Mass Movement: earth materials that move downhill by gravity's influence (landslides....)
     earth materials:  soil, loose sediment, rotten rock (aka regolith), broken rock, etc.
          looser materials move downhill easier, coherent materials resist motion
          properties of materials change with time, usually get less solid (due to weathering)

Gravity:  NOT a variable (gravity is constant) but a very important parameter/factor
     force of gravity = acceleration (about 10 meters/sec2) times mass (in kilograms)  F=ma
          force of gravity pulls mass straight down (gravitational force acts vertically)
         if mass sits on a slope, the force of gravity is "separated" into two parts:
              (1) force perpendicular to the slope, holding the object in place (holding in force)
              (2) force parallel to the slope, pushing the object downhill (pushing downhill force)
         steeper slopes have higher pushing down force and lower holding in force

Variables:  changeable factors that can influence the probability  of mass movement
     slope angle:  steeper slopes means more likely; ± no landslides on flat ground
          most slopes naturally decrease in angle with time due to erosion, mass movement,..
          slopes naturally steepened;  undercutting by streams and waves, tectonic uplift...
          slopes artificially steepened by humans; undercutting for roads and housing sites...
     earth materials:  loose and/or weak materials (dry sand, wet mud...) means more likely
     vegetation:  deep roots means more coherent materials, vegetation can dry out soils
     weight of material:  higher weight means more likely of mass movement
          weight can increase dramatically (20-30%) during heavy rains with ground saturated
          weight of vegetation usually not a factor; cut down trees to reduce landslides???

Mass Movement and Earthquakes: quakes can act as trigger for already unstable hillside
     ground shaking adds to gravity, increasing slope-parallel forces, triggers movement
     ground shaking weakens earth materials (e.g. liquefaction), triggers movement

Mechanisms of Mass Movement: how do masses get downslope? (just like glaciers........)
    sliding (S-words):  slides/slumps/slips (e.g. landslides, earthslumps)
          sliding as a coherent mass or block, mass not internally disturbed (until later)
    flowing (F-words):  earthflows, mud flows, debris flows, etc.
          flowing like a liquid, material thoroughly mixed.  usually water rich
          flows generally travel long distances because fluid like, slides only short distances
          not to be confused with lava flows!!!  no magma here, just mud etc.

Rates of Movement: can vary from centimeters per decade to 100 kilometers/hour
     slow = creep.  soil creep and solifluction.   freeze-thaw.   tilted retaining walls in Seattle
     slow to fast:  slides and flows; depends on angle of slope and materials involved
     fastest = falls.  rock falls,  free falling through the air.

Mass Movement and Volcanoes: lessons learned from the 1980 Mt. St. Helens eruption
     landslides on side of volcano can trigger eruptions; north bulge on Mt. St. Helens, e.g.
     eruptions on volcanoes can produce mudflows and debris flows (lahars)
          Osceola Lahar on Mt. Rainier about 5600 years ago (Kent, Auburn, Sumner areas)
          Toutle River Lahar on Mt. St. Helens in 1980 reached the Columbia River
 

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