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La Conchita Portuguese Bend Mt. Huascaran Cable Canyon Vaiont Dam
a CSU Long Beach, Department of Geological Sciences web site
The purpose of making this web site is to provide students and the general public with basic information on the different processes and types of mass wasting. The intent is to educate people about the potential and real hazards related to landslides without sensationalizing or downplaying their dangers.
What is mass wasting? Mass wasting, also referred to by the non-technical term "landslide", is the down-slope movement of a mass of sediment and/or rock due mainly to the force of gravity. The "mass" part of the name implies that a somewhat coherent grouping of sediment/rock begins moving downward due to the force of gravity, and usually in combination with some triggering mechanism such as an earthquake or rapid erosion of the base of a slope.
The "wasting" part of mass wasting means that a cliff or mountain slope is diminishing in size, or wasting away. This can occur suddenly with tremendous destructive force, or very slowly with only a gradual alteration of Earth’s surface over a period of many years. Given enough time and repetition, the different types of mass wasting can play significant roles in reducing a tall mountain to a mound of low rolling hills, or in widening a narrow canyon into a broad stream valley.
A dangerous place to stand, in Utah.
How is gravity a factor in mass wasting?
The force of gravity is downward, towards Earth’s center. As gravity pulls downward on material comprising a tilted or sloping portion of Earth’s surface, a translational force is formed within the slope sediment/rock. This force creates shear stress within the slope's material, reducing the slope's strength and making it more prone to mass wasting. So, since gravity is always in effect, there is always the possibility of mass wasting of a sloped surface. Note that the steeper the slope, the more in line its material components (sediment and/or rock) are with gravity, so the more likely is mass wasting of that slope. See the diagrams below to better visualize the effects of gravity on slope material.
Diagram 1 Mass wasting is unlikely on this illustration because little shear stress is being produced by gravity.
Diagram 2 Mass wasting is much more likely on the slope shown in this diagram because the slope is more in line with the force of gravity than in the top diagram.
What keeps the force of gravity from immediately flattening elevated portions of Earth's surface?
Rock and or sediment comprising a slope is held together in a variety of ways, giving that slope its shear strength. This shear strength resists the shear stress placed on the slope material by gravity. Listed below are some of the factors related to a slope's shear strength, and therefore its ability to resist mass wasting.
If slope material is composed of sediment then mass wasting can be resisted by:
1) friction between sediment grains in contact with each holds the loose grains together. The greater the friction between sediment grains, then the greater the shear strength of a slope.
2) the presence of a small amount of water which sticks sediment grains to each other. Note that too much water has an opposite effect, reducing a slope's shear strength.
3) plant roots which physically bind sediment grains together, and anchor the sediment to bedrock. The best situation is to have a combination of small plants which protect slope sediment from the impacts of rain drops and water runoff, and trees which send roots deeply into the sediment as well as underlying rock.
If slope material is composed of rock then mass wasting is resisted by:
1) the formation of natural cement which locks sediment grains together (present in sedimentary rocks)
2) interlocking mineral crystals within the rock (common to igneous and metamorphic rocks).
A slope composed of solid rock will be much more resistant to the many causes of mass wasting than a slope composed of sediment, no matter how many plants are binding the sediment together. So, given the option, always choose a building location underlain by solid rock.
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