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Cirque Glaciers
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Glacers fill mountain top bowls
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Internal Plastic (Ductile) Deformation
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-crystals may stretch or rotate and may shear past one another
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Cirque
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bowl shaped basin, high or mountain
Freezing and thawing during winter and spring help fracture the rock bordering the head of the glacier
Melts to form to form a lake (mountain lake) called a TARN
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Milankovitch Cycles
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Occur roughly 100,000, 40,000 to 20,000 years apart
-Found climate cycles with frequency predicted by Milankovitch
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Climate/Vegetation
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Climate and vegetation shifted southward
-Pollen found in bogs preserved evidence of the ice age
-Permafrost tundra shifted from 68 degrees to 48 degrees
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Short Term Causes
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Milutin Milankovitch showed several explanations:
Orbital Eccentricity
Titl of Earths Axis
Precession of Earth's Axis
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Toe Position
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Accumulation > Ablation...glacier toe advances
Accumulation < Ablation...toe will retreat
Accumulation=Ablation...Stays same place
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Varve
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Pair of thin layers deposited in a single year
-Made of silt brought in during spring floods
-Clay deposited during winter when lake freezes
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Interrupting the Global Heat Conveyor
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Decreasing evaporation decreases the salinity of the ocean which in turn can stop the system that brings warm water to high lattitudes (become colder)
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Tilt of Earth's Axis
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We have seaons because Earth's axis is not perpendicular to plane of orbit
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Loess
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Catabatic wind from cold air blows margin of glacier
-Picks up clay and silt and takes away from toe of glacier
-Where wind dies down and the sediment settles and forms a thick layer
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Polar Regions
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Flow out over ocean water
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Drumlins
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The flow of glaciers may mold till and other subglacial sediment into streamlined, elongated hills
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Glacial Lake Agassiz
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Gigantic glacial ice-margin lake formed
Huge area, 2700 years
Drained abruptly creating 250,000 square km
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Albedo
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Snow remaining all year or clouds in the sky reflect light making the Earth cooler
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Toe
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Leading edge of a glacier
-ice always flows downhill, even during retreat
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Glaciers
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Thick masses of recrystallized ice
-last all year, flow via gravity
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Ice Cap Glaciers
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Covers tall mountain peaks
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Piedmont Glaciers
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Glaciers spread out at the end of a valley
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Rate of Flow
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May vary widely (10 to 300 m/yr)
-rarely it may surge (20 to 100 m/day)
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Continental Glaciers
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Vast sheets covering large areas
Ice flows outward from thickest part of the sheet
2 left on Earth: Greenland and Antartica
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Long Term Causes
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Plate Tectonics
Atmospheric Chemistry
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Large Scale Conveyor Belts
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Pick up, transport and deposit
Always in downhill direction
Create an end moraine
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Tide Water Glaciers
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Valley glaciers enter sea
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FIRN
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Granular material resulting from melting and recrystallizing
-Over time firn becomes interlocking crystals of ice
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Thicknesses
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At their thickess, they obtained a 2 to 3 km thickness
-Each thinned towards their toe
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Hanging Valley
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Intersection of tributaries merge with trunk glaciers
Trunk glacier incises the bedrock deeper
Troughs have different elevations and waterfall results
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Glacial Outwash
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Sediments in transported meltwater
Muds removed
Size graded and stratified
Abraded and rounded
Dominated by sand and gravel
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Arete
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A "knife-edge" ridge - two cirques eroded toward each other
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Glaciations
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Times during which the glaciers grew
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Land Forms
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End/Terminal Moraines, Recessional Moraines, Drumlins, Ground Moraines, Kettle Lakes, Eskers
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Ice Shelves
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Continental glaciers enetering the sea
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Kettle Holes
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Circular depressions made when blocks of ice calve off the toe of the glacier, become buried by till, and then melt
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Animal Life
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Numerous species now extinct wandered forests and tundras of North America
-Giant Bears, Sloths, Mammoths, and Mastadons
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Equilibrium Line
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Zones meet here
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Glacial Abrasion
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-Sand paper effect on substrate
-pullverized to fine "rock flow"
-sand in moving ice abrades and polishes rocks
-large rocks dragged gouge striations
-boulders crack cross sections chatter marks in the bed rock
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Plastic
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Lower than 60 m
-Ductile flow occurs in deeper ice
-ice flow heals cracks
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Glacial Marine
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Sediment from melted glaciers when they reach the sea settle upon the muddy sediment on the sea floor
Drop stones- pebbles and large clasts
Ice rafted clasts and marine sediment
Sometimes water from base carries sediment
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Lateral Moraines
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Form along flanks of valleys
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Periglacial (perma-ice) Environments
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Unique envrionments
-regions of wide spread permafrost without ice or now
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End Moraine (Terminal Moraine)
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Forms at stable toe of glaciers
Forms at the farthest edge of the flow
Recessional moraines form as retreating ice stalls
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Erosion Valley Features
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Cirque, Tarns, Aretes, Horn, U-shaped Valley, Hanging Valley, Rock Mountain, Fjords
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Pleistocene Model
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-Long term cooling in the Cenzoic Era
-Plate tectonics causes drifting of Antartica and South America allowing cold circum-Antartic current to develop.
