How Does Earth Work

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Glaciers Rivers of Ice
Valley Glaciers in Alaska
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Why study glaciers?

• Glaciers cover 10 of Earths surface - the
  polar ice sheets account for most of this.
• Approximately 85 of Earths fresh water is
  stored as glacial ice.
• There have been numerous ice ages in the past,
  what about the future?
• They respond to modern climate changes.
• In many areas agriculture takes advantage of the
  fertile soil deposited by glaciers during the
  last ice age.

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Why study glaciers?
Remember Only about 3 of Earths total water
budget is fresh water.
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Glaciers Rivers of Ice

• What is a glacier?
• What are the types of glaciers and how do they
  form?
• How do glaciers flow?
• How do glaciers erode, transport, and deposit
  sediment?
• How do glaciers modify the landscape?
• What causes the ice ages?

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What is a glacier?

• A glacier is simply an accumulation of snow and
  ice that is thick enough to flow downhill under
  its own weight.

A valley glacier in the Swiss Alps.
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What are the types of glaciers and how do they
form?

• There are three types of glaciers
• Valley glaciers - long, narrow glaciers that are
  confined to valleys and flow downhill much like
  streams.
• Ice sheets - unconfined glaciers larger than
  50,000 km2. There are two ice sheets on Earth
  today, they are in Greenland and Antarctica -
  these are continental scale glaciers.
• Ice caps - unconfined glaciers that are smaller
  than ice sheets. Both ice sheets and ice caps may
  exhibit radial flow and have valley glaciers
  exiting them.

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What are the types of glaciers and how do they
form?

• Where do glaciers form?
• Anywhere snow persists all year, thus
  cold-climate areas, or high altitudes.
• High altitudes where temperatures are colder than
  the surrounding countryside.
• High latitudes where seasons are colder.
• Heavy yearly winter snowfall is essential.
• Snowline the elevation above which snow persists
  throughout the year.

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What are the types of glaciers and how do they
form?
Valley Glaciers in Alaska
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What are the types of glaciers and how do they
form?
Valley glaciers are usually 50-300 meters thick.
Fig 18.3
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What are the types of glaciers and how do they
form?
The Antarctic ice sheet glacier is up to 4 km
thick and covers the entire continent.
Fig 18.3
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What are the types of glaciers and how do they
form?
Ice cap glaciers are much smaller and cover,
e.g., local mountainous areas.
Fig 18.3
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What are the types of glaciers and how do they
form?

• Snow metamorphism snow becomes ice, i.e. it
  metamorphoses, at temperatures and pressures on
  the Earths surface.
• Snowflakes compact and turn into rounded ice
  grains due to the weight of snow accumulating
  year-round.
• Rounded ice grains continually recrystallize into
  denser ice crystals over time due to burial and
  increased pressure. Pore space is diminished.
• The result is interlocking crystals with a
  density of 0.9 gm/cm3 (close to that of water).

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What are the types of glaciers and how do they
form?
Fig 18.5
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How do glaciers flow?

• Glacial ice behaves like weak rock.
• Ice has cleavage (much like micas) and when
  stressed, ice crystals slip along the cleavage
  planes by plastic deformation.

• This occurs at a few tens of meters depth,
  typically 50 m.

• The upper reaches are brittle.
• Thus, there is a brittle-ductile transition in
  glaciers. The lower parts flow like plastic.

Fig 18.6
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How do glaciers flow?
lt50 m brittle fracture and crevasse formation
gt50 m internal flow and plastic deformation
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How do glaciers flow?
At higher elevations - the zone of accumulation,
where winter accumulation exceeds summer
melting. At lower elevations - the zone of
wastage, where summer melting exceeds
accumulation. The snowline separates the zone of
accumulation from the zone of wastage.
Fig 18.7
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How do glaciers flow?
Glaciers flow from the zone of accumulation to
the zone of wastage due to mass increase in the
zone of accumulation. If accumulation exceeds
wastage then glaciers advance. If wastage exceeds
accumulation then glaciers retreat.
Fig 18.8
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How do glaciers flow?
Glaciers move fastest in the center, away from
the bottom and sides.
Glaciers may either slide along at their base, or
not. Plastic flow dominates in the latter case.
Fig 18.9
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How do glaciers flow?

