Transcript Chapter 8

Table of Contents
Chapter: Erosional Forces
Section 1: Erosion by Gravity
Section 2: Glaciers
Section 3: Wind
Erosion by Gravity
1
Erosion and Deposition—
What wears away sediments?
• Erosion is a process that wears away surface
materials and moves them from one place to
another.
• An important erosional force is gravity.
• Gravity is the force of attraction that pulls all
objects toward Earth’s center.
• Other causes of erosion, also called agents of
erosion, are water, wind, and glaciers.
Erosion by Gravity
1
Erosion and Deposition—
What wears away sediments?
• Water and wind erode materials only when
they have enough energy of motion to do
work. A strong wind can move dust and
even larger particles.
• Glacial erosion works by slowly moving
sediment that is trapped in solid ice.
• As ice melts, sediment is deposited, or
dropped.
Erosion by Gravity
1
Dropping Sediments
• Agents of erosion drop the sediments they
are carrying as they lose energy. This is
called deposition.
• When sediments are eroded, they are not lost
from Earth—they are just relocated.
Erosion by Gravity
1
Mass Movement
• Rocks and other materials, especially on
steep slopes, are pulled toward the center of
Earth by gravity.
• A mass movement is any type of erosion that
happens as gravity moves materials
downslope.
Erosion by Gravity
1
Mass Movement
• Common types of
mass movement
include slump,
creep, rockfalls,
rock slides, and
mudflows.
• Landslides are
mass movements that can be one of these
types or a combination of these types of
mass movement.
Erosion by Gravity
1
Slump
• When a mass of material slips down along a
curved surface, the mass movement is called
slump.
Erosion by Gravity
1
Slump
• Sometimes a slump happens when water
moves to the base of a slipping mass of
sediment.
• Or, if a strong rock layer lies on top of a
weaker layer—commonly clay—the clay can
weaken further under the weight of the rock.
• A curved scar is left where the slumped
materials originally rested.
Erosion by Gravity
1
Creep
• Leaning trees and human-built structures
show another mass movement called creep.
• Creep occurs when sediments slowly shift
their positions downhill.
• Creep is common in areas of frequent
freezing and thawing.
Erosion by Gravity
1
Rockfalls and Rock Slides
• Rockfalls happen when blocks of rock break
loose from a steep slope and tumble through
the air.
• As they fall, these rocks crash into other rocks
and knock them loose.
• More and more rocks break loose and tumble
to the bottom.
Erosion by Gravity
1
Rockfalls and Rock Slides
• During the winter, when ice freezes in the
cracks of rocks, the cracks expand and extend.
• In the spring,
the pieces of
rock break
loose and fall
down the
mountainside.
Erosion by Gravity
1
Rockfalls and Rock Slides
• Rock slides occur when layers of rock—
usually steep layers—slip downslope
suddenly.
• They commonly
occur on
mountainous
areas or in areas
with steep cliffs.
Erosion by Gravity
1
Mudflows
• A mudflow is a thick
mixture of sediments
and water flowing
down a slope.
Erosion by Gravity
1
Mudflows
• Mudflows usually occur in areas that have
thick layers of loose sediments.
• When heavy rains fall on these areas, water
mixes with sediment, causing it to become
thick and pasty.
• Gravity causes this mass to flow downhill.
• When a mudflow finally reaches the bottom
of a slope, it loses its energy of motion and
deposits all the sediment and everything else
it has been carrying.
Erosion by Gravity
1
Mudflows
• Mudflows, rock slides, rockfalls, creep, and
slump are most likely to occur on steep slopes,
and they all depend on gravity to make them
happen.
• Also, all types of mass movement occur more
often after a heavy rain.
• The water adds mass and creates fluid pressure
between grains and layers of sediment.
Erosion by Gravity
1
Consequences of Erosion—
Building on Steep Slopes
• When people build homes on steep slopes,
they constantly must battle naturally
occurring erosion.
• Some steep slopes are prone to slumps
because of weak sediment layers underneath.
Erosion by Gravity
1
Making Steep Slopes Safe
• One of the best ways to reduce erosion is to
plant vegetation.
• Deep trees roots and fibrous grass roots
bind soil together, reducing the risk of mass
movement.
• Plants also absorb large amounts of water.
• Drainage pipes or tiles inserted into slopes
can prevent water from building up, too.
• Walls made of concrete or boulders also can
reduce erosion by holding soil in place.
Section Check
1
Question 1
What is erosion?
Answer
Erosion is a process that wears away
surface materials and moves them from
one place to another.
Section Check
1
Question 2
What type of mass movement
is shown in this illustration?
A. creep
B. deposition
C. mudflow
D. slump
Section Check
1
Answer
The answer is D. If a slope becomes too steep,
the base material may not be able to support
the rock and soil above it. Slump occurs when
material slips down slope as one large mass.
Section Check
1
Question 3
How does a rock slide differ from creep?
Section Check
1
Answer
Rock slides occur suddenly, are fast and move
layers of rock
down steep
slopes. Creep is
the slow shifting
of position of
sediments on a
slope.
