ESS 203 -Glaciers and Global Change · On a rocky bed: •Higher water pressure "floats" ice....
Transcript of ESS 203 -Glaciers and Global Change · On a rocky bed: •Higher water pressure "floats" ice....
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ESS 203 - Glaciers and Global Change
Outline for today• Volunteer for today’s highlights on Friday: ________• Summary of last Wednesday’s class – Lauren Fernandez
• Sliding, fracture• Glacial World
Lab next week• Glacial landforms
Wednesday January 29, 2020
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Mid-term #1It will be in one week - on Wednesday February 5.• 6 study questions are posted; 3 of these will form the
actual test.• Be sure to read the notes on the TESTS page “Writing a
test”
Study sessions?• Friday Jan 31 11:30 – 1:30 JHN 011• Monday Feb 3 10:30 – 1:30 JHN 011• Tuesday Feb 4 1:30 – 4:30 JHN 027
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ESS 203 - Glaciers and Global Change
Writing assignment due FridayNow is a good time to decide which question(s) you
will try first, and share with peers informally or at discussion sessions …
• Write out your current best answer to one Quiz study question that you will take to a study session.
• I won’t assign a grade to your preliminary answer. But it’s a C/NC contribution to class participation.
• It’s for your own good. Trust me. J
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Get the data directly from World Glacier Monitoring Service
https://wgms.ch/latest-glacier-mass-balance-data/
Are the glaciers growing or shrinking?
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https://wgms.ch/latest-glacier-mass-balance-data/
For more than a century, the World Glacier Monitoring Service (WGMS) and its predecessor organizations have been compiling and disseminating standardized data on glacier fluctuations. Thereto, the WGMS annually collects glacier data through its scientific collaboration network that is active in more than 30 countries. In close collaboration with the U.S. National Snow and Ice Data Center (NSIDC) and the Global Land Ice Measurements from Space (GLIMS) initiative, the WGMS runs the Global Terrestrial Network for Glaciers (GTN-G) in support of the United Nations Framework Convention on Climate Change (UNFCCC).
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Regelation (“refreezing” in French)
thescienceclassroom.wikispaces.org
www.studyblue.com
• Glacier sliding past small bumps
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Sliding SpeedSliding speed of glaciers increases if• Ice thickness increases• Ice surface slope increases• Water pressure at the bed increases
Recall demo: less force needed to make a block slide as water pressure under it increases
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On a rocky bed:• Higher water pressure "floats" ice.• Higher pressure reduces friction with the bedrock.• If bed is "rough", high pressure also reduces contact
area where sliding friction can occur.
Sliding Speed and Basal Water Pressure
On a "soft" bed:• High water pressure softens basal mud.• Softer mud deforms and flows faster. • Carries glacier along with it faster.
Hard bed rock
Ice Water in cavities
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Where When Sliding DeformationSpeed Speed (surface)
Nisqually Glacier Summer/fall 70 m a –1 15 m a –1
Mt RainierNisqually Glacier Winter/spring 35 m a –1 20 m a -1Blue Glacier summer 10 m a –1 40 m a –1
(lower reach)Variegated Glacier Surging 10,000 m a –1 40 m a -1Jakobshavn Isbrae All the time 8-12 km a-1 5 km a –1
(Greenland)Ice Stream Whillans All the time 500 m a –1 5 m a –1
(West Antarctica)
Examples of Sliding Speeds
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When a glacier changes its motion quickly (e.g. daily or weekly), the cause must be a change in the sliding speed.
• The internal deformation (quasi-viscous flow) cannot be radically altered until the thickness, or slope, or temperature of the glacier change significantly.
• All of these take a much longer time to change.
Rapid Changes in Speed
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Why does Water Pressure Change?
Variations in water level cause changes in basal water pressure.
Height of water in water tower controls water pressure in faucets.
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Origin of water• Melting of ice and snow at the surface • Rain • Frictional melting from ice sliding over bedrock.
Result• Large supply of water on hot summer days and
after rain storms.
Water Flow in Glaciers
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Rivers on the Greenland Ice SheetVibeke Gletscher, East Greenland
An opportunity to go rafting?
