mass movement

mass movement, bulk movements of soil and rock debris down slopes in response to the pull of gravity, or the rapid or gradual sinking of the Earth’s ground surface in a predominantly vertical direction. Formerly, the term mass wasting referred to a variety of processes by which large masses of crustal materials are moved by gravity from one place to another. More recently, the term mass movement has been substituted to include mass wasting processes and the sinking of confined areas of the Earth’s ground surface. Mass movements on slopes and sinking mass movements are often aided by water and the significance of both types is the part each plays in the alteration of landforms.

The variety of downslope mass movements reflects the diversity of factors that are responsible for their origin. Such factors include: weathering or erosional debris cover on slopes, which is usually liable to mass movement; the character and structure of rocks, such as resistant permeable beds prone to sliding because of underlying impermeable rocks; the removal of the vegetation cover, which increases the slope’s susceptibility to mass movement by reducing its stability; artificial or natural increases in the slope’s steepness, which will usually induce mass movement; earthquake tremors, which affect the slope equilibrium and increase the likelihood of mass movement; and flowing ground water, which exerts pressure on soil particles and impairs slope stability. These factors affecting slope conditions will often combine with climatic factors such as precipitation and frost activity to produce downslope mass movement.

The types of mass movements caused by the above factors include: the abrupt movement and free fall of loosened blocks of solid rock, known as rockfalls; several types of almost imperceptible downslope movement of surficial soil particles and rock debris, collectively called creep; the subsurface creep of rock material, known as bulging: the multiplicity of downslope movements of bedrock and other debris caused by the separation of a slope section along a plane of least resistance or slip surface, collectively called landslides; the separation of a mass along a concave head scarp, moving down a curved slip surface and accumulating at the slope’s foot, known as a slump; the saturation of debris and weathered material by rainfall in the upper section of a slope or valley, increasing the weight of the debris and causing a slow downslope movement, called an earthflow; a rapidly moving earthflow possessing a higher water content, known as a mudflow; a fast-moving earthflow in a mountainous region, called a debris flow or avalanche; and the downslope movement of moisture-saturated surficial material, known as solifluction, over frozen substratum material, occurring in sub-Arctic regions during seasonal periods of surface thaw.

Sinking mass movements occur in relatively rapid fashion, known as subsidence, and in a gradual manner, called settlement. Subsidence involves a roof collapse or breakdown of a subsurface cavity such as a cave. Extensive subsidence is evident in areas where coal, salt, and metalliferous ores are mined. Marine erosion sometimes causes the roof collapse of sea caves. Regions of karst topography will exhibit widespread subsidence in the form of sinkholes caused by underground drainage. Other types of subsidence caused by underground solutions have been found in chalk, gypsum, anhydrite, halite (salt), and loess terrains. The melting of ground ice also contributes to subsidence such as the formation of glacial kettles and depressions following the seasonal surface thaw of perennially frozen land. The chemical decomposition of subsurface rocks and ores is also a cause of subsidence. Another form of subsidence is the steep-walled depression, known as a volcanic sink, formed following the withdrawal of magma from below the ground surface.

The gradual settlement of confined areas of earth material occurs through consolidation of soil and rock by the squeezing or removal of fluids from the pore spaces, and by the collapse of the grain structure. The most widespread cause of consolidation is by surface loading such as the continued deposition of sediments in sea and lake beds or by loads imposed on land by glacial ice sheets or outwash deposits. Human-made structures also cause surface loading, consolidation, and settlement. Consolidation is also caused by the lowering of the ground water table. The extraction of pressurized water or oil from deep beneath the surface will cause a collapse of the pore spaces and consolidation of rock material. Grain structure collapse usually occurs from the wetting of rock materials such as clays and sands, which causes the structure of the grains to shift and settle in a more compact and dense configuration.