Agents of Erosion and Deposition

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Agents of Erosion and Deposition

• Chapter 12

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Shoreline Erosion and Deposition

• Chapter 12
• Section 1

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Water The Major Agent of Erosion

• What Is Erosion? Erosion is the process by which
  soil and sediment are transported from one
  location to another.
• Water and Erosion Water is a major agent of
  erosion. Rivers often carry eroded materials long
  distances.

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Shoreline Erosion and Deposition
The results of erosion can often be dramatic. For
example, this sinkhole formed in a parking lot in
Atlanta, Georgia, when water running underground
eventually caused the surface of the land to
collapse.

• Section 1

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Wave Energy

• When waves crash into rocks over long periods of
  time, the rocks are broken down into smaller and
  smaller pieces until they become sand.
• Waves usually play a major role in building up
  and breaking down the shoreline. A shoreline is
  the boundary between land and a body of water.

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Wave Energy, continued 2

• As the wind moves across the ocean surface, it
  produces ripples called waves. The size of a wave
  depends on how hard the wind is blowing, the
  distance over which it blows (fetch) and how long
  the wind blows.
• The wind that results from summer hurricanes and
  severe winter storms produces large waves that
  cause dramatic shoreline erosion.

This photo, taken while the Virginia Beach
Erosion Control and Hurricane Protection Project
was underway, shows the significant difference
between the unimproved area (top of photo) and
the area of the widened beach berm already
completed.
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Wave Energy, continued 3

• Wave Trains Waves travel in groups called wave
  trains. When wave trains reach shallow water, the
  bottom of the wave drags against the sea floor,
  slowing the wave down.
• The upper part of the wave moves more rapidly
  and grows taller, and begins to curl and break.
  These breaking waves are known as surf.
• The time interval between breaking waves is
  called the wave period.

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Section 1 Shoreline Erosion and Deposition
Chapter 12
Wave Period of Ocean Waves
Click below to watch the Visual Concept.
Visual Concept
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Wave Energy, continued4

• The Pounding Surf Tremendous energy is released
  when waves break. Crashing waves can break solid
  rock and throw broken rocks back against the
  shore.
• Breaking waves also wash away fine grains of
  sand, which are picked up by the waves and wear
  down and polish coastal rock.
• The process continues until rock is broken down
  in smaller and smaller pieces that eventually
  become sand.

Breaking waves crash against the rocky shore,
releasing their energy.
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Wave Erosion

• Shaping a Shoreline Wave erosion produces a
  variety of features along a shoreline. Much of
  the erosion responsible for coastal landforms
  takes place during storms.
• Sea cliffs are formed when waves erode and
  undercut rock to produce steep slopes.
• The next two slides show some of the major
  features that result from wave erosion.

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Wave Deposits

• Beaches are areas of the shoreline made up of
  material deposited by waves. Some beach material
  is also deposited by rivers.
• Waves carry a variety of materials, including
  sand, rock fragments, dead coral, and shells.
• The colors and textures of beaches vary because
  the type of material found on a beach depends on
  its source.

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Wave Deposits, continued2

• Wave Angle and Sand Movement Waves moving at an
  angle to the shoreline push water along the shore
  and create longshore currents.
• Longshore currents move sand in a zigzag pattern
  along the beach.

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Wave Deposits, continued3

• Offshore Deposits When waves erode material from
  the shoreline, longshore currents can transport
  and deposit the material offshore, which creates
  landforms in open water.
• A sandbar is an underwater or exposed ridge of
  sand, gravel, or shell material.
• A barrier spit is an exposed sandbar connected
  to the shoreline.

A barrier spit, such as Cape Cod, Massachusetts,
occurs when an exposed sandbar is connected to
the shoreline.
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Wind Erosion and Deposition

• Chapter 12
• Section 2

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Wind Erosion

• Normally the weakest, slowest type of erosion.
• Over 1000s of years however, winds can move vast
  quantities of sediment, and grind down even the
  strongest rock.

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The Process of Wind Erosion

• Saltation is the skipping and bouncing movement
  of sand or other sediments, caused by wind or
  water.
• Moving sand grains knock into one another,
  bounce up into the air, fall forward, and strike
  other sand grains, causing them to roll and
  bounce forward.

