The down-slope movement of material, whether it be bedrock, regolith, or a mixture of these, is commonly referred to as a landslide. All of these processes generally grade into one another, so classification of mass-wasting processes is somewhat difficult. We will use the classification used by your textbook, which divides mass wasting processes into two broad categories and further subdivides these categories.
A sudden failure of the slope resulting in transport of debris down hill by sliding, rolling, falling, or slumping. Sediment Flows- debris flows down hill mixed with water or air. Slope Failures
of slides wherein downward rotation of rock or regolith
occurs along a curved surface. The upper surface of each
slump block remains relatively undisturbed, as do the
individual blocks. Slumps leave arcuate scars or depressions
on the hill slope. Heavy rains or earthquakes usually
Rock falls and debris falls
Rock falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope. Debris falls are similar, except they involve a mixture of soil, regolith, and rocks. A rock fall may be a single rock, or a mass of rocks, and the falling rocks can dislodge other rocks as they collide with the cliff. At the base of most cliffs is an accumulation of fallen material termed talus. The slope of the talus is controlled by the angle of repose for the size of the material. Since talus results from falling large rocks or masses of debris the angle of repose is usually greater than it would be for sand.
Rock slides and debris slides
slides and debris slides result when rocks or debris
slide down a pre-existing surface, such as a bedding
plane or joint surface. Piles of talus are common at
the base of a rock slide or debris slide.
flows occur when sufficient force is applied to rocks
and regolith that they begin to flow down slope. A sediment
flow is a mixture of rock, regolith with some water.
They can be broken into two types depending on the amount
of water present.
Slurry flows are sediment flows that contain between about 20 and 40% water. As the water content increases above about 40% slurry flows grade into streams.
Granular flows are sediment flows that contain between 20 and 0% water. Note that granular flows are possible with little or no water. Fluid-like behavior is given these flows by mixing with air. Each of these classes of sediment flows can be further subdivided on the basis of the velocity at which flowage occurs.
Slurry flows solifluction
Flowage at rates measured on the order of centimeters per year of regolith containing water. Solifluction produces distinctive lobes on hill slopes . These occur in areas where the soil remains saturated with water for long periods of time.
These occur at higher velocities than solifluction, and often result from heavy rains causing saturation of the soil and regolith with water. They sometimes start with slumps and then flow down hill forming to lobes with an irregular surface consisting of ridges and furrows.
A highly fluid, high velocity mixture of sediment and water that has a consistency of wet concrete. These usually result from heavy rains in areas where there is an abundance of unconsolidated sediment that can be picked up by streams. Thus after a heavy rain streams can turn into mudflows as they pick up more and more loose sediment. Mudflows can travel for long distances over gently sloping stream beds. Because of their high velocity and long distance of travel they are potentially very dangerous.
Granular Flows Creep
The very slow, usually continuous movement of regolith down slope. Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent trees, offsets in roads and fences, and inclined utility poles.
Earthflows are usually associated with heavy rains and move at velocities between several cm/yr and 110s of m/day. They usually remain active for long periods of time. They generally tend to be narrow tongue-like features that begin at a scarp or small cliff
Usually form in relatively dry material, such as a sand dune, on a steep slope. A small disturbance sends the dry unconsolidated grains moving rapidly down slope.
These are very high velocity flows of large volume mixtures of rock and regolith that result from complete collapse of a mountainous slope. They move down slope and then can travel for considerable distances along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions.