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Landslides, Set 1
browse graphic This set of slides graphically illustrates the potential danger that major earthquakes pose to school structures and to the children and adults who happen to be inside at the time of the earthquake. It includes pictures from 1886 to 1988. The slide set includes nine destructive earthquakes that occurred in the U.S. and eight earthquakes that occurred in foreign countries. The slopes above streams and rivers are subjected to a variety of processes that cause them to recede and retreat from the river or stream channel. These processes, collectively called mass wasting, can be classified according to rapidity of movement and according to the type of materials that are transported. Gravity is the force behind all such downslope movement. Factors that enable the force of gravity to overcome the resistance of inertia and friction to move more material downslope include: saturation by water which acts as a lubricant, steepening of slopes by streams, waves, or road construction, alternate freezing and thawing, and earthquake vibrations. Mass wasting of surface material is widespread process that can be found in high mountains, desert hillsides, deep ocean shelves, steep ocean shores and even on the moon and other rocky planets. The major methods of mass movement include: Rockfalls: Large or small amounts of rock material break away from the face of a cliff as a result of weathering, and in the most rapid type of mass movement, free fall or bounce along an irregular slope to the base of the cliff forming talus. Rockslide: Rock material slides along a plane of structural weakness such as a bedding plane. Although they are most common on steep slopes, they can even occur on slopes of 15 degrees. Millions of tons of rock may plunge down slope at speeds greater than 160 km (100 miles per hour in what is often the most catastrophic form of mass wasting. Debris slide: Dry to moderately-wet, loose rock fragments and soil move rapidly over the surface of underlying bedrock. The interface of moving material and undrlying bedrock is dry in a debris slide. Debris avalanche: Loose earth on a steep slope becomes wet and slides to the bottom of the slope. Snow avalanche: Unstable snow breaks loose and plunges down slope carrying rock and debris, carving avalanche chutes. Debris flow: Rock fragments, mud, and water flow downslope as a thick viscous fluid. Debris flows may begin as slumps and continue as flows. Movement may be as slow as that of freshly poured concrete or as rapid as that of a river. Mudflow: Silt and clay particles with water content as high as thirty percent follow stream valleys until the terrain flattens. Then they spread out as fans. Mudflows are sometimes over 100 m (330 ft) thick; they can float large boulders and move houses from their foundations. The speed of movement depends on the slope and the water content of the flow. Landslide: Unconsolidated rock material and even the bedrock itself may be involved in what is usually a rapid movement of material beginning with the slumping of stream banks or sea cliffs, or the sliding of mountain sides. Landslides move as a unit or series of units along a definite plane (in contrast to debris flows which move as viscous fluids). The material moves downward and outward along a curved plane. Eventually the material breaks into fragments that slide over uneven ground at the base until friction overcomes the force of motion. Slumping: A resistant rock overlies a weaker rock layer. The weaker rock is eroded undermining the resistant rock and producing an unstable condition. Slump blocks can be as much as 5 km (3 mi) long and 150 m (495 ft) thick. They may move in a matter of seconds or gradually slip over a period of several weeks. SolifluctionThe upper zone of saturated soil flows slowly down even the most gentle slopes in arctic and subarctic regions where an impermeable permafrost area exists. Water can not percolate into this permafrost area so the thawed surface remains saturated and flows as a viscous fluid.Rock GlacierAngular rock debris resembling glaciers move as a body down slope at rates ranging from 3 cm (1.2 in) a day to 1 m (3.3 ft) a year. A considerable amount of ice exists in the pore spaces between the rock fragments and is responsible for much of the movement. The increased weight of rock fragments falling onto the flow cause the ice to flow. Steep cliffs and a cold climate that keeps ice permanently frozen are conditions that most often result in rock glaciers. A steep flow front, lobes, and concentric ridges on the flow are evidence of rock glaciers.CreepThe mantle on a slope moves downward almost imperceptibly under the constant pull of gravity. In areas subject to cold winters the water in the layers of soil or clay freezes and increases in volume. This lifts the rocks upward at right angles to the slope. However, when melting occurs the rocks fall vertically and so are moved downhill. Wetting and drying has the same effect since moisture causes expansion of clay materials. Burrowing organisms displace particles permitting the force of gravity to move them. Growing plant roots and the tramping of animals also force soil material downslope. Photograph credit: All photographs in this slide set were provided by B. Bradley of the University of Colorado's Geology Department.