• The bottom temperature of a glacier controls
  whether it slides or not, and sliding equals more
  erosion.
• If the base is frozen solidly to the rock then
  little motion can occur along the base. Motion
  is accomodated by plastic flow.
• If the base is at or near the melting
  temperature, then liquid water may exist there.
• Water acts as a lubricant, reducing cohesion and
  friction.
• Water is pressurized by the weight of the
  overlying ice. Water pressure at the base of some
  glaciers is nearly equal to the weight of the
  overlying ice. The glacier basically slides
  along on this thin layer of water.

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How do glaciers flow?
In the upper brittle zone crevasses may form.
These form where the glacier overrides higher
areas of bedrock.
Crevasses are the surface expression of changing
velocities.
Fig 18.10
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How do glaciers erode, transport, and deposit
sediment?

• Glaciers are rivers of ice, they flow downhill,
  eroding, transporting and ultimately depositing
  sediment.
• Glacial landforms are produced by these
  processes.
• Glaciers exert very large shear stress on the
  rocks they flow over due to the very large mass
  of thick ice.
• Thus, the erosional capacity of glaciers is many
  times greater than that of even the largest
  rivers.
• Freeze-thaw cycles cause glacial ice to attach to
  rock and pluck it off as the ice moves downhill.
• Plucked rock fragments act as an abrasive in the
  ice, further eroding the rock the glacier passes
  over. Glacial striations.

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How do glaciers erode, transport, and deposit
sediment?
Fig 18.12
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How do glaciers erode, transport, and deposit
sediment?
Moraines are formed by the bulldozing action of
moving glaciers.
Fig 18.15
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How do glaciers erode, transport, and deposit
sediment?
Till is sediment directly deposited by the
glacier. Cobbles are often have edges and
striations.
Outwash is sediment carried away by meltwater
from a glacier.
Fig 18.16
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Glacial Till Forms Moraines
Large Lateral Moraines - Sierra Nevada Mountains
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How do glaciers modify the landscape?

• In some areas glaciers are the dominant force for
  creating the landscape, especially in high, cold,
  mountainous regions and high latitude areas.
• Ice-age glaciers produced many of the landscape
  features over large areas of North America and
  northern Europe in areas that are now ice-free.

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• The five main glacial landscape features.
• Moraines a depositional feature already
  discussed.
• Important erosional features are
• U-shaped valleys
• Cirques
• Knife edge ridges and pointed peaks
• Hanging valleys

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How do glaciers modify the landscape?
Fig 18.26
A view of Alaska's Athabasca Glacier.
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How do glaciers modify the landscape?
When sea level rises into a U-shaped valley, the
result is a fjord.
A U-shaped valley.
Fig 18.27
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Fig 18.28
How a U-shaped valley is produced. Erosion is
highest in the center where velocity is the
fastest and ice thickness the greatest, resulting
in the U-shaped profile over time.
Fig 11.29
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Hanging valleys are produced by more rapid
erosion in the main glacier channel versus
tributary channels, causing a large vertical
difference between the channels once the glaciers
melt.
Fig 18.30
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What causes ice ages?

• The most likely explanation variations in
  Earths orbital parameters.
• The poles receive more summer sun and less winter
  sun when the axial tilt angle is high. The tilt
  changes with time.
• The Earth-Sun distance changes due to variations
  in the eccentricity of Earths orbit.
• The tilt and eccentricity, along with another
  parameter called precession, occur over different
  timescales, and thus may have an additive or
  subtractive effect (modulation).

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What causes ice ages?
Variation in axial tilt occurs over periods of
41,000 years.
Fig 18.46
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What causes ice ages?
Variation in orbital eccentricity occurs in
periods of 413,000 years.
Fig 18.46
Another orbital parameter called precession has a
period of 23,000 years.
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What causes ice ages?
The timing and relative effect of orbital
variations (Milankovich cycles) and their
relation to the ice age record.
Fig 18.47
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What About More Recent Warming?
The Portage Glacier in Alaska has receeded
dramatically in the past 100 years.
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What About More Recent Warming?
This is seen in glacial landscapes worldwide, it
is not a local event. These changes are
occurring over periods of lt100 years, much
quicker than glacial cycles driven by Earths
orbital cycles.