelling, it only increases the chances that it may experience mass wasting. Heavy rains can bring so much water that it saturates the soil, reducing its surface tension and causing it to slide—as occurred, for instance, in the area around Malibu, California, during the late 1990s.
The California mud slides and landslides are a dramatic example of mass wasting, but more often than not mass wasting takes the form of creep, which is detectable only over a matter of years. When creep occurs, the upper layer of soil moves, while the layer below remains stationary. One way to keep the upper layer in place is to plant vegetation that will put down roots deep enough to hold the soil.
This may create unintended consequences. During the 1930s, New Deal officials imported kudzu plants from China, intending to protect the hillsides of the American South from creep and erosion. The kudzu protected the slopes, but as it turned out, this voracious plant had a tendency to creep as well. Before communities began taking steps to eradicate it, or at least push it back, in the 1970s, kudzu seemingly threatened to cover the entire southern United States.
To prevent some of the more dramatic varieties of mass wasting, such as landslides in a residential area, a homeowner or group of homeowners may commission an engineer's study. The engineer can test the material of the slope, measure the stresses acting on it, and perform other calculations to predict the likelihood that a slope will succumb to a given amount of force. For this reason, zoning laws in areas with steep slopes are typically strict. These laws are geared toward preventing homeowners and builders from erecting structures likely to create a threat of mass wasting in a period of heavy rains.
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