Glaciers and Glacial Mechanics chapt' 9

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Glaciers and Glacial Mechanics(chapt. 9)
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Is this a glacier?
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Glacier definition

• A glacier is a MOVING body of ICE that has been
  formed on land by the COMPACTION and
  RECRYSTALIZATION of SNOW

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Water in the hydrosphere

• Oceans 97.2
• Ice sheets and glaciers 2.18
• Groundwater 0.6
• Lakes and streams 0.018
• Soil moisture 0.005
• Atmosphere 0.001
• 99 of water is UNAVAILABLE to us

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When do glaciers form?
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When do glaciers form?

• Glaciers form when the amount of snow falling in
  the winter is greater than the amount that melts
  in the summer

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Milankovitch proposed that ice sheets grow in the
northern hemisphere when tilt is low, and summer
solstice occurs at aphelion
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Milankovitch theory orbital variations produces
changes in insulation
Last Glacial Maximum 21,000 years ago
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Records of climate change preserved in ice core
records
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Records of climate change preserved in ice
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Records of climate change preserved in ice
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North America 21,000 years ago
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How do glaciers form?

• Part 1 changing snow to ice
• Snow becomes denser with time and compaction
• New snow density (0.05 g/cm3), FIRN (old snow)
  density 0.55 g/cm3
• Burial, pressure, and refreezing of meltwater
  leads to increase in crystal size
• As crystal sizes grow and spaces around snow and
  ice crystals disappear density increases
• Density increases to 0.85-0.9 g/cm3ice!

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Metamorphism of snow into ice (still preserve
annual layers!)
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Now we have icewhats next?
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How do glaciers grow? Glacial Mass Balance
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Glacier mass balance
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Glacier mass balance

• Accumulation addition of snow and ice to a
  glacier
• Ablation processes that remove snow and ice
• Melting, sublimation, calving

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How do glaciers grow? Glacial Mass Balance

• the Equilibrium Line Altitude (ELA) is the
  elevation on the glacier where the volumes of
  accumulation and ablation are equal (Balance
  point)
• This is also the long-term average position of
  the highest (late summer) firn line.
• The ELA is climate and location dependent
• In a small glacier, 66 of the glacial area is
  the accumulation area, and 33 is the ablation
  area. Why? Need larger area to accumulate snow
  in order to equal the amount that is melting below

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How do glaciers grow? Glacial Mass Balance
Border between old dirty snow and ice and new
snow firn line
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Accumulation and ablation
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How do glaciers move?
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Now we have icewhats next?

• ppgh
• pressuredensitygravityheight (thickness of
  ice)
• What does this mean? Pressure has an influence
  on the freezing point.

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How do glaciers move?

• Glaciers move via 2 processes
• Internal deformation of the ice (creep)
• Sliding of the glacier on its base
• Glacial movement pseudoplastic

plastic
stress
Strain rate
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How do glaciers move?

• Internal flow
• Ice crystals move by creep along internal planes
• Like deck of cards being pushed

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How do glaciers move?

• Internal flow
• Ice crystals move by creep along internal planes
• Like deck of cards being pushed
• Shear stresspgh -gt greatest stress at base of
  glacier

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Glacial movement-velocity profiles

• Similar concept to stream velocity
• Friction reduces velocity on sides of glacier
• Velocity at surface is the TOTAL of all the
  layers beneath
• Fastest velocity at surface of glacier (even
  though shear stress is greatest at base)
• Each layer is carried on top of the layer beneath

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Where is flow velocity greatest on a glacier?
Near the equilibrium line-highest ice discharge
in that area
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Velocity profile on glaciers
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How do glaciers move?

• Internal flow
• Ice crystals move by creep along internal planes
• Like deck of cards being pushed
• Basal sliding
• Melt water at base of glacier allows glacier to
  slide along its base
• Depends on temperature
• Regelation slip (melting and refreezing over
  obstacles)

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Regelation slip
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• http//www.homepage.montana.edu/geol445/hyperglac
  /systems1/

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Major types of glaciers (morphological
classification)

• Cirque glaciers
• Mountain glaciers
• Form in amphitheater-shaped depressions (called
  cirques!)
• Size-few kilometers
• Valley glaciers
• Streams of ice that flow downvalley from a cirque
• 100s of km
• Piedmont glaciers
• Glaciers that discharge ice from mountains onto
  broad lowlands
• Ice becomes unconfined
• Ice sheets
• Bread ice masses not confined to valleys, massive
  thicknesses of ice
• 1000s of km
• Creates its own topography, creates isostatic
  depression

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Cirque glacier
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valley glacier
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Valley glacier
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Ice sheet
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Wet based (temparate) vs. cold based (polar)
glaciers

• Wet-based or temperate glaciers
• Glaciers at pressure melting point that slide on
  a layer of liquid water (large component of basal
  sliding)
• Lots of meltwater, higher velocities, more
  erosion
• Cold-based or polar glaciers
• Glaciers are stuck to their bed. These glaciers
  are characteristic of cold, polar areas.
• No meltwater, minimal slippage along base, lower
  velocity, less erosion, less geomorphic work

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How do glaciers move material?

• (Glaciers dont bulldoze! )
• Glaciers are more like conveyor belts. Glacial
  deposits mark where a glacier has been in the
  same place for a period of time.
• Except glaciers can construct minor push
  moraines. MOST material is transported within
  and on top of the glacier

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