Probably not a good idea…
Hambrey and Alean, Glaciers.~300 m
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Streams on Glaciers
Water can melt channels into glacier ice.
• Water has lower albedo (darker) than ice, so water absorbs more sunlight than ice does.
• Water dissipates potential energy as it loses elevation.
• Water in contact with ice stays at 0oC.
• So what happens to that energy?
Austre Lovenbreen, Spitsbergen. Hambrey and Alean. Glaciers.
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Moulins
Water will find a way somehow to move down into a glacier.• a supraglacial stream will typically flow into a crevasse.
Mer de Glace, FranceHambrey and Alean. Glaciers.
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We can see the water coming out at a glacier terminus• But where has it been?
Tunnels
Fox Glacier, NZ.Hambrey and Alean. Glaciers.
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Schematic of Hydraulic System
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Linked CavitiesIn absence of subglacial tunnels, water flows through a linked-cavity system of interconnected basal cavities and narrow passage-ways.
• Drainage is inefficient.• High water pressure is needed to carry even limited amounts of water.
• Water will collect in crevasses and cavities until high pressures are reached.
(Hooke, Fundamentals of Glacier Mechanics)
Map
Vertical section
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Linked Cavities in Central Park
(http://www.swisseduc.ch/glaciers/)
Glaciated bedrock with ledges, cracks, and hollows
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TunnelsArborescent networks of tunnels can carry large volumes
of water at relatively low pressure compared to the pressure due to the weight of the overlying ice.
• Water flows from high pressure to low pressure.• Water will flow from linked cavities into tunnels.Ice also flows from high
to low pressure.• Ice also flows into the
tunnels. • Energy dissipated by the
flowing water melts off the walls as fast as they can close in.
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Early winter: little melt-water, low water pressure. • Tunnels close, linked cavities survive.Late winter: seepage, basal melt.• water pressure rises slowly, sliding increases slowly.Spring: high melt-water flux enters linked cavities.• Water pressure rises, sliding increases slowly.Early summer: too much water overloads cavities.• Sliding hits its peak.• Surface rises, cavities join, tunnels start to form.Late summer: water flows in low-pressure tunnels.• Basal water drains, sliding slows down.
Seasonal Water Cycle
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Flow of Nisqually Glacier
Nisqually River comes from under the glacier.
20
10
m/yr
m/yr100
50
75
54
3210
m3/sec
Surface speed• Measured
Speed due to viscous flow (deforming ice).
• Calculated from ice thickness and slope.
Sliding speed• Calculated as their
difference.
(Hodge, S., 1974. J.Glaciol.)
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Brittle Behavior in GlaciersCrevasses can form in the upper "skin" (about 30
meters, or 100 ft) of flowing glacier ice, when the ice tries to stretch rapidly.
Hambrey and Alean, 2005. Glaciers.
• At greater depths, the ice continually flows back in to close up the cracks.
• Crevasses almost always form at right angles to the direction of the maximum stretching.
Demo: silly putty
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HELP!COFFEE*
Hawley Extremely LightweightPortable !Crevasse Orientation Freehand Failure Estimation Equipment
[*Designed by former ESS 203 TA Bob Hawley]
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Where Crevasses tend to Form
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Crevasse on Griesgletscher Switzerland
Hambrey and Alean, 2005. Glaciers.
• How might this crevasse have formed?
• Since this is summer, you can see it easily.
• In winter it might be hidden by a thin snow bridge (falling through is bad for your health!).
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Crevasses on Fox Glacier, New Zealand
2006 ©Glaciers online · J. Alean · M. Hambrey
• How did these crevasses form?
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Crevasses on Arctic Piedmont Glaciers
http://www.swisseduc.ch/glaciers/
• Why did these crevasses form?
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Crevasses on an Antarctic Outlet Glacier
R.P. Sharp. 1988.Living Ice
Ice accelerated into a valley through the Transantarctic Mountains, then slowed down and spread sideways.
• When did each set of crevasses form?
Flow direction
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The Curious ScientistQuestions on flow, deformation, and
crevasses…
Five-minute questions for your group …• Recorders please turn in reports at end of class.
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1. Where are those Rocks Now?You put a straight line of 5 stones across a
glacier in its accumulation area. They got buried, and they will be carried along by the ice. Eventually the ice encasing them will melt in the ablation area.
