Tearing Down Mountains II: Glaciers, Glaciation,

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Tearing Down Mountains IIGlaciers, Glaciation,
Ice Ages

• Presented by Dr. Sridhar Anandakrishnan
• The Pennsylvania State University

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Glaciers and Ice Ages

• What glaciers are
• Why ice ages (and what evidence for them)
• Erosion by glaciers

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Glaciers

• A mass of snow and ice that deforms and moves
• Glaciers form where snowfall exceeds melting,
  known as the accumulation zone
• Usually in high, cold mountains and cold
  North/South
• But also possible if there is huge amount of
  snowfall, like in the Olympic Mountains
• If it is cold, but little snowfall, the
  groundremains frozen Permafrost

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Glacier Movement

• Glaciers move and deform because of gravity

Greater Force
Less Force
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Glacier Movement

• Glaciers move from where their surface is high to
  where their surface is low
• They can even move in an uphill direction
• If you pour pancake batter onto a griddle, it can
  lap up onto the raised edges...

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Glacier Cross Section
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Glacier Cycle

• Ice will flow from high to low spots
• Remember, high and low refer to the glacier
  surface
• Ice will deform most intensely at the bottom, and
  the upper ice rides on it... so the top of
  glacier is fast
• Sometimes the bottom of the glacier is wet and/or
  soft and the glacier slides

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Ablation Zone

• Ice flow from accumulation zone to ablation zone
• Ablation loss of ice by melt or by breaking off
  of big icebergs (known as calving)
• Ice always flows down the surface slope
• Ice flows even though solid because it is hot
• Ice at -30C is close to its melting point (0C)
• Ice at -30C is far from absolute zero (-273C)
• Iron at room temp is far from its melting
  point...

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Glacier Erosion

• Glaciers with lots of water at the bottom are
  good at eroding
• Plucking is when the glacier breaks loose small
  rocks and carries them
• Abrading is when the glacier drags those small
  rocks over other rocks and acts like sandpaper
• Any water flow under the glacier carries away the
  loose stuff

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• Smooth on one side (where the glacier came from).
• Rough on the other side
• Roche Moutonee (rock sheep)

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• Rocks are smoothed and scratched by the glacier...

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Alpine Landforms

• U-shaped valleys
• Hanging valleys
• Rounded bowls (cirques)
• Sharp ridges and sharp mountains

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Glacier Erosion

• Glaciers really good at eroding
• Great lakes, Finger lakes...
• U-shaped valleys
• Alpine terrain (cirques, arete, horns...)
• Streams make a very different landscape
• Sharp, V-shaped valleys
• Usually much steeper

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Hanging Valley

• When 2 glaciers meet, the bigger one digs a
  deeper hole
• So the smaller one is left perched way up high on
  the valley wall left behind by the bigger glacier

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Alpine Landforms

• U-shaped valleys
• Hanging valleys
• Rounded bowls (cirques)
• Sharp ridges and sharp mountains

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Big Ice

• 1/10th of land is covered by ice.
• Mainly Antarctica (South, penguins, etc.) and
  Greenland (North, polar bears, Inuit, etc..)
• In the past, 1/3rd of land was ice covered.
• How do we know that? And why did it happen? And
  could it happen again?

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Lots of Ice 20,000 Years Ago

• Ice pushed down the Earth in Canada Scandinavia
• It is slowly rising after the removal of that
  weight, 21,000 years ago
• That much water for the ice must have come from
  the oceans...
• ...so the oceans must have had less water, and
  been lower

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How Much? How Long?

• Ocean water has a lot of oxygen and hydrogen
  (which make up water - H2O)
• Most are normal, but some have a different
  atomic weight and are called isotopes
• Protons - number determine the element
• Neutrons - number determine the weight
• Isotopes have fewer (or too many) neutrons

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Isotope Ratio

• There is a certain ratio of number of light to
  heavy O isotopes (about 1 in 500 is heavy)
• When water evaporates, the lighter isotopes
  evaporate more easily
• When all that water evaporated to make the big
  ice in Canada, relatively more of the light
  isotopes evaporate and get trapped on the ice
  sheet
• The ratio of light to heavy isotopes changes

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The Record

• The layers of shells alternate between
  isotopically heavy and light
• Heavy light isotopes have evaporated light
  isotopes are frozen into ice sheets. And vice
  versa.
• For last 800,000 years, we have had 90,000 years
  of cold, with big ice sheets followed by 10,000
  years of warm with small ice sheets

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Why?

• Heat of the sun!
• The amount of sunlight varies according to
• Shape of earths orbit (eccentricity, changes on
  a 100,000 year cycle)
• Amount of tilt of earths axis (obliquity,
  changes on a 41,000 year cycle)
• Sirection of tilt of earths axis (precession,
  changes on a 19,000 year cycle)

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Ocean Critters Record Changes

• The oxygen in shells also changes depending on
  the amount of ice in the Big Ice sheets
• Those shells fall to the bottom of the ocean and
  stay there until we dig them up
• We can find a shell from 100,000 years ago and
  tell how big the ice sheets were

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Milankovich

• Predicted by Serbian mathematician, Milutin
  Milankovich in 1920s, long before these isotope
  records were available!
• The amount of sunlight in far Northern hemisphere
  seems to control ice ages. Lots of sunlight in
  Canada means warm, no ice.
• Changes in sunlight relatively small, but have
  big effects.
• Still dont understand all the changes.Some very
  rapid (10s of years?!).