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The Process of Wind Erosion, continued 2

• Deflation is a form of wind erosion in which
  fine, dry soil particles are blown away, removing
  the top layer of fine sediment or soil and
  leaving behind rock fragments that are too heavy
  to be lifted by the wind.
• Deflation may cause desert pavement, which is a
  surface consisting of pebbles and small broken
  rock.
• Scooped-out depressions in the landscape are
  called deflation hollows.

Desert pavement, such as that found in the
Painted Desert in Arizona, forms when wind
removes all the fine materials.
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The Process of Wind Erosion, continued3

• Abrasion is the grinding and wearing away of rock
  surfaces through the mechanical action of other
  rock or sand particles.
• Abrasion commonly happens in areas where there
  are strong winds, loose sand, and soft rocks.
• The blowing of millions of sharp sand grains
  creates a sandblasting effect, helping erode,
  smooth, and polish rocks.

Picture shows the powerful effect of wind
generated abrasion is the Double Arch from Arches
National Park.
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Wind-Deposited Materials

• Loess is a deposit of windblown, fine-grained
  sediment. Usually formed during glacial periods.
• Because wind can carry fine-grained material
  much higher and farther than it carries sand,
  loess deposits are sometimes found far from their
  source.
• Very rich and fertile.

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Loess
Section 2 Wind Erosion and Deposition
Chapter 12
Click below to watch the Visual Concept.
Visual Concept
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Wind-Deposited Materials, continued 2

• Dunes When the wind hits an obstacle, the wind
  slows down, depositing the heavier material. The
  material collects, creating an additional
  obstacle and eventually forming a mound that
  buries the original obstacle.
• The mounds of wind-deposited sand are called
  dunes. A dune keeps its shape, even though it
  moves.
• Dunes move in the same direction the wind does.

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Wind-Deposited Materials, continued 3

• The Movement of Dunes Different wind conditions
  produce dunes in various shapes and sizes. A dune
  usually has a gently sloped side and a steeply
  sloped side, called a slip face.

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Erosion and Deposition by Ice

• Chapter 12
• Section 3

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GlaciersRivers of Ice

• A glacier is a large mass of moving ice. They are
  capable of eroding, moving, and depositing large
  amounts of rock materials.
• Glaciers form in areas so cold that snow stays
  on the ground year-round. Because glaciers are so
  massive, the pull of gravity causes them to flow
  slowly, like rivers of ice.

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GlaciersRivers of Ice, continued 2

• Alpine Glaciers form in mountainous areas. One
  common type of alpine glacier is a valley
  glacier.
• Valley glaciers form in valleys originally
  created by stream erosion. As these glaciers
  slowly flow downhill, they widen and straighten
  the valleys into broad U shapes.

Valley Glacier a valley flowing glacier. These
glaciers may be the combination of several
smaller glaciers joining and flowing together
down a large valley.
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GlaciersRivers of Ice, continued 3

• Continental Glaciers are huge, continuous masses
  of ice that can spread across entire continents.
• The largest continental glacier in the world
  covers almost all of Antarctica. This ice sheet
  is approximately one and a half times the size of
  the United States, and is more than 4,000 m thick
  in some places.

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GlaciersRivers of Ice, continued 4

• Glaciers on the Move When enough ice builds up
  on a slope, the ice begins to move downhill.
  Thick glaciers move faster than thin glaciers,
  and the steeper the slope, the faster the
  glaciers will move.
• Glaciers move in two ways sliding and flowing.
  A glacier slides when its weight causes the ice
  at the bottom to melt. A glacier flows as ice
  crystals within the glacier slip over each other.

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Section 3 Erosion and Deposition by Ice
Chapter 12
Movement of Glaciers
Click below to watch the Visual Concept.
Visual Concept
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GlaciersRivers of Ice, continued 5

• Glacier movement is affected by climate. As the
  Earth cools, glaciers grow. About 10,000 years
  ago, a continental glacier covered most of North
  America.