• One year after you laid them out, which of the stones was closest to the terminus?
• Which stone will probably reappear first at the surface in the ablation area?
• Which stone will go the deepest into the glacier before it eventually starts to re-emerge?
• Which stone will reappear closest to the terminus?• What will happen to the stones after they reappear at the
surface?
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2. Crevasses on a Cascades GlacierCrevasses tend to open up at right angles to the direction where ice is stretched the most, i.e. like a rope of silly putty, the ice tends to break clean across the direction in which you are stretching it the most.
Suppose that you are climbing in the Cascades, and you have been told to watch out for crevasses on Glacier X near the equilibrium line.
• Using the figure showing how a line of stones across a glacier is deformed by flow, suggest which way the crevasses might run near the glacier edge, and why.
0
31
2
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3. Alaskan Glaciers
http://www.antarcticglaciers.org/modern-glaciers/structural-glaciology/splaying-crevasse/
• Locate examples of all 3 types of crevasses (transverse, marginal, and splay) and explain why they formed.
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4. Crevasses on Saskatchewan Glacier
Google Earth
• Locate examples of all 3 types of crevasses and explain why they formed.
Direction of flow
Direction of flow
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4. Crevasses on Saskatchewan Glacier
Google Earth
• Locate examples of all 3 types of crevasses (transverse, marginal, and splay) and explain why they formed.
Direction of flow
Direction of flow
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Ice-Age World
• What was it like … ?• How did it end … ?
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At 20 ka BP, Earth was in the grip of an Ice AgeOr, as Macdougall would remind us, taking the longer
view, Earth was in the grip of the most recent cold period of the current Ice Age.
• Thick ice sheets covered large areas.
• The ice sheets needed a combination of moisture and cold temperatures to form and survive.
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Where was the Ice?• Laurentide Ice Sheet (Eastern/central North America)• Cordilleran Ice Sheet (Western North America)• British Ice Sheet• Iceland• Greenland• Fennoscandian Ice
Sheet (Scandinavia)• Siberian Ice Sheet
(limited by low accumulation?)
• Barents Sea Ice Sheet (marine ice sheet - wet feet)
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Where was the Ice at 20 ka?
We know a lot about ice-sheet retreat during the past 20 ka, but not so much about earlier history.
• Why not?
Green colors indicate areas that were dry land.Sea level was much lowerthen (by ~120 meters).• Why?
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Ice Map - A Norwegian perspective
https://icemap.rhewlif.xyz/
https://icemap.rhewlif.xyz/
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Anderson and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet - 20kaIce sheet
Ice Shelf
Icebergs+Sea IceGlacier
Ice-dammed lakeOpen ocean
Sparse vegetationGlacial sedimentsGrass or brush
Grass or brush
Polar desert or tundra
Tundra or steppe
Partly forestedSteppe or parkland
Grass or brushGrass/brush+loessPartly forested
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Anderson and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet - 20ka
• Sea-ice cover to Spain (in winter).
• Fennoscandian and British ice sheets may have been joined.
• Europe covered by tundra and steppe, like northern Canada or Siberia today.
• Low sea level, English Channel dry.Why?
• Black and Caspian Seas are lakes. Why?
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Anderson and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet - 15ka
• Fennoscandian and British ice sheets have separated.
• Barents Ice Sheet is disintegrating.
• Sea level is still low.
• Black Sea is still a lake.
• Still few trees north of Pyrenees, Alps, or Caucasus.
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Anderson and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet – 12-11ka
Tundra/park tundra
Tundra/steppe park
Alpine vegetation
Steppe/parklandParkland+forestBirch forest
Birch/pine/spruce
Boreal:pine/spruceMixed boreal/broadleaf
Broadleaf /hazel
Allerod limit - birch
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Anderson and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet – 12-11ka• North Atlantic open in
summer ~12.5 ka.• Sea ice re-advanced in
Younger Dryas cold period 12-11 ka.
• British Ice Sheet is largely gone.
• Sea level is rising.• Barents Ice Sheet
disintegrating.• Baltic Sea is a lake.• Caspian Sea is now a
closed basin.• Black Sea still a lake.• Forests re-occupy
Europe.