Glaciers
2
How Glaciers Form and Move
• Glaciers form in regions where snow
accumulates.
• When snow doesn’t melt, it piles up.
• As it accumulates slowly, the increasing
weight of the snow becomes great enough
to compress the lower layers into ice.
Glaciers
2
How Glaciers Form and Move
• The mass slowly begins to flow in a thick,
plasticlike lower layer, and ice slowly moves
away from its source.
• A large mass
of ice and
snow moving
on land under
its own weight
is a glacier.
Glaciers
2
Ice Eroding Rock
• Glaciers are agents of erosion.
• As glaciers pass over land, they erode it,
changing features
on the surface.
• Glaciers then
carry eroded
material along
and deposit it
somewhere else.
Click image to view movie.
Glaciers
2
Plucking
• When glacial ice melts, water flows into
cracks in rocks.
• Later, the water refreezes in the cracks,
expands, and fractures the rock.
• Pieces of rock then are lifted out by the ice.
Glaciers
2
Plucking
• This process, called plucking, results in
boulders, gravel, and sand being added to
the bottom
and sides of
a glacier.
Glaciers
2
Transporting and Scouring
• A glacier can transport huge volumes of
sediment and rock.
• Plucked rock fragments and sand at its base
scour and scrape the soil and bedrock, eroding
the ground below even more.
• When bedrock is gouged deeply by rock
fragments being dragged along, marks are
left behind.
Glaciers
2
Transporting and Scouring
• These marks, called
grooves, are deep, long,
parallel scars on rocks.
• Shallower marks are
called striations.
Glaciers
2
Ice Depositing Sediment
• When glaciers begin to melt, sediment drops
or is deposited, on the land.
• When a glacier melts and begins to shrink
back, it is said to retreat.
• As it retreats, a jumble of boulders, sand,
clay, and silt is left behind.
• This mixture of different-sized sediments is
called till.
Glaciers
2
Moraine Deposits
• Till is also deposited at the end of glacier
when it is not moving forward.
• Rocks and soil are moved to the end of the
glacier, much like items on a grocery store
conveyor belt.
• Because of this, a big ridge of material piles
up.
• Such a ridge is called a moraine.
Glaciers
2
Outwash Deposits
• Material deposited by the meltwater from a
glacier, most often beyond the end of the
glacier, is called outwash.
• Meltwater carries sediments and deposits
them in layers.
Glaciers
2
Eskers
• Another type of outwash deposit looks
like a long, winding ridge.
• This deposit forms in a melting glacier
when meltwater forms a river within ice.
Glaciers
2
Eskers
• This river carries sand and gravel and
deposits them within its channel.
• When the glacier melts, a winding ridge of
sand and gravel, called an esker, is left behind.
Glaciers
2
Continental Glaciers
• Today, continental glaciers cover ten percent
of Earth, mostly near the poles in Antarctica
and Greenland.
• Continental glaciers are huge masses of ice
and snow.
• Continental glaciers are thicker than some
mountain ranges.
Glaciers
2
Climatic Changes
• In the past, continental glaciers covered as
much as 28 percent of Earth.
Glaciers
2
Climatic Changes
• These periods of widespread glaciation
are known as ice ages.
• During this time, glaciers advanced and
retreated many times over much of North
America.
• The average air temperature on Earth was
about 5°C lower during these ice ages than
it is today.
Glaciers
2
Valley Glaciers—Evidence of
Valley Glaciers
• Valley glaciers erode bowl-shaped basins,
called cirques, into the sides of mountains.
• If two valley
glaciers side by side
erode a mountain, a
long ridge called an
arête forms between
them.
Glaciers
2
Valley Glaciers—Evidence of
Valley Glaciers
• If valley glaciers erode a mountain from
several directions,
a sharpened peak
called a horn might
form.
Glaciers
2
Valley Glaciers—Evidence of
Valley Glaciers
• Valley glaciers flow down mountain slopes
and along valleys,
eroding as they go.
Glaciers
2
Valley Glaciers—Evidence of
Valley Glaciers
• Glacially eroded valleys are U-shaped because
a glacier plucks and scrapes soil and rock from
the sides as well as
from the bottom.
Glaciers
2
Importance of Glaciers
• Today, glaciers in polar regions and in
mountains, continue to change the surface
features of Earth.
• In addition to changing the appearance of
Earth’s surface, glaciers leave behind
sediments that are economically important.
• The sand and gravel deposits from glacial
outwash and eskers are important resources.
Section Check
2
Question 1
The mixture of different-sized sediments left
by a retreating glacier is called a __________.
A. moraine
B. outwash
C. pluck
D. till
Section Check
2
Answer
The answer is D. The sediments in outwash are
deposited in layers, with the heaviest sediments
dropping first.
Section Check
2
Question 2
__________ are winding ridges of sand and
gravel deposited by glacial meltwater streams.
