The Effect of Gravity on erosion and Deposition

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The Effect of Gravity on erosion and Deposition

• Chapter 3
• Section 4

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Angle of Repose

• Gravity is an agent of erosion and deposition. It
  influences the movement of water and ice, and it
  causes rocks and soil to move downslope.
• Mass movement is the movement of any material,
  such as rock, soil, or snow, downslope.

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Angle of Repose, continued 2

• Material such as rock, soil, or snow moves
  downhill until the slope becomes stable. The
  angle of repose is the steepest angle at which
  loose material will not slide downslope.
• The angle of repose is different for different
  surface material. Size, weight, shape, and
  moisture level determine at what angle material
  will move down-slope.

If the slope on which material rests is less than
the angle of repose, the material will stay in
place. If the slope is greater than the angle of
repose, the material will move downslope.
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Rapid Mass Movement

• Rock falls happen when loose rocks fall down a
  steep slope. The rocks can range in size from
  small fragments to large boulders.
• Mass movements, like rock falls, happen suddenly
  and rapidly, and can be very dangerous.

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Rapid Mass Movement, continued 2

• Landslides are sudden and rapid movements of a
  large amount of material downslope.
• .

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Slumps

• The most common type of landslide is a slump.
  Slumping occurs when a block of land becomes
  detached and slides downhill

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Rapid Mass Movement, continued 3

• Mudflows are rapid movements of large masses of
  mud. Mudflows happen when a large amount of water
  mixes with soil and rock. The water causes the
  slippery mass of mud to flow rapidly downslope.
• Mudflows commonly happen in mountainous regions
  when a long dry season is followed by heavy
  rains.

Mudflow-damaged house along the Toutle River. The
height of the mudflow is shown by the
"bathtub-ring" mudlines seen on the tree trunks
and the house itself. Caused by eruption of Mt.
St. Helens May 18, 1980.
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Rapid Mass Movement, continued 4

• Lahars are mudflows caused by volcanic eruptions
  or heavy rains on volcanic ash. Lahars can travel
  at speeds grater than 80 km/h and can be as thick
  as cement.
• On volcanoes with snowy peaks, an eruption can
  suddenly melt a great amount of ice. Water from
  the ice liquefies the soil and volcanic ash to
  produce a hot mudflow that rushes downslope.

A lahar overtook this area on the island of
Kyushu in Japan.
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Slow Mass Movement

• Creep is the slow mass movement of material
  downslope.
• Although rapid mass movements are visible and
  dramatic, slow mass movements happen a little at
  a time. However, slow mass movements occur more
  frequently, and more material is moved
  collectively.

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Chapter 12

• Click on the Links below for video clips on
  various forms of Mass movement.
• Avalanche/landslide
• Mudflows
• Lahars
• Rock Falls
• Slump