Mass Wasting

Mass wasting is a term for a wide variety of motions of rock, debris, earth, and organic matter that are moved downhill by gravity with various fluids entrained, such as water and air as well as ice. The concept of mass wasting is commonly supplanted by several other terms, such as mass movement or slope failure, and it also includes a tremendous diversity of things such as the various forms of creep, talus accumulation, flow of dry loess, burst of wet peat bogs, or block streams and rock glaciers as well. Classification of such diverse phenomena has been refined over the past century and a half, ultimately with emphasis on the types of material and types of movement, with a present-day consensus classification using rates of movement and the amounts and phase of the entrained H2O system (water, ice, steam). The result is a matrix classification of mass-movement phenomena that enables understanding, although it is recognized that fine gradations of process type can occur in various continua between one form and another.

The types of material subject to mass wasting includes rock, as large masses or many smaller fragments; fine-grained earth, which is sand, silt, and clay; and fine organic matter, such as peat or humus in soil. Debris is a mixture of rock and earth in various proportions. Types of motion in discrete landslides are falls, topples, slides, flows, and lateral spreads, whereas pervasive nondiscrete movements on slopes can include creep as well as isolated unit rock falls or slides that lead to accumulations of talus. Rates of motion range from the imperceptibly slow, where a few millimeters or centimeters a year occur, to hundreds of kilometers an hour, or even faster than the speed of sound, where lateral blasts from volcanoes impart exceptional velocities in certain cases. All phases of the H2O system can be involved, from the merely ancillary presence of minor amounts of H2O up to and including one phase or another of the H2O as the primary motivating mechanism.

Falls of rock, debris, and earth (rock fall, debris fall, earth fall) from cliffs involve relatively simple accelerations through the air to a point of impact, at which point some further bouncing, rolling, and gliding of particles can occur to a certain extent. Topple failure occurs where a block of rock, debris, or earth rotates forward around a pivot zone at its base until it then tumbles over completely, usually breaking up in the process. Slides, such as the common rock slide, debris slide, or earth slide, can occur where rock debris or earth moves as one large block or breaks up into many separate units. They involve linear or translational slipping movements over inclined bedding planes or planar glide surfaces (as a glide block), or down curvilinear shear surfaces (as a slump block) with backward rotation. Flowing motions can be quite complex because there can be many laminar and turbulent actions in dry or wet media, where the particles move past each other in a variety of irregular motions that constitute distributed shear. For example, whole mountains can collapse internally under the force of gravity as a kind of rock flow or deep creep along many internal small shear planes until a ridge-top graben or ditch results in a sackung (sagging) failure with antislope scarps lower down as the mountain bulges outward. Or as sediment concentrations increase in mixtures of clastic particles of debris with water, forming a slurry that resembles wet concrete, various forms of rapid, wet, debris flow result. If the material is all fine grained, yet full of water, a mudflow occurs. If a dry mass of wind-blown dust or loess is shaken strongly by an earthquake, for example, all of the weak bonds between its particles may be ruptured, [Page 1869]and it can become a fluidized granular flow that is like loose talcum powder, as a loess flow. Or where a peat bog of plant-matter accumulation undergoes a torrential rainstorm, it may become supersaturated and break out as a bog burst. An earthflow results where fine-grained and damp clastic accumulations flow slowly downslope as viscous granular flows. On steep mountains where surficial soil layers, rock fragments, and vegetation are accelerated off the slopes in torrential rainstorms, a debris avalanche results. On the other hand, where a dry mass of rock fragments is detached from a steep mountain slope in what would normally be considered just a rockslide, if the acceleration is rapid enough, it may become a high-speed-flow sturzstrom landslide with a long runout zone.

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