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Glaciers and Glacial Erosion – GLY 2010- Summer 2012 -Lecture 20
• Ice Margin, Commonwealth Glacier, Antarctica
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Glacier
• Moving mass of ice that forms when snowfall exceeds snowmelt over a long period of time
• Movement is downhill, due to gravity• Form at or above the snowline, the lowest
altitude at which snow commonly forms in the mountains
• As climates warm, the snowlines are expected to move higher in elevation, and glaciers to retreat
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Types of Glaciers
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Continental Ice Sheet
• Large ice masses that blanket a sizable part of a continent
• Ice may be kilometers thick, and movement is limited, occurring mainly in local areas or very slowly over time
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Present Day Continental Ice
Sheets
• Greenland and Antarctica currently are occupied by continental ice sheets
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Snowfield and Ice Cap
• Large mass of snow and ice on a flat surface, topped by recent snow
• Ice caps show little movement
• They occupy the tops of mountains
• Outlet glaciers may flow downward from the ice cap
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Snowfield and Icecap Above Byrd Glacier, Antarctica
• Snow blankets and fills the valleys between the nunataks in the foreground
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Fox Glacier, New Zealand
• Outlet glacier fed by Ice Cap
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Mountain Valleys
• High in the mountains, snow may accumulate
• Glaciers are formed in stages Snow Firn Glacial Ice
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Snow
• Snow occurs in many forms, for example wet or dry
• Snow accumulates with a great deal of air trapped inside (you need to pack snow to make a decent snowball)
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Firn
• During the first summer following snowfall, some of the snow melts
• Meltwaters trickles downward, helping to compact the snow
• As winter approaches, resulting mass freezes together to form firn
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Formation of Glacier Ice
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Glacial Ice
• Repeated years of thaw/freeze cycles, and the weight of accumulated firn and snow, transform the lower layers to ice
• There are twelve known structural forms of water ice, at least half of which occur in nature
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Cirque Glacier
• Ice movement erodes a depression near the head of the glacier
• Glacier occupies the hole for a prolonged period, creating a bowl-shaped depression under the glacier
• After the cirque glacier melts, the depression remains
• Cirque may be filled with water, and is called a tarn lake, or it may be dry
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Cirque Glacier Photo
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Cirque
• Photo by Dr. Michael Hambrey
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Alpine or Valley Glacier• Confined by surrounding bedrock
highlands• Generally move down steep to very
steep surfaces• As cirque glaciers expand, they flow into
pre-existing stream cut channels, enlarging and changing the shape of these valleys
• Stream valleys have V-shaped profiles, whereas glacial valleys are U-shaped
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Valley Glacier Photo
Valley glacier flowing through mountains in Alaska
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Piedmont Glaciers• One or more valley
glaciers flowing from the confines of valley walls and spread out to form broad sheets
• Piedmont literally means foot of the mountain
• Malaspina Galcier (Alaska) is a classic large piedmont glacier that descends to tidewater from several mountain sources
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Tidewater Glacier, Alaska
• Tidewater glaciers flow into the sea, calving icebergs
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Calving Glacier Video
• Video was shot from a small ship July 1993 about one-half mile from the calving glacier by the instructor
© David Warburton, 2006
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Glacial Zones
• Accumulation
• Wastage (Ablation)
• Fracture
• Flowage
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Glacial Anatomy
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Zone of Accumulation
• Snowfall exceeds snowmelt, on average over many years
• Upper portion of the glacier, at all depths
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Zone of Wastage (Ablation)
• Snowmelt exceeds snowfall, on average over many years
• Glacier will retreat unless gravitational movement of glacier downhill replaces glacial ice as fast as it melts
• 85% of world’s glaciers are currently retreating
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Glacial Cross-Section
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Zone of Fracture
• Upper surface of glacier is under little pressure
• Near toe of glacier, glacier moves faster, and pulls ahead of the ice uphill
• Tensional cracks develop (crevasses)
• Ice behaves as a brittle solid
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Zone of Flowage
• Lower part of the glacier, except near the toe, where glacier is thin
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Glacial Advance and Retreat
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Crevasses, Exit Glacier, Alaska
• Extensional crevasses
• Note that they have the same shape as normal fault blocks
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Inside a Crevasse
• Photos by Kristina Ahlnas, University of Alaska
• Glacial ice is so blue because the dense ice of the glacier absorbs every other color of the spectrum except blue--so blue is what we see!
