28 terms

Mass Wasting

Geology Exam Review for Trimester A: Mass Wasting
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Mass Wasting
downslope movement of rock, regolith, and soil under direct influence of gravity; does not require transporting a medium
Water
mass wasting is triggered when heavy rains or periods of snow saturate surface materials; pores in sediment become filled with this, cohesion among particles is destroyed allowing them to slide past one another with relative ease; it adds weight as well
Oversteepened Slopes
another trigger of mass wasting; streams undercutting valley walls, waves pounding against base of cliff, angle of repose is adjusted, and rock debris responds by moving downslope
Angle of Repose
unconsolidated, granular particles assume stable slope; the steepest angle which material remains stable
Removal of Vegetation
plants protect against erosion and contribute to stability of slopes because their root systems bind soil and regolith together; shield soil surface from erosional effects of raindrop impact; where plants are lacking, mass-wasting is enhanced
Earthquakes as Triggers
conditions favoring mass-wasting may exist in an area for a long time without movement occurring; this additional factor is needed, which can dislodge enormous volumes of rock and unconsolidated material
Liquifactoin
intense ground shaking during EQs causes well saturated surface materials to lose their strength and behave as fluidlike masses that flow
fall, slide, or flow
types of motion in mass wasting
fall
movement involves freefall of detached individual pieces of any size; common form of movement on slopes that are too steep for loose material to remain on the surface
talus slopes
formed from rock falls; mostly with ice wedging
slides
mass movements in which there is a distinct zone of weakness separating slide material from the more stable underlying material
rotational slides
slide where surface of rupture is concave-upward curve that resembles the shape of a spoon and descending material exhibits downward and outward
transitional slides
slide where mass material moves along a relatively flat surface (joint, fault, or bedding plane); exhibits little rotation or backward tilting
flow
material moves downslope as a viscous fluid; most are saturated with water and typically moves as lobes or tongues
rock avalanche
most rapid type of mass movement; most literally float on air as it moves downslope
slump
downward sliding of a mass of rock or unconsolidated material moving as a unit along a curved surface; usually does not travel spectacularly fast nor very far;
scarp
scar on the suface/face of landform
rockslides
occur when blocks of bedrock break loose and slide down a slope; fastest and most destructive of mass movements
debris slides
if the material in a rockslide is largely unconsolidated, this term is used
Gros Ventre
1925; 38 million cubic meters of debris created 70 m high dam on river; because river was blocked, a lake formed, it filled so quickly that it flooded the area; the event was inevitable
debris flow
relatively rapid type of mass wasting; involves flow of soil and regolith containing large amounts of water
mud flow
another name for debris flow; most characteristic of semiarid mountainous regions and on slopes of some volcanoes
lahars
debris flow composed mostly of volcanic materials on flanks of volcanoes; deadly volcano hazard
earthflows
form on hillsides in humid areas during times of heavy precipitation or snow melt; when water saturates soil, material may break away leaving scar on slope and forming a tongue or teardrop shaped mass downslope
Creep
type of mass wasting that involves gradual downhill movement of soil and regolith; alternate expansion and contraction of surface material from freezing/thawing and wetting/drying; soil loses internal cohesion and gravity moves it downslope
Solifluction
type of mass wasting that occurs in highest elevations due to extreme conditions; movement of soggy mass of soil downslope at a rate of a few millimeters per day; common where water cannot escape from saturated surface layer; common in regions with permafrost
permafrost
permanently frozen ground that occurs in association with harsh tundra and ice cap climates
active layer
layer above permafrost where soil thaws to a depth of about 1 meter during brief high latitude summer and refreezes in winter; in summer, water is unable to percolate permafrost, so layer becomes saturated and flows downslope
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