DEFINITION OF PHYSICAL CONDITIONS THAT CONTROL MASS WASTING

Whether or not mass wasting occurs—and the type of resulting

mass wasting—is controlled by many factors. These

include characteristics of the regolith and bedrock, and the

presence or absence of water, overburden, angle of the slope,

and the way that the particles are packed together.

Mass wasting in solid bedrock terrain is strongly influenced

by preexisting weaknesses in the rock that make movement

along them easier than if the weaknesses were not

present. For instance, bedding planes, joints, and fractures, if

favorably oriented, may act as planes of weakness along

which giant slabs of rock may slide downslope. If the rock or

regolith has many pores, or open spaces between grains, it

will be weaker than a rock without pores. This is because

there is no material in 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 and making the rock weaker.

Water may act to 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.

This surface tension is able to bond 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 may

actually exert 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.

The loading from water in pore spaces is in many cases

enough so that the strength of the underlying rocks and soil

is exceeded, and 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 about 33°–37°, depending on the specific

characteristics of the material. The way that the particles are

arranged or packed in the slope is also a factor; the denser

the packing, the more stable the slope.

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