Glaciers (Part I)
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Transcript of Glaciers (Part I)
Glaciers (Part I)
• What is a glacier?
• Where are glaciers found?
• What is climate effect on glaciers?
What is a glacier?
• Mass of moving glacial ice created by the accumulation of snow
• glaciers always moving forward at terminus
• ice & water move forward
Typical glacier system inCordillera Blanca, Peru
Glacier landscape in Nepal Himalayas
Shorong Yul-lha glacier,Nepal Himalayas
How are glaciers formed?
• where average temperatures < O deg C. • Snow accumulates and compressed by
weight of layers• buried layers slowly form a thickened mass
of ice • snow grains squashed together-- snow
metamorphosis
Glacial Ice formation
• SNOW: seasonal snow void spaces
• FIRN (névé): snow that has lasted more than one year less void
space• ICE: compacted, air pores not connected
Air bubblesdensity > 860 kg/m3
Transformation of SNOW --> ICE
• Rate of transformation dependent on temperature and accumulation rate
• Rate with load • Rate with Temperature (for a given load)
– temperature determines size of crystals and amount of snowfall
Thermodynamic classification of glaciers
• “Cold” glaciers: – frozen to the rock of their beds– ice below pressure melting point– remain well frozen; melting only at surface
• “Warm” glaciers: – warm based– thawed from their bed– slide and flow
Glacier movement
• When glacier reaches critical mass (>20m thick)
• flow occurs
Ice is solid but it flows!
How does ice move?
Ffaster
Two ways of glacier movement
• PLASTIC DEFORMATION
• BASAL SLIDING
Gravity main driving force, gh*sin
1. Internal deformation
• Ice > 60m thick• specific for cold-based
glaciers (frozen to bed)
velocity
Factors controlling rate of deformation:•depth of ice•temperature•slope
Compaction weight
Pressure melting point
2.Basal sliding
• only “warm-based” glaciers
• glacier slips over the rock surface
• H2O as lubricator• less friction
• velocities: 0-300m/day
-water-sliding
Glacier surge
• velocities of 100m/day
• dramatic increase in flow rate, 10-100 hundred times faster than its normal rate
Structures within glacial ice
• Crevasses: – cracks in the ice due to different velocities
between center and edges of
glacier– formed perpendicular to
direction of flow
• Bergschrund:crevasse that separates flowing ice from stagnant ice at the head of a glacier
Glacier on ShorongYul-lha, Nepal
Crevasses
3. Icefall: steep, fast-flowing section of glacier with cracked and jumbled surface
Khumbu Ice fall, Everest
• Ogives: alternate bands of light and dark ice on a glacier(summer)
(winter)
Geographical and Climatic conditions
• high snowfall in winter• cool temperatures in summer
• Moisture important!!!– Eg: Siberia and parts of Antarctica: low temperatures
meet glacier growth requirements, but lack of adequate precipitation prevents glacier development
Glacier distribution & importance
• 10% of earth covered by ice
– 85% Antarctica– 11% Greenland– 4% elsewhere
• Glaciers store about 75% of the world's freshwater
Glacier Mass Balance
• (Net)Accumulation zone = area where ice accumulates
• (Net) Ablation zone= area where glacial ice melts
• Equilibrium line where accumulation=ablationbalance = 0 (at equilibrium)
Where is the ELA?
Where is the ELA?
Mass balance
• NEGATIVE: glacier gets smaller• POSITIVE: glacier gets larger• ZERO MASS BALANCE:
– no change in glacier size (mass,volume)– GLACIER STILL MOVING FORWARD!!!
ELA and climate
• Cooling -- ELA lower• Warming -- ELA higher
• Polar glaciers: ELA lower• Tropical glaciers: ELA higher
How do glaciers reflect climate change?
• Climate change:– changes in temperature– changes in amount of moisture – Glaciers sensitive to temperature fluctuations
• climate change can cause glaciers to melt • but the relationship is not straightforward,
– eg. Antarctica:climate change-->warmer-->more evaporation from ocean ->more water vapor -> more snowfall!
Ice ages• Ice ages return every 100,000 years• approx. 20 ice ages• Pleistocene = most recent ice age, that
started about 2 million years ago and ended ~10,000 yrs ago
• 4 major advances of ice, most recent ones:– Laurentide: ended 20,000 yrs ago– Wisconsin: ended 100,000 yrs ago– Presently we are in an interglacial period
Causes of ice ages? Milankovic cycles
long term variations in Earth’s orbit around the Sun:
Glacial ages
• During the last Ice Age, glaciers covered 32% of the total land area.
• Little Ice Age: – 17th century - late 19th century – consistently cool temperatures– significant glacier advances.
Earth’s climate record
Glaciers sensitive to climate changes: a few facts
• strong warming over the last 50-200 yrs• increasing CO2 levels• Alpine glaciers have been experiencing
rapid retreat• Ice cap on Mt. Kilimanjaro has been
decreasing by 82% in the last 88 years• Glaciers in the Alps decreased by 50% in
volume
Ice-albedo (positive) feedback
Global warming
Glacial melt
Decrease in albedo
More energy absorbed
Decrease in surfaceof ice
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Climatic responses- scenarios
• winter temperature:– less, not more, snow– polar areas get little precip. (cold air)– if summer ablation same -- glacier retreats
• summer temperature:– more cloud cover– less summer ablation– if winter accumulation same -- glacier grows
Climatic response (cont’d)
• winter precipitation (snowfall)– if no change in temperature– some snow survives over summer– glacier advances
– temperature crucial factor-
Glacier response -summary
• Alpine glaciers and N.Hem. Ice caps expected to retreat under global warming scenario
• NOTE: Antarctica expected to grow due to possible increase in humidity
Monitoring glaciers
• Field measurements
• Aerial photography
• Satellite images
Radar measurements - ice thickness
Aster image, Patagonia, Chile
ASTER Image courtesy of: NASA EROSData Center, Sept. 9, 2001
Indian HimalayasIndian Himalayas::
Glacier ablation at Glacier ablation at Gangotri, source ofGangotri, source ofthe holy Gangesthe holy Ganges
• glacier terminus retreated by glacier terminus retreated by 3km3km
Climate reconstructions
• Ice core drilling
Drilling tent on the summit of Cerro Tapado, Chile
Ice Core drilling, Coropuna, 2003
Having fun at high altitide...
..trying to get the generatorto work
Oxygen isotopes: 18O and 16O
• clues of temperature in the areas where ice formed
• Ratio of 18O and 16O indicator of temperature– 18O/ 16O > - warming signal– 18O/ 16O < - cooling signal
1.drilling the ice core
2. Taking the ice core out
3.measuring and storing the ice core
Isotope record
Glacier mummies: climate records?
500-year old mummy found in Peruvian Andes
Otzi- 5,000 year old mummy found in Tyrolean Alps, ItalyGlacier retreat revealed mummy