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Andersen and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet – 9.5ka
• Yoldia Sea in Baltic area
• UK still connected to Europe
• Sea ice rare around Europe
• Less ice on Iceland
• Forests moving north
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Andersen and Borns, 1994. The Ice Age World
Fennoscandian Ice Sheet - 8ka
• Only a small ice cap left on Norwegian mountain spine.
• Only a few small ice caps in Iceland.
• Sea ice rare around Europe.
• Baltic Lake• Sea level still lower
than today • parts of North Sea
and English Channel are dry
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Andersen and Borns, 1994. The Ice Age World
North America 20-18ka
GlacierIce sheet
Ice shelfIce-dammed
Open ocean
Pluvial lake
Sea ice
Park tundraDesert/tundra
ConifersBroadleaf
• Cordilleran ice sheet (C) operated independently from Laurentide (L) and Keewatin (K) domes.
• Connection to Greenland Ice Sheet(?)
• Ice-free areas in Alaska (why?)
• Bering Strait is dry.
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Andersen and Borns, 1994. The Ice Age World
North America 15ka
GlacierIce sheet
Ice shelfIce-dammed
Open ocean
Pluvial lake
Sea ice
Park tundraDesert/tundra
ConifersBroadleaf
• Southern margin retreating.
• Surge lobes advance on soft wet bed
• Cordilleran and Keewatin Ice Sheets starting to separate (Why?)
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Andersen and Borns, 1994. The Ice Age World
North America 11ka
• Forests advancing northward
• Baffin Bay free of glacier ice
• Ice-marginal lakes (Why?)
• Cordilleran and Keewatin/Laurentide ice sheets separated
• Importance for humans?
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Dyke and Prest, 1989.
Laurentide Ice Sheet – 8.4ka
• Why are there big lakes along southern margin?
• Cochrane Lobe surge into Lake Ojibway.
• Hudson Bay Dome collapses.
• Drainage of marginal lakes Agassiz and Ojibway.
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Dyke and Prest, 1989.
Laurentide Ice Sheet – 7.9ka
• Sea level was still lower than today.
• Sea enters Hudson Bay -extensive flooding• How could there be
flooding?
• Remnants of ice sheet remain in Ungava (northern Quebec) and on Baffin Island/Foxe Basin.
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Laurentide Ice Sheet and Climate
A huge lump of ice several km high can affect persistent low-pressure systems, wind patterns, and jet stream.
• Jet stream diverted south, takes Seattle weather to California.
• Jet diverted north, then sweepssouth from Arctic down over Atlantic, cooling Europe even more.
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Great Ocean Conveyor
(From the IPCC Report 2001)
Ocean “conveyor” driven by formation of sea ice in North Atlantic and around Antarctica
Conveyor may have been off frequently during the Ice Age, contributing to very cold European climate.
• Salt excluded from freezing sea ice makes the water dense.
• It sinks, and new warm water is drawn in to replace it.
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Western Lakes
• Large pluvial lakes(formed by rainfall) in USA where there are only deserts today.
• Lahontan, Bonneville (Salt Lake) drained to Columbia River.
• Their shorelines are still visible.
• Why was that area so different from today? (the continental ice sheet was hundreds of miles away, and did not drain into these lakes.) Andersen and Borns, 1994. The Ice Age World
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Lake Bonneville and Lake Provo shorelines
Andersen and Borns, 1994. The Ice Age World
330 m above Great Salt Lake
Provo Valley, Utah
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Mendelova, M. et al. (2017) Geographical Res. Letters 43(2), 719-0750
High precipitation from strong Westerlies
Ice-age Patagonia
Ice Age ice sheet
Modern ice caps
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Mendelova, M. et al. (2017) Geographical Res. Letters 43(2), 719-0750
Big glaciers excavated the
modern Argentine
lakes
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Antarctica leaves the Ice Age• Larger West Antarctic Ice Sheet (WAIS) at 20ka BP. • Embayments with floating ice shelves today may have had
thick, grounded ice.
• East Antarctica probably saw minor change, • It could not expand very far, because of narrow continental shelf.
• West Antarctic grounding-line retreat may have been near-continuous ever since ~8 ka BP, and it is likely to continue to shrink.