Geography · Ch 11 — Geomorphic Processes
Mass Movements
Mass Movements
Mass movements transfer rock debris down slopes under the direct pull of gravity. The key distinction here is that air, water, or ice do not carry the debris; instead, the moving debris may itself carry air, water, or ice along with it. These movements range from slow to rapid and can affect shallow to deep columns of material. The four broad categories are creep, flow, slide, and fall.
Gravity acts on all matter, both bedrock and the products of weathering. This means weathering is not a prerequisite for mass movement, though it certainly aids it. Mass movements are far more active over weathered slopes than over unweathered materials. Importantly, no geomorphic agent — running water, glaciers, wind, waves, or currents — participates in mass movement. So even though material shifts from one place to another, mass movement is not considered erosion because gravity alone drives the shift.
Materials on slopes have their own resistance to disturbing forces. They yield only when the applied force exceeds the shearing resistance of the material. Several conditions favour mass movements: weak unconsolidated materials, thinly bedded rocks, faults, steeply dipping beds, vertical cliffs or steep slopes, abundant precipitation, torrential rains, and scarcity of vegetation.
Before a mass movement actually occurs, certain activating causes set it off. These include:
- removal of support from below the materials, whether natural or artificial
- increase in gradient and height of slopes
- overloading through natural addition of materials or artificial filling
- overloading due to heavy rainfall, which saturates and lubricates slope materials
- removal of material or load from over the original slope surfaces
- occurrence of earthquakes, explosions, or machinery
- excessive natural seepage
- heavy drawdown of water from lakes, reservoirs, and rivers, causing slow outflow of water from under slopes or river banks
- indiscriminate removal of natural vegetation
The three forms of movement are heave (the heaving up of soils due to frost growth and other causes), flow, and slide.
Landslides
Landslides are relatively rapid and perceptible movements. The materials involved are relatively dry. The size and shape of the detached mass depend on the nature of discontinuities in the rock, the degree of weathering, and the steepness of the slope. Depending on the type of movement, several distinct types are identified.
Slump is the slipping of one or several units of rock debris with a backward rotation with respect to the slope over which the movement takes place.
Debris slide is the rapid rolling or sliding of earth debris without backward rotation of the mass.
Debris fall is nearly a free fall of earth debris from a vertical or overhanging face.
Rockslide is the sliding of individual rock masses down bedding, joint, or fault surfaces. Over steep slopes, rock sliding is very fast and destructive.
Rock fall is the free falling of rock blocks over any steep slope, keeping itself away from the slope. Rock falls occur from the superficial layers of the rock face — this distinguishes it from rockslide, which affects materials up to a substantial depth.
Slides occur as planar failures along discontinuities like bedding planes that dip steeply.