Chapter 18 Glaciers and Glaciation

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Chapter 18 Glaciers and Glaciation
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Glaciers

• Glaciers are parts of two basic cycles
• Hydrologic cycle
• Rock cycle
• Glacier a thick mass of ice that originates on
  land from the accumulation, compaction and
  recrystallization of snow

NOTE Glaciers cover 10 of Earths land surface
today.
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Glaciers are powerful agents of erosion
Fig 18.1
Example Glacier National Park, Montana - a
classic glaciated landscape
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Glaciers

• Types of glaciers
• 1/ Valley (or alpine) glaciers
• Exist in mountainous areas
• Flow down a valley from an accumulation center at
  its head
• 2/ Ice sheets
• Exist on a larger scale than valley glaciers
• Two largest ice sheets on Earth are over
  Greenland and Antarctica

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Fig 18.2 Valley glacier in Alaska
Fig 18.3 The Greenland Antarctic Ice-sheets
(the Antarctic Ice-sheet occupies 14 million sq.
kms)
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Glaciers

• Types of glaciers
• Ice sheets
• Often called continental ice sheets
• Ice flows out in all directions from one or more
  snow accumulation centers
• Other types of glaciers
• Ice caps - e.g. Iceland
• Outlet glaciers - tongue of ice that enters the
  ocean
• Piedmont glaciers - broad sheet of ice in broad
  plain at base of mountains

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Glaciers

• What if all the ice on Earth melted?
• Slightly more than 2 of the worlds water is
  tied up in glaciers
• Antarctic ice sheet
• 80 of the worlds ice
• Nearly two-thirds of Earths fresh water
• Covers almost one and one-half times the area of
  the United States
• If melted, sea level would rise 60 to 70 meters

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Fig 18.6 Predicted coastline if current
ice-sheets melted
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Formation of glacial ice

• Glaciers form in areas where more snow falls in
  winter than melts during the summer
• Steps in the formation of glacial ice
• Air infiltrates snow, outer part evaporates
  central part contains vapor that condenses
• Snowflakes become smaller, thicker, and more
  spherical
• Air is forced out

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Formation of glacial ice

• Steps in the formation of glacial ice
• Snow is recrystallized into a much denser mass of
  small grains called firn
• Once the thickness of the ice and snow exceeds 50
  meters, weight forces firn to fuse into a solid
  mass of interlocking ice crystals glacial ice

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The transformation of snow to glacial ice
Fig 18.7
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Movement of glacial ice

• Overall movement is referred to as flow
• Two basic types
• Plastic flow
• Occurs within the ice
• Under pressure, ice behaves as a plastic material
  (ie. bonds between layers are weaker than within
  layers, stress applied and slide in response)
• Basal slip
• Entire ice mass slipping along the ground
• Meltwater acts as lubricant between ice rock
• Most glaciers are thought to move this way by
  this process

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Glaciers move by basal sliding and internal flow
Fig 18.8
NOTE Ice in Zone of fracture is carried along
piggy-back style
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Movement of glacial ice

• Overall movement is referred to as flow
• Zone of fracturing - in the uppermost 50 meters,
  ice acts in brittle manner and carried along by
  deeper ice
• Travel over irregular terrain causes tension
  large cracks (or crevasses) form in brittle ice
• Rates of glacial movement
• Average velocities vary considerably from one
  glacier to another but generally several meters
  /day

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Fig 18.9 Crevasse in the zone of fracturing
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NOTE Some glaciers exhibit extremely rapid
movements in short periods of time called surges,
growth is 20 to 50 times faster than normal.
E.g. surge of Variegated Glacier in Alaska,
1964-65
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Movement of glacial ice

• Budget of a glacier
• Zone of accumulation the area where a glacier
  forms

• Zone of wastage the area where there is a net
  loss to the glacier due to
• 1/ Melting
• 2/ Calving the breaking off of large pieces of
  ice (icebergs where the glacier has reached the
  sea)
• NOTE The snowline separates the zone of
  accumulation and the zone of wastage. Elevation
  of the snowline varies greatly depending on many
  factors.