A. aretes
B. cirques
C. eskers
D. moraines
Section Check
2
Answer
The answer is C. Eskers are glacial deposits
formed by meltwater rivers and streams.
Section Check
2
Question 3
In which country would you expect to find
continental glaciers?
A. Australia
B. Denmark
C. Greenland
D. South Africa
Section Check
2
Answer
The answer is
C. Continental
glaciers are
found mostly
near the poles
in Antarctica
and Greenland.
Wind
3
Wind Erosion
• Air differs from other erosional forces
because it usually cannot pick up heavy
sediments.
• Wind carries and deposits sediments over
large areas.
Wind
3
Deflation
• Wind erodes Earth’s surface by deflation and
abrasion.
• When wind erodes by deflation, it blows
across loose sediment, removing small
particles such as silt and sand.
Wind
3
Abrasion
• When windblown sediment strikes rock,
the surface of the rock gets scraped and
worn away by a process called abrasion.
• Wind acts like a sandblasting machine,
bouncing and blowing sand grains along.
• The rocks becomes pitted and are worn
down gradually.
Wind
3
Abrasion
• These sand grains strike against rock and
break off small fragments.
• The rocks
becomes
pitted and
are worn
down
gradually.
Wind
3
Abrasion
• Deflation and abrasion happen to all land
surfaces but occur mostly in deserts,
beaches, and plowed fields.
• These areas have fewer plants to hold the
sediments in place.
• When winds blow over them, they can be
eroded rapidly.
Wind
3
Sandstorms
• When the wind blows forcefully in the
sandy parts of deserts, sand grains bounce
along and hit other sand grains, causing
more and more grains to rise into the air.
• These windblown sand grains form a low
cloud just above the ground.
Wind
3
Dust Storms
• Silt and clay particles are small and stick
together.
• A faster wind is needed to lift these fine
particles of soil than is needed to lift grains
of sand.
• However, after they are airborne, the wind
can carry them long distances.
• Where the land is dry, dust storms can cover
hundreds of kilometers.
Wind
3
Dust Storms
• These storms blow topsoil from open fields,
overgrazed areas, and places where vegetation
has disappeared.
Wind
3
Reducing Wind Erosion—
Windbreaks
• One of the best ways to slow or stop wind
erosion is to plant vegetation.
• For centuries, farmers have planted trees
along their fields to act as windbreaks that
prevent soil erosion.
• As the wind hits the trees, its energy of
motion is reduced.
• It no longer is able to lift particles.
Wind
3
Roots
• Plants with fibrous systems, such as grasses,
work best at stopping wind erosion.
• Grass roots are shallow and slender with
many fibers.
• They twist and turn between particles in the
soil and hold it in place.
Wind
3
Deposition by Wind—Loess
• Wind deposits of fine-grained sediments are
known as loess.
• Strong winds that blew across glacial outwash
areas carried the sediments and deposited
them.
Wind
3
Deposition by Wind—Loess
• The sediments settled on hilltops and in
valleys.
• Once there, the particles packed together,
creating a thick, unlayered, yellowish-browncolored deposit.
• Loess is as fine as talcum powder.
Wind
3
Dunes
• A dune is a mound of
sediments drifted by
the wind.
• Dunes are common in
desert regions.
Wind
3
Moving Dunes
• A sand dune has two sides.
• The side facing the wind has a gentler slope.
• The side away from the wind is steeper.
• Most dunes move, or migrate away from the
direction of the wind.
• As they lose sand on one side, they build it
up on the other.
Wind
3
Dune Shape
• The shape of a dune depends on the amount
of sand or other sediment available, the wind
speed and direction, and the amount of
vegetation present.
• One common dune shape is a crescent-shaped
dune known as a barchan dune.
• This type of dune forms on hard surfaces
where the sand supply is limited.
Wind
3
Dune Shape
• Another common type of dune, called a
transverse dune, forms where sand is
abundant.
• In regions
where the
wind changes,
star dunes,
form pointed
structures.
Wind
3
Shifting Sediments
• When dunes and loess form, the landscape
changes.
• Wind, like gravity, running water, and
glaciers, shapes the land.
• New landforms created by these agents of
erosion are themselves being eroded.
• Erosion and deposition are part of a cycle of
change that constantly shapes and reshapes
the land.
Section Check
3
Question 1
__________ is erosion by wind in which
small particles are removed, leaving heavier
material behind.
A. abrasion
B. deflation
C. glaciation
D. loess
Section Check
3
Answer
The answer is B. When wind erodes by
deflation, it blows across loose sediment
and removes small particles.
Section Check
3
Question 2
Mounds of sediment drifted by the wind are
called __________.
A. dunes
B. loess
C. shifters
D. windbreaks
Section Check
3
Answer
The answer is A. Dunes form when wind sweeps
around an obstacle and drops sediment behind it.
Section Check
3
Question 3
Large wind deposits of fine grained sediments
are known as __________.
A. dunes
B. loess
C. outwash
D. till
Section Check
3
Answer
The answer is B. Loess deposits are made up
of tightly-packed, fine-grained sediments.
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