Transcript Mass Movements - rstepneysciencekhs

Chapter 5.3
Mass Movements
Triggers of Mass Movements
 What force causes
material to move
down slope?
 The transfer of
rock and soil down
slope due to
gravity is called
mass movement
Mass Movement
 This home in
Pacific Palisades,
California, was
destroyed by a
landslide triggered
by the January 1994
Northridge
earthquake
Q&A
 Are snow avalanches a type
of mass movement?
Yes. These thundering down
slope movements of snow and
ice can also transport large
quantities of rock, soil, and
trees. 10,000 snow avalanches
occur each year in the
mountainous western United
States.
Triggers of Mass Movement
 Water

Heavy rains and rapid
melting of snow can
trigger mass movement
by saturating surface
materials with water
Triggers of Mass Movement
 Oversteepened Slopes

An oversteepened slope
can result when a stream
undercuts a valley wall or
waves pound against the
base of a cliff
Triggers of Mass Movement
 How do oversteepened slopes trigger mass
movements?
Loose soil particles can maintain a relatively
stable until a certain angle. When this angle is
exceeded mass movements are more likely to
occur.

Triggers of Mass Movements
 Plants make slopes
more stable because
their root systems
bind soil and regolith
together
 When plants are
removed by human
activities the
likelihood of mass
movement increases
Triggers of Mass Movement
 Earthquakes are one of
the most dramatic triggers
of mass movements
 An earthquake and its
aftershocks can dislodge
enormous amounts of
rock and unconsolidated
material
Types of Mass Movements
 Geologists classify mass movements based on the
kind of material that moves, how it moves, and the
speed of movement
 Five kinds





Rockfalls
Slides
Slumps
Flows
Creep
Rockfalls
 A rockfall occurs when
rocks or rock fragments fall
freely through the air
 Common on slopes that are
too steep for loose material
to remain on the surface
Slides
 In a slide, a block of material moves suddenly along a flat,
inclined surface
 Slides that include segments of bedrock are called rockslides
 Often occur in high mountain areas
 Reaching speeds of over 124 mph
Slumps
 A slump is the downward movement of a block of
material along a curved surface
 As the block moves, its upper surface sometimes
tilts backward and leaves a crescent-shaped cliff
just above the slump
Flows
 Flows are mass movements of material containing a
large amount of water that move downslope as a
thick fluid
 Mudflows move quickly and carry a mixture of
soil, rock, and water that has a consistency of
wet concrete
 Earthflows move relatively slowly and carry
clay-rich sediment (They move about a millimeter
per day to several meters per day)
Creep
 Creep the slowest, downhill movement of
soil
and regolith (only travels a few millimeters
or centimeters per year)
 Alternates freezing and thawing


Freezing expands the water in the soil, lifting
soil particles at right angles to the slope
Thawing causes contraction, which allows the
particles to fall back to a slightly lower level
Creep
Review
 What is mass movement?
The transfer of rock and soil downslope due to gravity
 How does water trigger mass movements?
Water fills the pores in sediment, allowing the particles
to slide past one another more easily
 How does a rockfall differ from a rockslide?
In a rockfall, rocks or rock fragments fall freely through
the air. In a rockslide, a block that includes segments of
bedrock moves suddenly along a flat, inclined surface
Review
 What is the slowest type of mass movement?
Creep
 When highway engineers build a road in a
mountainous area, they insert drainage pipes
into the slopes alongside the road. Explain
why.
The pipes allow water to drain out of the soil and regolith in
the slopes, reducing the chance that a mass movement will
carry slope material onto the road.