2 Mass Movements What is mass movement? It is the transfer of rock and soil downslope due to gravity.The force of gravity causes material to move downslope.
3 Mass MovementsSome types of mass movement are so slow that you cannot see them. Others, are very sudden.
4 Mass Movements: Triggers What triggers mass movements?Saturation of surface materials with water;Oversteepening of slopes;Removal of vegetation; andEarthquakes.
5 WaterHeavy rains and rapid melting of snow can trigger mass movement by saturating surface materials with waterWhen the pores in sediment become filled with water, the particles slide past each other more easily.Moist sand can hold its shape but, when saturated, it will ooze downhill.
6 Oversteepened slopesLoose particles can maintain a stable slope up to a certain angle. If the steepness of a slope exceeds the stable angle, mass movements become more likely.
7 Causes of oversteep slopes When a stream undercuts a valley wallWaves pounding against the base of a cliffExcavating during construction
8 Removal of vegetationPlants make slopes more stable. Their roots bind the soil together. When plants are removed by forest fires or human activity the likelihood of mass movement increases.
9 Earthquakes (trigger) Earthquakes are one of the most dramatic triggers of mass movements. An earthquake and its aftershocks can dislodge enormous amounts of rock and other material.
10 Types of Mass Movements Geologists classify mass movements based on:The kind of material that movesHow it movesThe speed of the movement
11 Types of Mass Movements We will discuss the following types of mass movements:Rockfalls;Slides;Slumps;Flows; andCreep.
12 What type of weathering might cause a rock to break free and fall? RockfallsA rockfall occurs when rocks or rock fragments fall freely through the air.This type of mass movement is common on slopes that are too steep for loose material to remain on the surface.What type of weathering might cause a rock to break free and fall?
14 SlideIn a slide, a block of material moves suddenly along a flat, inclined surface.Rockslides contain segments of bedrock.One of the fastest type of mass movement. (can move up to 124 mph!)What can trigger a slide?
15 SlumpsA slump is a downward movement of a block of material along a curved surface.Does not travel very fast or farSlumps leave a crescent-shape cliff just above the slump.What might trigger a slump?
16 FlowsFlows are mass movements of material containing a large amount of water, which move downslope as a thick fluid.
17 Mudflows Move quickly Mixture may have the consistency of wet concrete Will take most everything in its path down with it
18 Earthflows Move slowly Occur most often on hillsides in wet climates Water saturates the soil, the material breaks away and leaves a tongue-shaped mass
20 Creep Creep is the slowest type of mass movement Travels only a few centimeters per yearFreezing and thawing of the ground results in gradual downhill movement of the soil.Structures that were once vertical will tilt downhill
21 CreepCreep evidence apparent at the base of the tree trunks.
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