PCC - Mass Wasting Lesson
Mass Wasting
Mass wasting refers to the movement of geologic material (rock, soil, sand, etc.) downslope due to gravity, and usually assisted through the actions of water. Mass wasting may occur gradually or suddenly, depending on the conditions, and is categorized into three broad groups - falls, slides, and flows.
Falls describe a mass wasting down a vertical surface, such as a cliff. Falls can produce very fast-moving material, and can range from a single rock falling to thousands of rocks. These rocks may fragment upon impact, forming a pile at the base of the cliff. Over time, multiple rock fragmentations will form a slope of fragmented material at the base of the cliff, known as a talus.
Mass wasting events that occur along slopes are generally known as slides or slumps. In both cases, the material that erodes will travel downslope as a single block (even if it is fine particles of dirt). Slides occur when the unstable surface shears away from more stable material underneath, usually due to a pre-existing area of weakness (such as the boundary between layers in sedimentary rock). Slides can be triggered by an earthquake or when the surface material becomes saturated by water (which increases gravitational pull on the material). Slumps are similar to slides, but are formed when the lower portion of the weak area is removed due to erosion or human efforts. Without this lower material in place for stabilization, the uphill material shears off and moves downslope due to the pull of gravity. Many seaside cliffs form slumps as the ocean waves erode away at the base of the cliff, which causes the upper material to eventually break free.
The final classification of mass wasting is known as a flow. Flows can be rapid or gradual, and can often result after the initial movement caused by either a fall or a slide. Flows differ from slides and slumps in that the individual fragments (from sand and silt all the way up to boulders) do not travel downslope as a cohesive unit; instead, each fragment has its own internal energy, resulting in individual movements. For example, if a large block of rock were to break off of a vertical cliff, it would be classified as a fall. However, if it were to break into pieces when it reached the talus below and then the individual fragments tumbled downslope for a period of time, it would then be classified as a rock flow (also known as an avalanche).
Gradual flows occur on gentler slopes, and typically involve small-grained fragments (such as sand or silt) being moved downslope through the action of gravity, along with saturation and temperature changes on the grains. In areas saturated with water, the freeze-thaw cycle can cause frozen water in the ground to expand nearly 10 percent. This can lift surface grains, which are then pulled downslope as the water eventually thaws. In areas with little previous water saturation, the addition of more water (such as heavy rainstorms) in the ground can cause the ground to swell, lifting grains upward and allowing them to move downhill due to gravitational force. This gradual movement of fine-grained sediments is often referred to as "creep".
Since it is a very slow process, creep can often be indicated by telephone poles, fenceposts, or retaining walls that lean downslope, as they are literally being pushed over by the slow-moving sediments. Trees with bends in their lower trunks can also indicate an area where creep has been occurring; the roots anchor the tree in stable bedrock, but the surface sediment pushes the tree trunk downslope. However, since tree trunks are constantly growing towards sunlight, the new growth bends the tree back to vertical, forming a distinct bend in the base of the tree.
Complete the mass wasting review activity below:
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