Mass Movements

Mass Movements

Movement of material downslope under the influence of gravity is called mass movement. From a geographical perspective, the place to consider mass wasting is on hillslopes, which are land surfaces inclined at various angles from the horizontal. This process takes place when the downward force of gravity overcomes the resisting force of the slope material’s shearing strength—in other words, its cohesiveness and internal friction. The point at which sediment becomes unstable is called the threshold pointand depends a great deal on the material’s angle of repose. This angle is the natural maximum slope that any deposit of a particular kind of sediment—for example, sand—can achieve without moving under the force of gravity. The angle of repose ranges between about 25° for fine sands and approximately 40° for angular pebbles and cobbles.

Kinds of Mass Movements

Many types of mass movement (Fig 3.7.) can be recognized on the basis of the behaviour of the material and the mechanics of movement.

RAPID MOVEMENTS

  1. Landslides—these are by far the most spectacular and violent of all mass movements. Landslides are characterized by the sudden movement of great quantities of rock and soil downslope. Such movements typically occur on steep slopes that have large accumulations of weathered material. Precipitation in the form of rain or snow may seep into the mass of steeply sloping rock debris, adding sufficient weight to start the entire mass sliding.
  2. Slumps—these special landslides occur along a curved surfaces. The upper surface of each slump block remains relatively undisturbed, as do the individual blocks. Slumps leave arcuate (Latin, curved like a bow) scars or depressions on the hillslope. Heavy rains or earthquakes usually trigger slumps. Slump is a common occurrence along the banks of streams or the walls of steep valleys.
  3. Mudflows—these are highly fluid, high-velocity mixtures of sediment and water that have a consistency of wet concrete. Mass wasting of this type typically occurs as certain arid or semiarid mountainous regions are subjected to unusually heavy rains.
  4. Earthflows—these are usually associated with heavy rains and move at velocities between several centimeters and hundreds of meters per year. They usually remain active for long periods of time. They generally tend to be narrow, tonguelike features that begin at a scarp or small cliff.
  5. Rockfall—the easiest kind of mass-wasting process to envision is rockfall. As the name implies, rockfall occurs when rocks fall quickly down a hillslope under the force of gravity. Rockfall is usually most prominent in places where extensive rock outcrops occur on steep hillslopes or canyon walls.
  6. Debris slide—A debris slide occurs when slope failure occurs along a plane that is roughly parallel to the surface.
  7. Avalanches— Another mass-wasting process that you have probably heard of is an avalanche, which is a large mass of snow or rock that suddenly slides down a mountainside. Avalanches are most commonly associated with snow and typically occur when thick deposits of snow accumulate at steep angles on mountain slopes. These steep-angled snow deposits can become unstable when a subsequent large storm produces another layer of snow of lighter density than the snow it buries. Under these conditions, an avalanche will occur because the two layers of snow are not firmly bound together to form a uniform mass. When the uppermost layer is somehow destabilized, a section will break free as a slab avalanche.

SLOW MOVEMENTS

  1. Soil creep—this continuous movement, usually so slow as to be imperceptible, normally occurs on almost all slopes that are moist but not steep enough for landslides. Soil creep is usually accelerated by frost wedging, alternate thawing and freezing, and certain plant and animal activities. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles.
  2. Solifluction—this downslope movement is typical of areas where the ground is normally frozen to considerable depth—arctic, subarctic, and high mountain regions. The actual soil flowage occurs when the upper portion of the mantle rock thaws and becomes water saturated. The underlying, still frozen subsoil acts as a slide for the sodden mantle rock that will move down even the gentlest slope.

FACTORS INFLUENCING MASS MOVEMENT

Gravity is the driving force for the downslope movement of material, but several factors are important in causing movement to occur. The most important are (1) saturation of material with water, (2) vibrations from earthquakes, (3) oversteepening of slopes by undercutting, (4) alternating freezing and thawing, and (5) strength of the slope materials.


(A) Creep is the slow downslope migration of soil and loose rock fragments. /
(B) A debris flow is the rapid flow of a mixture of rock fragments, soil, mud, and water.

(C) Slump is the slow or moderately rapid movement of a coherent body of rock along a curved rupture surface. /
(D) A rockslide is the rapid downslope movement of rock material.

(E) A rockfall is the free-fall of rock from steep cliffs. /
(F) Subaqueous slope failures form on steep submarine slopes involving clasts of all sizes.

Fig. 3.7: Various forms of Mass movements

Lecture Delivered By: Dr. M. Imran Malik