Physical Conditions That Control Mass Wasting

Many factors control whether or not mass wasting occurs and, if it does, the type of mass wasting. These include characteristics of the regolith and bedrock, the presence or absence of water, overburden, angle of the slope, and the way that the particles are packed together.

Preexisting weaknesses in the rock that facilitate movement along them strongly influence mass wasting in solid bedrock terrain. For instance, bedding planes, joints, and fractures, if favorably oriented, can act as planes of weakness along which giant slabs of rock may slide downslope. Rock or regolith containing many pores, or open spaces between grains, is weaker than a rock without pores, because no material fills the pores, whereas if the open spaces were filled the material in the pore space could hold the rock together. Furthermore, pore spaces allow fluids to pass through the rock or regolith, and the fluids may further dissolve the rock, creating more pore space, and further weakening the material. Water in open pore space may also exert pressure on the surrounding rocks, pushing individual grains apart, making the rock weaker.

Water can either enhance or inhibit movement of regolith and rock downhill. Water inhibits downslope movement when the pore spaces are only partly filled with water and the surface tension (bonding of water molecules along the surface) acts as an additional force holding grains together. The surface tension bonds water grains to each other, water grains to rock particles, and rock particles to each other. An everyday example of how effective surface tension may be at holding particles together is found in sand castles at the beach—when the sand is wet, tall towers can be constructed, but when the sand is dry, only simple piles of sand can be made.

Water more typically acts to reduce the adhesion between grains, promoting downslope movements. When the pore spaces are filled, the water acts as a lubricant and exerts forces that push individual grains apart. The weight of the water in pore spaces also exerts additional pressure on underlying rocks and soils, known as loading. When the loading from water in pore spaces exceeds the strength of the underlying rocks and soil, the slope fails, resulting in a downslope movement.

Another important effect of water in pore spaces occurs when the water freezes; freezing causes the water to expand by a few percent, and this expansion exerts enormous pressures on surrounding rocks, in many cases pushing them apart. The freeze-thaw cycles found in many climates are responsible for many of the downslope movements.

Steep slopes are less stable than shallow slopes. Loose unconsolidated material tends to form slopes at specific angles that range from about 33-37 degrees, depending on the specific characteristics of the material. The arrangement or packing of the particles in the slope is also a factor; the denser the packing, the more stable the slope.

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