Chapter 8, Section 1
EROSION BY GRAVITY
EROSION AND DEPOSITION

Erosion



Process that wears away surface
materials and moves them from one place
to another.
Erosion only occurs when the eroding
agent has enough energy to move
sediments.
Eroded sediments are not lost, but simply
recycled.
EROSION AND DEPOSITION

What wears
away
sediments?

Agents of erosion:

Gravity
Force of attraction that pulls objects towards
the center of the Earth.
 The greater the mass of an object, the greater
its gravitational pull.




Water
Wind
Glaciers
EROSION AND DEPOSITION

Dropping
Sediments

Deposition

Dropping of sediments by eroding agents
when they lose energy.
MASS MOVEMENT

Mass
movement



Any type of erosion that happens when gravity
moves materials downslope.
Can be very slow or very quick.
Usually occurs on steep slopes and after heavy
rains.


Water adds mass and creates a fluid pressure
between the grains and layers of sediment causing the
sediment to expand and weaken.
Examples:





Slump
Creep
Rockfalls
Rock slides
Mud flows
MASS MOVEMENT

Slump



Occurs when a mass of material slips
down along a curved surface.
Leaves a curved scar where the
slumped material originated.
Often occurs when:



The slope becomes so steep that the base
material cannot support the material above
it.
The base material is weaker than the
material above it (ex. clay)
Water moves to the base of a slipping mass
of sediment.
MASS MOVEMENT

Creep



Occurs when sediments slowly shift their
positions downhill.
Common in areas of freezing and
thawing.
Causes structures and trees to “lean”
uphill.
MASS MOVEMENT

Rockfalls


Occur when blocks of rock break
loose from a steep slope and tumble
through the air.
As the rocks move down the slope,
they break other rocks loose.
MASS MOVEMENT

Rock Slides



Occur when layers of rock slip
downslope suddenly.
Commonly occur on steep slopes in
mountainous areas or areas with
steep cliffs.
Happen most often after heavy rains
or earthquakes.
MASS MOVEMENT

Mudflows

Occur when heavy rains mix with
sediment causing the sediment to
become thick and pasty and then move
downhill under the force of gravity.



The materials of the mudflow are
deposited at the base of the slope…often
in a fan shape.
Usually occur in areas that have thick
layers of loose sediments.
Often happen after vegetation has
been removed by fire.
CONSEQUENCES OF EROSION

Building
on Steep
Slopes

Building on slopes can speed up erosion
by making the slope steeper or removing
vegetation.
CONSEQUENCES OF EROSION

Making Steep
Slopes Safe



Plant vegetation to anchor the soil
and absorb water.
Use drainage pipes or ceramic tiles
inserted in slopes to prevent the
build up of water.
Use walls of concrete or boulders to
hold soil in place.