Topic 20: SLIP SLIDIN' AWAY: Ground Failure Mass wasting - is movement in which bed rock, rock debris, or soil moves downslope in bulk, or as a mass, because of the pull of gravity. Landslides is a general term for the slow-to-very rapid descent of rock or soil. Regolith - Layer of all rocks and mineral fragments. Debris - As applied to mass wasting processes, debris are any unconsolidated material at the earth's surface, such as soil and rock fragments (weathered or unweathered) of any size. Types of mass wasting: Flow - The debris is moving downslope as a viscous fluid. A mudflow is a flowing mixture of debris and water, usually moving down a channel. Slump - involves movement along a curved surface, the upper part moving downward while the lower part moves outward. Slide - A descending rock mass remaining relatively coherent, moving along one or more well defined surfaces. A rock slide is the rapid sliding of a mass of bed rock along an inclined surface of weakness such as bedding plane or a major fracture. In contrast, a rock avalanche is a very rapidly moving, turbulent mass of broken-up bed rock. Fall - occurs when material free-falls or bounces down a cliff. Controlling Factors in Mass Wasting: Water - When debris is saturated with water (as from a heavy rain or melting snow), it becomes heavier and is more likely to flow downslope. Interestingly when a small amount of water in soil forms a thin film around each grain the surface tension of the water holds the grains together. It is surface tension which makes wet beach near the surf zone relatively hard and strong. In contrast, dry sand further up the beach has little strength because the surface film is gone. Likewise, when sand become saturated (water fills all the pore space), the surface tension no longer acts and the grains may be forced apart by the water. In this latter case, the grains can slide easily and, so, the sand or soil become very weak can easily slide down a slope. Local relief - Steeper relief always speeds mass wasting. Thickness of debris - Great thicknesses of debris favor downslope movement. Creep - is a very slow, continuous, downslope movement of soil or unconsolidated debris. Earthflow - debris moves downslope as a viscous fluid. Solifluction - is the flow of water-saturated debris over impermeable material. The impermeable material causes the cover to become saturated so that surface tension bonds between grains are removed. Permafrost - is ground that remains frozen for many years. This type of ground is impermeable the is commonly associated with solifluction in Arctic areas. Triggering of Mass-wasting- Several types of events can trigger mass wasting including earthquake shocks, slope modification by erosion or man-made cuts, exceptional precipitation, and volcanic eruptions.