Landslide, the relatively rapid downslope movements of large masses of soil and rock debris in response to the pull of gravity. Landslides are a kind of mass wasting, a downward movement of unconsolidated surface rocks and soils. In general, landslides are sudden and fast, unlike creep and solifluction, which involve gradual and slow movement.
All slides involve the failure of earth materials under shear stress (a stress that tends to make part of a block of material slide with respect to an adjoining part). The material can fall, slide, or flow; it may move a few feet or several miles. Gravity provides the energy to displace the soil and bedrock, but other factors contribute to an increase in stress and decrease in strength. An increase in the water content of soil may reduce its cohesion and internal friction, thus lowering its resistance to movement. To a lesser degree, the added weight of waterlogged soil can also hasten downslope movement. The removal of lateral support by streams, glaciers, waves, and construction or mining activities is another factor. Construction activity may have triggered the massive landslide that killed 70 people in the coal-mining village of Frank, Alberta, in 1903. But natural causes were basically responsible for the increased shear stress involved.
Vibrations from blasting, heavy traffic, and earthquakes also can contribute to shear stress and precipitate slides. An earthquake-induced slide clogged Montana's Madison River Canyon on Aug. 17, 1959. The slide tore some 40 million cubic yards (30,000,000 cu km) of rock from the canyon wall. Moving at an estimated speed of 100 miles (160 km) per hour, the slide buried 26 people and formed a natural dam creating a lake 6 miles (9.7 km) long and 180 feet (55 meters) deep.
Air may be another factor in moving masses of earth material down steep slopes. Confined by falling rock debris, air may act as a cushion to reduce the friction between the sliding material and the underlying rock or ground.
Types of Landslides. Landslides and related phenomena are typically classified according to the types of material transported and the way in which it moves. Rockslides involve movements of newly detached blocks of bedrock along tilted, water-lubricated joints, cracks, or bedding surfaces. Rockslides are frequent in folded mountain ranges such as the Alps, Appalachians, and Canadian Rockies. Mudflows are large flowing masses of rock, soil, and water of mudlike consistency. They may occur in arid or semiarid mountainous regions where they course down steep-walled gulches or canyons. Conditions conducive to mudflows include loose, weathered surface rock; steep, barren hillslopes; and heavy rain. Because they can carry huge quantities of debris¡Xeven houses and automobiles¡Xmudflows do millions of dollars in damage annually. Suburban communities in hilly parts of Southern California are especially subject to such disasters. Unlike mudflows, which are confined to channels, earthflows contain less water and thus flow more slowly; they also move shorter distances downslope than mudflows.
Slump, a special type of landslide, occurs as large masses of the slope move downward and outward due to gravitational pull. Slump more typically develops in unconsolidated material, and is commonly caused by undercutting or steepening of the slope to the extent that it can no longer support its own weight. Slumps often occur along eroded stream banks, sea cliffs, or human-excavated slopes. Sometimes called slope failure, a slump may involve one block or a series of slump blocks. The blocks move along a curved slip surface in a rotational motion that tilts the blocks backward.
Debris slides consist of a moving mass of mud, soil, and rock fragments mixed with water. They contain more solid material than water and typically occur in hilly or mountainous regions after heavy rains. Such slides that move suddenly and at great speeds are called debris avalanches. In the Peruvian Andes in 1977, a debris avalanche moved some 20 to 45 million cubic yards of earth material at speeds of almost 100 miles (160 km) per hour. This massive slide buried an entire city, killing most of its 19,000 inhabitants. The huge landslide that followed the eruption of Mount St. Helens in Washington in 1980 was a debris landslide.
The economic impact of landslides is difficult to determine, but losses are estimated to be more than a billion dollars per year. Because most damaging landslides are caused by human activities, many slides can be prevented. Techniques successful in some areas include proper drainage; planting slopes with grasses, shrubs, and trees; terracing; and the construction of retaining walls.
William H. Matthews III, Lamar University
Source: "Landslide." Encyclopedia Americana. Grolier Online http://ea.grolier.com/cgi-bin/article?assetid=0238220-00
(accessed August 13, 2007).