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Movement of glacial ice

• Budget of a glacier
• Balance, or lack of balance, between accumulation
  at the upper end of the glacier, and loss at the
  lower end is referred to as the glacial budget
• If accumulation exceeds loss (or ablation), the
  glacial front advances
• If ablation increases and/or accumulation
  decreases, the ice front will retreat

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The glacial budget
Fig 18.11
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Glacial erosion

• Glaciers are capable of great erosion and
  sediment transport
• Glaciers erode the land primarily in two ways
• 1/ Plucking lifting of rocks
• 2/ Abrasion
• Rocks within the ice act like sandpaper to smooth
  and polish the surface below

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Fig 18.13 Glacial erratic dumped by retreating
glacier
Fig 18.14 Glacial striations (or grooves) and
polishing are distinctive glacial features
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Glacial erosion

• Glacial erosion
• Glacial abrasion produces
• Rock flour (pulverized rock)
• Glacial striations (grooves in the bedrock)
• Landforms created by glacial erosion
• Erosional features of glaciated U-shape valleys
• Glacial trough
• Truncated spurs
• Hanging valleys
• Cirques aretes
• Horn

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Glaciated topography
Fig 18.15 AB
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The Matterhorn in the Swiss Alps
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Glacial deposits

• Glacial drift refers to all sediments of
  glacial origin
• Types of glacial drift
• Till material that is deposited directly by the
  ice, a very poorly-sorted mix of debris
• Stratified drift sediments laid down by glacial
  meltwater

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Glacial till - typically unstratified and unsorted
Fig 18.19
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Glacial deposits

• Landforms made of till
• Moraines
• Layers or ridges of till
• Moraines produced by glaciers
• Lateral moraine
• Medial moraine
• Terminal moraine
• Ground moraine

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Glacial depositional features
Fig 18.25
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Glacial deposits

• Landforms made of till
• Drumlins
• Smooth, elongated, parallel hills of bedrock
• Steep side faces the direction from which the ice
  advanced
• Occur in clusters called drumlin fields

A drumlin in upstate New York
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Glacial deposits

• Landforms made of stratified drift
• Outwash plains (from ice sheets)
• Broad ramp-like surface composed of stratified
  drift deposited by meltwater leaving a glacier
• Located adjacent to the downstream edge of most
  end moraines
• Often pockmarked with depressions called
  kettle-holes (originally large blocks of stagnant
  ice)

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Glacial deposits

• Landforms made of stratified drift
• Ice-contact deposits
• Deposited by meltwater flowing over, within, and
  at the base of motionless ice
• Features include
• Kames (steep-sided hill or mound)
• Kame terraces (along sides of a valley)
• Eskers - long sinuous ridge of sand gravel

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Esker
Kame
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Glaciers of the past

• Last Ice Age - last 3 million yrs
• Four major stages recognized in North America
• Nebraskan
• Kansan
• Illinoian
• Wisconsinan
• Most recent Ice Age peaked 18 000 years ago and
  ice covered 30 of Earths land area

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Maximum extent of ice during the Ice Age
Fig 18.27
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Glaciers of the past

• Ice Age
• The Ice Age began between two million and three
  million years ago
• Most of the major glacial stages occurred during
  a division of geologic time called the
  Pleistocene epoch

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Glaciers of the past

• Indirect effects of Ice Age glaciers
• Forces migration of animals and plants
• Changes in stream courses
• Rebounding upward of the crust in former centers
  of ice accumulation
• Worldwide change in sea level
• Global climatic changes

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Crustal rebound following the removal of glacial
ice
Fig 18.28
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Causes of glaciation

• WHY? Any successful theory must account for what
  causes the onset of glacial conditions.

• Some possible causes of glaciation
• Plate tectonics
• Continents were arranged differently in the past
• Changes in oceanic circulation
• BEST THEORY The Milankovitch hypothesis
• Changes in climate over the past several hundred
  thousand years are closely associated with
  variations in the geometry of Earths orbit

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Causes of glaciation

• Some possible causes of glaciation
• Milankovitch hypothesis
• Shape (eccentricity) of Earths orbit varies
• Angle of Earths axis (obliquity) changes
• Earths axis wobbles (precession)
• All of these factors vary the amount of
  INSOLATION (incoming solar radiation)
• Other factors are probably also involved

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Fig 18.32A
Shape of Earths orbitals changes from spherical
to elliptical on a cycle of 100 000 years
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Fig 18.32B
The angle of tilt of the Earth varies from 22 to
25 on a cycle of 41 000 years. Today,
the angle of tilt on the axis of rotation is
23.5.
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Fig 18.32C
Precession The Earths axis wobbles like a
spinning top. Thus, the axis points to different
parts of the sky on a cycle of 26 000 years