2. Two basic types of movement may be involved in mass-wasting the sudden failure of a slope, resulting in the downslope transfer of relatively coherent masses of rock or rock debris by slumping. falling, or sliding-, and the downslope flow of mixtures of sediment, water, and air.
3. Slumps involve rotational movement of blacks of material; falls involve vertical free-fall of material; and slides involve translational movement along a well-defined plane.
4. Flows can be divided into slurry (water-saturated) and granular (unsaturated) flows. The water content of a flow may range from heavily sectiment-laden stream flow to dry grain flow in a sand dune. Flows also vary in velocity, from imperceptibly slow (creep, solifluction) to extremely rapid (mudflows, debris avalanches)
5. Mass-wasting is especially prevalent at high latitudes and high altitudes, where average temperatures are very low and frost-heaving is an important force. Subaqueous (underwater) mass-wasting is also very common, especially on continental slopes and in major marine deltas.
6. The main factors that influence slope stability are (a) the force of gravity, and therefore the gradient of the slope; (b) water, and therefore the hydrologic characteristics of the slope; (c) the presence of troublesome Earth materials; and (d) the occurrence of a triggering event.
7. In any body of rock or rock debris located on a slope, two opposing forces-shear stress and shear strength determine whether the body will move or remain stationary. As long as shear strength exceeds shear stress, the rock or debris will not move. However, as these two forces approach a balance, the likelihood of movement increases.
8. Water plays a variety of important roles in mass-wasting of both solid rock and regolith. Water can decrease slope stability by reducing the natural cohesiveness be. between grains or reducing friction at the base of a rock mass through increased water pressure. Water can also contribute to slope instability through wave action undercutting the base of the slope, or by altering the chemical composition of sediment.
9. Some types of Earth materials arc considered "troublesome" because they are particularly susceptible to fail under certain conditions. These include sensitive soils, expansive clays, and hydrocompacting clays.
10, Among the most common triggering events are earthquakes, volcanic eruptions, slope modifications, and changes in the hydrologic characteristics of an area (including the effects of prolonged or exceptionally intense rainfall).
11. Scientific understanding of the types of rock and characteristics of slopes in a given area can be combined with weather analysis to delineate landslide hazards and, sometimes, to issue predictions and early warnings of major landslides.
12. With careful planning, building regulations, and zoning laws, and the use of appropriate stabilization techniques, the impacts of mass-wasting processes on humans can often be reduced. Commonly used techniques include retaining devices (concrete, rock bolts, gabions), drainage pipes, grading, and diversion walls.