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Meltwater
• As glaciers melt, water trickles through the glacier, getting under it
• May form under-ice stream channels
• Channels form tunnels under the ice
• Tunnels emerge at snout of glacier
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Ice Cave
• Video of tidewater glacier, Prince William Sound, Alaska, July, 1993 © David Warburton, 2006
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Work of Glaciers
• Like rivers, glaciers alter the landscape
• They may erode the landscape, but they also deposit large amounts of material
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Glacial Erosion
• Abrasion• Quarrying• U-shaped valleys• Hanging valleys• Cirque• Arěte• Horn• Roche moutonnée
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Abrasion
• Rock fragments imbedded in the base of the glacier scrap, and polish underlying rock, and in some cases create long striations (thin) or grooves (thicker) in the bedrock
• Stria and grooves indicate the direction of glacier movement
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Kelley's Island Grooves, Ohio
• Grooves may be of glacial or fluvial origin
• View is in direction of ice flow
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Glacial Striations
• Striations are smaller than grooves
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Quarrying
• Large masses of bedrock are lifted away from the bed after water from the glacier seeps into cracks and refreezes (frost wedging)
• Rock is incorporated into the glacier
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New Zealand Glacier
• Block to the right has been partially quarried
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U-Shaped Valleys
• Glacial ice follows previously cut stream valleys
• Stream valleys have V-shaped profiles
• Glacial erosion changes the shape to a U
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Glacial Valley Development I
• Typical, meandering V-shaped river valley
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Glacial Valley Development II
• Running water erodes and deepens the V - shape
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Glacial Valley Development III
• Glacier fills the river valley
• Channel is widened and straightened
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Glacial Valley Development IV
• Melting of glacier reveals a U – shaped valley
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Glacial Trough
• The U-shaped glacial trough seen here is in Glacier National Park, Montana
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Hanging Valleys
• Where tributary glaciers flow into the trunk glacier, they are often unable to erode as fast as the heavier trunk glacier
• When the ice melts, a hanging valley is left
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Yosemite Valley
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Waterfalls in a Hanging Valley
• Yosemite Falls occupies a glacial valley
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Cirque
• Cirques remain long after glaciers disappear
• May fill with water to become tarn lakes
• Bowl-shaped depression near the head of an alpine glacier
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Tarn Lake
• Lake Ann, North Cascades National Park, is a tarn lake, occupying a cirque
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Arěte
• Knife-edged ridge of land, formed by parallel erosion of two alpine glaciers
• Another visible sign of previous glaciation
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Horn
• A three or four-sided mountain
• Cut by glaciers flowing off an isolated mountain in several directions
• Matterhorn in Switzerland is an excellent example
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Roche Moutonnée
• From the French, meaning sheep rock
• Glacial abrasion smoothes the slope facing the oncoming ice
• Glacial quarrying plucks rocks from the opposite slope, steepening it
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Liberty Cap, Yosemite
National Park
• Liberty Cap is a Roche moutonnée
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Alpine Glacial Erosion
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Glacial Deposition
• Till
• Drift
• Erratics
• Rock flour
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Glacial Till
• Deposits directly from glacial ice, with no sorting
• Light rocks are cobbles and pebbles
• Dark tan "matrix" is a mixture of sand, silt, and clay
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Formation of a Till Deposit
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Glacial Drift
• Includes glacial till, material dumped by glacier when melting
• Deposits from meltwater flowing out under a glacier
• Meltwater deposits are moderately to well-sorted
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Rock Flour
• Finally ground sediment produced under a glacier
• Meltwater streams carry the flour to lakes like this one in Alberta, Canada
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Moraines
• Masses of glacial drift left behind by a glacier
• Types of Moraine Terminal Lateral Medial
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Terminal Moraine
• Left at the end of a glaciers advance
• May act as a natural earth-fill dam
