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23.2 Weathering and Mass Movement
• A beach has sand
(most of them do
anyways). Where
did the sand come
from?
• Be specific as to
where and tell me
how sand is
formed.
23.2 Weathering and Mass Movement
Incredible rock formations
called hoodoos can be
seen at Bryce Canyon
National Park in Utah.
23.2 Weathering and Mass Movement
Erosion
What are the agents of erosion?
Erosion acts through weathering,
the force of gravity, and through
the movement of streams,
groundwater, glaciers, wind, and
waves.
23.2 Weathering and Mass Movement
Erosion
Erosion is the process that wears down and
carries away rock and soil.
• Erosion is the destructive process that
has shaped Earth’s surface over hundreds of
millions of years.
• The end product of erosion is
sediment.
• The opposite of erosion is
deposition.
23.2 Weathering and Mass Movement
Weathering
What causes mechanical and chemical
weathering?
Weathering is the process by which rocks
are chemically altered or physically broken
down into fragments at or near Earth’s
surface.
There are two forms of weathering:
mechanical and chemical. They cause rocks
to disintegrate or decompose.
23.2 Weathering and Mass Movement
Weathering
Mechanical Weathering
Mechanical weathering is the process of
physically breaking rock into smaller
fragments.
Mechanical weathering occurs through frost
wedging, abrasion, growth of plant roots, and
other processes.
23.2 Weathering and Mass Movement
Weathering
When water expands to form ice, it can pry open
cracks in rock. This is called frost wedging.
23.2 Weathering and Mass Movement
Weathering
Abrasion is a form of mechanical weathering
that occurs when rocks scrape or grind
against one another.
For example, abrasion occurs when sand
carried by water or wind causes bits of rock
to break off as if they were being
sandblasted.
23.2 Weathering and Mass Movement
Weathering
Mechanical weathering also occurs in other
ways.
• Plant roots can grow into cracks in a rock.
Roots exert a powerful force that can slowly pry
the rock apart.
• Erosion removes material from the surface of a
mass of rock, reducing pressure on the rock
that remains. The rock expands, causing the
outside of the rock to crack and flake off.
23.2 Weathering and Mass Movement
Weathering
Chemical Weathering
Chemical weathering is a process in
which rock is broken down by
chemical reactions.
• Chemical reactions dissolve the minerals making up
rock or change them into new minerals.
• Eventually, the rock crumbles and disintegrates.
23.2 Weathering and Mass Movement
Weathering
Water is the main agent of
chemical weathering.
• Water is an effective solvent.
• All minerals dissolve in water, though most do
so very slowly.
• Chemical weathering also occurs because rain
is slightly acidic.
23.2 Weathering and Mass Movement
Weathering
A. One form of chemical weathering occurs
through oxidation. Iron-rich minerals
become rusted like this old bicycle.
23.2 Weathering and Mass Movement
Weathering
B. Chemical weathering also occurs when
rainwater dissolves or reacts with the minerals
in rocks, as with this statue of a lion.
23.2 Weathering and Mass Movement
Rates of Weathering
What factors affect the rate at which rocks
weather?
The rate at which mechanical and
chemical weathering take place
depends on three main factors:
temperature, the availability of water,
and the type of rock.
23.2 Weathering and Mass Movement
Rates of Weathering
Chemical weathering occurs more rapidly
in places with high temperatures and
abundant rainfall. These conditions
generally speed up chemical reactions.
Mechanical weathering occurs faster in
places where temperature
conditions frequently alternate
between freezing and thawing.
23.2 Weathering and Mass Movement
Rates of Weathering
Some rocks, such as limestone and marble,
undergo rapid chemical weathering.
These rocks are composed primarily of
calcite, a mineral that reacts readily with
carbonic acid.
23.2 Weathering and Mass Movement
Rates of Weathering
Different rocks
weather at
different rates.
Even though
the slate
tombstone is
older, it is much
less weathered
than the marble
one (left).
23.2 Weathering and Mass Movement
Mass Movement
What force causes mass movement?
Through the process of mass
movement, gravity moves loose material
down a slope.
23.2 Weathering and Mass Movement
Mass Movement
Once weathering loosens particles of rock,
the particles, or sediment, do not stay in the
same place.
Mass movement is the
downward movement of rock
and soil due to gravity.
23.2 Weathering and Mass Movement
Mass Movement
Landslides
The rapid movement of large amounts of rock and
soil is a landslide.
Landslides often occur after heavy rains or after
earthquakes loosen materials on a steep slope.
23.2 Weathering and Mass Movement
Mass Movement
Mudflows
Rapid mass movements of soil and other
sediment mixed with water are called mudflows.
Mudflows tend to occur in areas where fine
sediment has collected in thick layers.
When it rains, water loosens the sediment and
increases its weight.
23.2 Weathering and Mass Movement
Mass Movement
Creep
Creep occurs when soil gradually moves down a
slope.
Creep often occurs because of the formation of
ice. Each time the ground freezes, the soil
expands outward.
Creep happens so slowly that it’s hard to notice.
23.2 Weathering and Mass Movement
Mass Movement
Creep can be caused by the alternate
freezing and thawing of water in soil.
Slumping often occurs when soil that is rich in
clay is soaked by water.
23.2 Weathering and Mass Movement
Mass Movement
Slumping
Slumping occurs when weak layers of soil or rock
suddenly move downslope as a single unit.
Gravity acting on water-saturated soil and rock
causes slumping. Slumping often leaves a curved
scar.
23.2 Weathering and Mass Movement
Assessment Questions
1. Which of these factors tends to increase the rate
of chemical weathering?
a.
b.
c.
d.
freezing temperatures
strong winds
abundant rainfall
presence of minerals such as quartz
23.2 Weathering and Mass Movement
Assessment Questions
1. Which of these factors tends to increase the rate
of chemical weathering?
a.
b.
c.
d.
freezing temperatures
strong winds
abundant rainfall
presence of minerals such as quartz
ANS: C
23.2 Weathering and Mass Movement
Assessment Questions
2. Which type of mass movement involves the
gradual movement of soil down a slope?
a.
b.
c.
d.
landslides
mudflows
creeps
slumping
23.2 Weathering and Mass Movement
Assessment Questions
2. Which type of mass movement involves the
gradual movement of soil down a slope?
a.
b.
c.
d.
landslides
mudflows
creep
lumping
ANS: C
23.2 Weathering and Mass Movement
Assessment Questions
1. Erosion breaks rocks down by two processes:
mechanical weathering and chemical weathering.
True
False
23.2 Weathering and Mass Movement
Assessment Questions
1. Erosion breaks rocks down by two processes:
mechanical weathering and chemical weathering.
True
False
ANS:
T