Glacial Ice

1
Glacial Ice

• Lecture Outline
• Introduction to glacial geology
• Water
• Ice as a mineral
• Ice as a rock
• Metamorphism of snow
• Snow hazards and resources

2
Introduction to Glacial Geology

• We live in an interglacial period
• Ice covers 11 of continents
• Outside of glaciated areas 11 of earth is
  permafrost
• 12 of surface water is frozen
• Ice in atmosphere
• Global changes of just a few degrees one way or
  other can move earth into interglacial period or
  ice age

3
Introduction to Glacial Geology

• Glacial studies
• Sensitive recorder of paleoclimates
• Sensitive indicators of current climate change
• Significant portion of earths surface has been
  modified directly/indirectly by glaciers

4
Introduction to Glacial Geology

• Glacial studies
• Glaciology How glaciers form and behave
• Glacial geomorphology
• how glaciers and glacial meltwater erode,
  transport, deposit sediments
• Landforms created by glaciers and glacial
  meltwater
• Glacial geology
• sediment left behind by glaciers
• History of glaciation recorded by glaciers

5
Introduction to Glacial Geology

• Glacial record on Earth

6
Water

• The water molecule
• 2 positive univalent hydrogen ions asymmetrically
  bonded to one bivalent oxygen ion (covalent
  bond)
• Causes small residual negative charges at oxygen
  end and slight positive charges at hydrogen end
  (polar)
• Hydrogen end of one molecule attracts oxygen end
  of another molecule ? weak electrostatic bond
  (hydrogen bond)

7
Ice as a Mineral

• Ice differs from water in structure, physical
  properties, isotopic composition
• Crystallice ice that forms glaciers is
• Hexagonal
• Exists at temps 0oC. to 70oC. And at pressures
  as high as 2000 atm
• Other types of ice not found on earth

8
Ice as a Mineral

• Ice is a mineral
• Relatively stable crystalline structure within
  certain range of pressure-temperature
• Hexagonal structure revealed by X-ray diffraction
  analysis of position of oxygen atoms in space (H
  atoms too small)

9
Ice as a Mineral

• Physical properties of ice defined at pressure
  melting point (0oC. At 1 atm)
• Melting point decreases with increasing pressure
• Specific gravity of ice lt water, but volume is
  higher
• Because ice has a 3-D structure more open than
  that of water
• water is well-packed chains of H2O)

10
Ice as a Mineral

• See text for discussion of
• Specific heat
• Thermal conductivity
• Heat content (enthalpy)
• Latent heat heat absorbed/freed by unit weight
  of water or ice during phase change

11
Ice as a Mineral

• Hardness of ice crystals increases as temperature
  decreases
• At pressure melting point ice hardness 1.5 on
  Mohs Scale (similar to talc, gypsum)
• At 15oC. hardness 2-3 (similar to
  gypsum-calcite)
• Thus, too soft to abrade most rocks

12
Ice as a Mineral

• Nature of ice deformation depends on amount
  rate that stress is applied
• With increasing stress elastic deformation
  followed by breakage
• With constant stress applied over long time ?
  plastic deformation
• Deformation of ice crystal depends on direction
  that stress is applied

13
Ice as a Sediment and Rock

• Two major forms of ice rock
• Congelation ice formed from freezing of water
• Metamorphic glacier ice formed through
  densification of snow

14
Ice as a Rock

• Gas impurities
• Mostly as air but can also trap CH4, H2S, CO2
• Percent volume as air
• Up to 99 in fresh snow
• Up to 56 in firn
• Air reduces thermal conductivity of snow and ice
  snow is a good insulator
• Air reduces transparency of ice

15
Ice as a Rock

• Liquid impurities
• Liquids found in ice as a response to heat,
  pressure, salt
• As ice melts water initially develops between
  ice crystals

16
Ice as a Rock

• As ice melts into water water temperature
  remains at 0oC. until all of ice is gone
• Only after that will water temperature rise

17
Ice as a Rock

• Salt impurities
• Salts lower freezing point of water
• Until eutectic point (see E in figure) when
  everything freezes

18
Ice as a Rock

• Rock impurities in ice
• Rocks absorb radiant energy ? heat up and melt
  surrounding ice

19
Ice as a Rock

• Rock impurities in ice
• Thick deposits of rock serve as an insulator and
  protect ice from melting

