The Downslope Movement of Material
Mass wasting, also known as mass movement, is the downslope movement of rock, regolith (soil and loose rock), and earth materials under the direct influence of gravity. It is a major component of the erosional process, moving material from higher elevations to lower elevations where it can then be transported by water or wind.
The Force of Gravity
Gravity is the ultimate driving force behind all mass wasting. On any slope, gravity has a component that acts parallel to the slope, pulling material downwards. The processes of weathering weaken surface materials, making them more susceptible to the pull of gravity.
Factors Controlling Slope Stability
Several factors control whether a slope will be stable or unstable:
1.Material and Structure: Unconsolidated materials (like soil and sand) are less stable than solid bedrock. The orientation of layers or fractures in the rock can also create planes of weakness.
2.Angle of Repose: This is the steepest angle at which a pile of loose, unconsolidated material will remain stable. For most dry sand, it's about 34 degrees. If the slope is steeper than the angle of repose, it is unstable.
3.Water Content: The effect of water is complex. A small amount of water can increase cohesion (think of a sandcastle). However, saturating the material with water lubricates the particles, reduces friction, and adds weight, dramatically reducing slope stability and often triggering failure.
4.Vegetation: The root systems of plants can bind soil and regolith together, increasing the slope's stability. Removing vegetation (e.g., through logging or forest fires) often leads to an increase in mass wasting events.
5.Triggers: While a slope may be unstable, it often needs a trigger to start moving. Common triggers include earthquakes, heavy rainfall, and human activities like road construction that over-steepen a slope.
Types of Mass Wasting
Mass wasting events are classified based on the type of material moving and the speed of the movement.
Creep: The slowest form of mass wasting. It's the gradual, almost imperceptible downslope movement of soil and regolith. Evidence includes tilted fences, trees with curved trunks, and leaning utility poles.
Slump: The rotational sliding of a mass of rock or regolith along a concave, curved surface. It leaves a crescent-shaped scarp at the head of the slump.
Flows: Material moves downslope as a viscous fluid.
Debris flow / Mudflow: A rapid flow of soil, rock, and a large amount of water, often funneled into canyons.
Slides and Falls: Material moves as a coherent mass along a planar surface (a slide) or falls through the air (a fall).
Rockslide / Rockfall: The rapid movement of a large block of rock or the free-fall of individual rocks from a cliff. These are the fastest and most dangerous types of mass wasting.