Mass movements are of three basic types, distinguished from each other by the way that the rock, soil, water, and debris move. slides move over and in contact with the underlying surface, while flows include movements of regolith, rock, water, and air in which the moving mass breaks into many pieces that flow in a chaotic mass movement. Falls move freely through the air and land at the base of the slope or escarpment. A continuum exists between different processes of mass wasting, but many differ in terms of the velocity of downslope movement and also in the relative concentrations of sediment, water, and air. A landslide is a general name for any downslope movement of a mass of bedrock, regolith, or a mixture of rock and soil and indicates any mass wasting process.
A slump is a type of sliding slope failure in which a downward and outward rotational movement of rock or regolith occurs along a concave upslip surface. This produces either a singular or a series of rotated blocks, each with the original ground surface tilted in the same direction. slumps are especially common after heavy rainfalls and earthquakes and are common along roadsides and other slopes that have been artificially steepened to make room for buildings or other structures. Slump blocks can continue to move after the initial sliding event, and in some cases this added slippage is enhanced by rainwater that falls on the back-tilted surfaces, infiltrates along the fault, and acts as a lubricant for added fault slippage.
A translational slide is a variation of a slump in which the sliding mass moves not on a curved surface, but moves downslope on a preexisting plane, such as a weak bedding plane or a joint. Transla-tional slides may remain relatively coherent or break into small blocks forming a debris slide.
When mixtures of rock debris, water, and air begin to move under the force of gravity, they are said to flow. This is a type of deformation that is continuous and irreversible. The way in which this mixture flows depends on the relative amounts of solid, liquid, and air, the grain size distribution of the solid fraction, and the physical and chemical properties of the sediment. Mass-wasting processes that involve flow are transitional within themselves, and to stream-type flows in the amounts of sediment/ water and in velocity. Many names for the different types of sediment flows include slurry flows, mud-flows, debris flows, debris avalanches, earthflows, and loess flows. Many mass movements begin as one type of flow and evolve into another during the course of the mass-wasting event. For instance, flows commonly begin as rock falls or debris avalanches and evolve into debris flows or mudflows, as the flow picks up water and debris and flows over differing slopes along its length.
Creep is the slow, downslope-flowing movement of regolith; it involves the very slow plastic deformation of the regolith, as well as repeated microfractur-ing of bedrock at nearly imperceptible rates. Creep occurs throughout the upper parts of the regolith, and there is no single surface along which slip has occurred. The rates range from a fraction of an inch (about a centimeter) per year up to about two inches (five cm) per year on steep slopes. Creep accounts for leaning telephone poles, fences, and many of the cracks in sidewalks and roads. Although creep is slow and not very spectacular, it is one of the most important mechanisms of mass wasting, accounting for the greatest total volume of material moved downhill in any given year. One of the most common creep mechanisms is through frost heaving, an extremely effective means for moving rocks, soil, and regolith downhill. The ground freezes and ice crystals form and grow, pushing rocks upward perpendicular to the surface. As the ice melts in the freeze-thaw cycle, gravity takes over and the pebble or rock moves vertically downward, ending up a fraction of an inch (centimeter) downhill from where it started. other mechanisms of surface expansion and contraction, such as warming and cooling, or the expansion and contraction of clay minerals with changes in moisture levels can also initiate creep. In a related phenomenon, the freeze-thaw cycle can push rocks upward through the soil profile, as revealed by farmers' fields in New England and other northern climates, where the fields seem to grow boulders. The fields are cleared of rocks, and years later, the same fields are filled with numerous boulders at the surface. In these cases, the freezing forms ice crystals below the boulders that push them upward; during the thaw cycle, the ice around the edges of the boulder melts first, and mud and soil seep down into the crack, finding their way beneath the boulder. This process, repeated over years, is able to lift boulders to the surface, keeping the northern farmer busy.
The operation of the freeze-thaw cycle makes rates of creep faster on steep slopes than on gentle slopes, with more water, and with greater numbers of freeze-thaw cycles. Rates of creep of up to half an inch (1 cm) per year are common.
Solifluction, the slow viscous downslope movement of water-logged soil and debris, is most common in polar latitudes where the top layer of permafrost melts, resulting in a water-saturated mixture resting on a frozen base. This process is also common to very wet climates, as found in the Tropics. Rates of movement are typically an inch or two (2.5-5 cm) per year, slightly faster than downslope flow by creep. solifluction results in distinctive surface features, such as lobes and sheets, carrying the overlying vegetation; sometimes the lobes override each other, forming complex structures. Solifluction lobes are relatively common sights on mountainous slopes in wet climates, especially in areas with permafrost. The frozen layer beneath the soil prevents drainage of water deep into the soil or into the bedrock, so the uppermost layers in permafrost terrains tend to be saturated with water, aiding solifluction.
A slurry flow is a moving mass of sediment saturated in water that is transported with the flowing mass. The mixture is so dense that it can suspend large boulders or roll them along the base. When slurry flows stop moving, the resulting deposit therefore consists of a nonsorted mass of mud, boulders, and finer sediment.
Debris flows involve the downslope movement of unconsolidated regolith, most of which is coarser than sand. Some debris flows begin as slumps, but then continue to flow downhill as debris flows. They fan out and come to rest when they emerge out of steeply sloping mountain valleys onto lower-sloping plains. Rates of movement in debris flows vary from several feet (1 m) per year to several hundred miles (kilometers) per hour. Debris flows are commonly shaped like a tongue with numerous ridges and depressions. Many form after heavy rainfalls in mountainous areas, and the number of debris flows is increasing with greater deforestation of mountain and hilly areas. This is particularly obvious on the island of Madagascar, where deforestation in places is occurring at an alarming rate, removing most of the island's trees. What was once a tropical rain forest is now a barren (but geologically spectacular)
landscape, carved by numerous landslides and debris flows that bring the terra rossa soil to rivers, making them run red to the sea.
Most debris flows that begin as rock falls or avalanches move outward in relatively flat terrain less than twice the distance they fell. Internal friction (between particles in the flow) and external friction (especially along the base of the flow) slow them. Some of the largest debris flows that originated as avalanches or debris falls travel exceptionally large distances at high velocities—these are debris avalanches and sturzstrom deposits.
Mudflows resemble debris flows, except that they have higher concentrations of water (up to 30 percent), making them more fluid, with a consistency ranging from soup to wet concrete. Mudflows often start as a muddy stream in a dry mountain canyon. As it moves it picks up more and more mud and sand, until eventually the front of the stream is a wall of moving mud and rock. When this comes out of the canyon, the wall commonly breaks open, spilling the water behind it in a gushing flood, which moves the mud around on the valley floor. These types of deposits form many of the gentle slopes at the bases of mountains in the southwestern United States.
Mudflows have also become a hazard in highly urbanized areas such as Los Angeles, where most of the dry riverbeds have been paved over and development has moved into the mountains surrounding the basin. The rare rainfall events in these areas then have no place to infiltrate, and they rush rapidly into the city, picking up all kinds of street mud and debris and forming walls of moving mud that cover streets and low-lying homes in debris. unfortunately, after the storm rains and waters recede, the mud remains and hardens in place. Mudflows are also common with the first heavy rains after prolonged droughts or fires, as many residents of California and other western states know. After the drought and fires of 1989 in Santa Barbara, California, heavy rains brought mudflows down out of the mountains, filling the riverbeds and inundating homes with many feet of mud. Similar mudflows followed the heavy rains in Malibu in 1994, which remobilized barren soil exposed by the fires of 1993. Three to four feet (more than a meter) of mud filled many homes and covered parts of the Pacific Coast highway. Mudflows are part of the natural geologic cycle in mountainous areas, and they serve to maintain equilibrium between the rate of uplift of the mountains and their erosion. Mud-flows are catastrophic only when people have built homes, highways, and businesses in their paths.
Volcanoes, too, can produce mudflows. Rain or an eruption easily remobilizes layers of ash and volcanic debris, sometimes mixed with snow and ice, that can travel many tens of miles (kilometers). Vol canic mudflows are known as lahars. Mudflows have killed tens of thousands of people in single events and have been some of the most destructive of mass movements.
Granular flows are unlike slurry flows, in that, in granular flows, the full weight of the flowing sediment is supported by grain to grain contact between individual grains. Earthflows are relatively fast granular flows with velocities ranging from three feet per day to 1,180 feet per hour (1 m/day to 360 m/hour).
Rockfalls are the free falling of detached bodies of bedrock from a cliff or steep slope. They are common in areas of very steep slopes, where rockfall deposits form huge deposits of boulders at the base of the cliff. Rockfalls can involve a single boulder, or the entire face of a cliff. Debris falls are similar to rockfalls, but these consist of a mixture of rock and weathered debris and regolith.
Rockfalls have been responsible for the destruction of parts of many villages in the Alps and other steep mountain ranges, and rockfall deposits have dammed many a river valley, creating lakes behind the newly fallen mass. Some of these natural dams have been extended and heightened by engineers to make reservoirs, with examples including Lake Bonneville on the Columbia River and the Cheaka-mus Dam in British Columbia. Smaller examples abound in many mountainous terrains.
Rockslide is the term given to the sudden downslope movement of newly detached masses of bedrock (or debris slides, if the rocks are mixed with other material or regolith). These are common in glaciated mountains with steep slopes and also in places having planes of weakness, such as bedding planes or fracture planes that dip in the direction of the slope. Like rockfalls, rockslides can form fields of huge boulders coming off mountain slopes. The movement to this talus slope is by falling, rolling, and sliding, and the steepest angle at which the debris remains stable is known as the angle of repose, typically 33-37 degrees for most rocks.
Debris avalanches are granular flows moving at very high velocity and covering large distances. These rare, destructive (but spectacular) events have ruined entire towns, killing tens of thousands of people in them without warning. Some have been known to move as fast as 250 miles per hour (400 km/hr). These avalanches thus can move so fast that they move down one slope, then thunder right up and over the next slope and into the next valley. One theory of why these avalanches move so fast is that when the rocks first fall, they trap a cushion of air, and then travel on top of it like a hovercraft. Two of the worst debris avalanches in recent history originated from the same mountain, Nevados Huascaran, the highest peak in the Peruvian Andes. More than
22,000 people died in these two debris avalanches. Numerous debris avalanches were also triggered by the May 12, 2008, magnitude 7.9 earthquake in China that killed nearly 90,000 people.
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