Mass Movement
(or Mass Wasting)
• Definition: the down slope movement of
materials under the influence of gravity (therefore
a type of erosion)
• Occurs when the stress imposed is greater than
the strength of the material to hold it together.
Mass Movement
• Water affects mass movement by not only
increasing mass, but also decreasing viscosity.
• Movement occurs at specific often well-defined
planes. (usually between regolith and bedrock)
• This does NOT always mean landslides. It also
includes such processes as creep, slumps,
mudflow, and rock fall,.
Soil Creep
• Very slow
(imperceptible)
movement of the top
layer of soil.
• Generally occurs due to
expansion and
contraction of the soil.
– Expansion pushes
particles perpendicular
to the surface. When it
contracts, gravity pulls
the particles straight
down, resulting in slight
downward motion.
Slumps
• Blocks of land, usually on
curving slopes, can
fracture from the
surrounding terrain and
move downhill. (note: all
one part)
• Often results when land at
the bottom of the slope is
removed, therefore it can’t
support soil at the top of
the slope.
• May move very slowly
(m/s), moderately (m/yr),
or quickly (ie. during
earthquakes)
Rock Falls
• Chunks of rocks near the top of a cliff or slope
detach and fall to the base of the slope.
Mudflows
• Mud is essentially water that is full of
suspended small rocky particles (clay and
silt).
• When an sloped area that is normally dry
experiences large rainfall, the ground is not
equipped to drain the vast quantities of
water. The area becomes muddy, heavy, and
can slide down the slope.
• The force of the mud, much like a tidal
wave, can carry away even large rocks and
trees.
• How do all of these differ from landslides?
– Landslides involve relatively quick movement of
unconsolidated material (not packed together).
– Mudflows…
• are also unconsolidated, but involve mud (imagine
that!)
– Slumps…
• move as one unit.
– Rockslides
• involve consolidated material rocks
– Soil creep
• is very slow (imperceptible)
• HW: Read section 12.3 (again, take notes)
• Do #2, 4 p. 270