• Multiple terminal moraines may form a series of hills, running parallel to a ridge of mountains
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Moraine Formation
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Terminal Moraine Formation
• Terminal moraine is a more common name for what they call end moraine
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Cape Cod From Space
• Cape Cod is a terminal moraine
• It marks the farthest point that the glaciers reached during the most recent "ice age" in North America
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Southern New England Moraines
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Lateral Moraine
Photo• Lateral
moraines are visible to either side of the glacier
Athabaska Glacier, Jasper National Park, Canada
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Medial Moraine Formation
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Medial Moraine Photo
• Wrangell National Park, Alaska• Lateral moraines merging to form a complex of
medial moraines
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Glacier Junction, Southern Alps, New Zealand
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Twentymile Glacier, Alaska
• Two tributaries meet in a complex of medial moraines
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Outwash, Tasman Glacier, New Zealand
• Running water re-works the glacial gravel into the outwash
• Note the car in the lower center of the image
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Braided Outwash Channels, Toklat River, Alaska
• Braided channels are constantly changing
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Glacial Erratics
• Large glacial erratic• Ice is capable of carrying all sizes of material
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Kettle Lakes
• Form when a large chunk of glacial ice is buried as glacier passes over
• Later, it melts, and forms a lake• Bear and Nymph Lakes, Rocky Mountain NP
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Drumlin
• Gently rounded hills formed by a massive ice sheet (thicker then a glacier) overriding a moraine
• Massive ice reshapes the moraine into elongated hills
Aerial View of drumlin field
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Drumlin Formation
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Esker Formation• Meltwater streams, flowing
within or under active glaciers deposit sand and gravel in curving channels
• When the glaciers melt the eskers are exposed as topographically positive features
• Long, linear features, sometimes in a network
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Esker Photo
• Esker visible as a sinuous ridge (arrow) in this aerial photo
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Kame
• Formed when meltwater washes sediment into openings in a stagnant wasting glacier terminus
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Glacial Features
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Glacial Subsidence and Rebound
• Continental ice sheets are so heavy they depress the rock under them (subsidence, due to isostasy)
• When the ice sheets melt, the land begins to slowly rise
• In the Canadian Shield region, the glacial rebound rate is about one foot per century
• Rebound is sometimes visible along lake-shores, where older beaches and wave-cut terraces are now considerably higher then the current lakeshore
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Glacial Isostasy
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Crustal Depression
• Crust bulges on either side of glacier (isosastacy)
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Bulging Produces Raised Beaches
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Postglacial Effects
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Creation of Lake
Missoula
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Ice Dam
Breaks
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Bretz Floods
• The resulting floods, which recurred numerous times, are called Bretz floods after J Harlan Bretz, who first realized the significance of the gravel ridges meters high, and sometimes with wavelengths of a hundred meters, as Ripple Marks!
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Giant Ripple Marks
• Aerial view of giant ripple marks
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Catastrophic Flood
• Flooding occurred about 13,000 to 15,000 years ago
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Dry Falls
• Dry channels occur hundreds of feet above the present river channels, and former cataracts existed in many areas
• The best known of these, Dry Falls, is a 3.5 mile wide former cataract that dropped 400' over vertical cliffs of basalt
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Scablands
• Floods gouged and tore at the land removing surface cover
• Left masses of basaltic lava as remnants, like scabs on a wound
• Area is now called the channeled scablands
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Palouse Falls, Washington
• Note the very large channel behind the falls
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Pluvial Lakes
• During glacial times, the climate was cooler, and evaporation rates were much lower in arid and semi-arid regions
• Many pluvial lakes formed from rainwater which did not evaporate
• Pluvial comes from Latin pluvia meaning rain