-Uplift of Himalayas and Tibet diverted winds that made cooling of climate
-Closing of Isthmus of Panama diverted warm air toward cap, allowing snow and ice formation
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Dry-bottom Glaciers
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Are frozen to the substrate
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Plastic Deformation
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Movement by internal plastic deformation of the ice
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Lattitude
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Controls the elevation of glaciers
-Polar regions form at sea level
-Equitorial Region- form 5km above sea level
Elevation is marked by the "SNOW LINE"
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Erractics
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European farmers broke plows on large rocks
-buried in the soil
-100s kms away
Called Erratics
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Oxygen Isotopes
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Way in which to study warming periods
-During colder climates more Calcium carbonate absorbed in plankton
-Less during warmer climates
-Striations tell patterns, 20 to 30 during last 3 mil
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Glacial Rebound
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The underlying surface begins to rise back up when the ice melts
Asthenosphere flows back upward to fill the space
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Ice: The Water Mineral
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Solid water (h20)
-cools below freezing point
-naural ice is a mineral, grows in hexagonal form
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Drainages
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Glaciation replenishes river systems
-ice and glacial drift blocks pre-existing drainages
-After melting, altered river course remains
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Pluvial Lakes
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Rainfall accumulates in low-lying land at great distance from the ice front
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U-Shaped Valley
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Erosion makes distinct trough, unlike u-shaped fluvial
-Smooth and rounded
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Polar Glaciers
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Ice is well below melting temperature
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Holocene
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Time since the last glaciation
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End Moraine
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Debris at the end of of a toe
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Snow Transformed to Ice
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-Flakes accumulate and our buried
-They compress and air is expelled
-Burial pressures cause melting and recrystallizing
-Become FIRN
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Sea Ice
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Polar Regions
Tide water glaciers
Ice Shelves
Sea Ice
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Wet-bottom
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Water flows along the base of the glacier
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Interglacial Periods last 10,000 years
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...
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Brittle
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Uppermost 60 m
-Tension initiates cracking of ice
-Crevasse(s) may open and close with movement
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Pleistocene Glaciers
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Young, 2 million years ago, abdunant remenants
-North america, Scandanavia, Europe, Asia
Landscapes distinctual,glacial
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Zone of Accumulation
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Area of net snow addition
Colder temperature prevents melting
-Snow remains all year (summer too)
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Kame
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Stratified sequence of sediment from a lateral moraine
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Wet-bottom Glaciers
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Slide over melted slurry
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Climatic Changes
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Wather patterns were different then
-Large lakes in the deserts
-Great salt lakes were once the giangantic Lake Bonnesville
-Fossil lakeshores ring basins
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Advance and Retreat
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Behaves like bank accounts
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Pleistocene Glaciers
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From mapping of glacial striations and deposits, geologists have determined where the ice sheets originated and flowed from.
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Sea Ice
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Nonglacial ice formed of frozen sea water
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Biological Processes
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Greater amounts of plankton can absorb more carbon dioxide removing it from atmosphere (cooling)
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Plate Tectonics
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Control factors influencing glaciation
-movement controls higher lattitudes
-Controls where warmer oceanic currents are
-Sea level fluctuation by the mid ocean ridge creaters volcanic changes
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Deposition
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...
