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massmovements - Mass Movements EENS 1110 Physical Geology...

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This page last updated on 21-Mar-2011 EENS 1110 Physical Geology Tulane University Prof. Stephen A. Nelson Mass Movements Mass movements (also called mass-wasting) is the down-slope movement of Regolith (loose uncemented mixture of soil and rock particles that covers the Earth's surface) by the force of gravity without the aid of a transporting medium such as water, ice, or wind. Still, as we shall see, water plays a key role. Mass movements are part of a continuum of erosional processes between weathering and stream transport. Mass movement causes regolith and rock to move down-slope where sooner or later the loose particles will be picked up by another transporting agent and eventually moved to a site of deposition such as an ocean basin or lake bed. Mass movement processes are occurring continuously on all slopes; some act very slowly, others occur very suddenly, often with disastrous results. In this discussion, we hope to answer the following questions: 1. What determines whether or not a slope is stable or unstable? 2. How are the different mass movement processes classified? 3. How are mass movement events triggered? 4. What observations might suggest that the area is unstable and may start to move? 5. How can we mitigate against mass movement hazards? We start with a discussion of the forces acting at the surface that cause mass movements. Gravity Gravity is the main force responsible for mass movements. Gravity is a force that acts everywhere on the Earth's surface, pulling everything in a direction toward the center of the Earth. On a flat surface, parallel to the Earth's surface, the force of gravity acts downward. So long as the material remains on the flat surface it will not move under the force of gravity. Of course if the material forming the flat surface becomes weak or fails, then the unsupported support mass will move downward. Mass Movements 3/21/2011 Page 1 of 12
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On a slope, the force of gravity can be resolved into two components: a component acting perpendicular to the slope, and a component acting parallel to the slope. z The perpendicular component of gravity, g p , helps to hold the object in place on the slope. z The component of gravity acting parallel to the slope, g s , causes a shear stress parallel to the slope and helps to move the object in the down-slope direction. z On a steeper slope, the shear stress component of gravity, g s, increases, and the perpendicular component of gravity, g p , decreases. z Another force resisting movement down the slope is grouped under the term shear strength and includes frictional resistance and cohesion among the particles that make up the object. z When the sheer stress becomes greater than the combination of forces holding the object on the slope, the object will move down-slope.
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