Test 3 Study Guide Mass Wasting Understand slopes and all the factors that affect mass wasting i e what factors influence the stabilities of how they affect it Such factors include Nature of the materials unconsolidated sediment soil or consolidated rock Unconsolidated materials such as soil and sediment tend to be more prone to slope failure than rocks Sand rich sediment tends to be the least stable because there is more void space and the packing of individual grains is not as close as sediment with clay sand and silt Layered rocks are more stable than sediment or soil but are less stable than massive igneous rocks Layered rocks tend to be more friable and can be fragmented and broken away from the bedrock Massive rocks tend to be less prone to fracturing and fragmentation except when they are highly fractured Steepness of slope The angle at which material slopes is the major determining how much of the force of gravity is directed downslope Water Water can play a significant role in mass wasting sometimes acting as the key component to a mass wasting event or serving as a lubricant within a mass of sediment and rock enabling it to travel faster and further than it would otherwise Vegetation Vegetation has an anchoring effect on slopes The roots of the vegetation form a physical anchor for the soil and sediment along the slope Vegetation also reduces the amount of water in the pore spaces for nutrition When human development occurs vegetation is often stripped from slopes increasing the risk of slope failure Presence and orientation of planes of weakness bedding joints metamorphic foliations i e potential slide surfaces Planes of weakness in the material can facilitate mass wasting if the planes are oriented parallel to the slope Planes of weakness include bedding planes in sedimentary rocks and foliation planes in metamorphic rocks The orientation of rock layers has a significant effect on the stability of the slopes containing layered rock units When rock layers dip in the same direction as the slope failure of the slope is most likely Horizontal layering is a somewhat more stable configuration The most stable configuration is when the rock layering dips into the slope in the opposite direction of the slope direction Climate in addition to precipitation freeze thaw cycles are important The creep of soil and other unconsolidated material can be a grain by grain process facilitated by freeze thaw cycles As water between grains crystallizes to ice and expands a grain is pushed away from the surface of the ground On thawing that grain falls under the influence of gravity ever so slightly farther down slope than it was Think about some ways in which human activity can influence these factors and what the results of such activity might be Like it says above when human activity eliminated vegetation from an area it increases the risk of slope failure which can lead to unnecessary mass wasting Because the steepness of a slope is one of the primary factors contributing to slope stability anything that people do to cause a slope to become steeper will generally decrease that slope s stability Some natural processes that over steepen slopes include stream erosion wave erosion tectonic uplift volcanic activity Mass wasting events are manifested in different ways and at different rates depending on the types of materials involved such as rock earth mud debris and the type of motion involved such as fall slide flow To what do the terms creep slump and avalanche refer what about the rates at which they occur Creep flow occurs at the slowest rate of mass wasting processes often at a cm year or less Bent tree trunks fence posts utility poles as well as gravestones tilted downslope indicate that creep has occurred Slump slide occurs at a faster rate than creeps but are still fairly slow at roughly 1 km hr Slumps occur when a coherent mass of loosely consolidated materials or rock layers moves a short distance down a slope Movement is characterized by sliding along a concave upward or planar surface Avalanche flow occurs at the fastest rate at around 5 km hr Avalanches occur when a mass of material breaks loose from its surroundings causing it to move rapidly down a slope Landslide is a general term referring to all slides flows even falls that occur at a moderately fast rate Be familiar with the details of creep and the structure geometry and behavior of slumps Soil creep is a long term process The combination of small movements of soil or rock in different directions over time are directed by gravity gradually downslope The steeper the slope the faster the creep The creep makes trees and shrubs curve to maintain their perpendicularity and they can trigger landslides if they lose their root footing The surface soil can migrate under the influence of cycles of freezing and thawing or hot and cold temperatures inching its way towards the bottom of the slope forming terracotta s This happens at a rate that is not noticeable to the naked eye Slumps also known as rotational slides involve the movement of relatively intact masses of rock or sediment downslope along a curved concave upward failure plane The material involved in the slump rotates along the failure surface as it slides down the slope In many cases small flows of unconsolidated sediment or soil move out in front of the slump Slumps usually occur in rock units where there are some unconsolidated or weak rock layers How can you recognize evidence of past mass wasting events Creeps can be recognized by bent tree trunks Slumps can be characterized by an arched scarp or cliff at the top and an irregular surface at the base What if anything can be done to lessen the possibility of future such events in areas prone to mass wasting Construction of retaining walls putting in drains through the walls reducing the angle of slope through construction of terraces and planting grasses or other plants whose roots anchor the slope are sometimes effective means of slowing downslope movement Streams Recall that precipitation that does not evaporate either infiltrates the ground or runs off the surface into the ocean or inland lakes Remember what factors affect the infiltration capacity of the ground Runoff is either in the form of sheet flow such as during floods or it is confined to channels Channelized runoff is stream flow Factors that affect the infiltration capacity of the ground include the permeability of the surficial material the slope of the
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