Mass Wasting Essay

Mass Wasting (also Mass Movement): – is the down slope movement of earth materials under the influence of gravity. The detachment and movement of earth materials occurs if the stress imposed is greater than the strength of the material to hold it in place. – Mass movement is a naturally occurring process that contributes to the cycle of tectonic uplift, erosion, transportation, and deposition of sediments. They are responsible for the topography of mountain ranges and river canyons that has developed over geologic time. Types of mass wasting: A. Slide involves movement of coherent blocks of material along a well-defined surface 1.

Rockslide • Also called debris slides or “landslides”. Occurs when blocks of rock, or masses of unconsolidated material slide down a slope. These are among the most destructive of mass movements. May be triggered by rain or melting snow, or earthquakes. 2. Slump • Slumps involve a mass of soil or other material sliding along a curved, rotational surface. (Shaped like a spoon. ) Slumps are sometimes seen along interstate highways where the graded soil on the sides of the road is a little too steep. 3. Creep • A SLOW downhill movement of soil and regolith.

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Creep results in tree trunks that are curved at the base, tilted utility poles, fence posts, and tombstones, and causes retaining walls to be broken or overturned. B. Fall involves free fall of material (no contact with any surface except to bounce) Rock fall -The free fall of detached pieces of material of any size; may fall directly downward or bounce and roll. May occur as result of freeze-thaw, or the loosening action of plant roots. Causes the formation of talus slopes. Signs along highways warn of rolling rock in mountainous areas where the road has been cut into the hillside.

C. Flow involves continuous movement of material as a viscous fluid 1. Debris flow or mudflow • Commonly occur in volcanic areas, where they are called lahars. Mudflows generally follow established drainage patterns (valleys). 2. Earthflow • Form in humid areas on hillsides following heavy rain or melting snow, in fine-grained materials (clay and silt). Also occurs at the toe of slumps. Rate of movement varies (less than 1 mm per day to several meters per day), but may be long-lived (days to years). Includes the liquefaction associated with earthquakes. 3. Solifluction also known as soil fluction or soil creep, where waterlogged sediment slowly moves downslope over impermeable material. It can occur in any climate where the ground is saturated by water, though it is most often found in periglacial environments where the ground is permanently frozen (permafrost) Forces involved in mass wasting are: 1. gravity, a vertical force that can be split into vectors parallel to (tangential) and perpendicular to a surface(normal) 2. friction on the surface or between grains 3. shear strength, a measure of material strength and cohesion Kinds of Material Moved 1. Bedrock. . Soil (Regolith) 3. Water Mass movements are caused by various conditions: • Volcanic activity many times causes huge mudflows when the icy cover of a volcano melts and mixes with the soil to form mud as the magma in the volcano stirs preceding an eruption. • During heavy rainfall or rapid snow melt, water rapidly accumulates in the ground, changing the earth into a flowing river of mud. • Earthquake shocks cause sections of mountains and hills to break off and slide down. • Human modification of the land or weathering and erosion help loosen large chunks of earth and start them sliding downhill. Vibrations from machinery, traffic, weight loading from accumulation of snow; stockpiling of rock or ore; from waste piles and from buildings and other structures. • Gravitational pull of the earth on soil, rocks, and mud is the force behind mass movements. Factors Affecting Mass Wasting Several factors that determine the extent of mass wasting are: 1. Slope stability. The steeper the slope, the less stable it is. If the angle on the slope is great than the angle of repose, the slope will fail, resulting in mass wasting. Angle of Repose -maximum angle at which unconsolidated material on slope is stable • Slope stability depends on the nature of material • Slope stability also depends on the driving and resisting forces that act on the slope • 2. The degree of chemical weathering. In regions where there is more chemical weathering, there is a greater chance for mass wasting. • 3. The water content. Water adds weight to the slope, increase pore spaces in between grains, makes it easier for material to slide down the slope. • . Vegetation. The amount of vegetation can help reduce the rate of mass wasting. The roots help absorb water and help keep the soil in place. Removal of vegetation will speed up mass wasting. • 5. Overloading. Adding too much weight on a slope can increase water pressure. • 6. Geological features. When bedding is dipping in the same direction of the slope, there is a greater chance for mass wasting. Joints in the rocks also allow more water to seep into the ground. The type of rocks and their composition also affects mass wasting. Rate of Movement Slow movements primarily affect unconsolidated material at depths less than 1 meter, where movement rates average from 1 mm/yr to 1 mm/day: • Moderate velocity movements have movement rates that average from 1 cm/day to 1 cm/s. Mass movements of this type include: • Rapid movements, which have movement rates ranging from meters/second to 100 km/hr, occur on steep slopes. Social and Economical Impact of Mass Movements Mass movements produce a variety of effects. 1. loss of life 2. floods, damming up bodies of water 3. destruction to habitable land 4. Damages to structures or property . loss of tax revenues on devalued properties 6. reduced real estate values in landslide prone areas 7. loss of productivity of agricultural lands affected by landslides 8. loss of industrial productivity because of interruption of transportation systems by landslides 9. damage to railroads, building structure and underground pipes Preventive Measures 1. Bridge 2. Slope reduction and weight reduction 3. Retention Structures – walls or ground covers 4. Fluid removal – bore holes, subsurface drainage 5. Vertical piles driven into slide 6. Rock bolts 7. Flood control channels

Mass wasting Essay

Mass wasting is defined as, “The down slope movement of rock and regolith near the Earth’s surface mainly due to the force of gravity.” (Nelson, 2007) In simpler terminology, mass wasting is knows as landslide. Basically, mass wasting occurs when the force of gravity forces sediments and mass to move from higher elevation to lower elevation. The term “mass” is used since the sediments are grouped together and large amounts of sediments or rocks move down a slope. On the other hand, Perry (n.d) explains the use of the term wasting:

“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.”