20
Ice as a Rock

• Rock impurities in ice
• Melting occurs when basal ice is at pressure
  melting point and glacier passes over rock
• Rock in ice increases rigidity of glacier

21
Metamorphism of Snow

• Snow
• Snowflakes are ice crystals that have combined in
  hexagonal structure
• New snow density is 0.1 (90 air) to 0.3 g/cc
• Old snow
• Coarse granular snow rounded-off crystals
• Density 0.3 to 0.45 g/cc

22
Metamorphism of Snow

• Firn
• Snow that has survived at least one melt season
• Loosely consolidated, randomly-oriented ice
  crystals, 1-4 mm diameter
• Interconnected air pockets
• Density 0.45 to 0.75 g/cc

23
Metamorphism of Snow

• Firn ice
• Mix of partially altered firn and white bubbly
  glacier ice
• Density 0.75 to 0.88 g/cc

24
Metamorphism of Snow

• Glacier ice
• Crystals in firn grow and air pockets become
  isolated air cannot be blown into it!

25
Metamorphism of Snow

• Glacier ice
• Crystals can grow quite large gt10 cm

26
Metamorphism of Snow

• Glacier ice
• Crystals can grow quite large gt10 cm
• Density 0.88 to 0.90 g/cc
• Less than water
• As more air is expelled white bubbly ice becomes
  blue and eventually black ice
• Water molecules between ice refract blue
  wavelengths
• Other colors are refracted but they interfere and
  we dont see them

27
Metamorphism of Snow
28
Metamorphism of Snow
29
Metamorphism of Snow

• Depth of firn needed to form glacier ice depends
  on temperature and water equivalent of snow
• 80 meters in Antarctica
• 13 meters in SE Alaska

30
Snow Hazards and Resources

• Snow hazards
• Hazard for humans road ice, white-outs, snow
  bridges, snowmelt flooding
• Hazard for animals
• Hazard for plants abrasion, bending of branches,
  reduced soil productivity

31
Snow Hazards and Resources

• Costs of snow removal

32
Snow Hazards and Resources

• 25 deaths per year in Switzerland from snow
  avalanches
• Hannibals Army of 18,000 wiped-out by avalanches
  in Alps in 218 B.C.
• During WWII 40,000 Italian Austrian troops
  killed by avalanches in Dolomite Mts., Italy

33
Snow Hazards and Resources

• Avalanches can start at a point or as a slab
• May reach high velocity 300 kph
• 10,000 per year in mountains of western USA
• 1 of USA avalanches cause fatalities and/or
  damage 7 deaths 300,000 in damage per year

34
Snow Hazards and Resources

• Contributing factors
• Cornice overhang
• 1-2 feet of new snow ? weight
• Sun crust forms ? future sliding plane
• Rounding-off of crystals in old snow ? loose
  interlocking crystals
• Depth hoar crystals from sublimation of water
  vapor (sugar snow) ? act like ball bearings

35
Snow Hazards and Resources

• Avalanches of 1951 in Alps
• Heavy snowfall for 3 weeks in Jan.-Feb.
• Up to 4 feet in one day
• 500,000 tonnes of wet snow let loose above
  village of Vallascia
• Killed 10 who had ignored instructions to
  evacuate

36
Snow Hazards and Resources

• Collapse of ice summit on Huascaran (21,831 ft.)
  in Peru
• Rock displaced by falling ice
• Debris flow in valley with ice-rock-water moved
  200 mph
• Demolished 3 towns 25,000 died
• Smaller scale event in 1962 had claimed 4000
  deaths

37
Snow Hazards and Resources

• Juneau, Alaska
• 6 major avalanches in last 100 years
• None in last 25 years
• New subdivision has been constructed across
  avalanche chute
• Threat to 30 homes, school, hotel, marina

38
Snow Hazards and Resources

• Prediction mitigation
• Need adequate network of weather stations for
  real-time prediction
• Avalanche mapping
• Artificial triggering with explosives
• Avalanche barriers or energy dissipaters

39
Snow Hazards and Resources

• Snowmelt and flooding esp. from regional warm
  rain-on-snow events

40
Snow Hazards and Resources

• Snow as a resource
• Insulation inhibits deep frost penetration and
  prevents damage to crops, gardens, buried
  pipelines
• Water supply
• Transportation skis, snowmobiles, packed snow
  roads in arctic
• Recreation