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Erratics
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Boulders and cobbles dropped by glacial ice
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Moraines
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Unsorted debris dumped by a glacier
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Sediment Transport
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Carries large rocks to fine materials
-Some falls on the ice surface from cliffs
-Some plucked and lifted from the subtrate and brought into the ice
-When the ice melts, sediments drop out
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Temperate
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Glacier ice is at or near melting temperature
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Ice flow
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Ice flows downhill via gravity
Flows away from the thickest part of of the continental glaciers
-Analogous to honey flowing away from thickest zone
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Two Types of Mechanical Behavior
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1) Brittle
2) Plastic
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Stratified Drift
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Water sorted,
Unstratified- not water sorted
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Thermal Categories
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Classify glaciers, climate determines this
Temperate
Polar Glaciers
Wet-bottom
Dry-bottom
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Rate Controlled By
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Slope severity - steeper =faster
Basal Water- Wet bottom is faster
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Zone of Ablation
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Area of net ice loss
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Flow
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Ice in zone of accumulation slowly buries
Ice in zone of ablation slowly move to surface
Individual ice crystals followed curved trajectory
Antartic meteorites found at end of a glacial flows
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North America
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Laurentide Ice Sheet orginated in Northeastern Canada
Merged with the Kweewatin ice sheet in northwestern Canada
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Erodes
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Substrates are eroded in several ways
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Basal Slipping (Sliding)
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Melt water/slurry allows the glacier to slide
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Forms
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Quickly - 10s of years
Slowly - 1000s of years
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North America
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Glaciation completely changed drainage
-used to go north
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Sea Level Changes
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Rises and falls with the ice ages
Falls during the ice age and stored in the ice on land
Deglaciation returns the water to the oceas and the sea level rises
If the ice melts, the coastal regions will be flooded
Sea level was 100 m lower during Wisconsan Age
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Medial Moraines
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Mid-ice moraines from merging lateral moraines
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Movement of Glaciers
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Wet-bottom
Dry-bottom
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Plucking
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-Ice breaks off chunks of rock
-Ice melts by pressuge against up-side ice of an obstruction
-Enetrs cracks and refreezes to ice
-Glacier plucks away bedrock chunks
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Precession of Earth's Axis
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Axis wobbles like a top
-wobble or motion
-determines relations between timing of seasons and position of earth
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Glacial Drift
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Glacial till, erractics, moraine sediments, glacial outwash, lake-bed sediments, loess
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Orbital Eccentricity
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Showed Earth's Orbit gradually changes from circular to elliptical
100,000 to 300,000 cycle
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Horn
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Pointed mountain peak (3 or more cirques that coalesce)
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Glacial Lakebed Sediment
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-Lakes abdunant in glaciated landscape
-Fine clasts and rock flour settles in meltwater lakes making a layer of glacial lake sediment
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Four Major Glaciations
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Wisconsinan
Illinoian
Kansan
Nebraskan
-Last two are poorly preserved
-Used radiometric dating for young glaciations
-Fossils for older glaciations
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Fjords
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U-shaped glacial troughs flooded by the sea
Accentuated by rebound as the sea has refills them
Norway, New Zealand, Alaska, Chilie
Walls rise directly from the sea sometimes
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Interglacials
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Times between glacial periods
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Louis Aggassiz
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Agents of landscape change (1837)
Ice age covered Europe
-Ice Sheets on land
-carried and dropped erratic boulders and fine grained soil
1850- people began to believe and accept
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Eskers
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When a glacier eventually melts away, ridges of sorted sand and gravel, deposited in subglacial meltwater tunnels, snake across the ground moraine
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Three Conditions to Form
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-Cold local climate (polar lattitude or high elevation)
-Snow must be abundant; more fall than melts
-Snow must not be removed by avalances or wind
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Permafrost
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-year round frozen ground
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Mountain Glaciers
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Flow high to low elevation in mountainous settings
Types: Ice Cap, Cirque,Valley, Piedmont
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Glacial Effects
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Important forces of landscape change
-Erosion
-Transport
-Deposition
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Ice Ages
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Glaciers - 10% Earth Today
In Ice Ages - 30% covered Earth
Most recent Ice age- 11 thousand years ago
-NY, Montreal, London, Paris
-100s to 1000s meters thick
Current Ice Age is one of many
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Flow location within the Glacier
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Greater velocity in the center
Friction slows ice at the margins
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Atmospheric Chemistry
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Change of carbon dioxide can determine whether or not ice age can occur
-CO2 and Greenhouse gases
-changes in marine organisms that extract CO2
-Methane output
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Valley Glaciers
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Flow like rivers down valleys
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Glacial Subsidence
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Weight of ice sheet causes the surface of the lithosphere to sink.
Asthenosphere is soft enough to flow out of the way
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Glacial Till
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-Sediments dropped by ice
-All types of grain sizes
-Aka- "boulder clay"
-No water- so its unsorted, unstratified
-Forms under ice, at toe of glacier, along flanks
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Lodgment Till
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Till that has been released at the base of a flowing glacier and remains after it has melted away
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Ground Moraines
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The till left behind during rapid succession forms a thin, humocky layer on the land surface; this till along with lodgment till forms a landscape known as ground moraine
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Dry-bottom
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Cold base frozen to the substrate
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