Transcript Ch. 5: “Weathering, Soil, and Mass Movements”

Ch. 5: “Weathering, Soil, and
Mass Movements”
5.3: “Mass Movements”
1
What Is Mass Movement?
• The transfer of rock and
soil downslope due to
gravity is called mass
movement.
– Some types are so slow
that you cannot see them.
– Others, such as landslides,
are very sudden.
– The combined actions of
weathering and mass
movement produce most
landforms.
2
Triggers of Mass Movements
• Water
Heavy rains or melting
snow saturate the
ground.
Water fills in pores in
sediment, allowing
particles to slide past
one another more easily.
• Oversteepened Slopes
Slopes that are above 25
to 40 degrees (depends
on size & shape of
particles).
This can result when a
stream undercuts a
valley wall or waves
pound against the base
of a cliff.
3
Triggers of Mass Movements
• Removal of Vegetation
Plants make slopes more
stable because their root
systems bind soil and
regolith together.
When plants are removed
by forest fires by human
activities the likelihood of
mass movement
increases.
• Earthquakes
These are one of the most
dramatic triggers of
mass movements.
An earthquake and its
aftershocks can dislodge
enormous amounts of
rock & unconsolidated
material.
4
Types of Mass Movements
• Rockfalls
Rock or rock fragments fall
freely through the air.
Many result from
mechanical weathering.
These are common on
steep slopes.
They can trigger other types
of mass movements.
• Slides
A block that includes
segments of bedrock
moves sudden along a
flat, inclined surface.
Rockslides are among the
fastest mass movements,
reaching speeds of over
200 km/hr.
5
Types of Mass Movements
• Slumps
This is the downward movement
of a block of material along a
curved surface.
Doesn’t travel very fast or far.
Leaves crescent-shaped cliff
above the slump.
Common on oversteepened
slopes slopes that have much
clay.
• Flows
The mass movement of material
containing large amounts of water.
* mudflows: flows that move
quickly
* earth flows: flows that move
slowly
• Creep
Slowest type of mass movement.
Continual freezing and thawing
contributes to it.
Too slow to observe directly.
- May notice poles, fence posts,
etc begin to lean over time.
6
Heavy Rains Can Trigger Slumps
7
Creep
8