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For mass wasting to actually occur, a trigger mechanism is necessary. Both natural and man-made events can trigger mass wasting. Gravity alone is often times insufficient to cause mass wasting. Gravity must work together with a certain event. Some common natural events that can lead to mass wasting are earthquakes, typhoons or storms, and the rapid erosion of a slope. Man-made events can also trigger a landslide. For instance, mining activities can cause sudden rock movement that can eventually lead to a landslide. Furthermore, the continuous misuse of the Earth’s resources, specifically the denudation of forests can lead to disastrous landslides.

There are several types of mass wasting. The two primary categories of mass wasting are slope failures and sediment flows. Slope failures refer to abrupt failures of a slope that leads to the downward movement of debris by sliding, rolling, falling, or slumping. Sliding of rocks or sediments occurs when sediments or rocks move or slide down a certain surface. Some common surfaces where the rocks slide down are a bedding plane, a foliation surface or a joint plane.  Rocks fall when they are dislodged from a steep slope. Debris can likewise fall in the same manner. Lastly, Nelson (2007) describes slumps as, “Types of slides wherein downward rotation of rock or regolith occurs along a concave-upward curved surface.” Furthermore, Nelson notes that slumps commonly form because of human activities. Erosion and heavy rainfall can often times trigger slumping.

The other type of mass wasting is sediment flow.  Sediment flows occur when sufficient force is applied to rocks and regolith that they begin to flow down slope. There are two main types of sediment flows, slurry flows and granular flows.

One way to know if mass wasting is likely to occur is by considering the water level. Although water is necessary to keep sediments, particularly soil together, too much water can also cause sediments to loosen up and eventually slide or flow down the slope. As such, heavy rainfall is a valuable indicator of how likely mass wasting is to occur.

As earlier mentioned, the force that causes mass wasting is gravity. Gravity pulls the sediments or rocks towards the earth. When gravity exerts force on a sloping surface of the earth, it creates a translational force the slope sediment/rock. (Monroe, 2005) Translational force then creates sheer stress which basically means that the slope is weakened and made more prone to mass wasting. Since gravity is always present, there is always a chance that mass wasting may occur. However, steeper slopes are more prone to mass wasting since the materials are more aligned with the downward pull of gravity.

There are several ways by which the force/pull of gravity is countered. The law of physics suggests that friction is the counter-force to any push or pull. The same law applies to sediment and rock movement. Basically, friction reduces the effect of gravity on a certain slope. Friction holds materials together. It keeps sediments cohesive. Thus, when there is greater friction between the sediments, the slope has greater sheer strength. In turn, greater sheer strength means that the pull of gravity is weaker and thus, the likelihood of mass wasting to occur is lesser as well.

Another factor that helps determine the possibility of mass wasting to occur is the angle of repose. This refers to the angle of the slope at which the sediments sit at rest. Commonly, the angle of repose is between 25 to 40 degrees. Another factor that influences mass wasting is the critical angle of slope. This refers to the steepest angle that a slope can have before materials slide or fall down the slope. The steeper the critical angle, the more prone the slope is to mass wasting.

Finally, the characteristics of the materials themselves can also determine how likely it is for mass wasting to occur. According to Gore (2006), “Some types of materials shrink and swell as they are wetted and dried. For example, a type of clay called montmorillonite clay expands when wet and shrinks when dry. It causes soil instability and can lead to mass wasting or downslope movement of soil.”

Mass wasting is not simply caused by gravity. Several factors can determine the how likely mass wasting is to occur.

 

References

Gore, P. (2006). Mass Wasting. Georgia Perimeter College. Retrieved 29 April 2008 from: http://facstaff.gpc.edu/~pgore/geology/geo101/masswasting.html.

Monroe, Wicander (2005). The Changing Earth: Exploring Geology and Evolution. Thomson Brooks/Cole.

Nelson, S. (2007). Mass wasting and mass wasting processes. Tulane University. Retrieved 29 April 2008 from: http://www.tulane.edu/~sanelson/geol204/masswastproc.htm.

Perry, B. (n.d.) Introduction to mass wasting. California State University, Long Beach. Retrieved 29 April 2008 from: http://seis.natsci.csulb.edu/bperry/Mass%20Wasting/Introduction_to_Mass_Wasting.htm.

Selby, M.J. (1993). Hillslope Materials and Processes, 2e. Oxford University Press.

 

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