Title: Glacial Ice
1Glacial Ice
- Lecture Outline
- Introduction to glacial geology
- Water
- Ice as a mineral
- Ice as a rock
- Metamorphism of snow
- Snow hazards and resources
2Introduction 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
3Introduction 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
4Introduction 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
5Introduction to Glacial Geology
6Water
- 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)
7Ice 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
8Ice 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)
9Ice 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)
10Ice 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
11Ice 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
12Ice 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
13Ice 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
14Ice 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
15Ice 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
16Ice 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
17Ice as a Rock
- Salt impurities
- Salts lower freezing point of water
- Until eutectic point (see E in figure) when
everything freezes
18Ice as a Rock
- Rock impurities in ice
- Rocks absorb radiant energy ? heat up and melt
surrounding ice
19Ice as a Rock
- Rock impurities in ice
- Thick deposits of rock serve as an insulator and
protect ice from melting
20Ice 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
21Metamorphism 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
22Metamorphism 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
23Metamorphism of Snow
- Firn ice
- Mix of partially altered firn and white bubbly
glacier ice - Density 0.75 to 0.88 g/cc
24Metamorphism of Snow
- Glacier ice
- Crystals in firn grow and air pockets become
isolated air cannot be blown into it!
25Metamorphism of Snow
- Glacier ice
- Crystals can grow quite large gt10 cm
26Metamorphism 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
27Metamorphism of Snow
28Metamorphism of Snow
29Metamorphism 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
30Snow 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
31Snow Hazards and Resources
32Snow 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
33Snow 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
34Snow 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
35Snow 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
36Snow 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
37Snow 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
38Snow 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
39Snow Hazards and Resources
- Snowmelt and flooding esp. from regional warm
rain-on-snow events
40Snow 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