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Landforms Carved by Glaciers

• Continental glaciers and alpine glaciers produce
  landscapes that are very different from one
  another.
• Continental glaciers smooth the landscape by
  scraping and eroding features that existed before
  the ice appeared.
• Alpine glaciers carve out large amounts of rock
  material and create spectacular landforms.

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Landforms Carved by Glaciers 2
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Glacial Landscape Features
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Types of Glacial Deposits

• As a glacier melts, it drops all the material it
  is carrying. Glacial drift is the general term
  used to describe all material carried and
  deposited by glaciers.
• Glacial drift is divided into two main types,
  till and stratified drift.

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Types of Glacial Deposits, continued 2

• Till Deposits Unsorted rock material that is
  deposited directly by the ice when it melts is
  called till. Unsorted means that the till is made
  up of rock material of different sizes.
• The most common till deposits are moraines.
  Moraines generally form ridges along the edges of
  glaciers.

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Moraines
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Types of Glacial Deposits3

• Stratified drift is a glacial deposit that has
  been sorted and layered by the action of streams
  or meltwater.
• Streams carry sorted material and deposit it in
  front of the glacier in a broad area called an
  outwash plain.
• Sometimes, a block of ice is left in an outwash
  plain when a glacier retreats. As the ice melts,
  sediment builds up around the block of ice,
  forming a depression called a kettle.

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Section 3 Erosion and Deposition by Ice
Chapter 12
Glacial Drift Stratified Drift and Till
Click below to watch the Visual Concept.
Visual Concept
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The Effect of Gravity on Erosion and Deposition

• Chapter 12
• Section 4

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Angle of Repose

• Gravity is an agent of erosion and deposition. It
  influences the movement of water and ice, and it
  causes rocks and soil to move downslope.
• Mass movement is the movement of any material,
  such as rock, soil, or snow, downslope.

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Angle of Repose, continued 2

• Material such as rock, soil, or snow moves
  downhill until the slope becomes stable. The
  angle of repose is the steepest angle at which
  loose material will not slide downslope.
• The angle of repose is different for different
  surface material. Size, weight, shape, and
  moisture level determine at what angle material
  will move down-slope.

If the slope on which material rests is less than
the angle of repose, the material will stay in
place. If the slope is greater than the angle of
repose, the material will move downslope.
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Rapid Mass Movement

• Rock falls happen when loose rocks fall down a
  steep slope. The rocks can range in size from
  small fragments to large boulders.
• Mass movements, like rock falls, happen suddenly
  and rapidly, and can be very dangerous.

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Rapid Mass Movement 2

• Landslides are sudden and rapid movements of a
  large amount of material downslope.
• .

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Slumps

• The most common type of landslide is a slump.
  Slumping occurs when a block of land becomes
  detached and slides downhill

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Rapid Mass Movement 3

• Mudflows are rapid movements of large masses of
  mud. Mudflows happen when a large amount of water
  mixes with soil and rock. The water causes the
  slippery mass of mud to flow rapidly downslope.
• Mudflows commonly happen in mountainous regions
  when a long dry season is followed by heavy
  rains.

Mudflow-damaged house along the Toutle River. The
height of the mudflow is shown by the
"bathtub-ring" mudlines seen on the tree trunks
and the house itself. Caused by eruption of Mt.
St. Helens May 18, 1980.
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Rapid Mass Movement 4

• Lahars are mudflows caused by volcanic eruptions
  or heavy rains on volcanic ash. Lahars can travel
  at speeds grater than 80 km/h and can be as thick
  as cement.
• On volcanoes with snowy peaks, an eruption can
  suddenly melt a great amount of ice. Water from
  the ice liquefies the soil and volcanic ash to
  produce a hot mudflow that rushes downslope.

A lahar overtook this area on the island of
Kyushu in Japan.
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Slow Mass Movement

• Creep is the slow mass movement of material
  downslope.
• Although rapid mass movements are visible and
  dramatic, slow mass movements happen a little at
  a time. However, slow mass movements occur more
  frequently, and more material is moved
  collectively.

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Creep
Section 4 The Effect of Gravity on Erosion and
Deposition
Chapter 12
Click below to watch the Visual Concept.
Visual Concept