11

Glaciers and Glacial Erosion – GLY 2010- Summer 2012 -Lecture 20

• Ice Margin, Commonwealth Glacier, Antarctica

22

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

33

Types of Glaciers

44

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

5

Present Day Continental Ice

Sheets

• Greenland and Antarctica currently are occupied by continental ice sheets

66

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

77

Snowfield and Icecap Above Byrd Glacier, Antarctica

• Snow blankets and fills the valleys between the nunataks in the foreground

88

Fox Glacier, New Zealand

• Outlet glacier fed by Ice Cap

9

Mountain Valleys

• High in the mountains, snow may accumulate

• Glaciers are formed in stages Snow Firn Glacial Ice

1010

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)

1111

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

1212

Formation of Glacier Ice

1313

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

1414

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

1515

Cirque Glacier Photo

1616

Cirque

• Photo by Dr. Michael Hambrey

1717

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

1818

Valley Glacier Photo

Valley glacier flowing through mountains in Alaska

1919

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

2020

Tidewater Glacier, Alaska

• Tidewater glaciers flow into the sea, calving icebergs

2121

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

2222

Glacial Zones

• Accumulation

• Wastage (Ablation)

• Fracture

• Flowage

23

Glacial Anatomy

2424

Zone of Accumulation

• Snowfall exceeds snowmelt, on average over many years

• Upper portion of the glacier, at all depths

2525

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

2626

Glacial Cross-Section

2727

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

2828

Zone of Flowage

• Lower part of the glacier, except near the toe, where glacier is thin

29

Glacial Advance and Retreat

3030

Crevasses, Exit Glacier, Alaska

• Extensional crevasses

• Note that they have the same shape as normal fault blocks

3131

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

34

Work of Glaciers

• Like rivers, glaciers alter the landscape

• They may erode the landscape, but they also deposit large amounts of material

3535

Glacial Erosion

• Abrasion• Quarrying• U-shaped valleys• Hanging valleys• Cirque• Arěte• Horn• Roche moutonnée

3636

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

3737

Kelley's Island Grooves, Ohio

• Grooves may be of glacial or fluvial origin

• View is in direction of ice flow

3838

Glacial Striations

• Striations are smaller than grooves

3939

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

4040

New Zealand Glacier

• Block to the right has been partially quarried

4141

U-Shaped Valleys

• Glacial ice follows previously cut stream valleys

• Stream valleys have V-shaped profiles

• Glacial erosion changes the shape to a U

4242

Glacial Valley Development I

• Typical, meandering V-shaped river valley

4343

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

46

Glacial Trough

• The U-shaped glacial trough seen here is in Glacier National Park, Montana

47

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

48

Yosemite Valley

49

Waterfalls in a Hanging Valley

• Yosemite Falls occupies a glacial valley

5050

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

51

Tarn Lake

• Lake Ann, North Cascades National Park, is a tarn lake, occupying a cirque

5252

Arěte

• Knife-edged ridge of land, formed by parallel erosion of two alpine glaciers

• Another visible sign of previous glaciation

5353

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

54

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

55

Liberty Cap, Yosemite

National Park

• Liberty Cap is a Roche moutonnée

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Alpine Glacial Erosion

5757

Glacial Deposition

• Till

• Drift

• Erratics

• Rock flour

5858

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

5959

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

65

Terminal Moraine Formation

• Terminal moraine is a more common name for what they call end moraine

6666

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

6969

Medial Moraine Formation

70

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

7272

Twentymile Glacier, Alaska

• Two tributaries meet in a complex of medial moraines

7373

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

7474

Braided Outwash Channels, Toklat River, Alaska

• Braided channels are constantly changing

7575

Glacial Erratics

• Large glacial erratic• Ice is capable of carrying all sizes of material

76

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

7777

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

7878

Drumlin Formation

7979

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

8080

Esker Photo

• Esker visible as a sinuous ridge (arrow) in this aerial photo

81

Kame

• Formed when meltwater washes sediment into openings in a stagnant wasting glacier terminus

82

Glacial Features

8383

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

84

Glacial Isostasy

8585

Crustal Depression

• Crust bulges on either side of glacier (isosastacy)

8686

Bulging Produces Raised Beaches

8787

Postglacial Effects

8888

Creation of Lake

Missoula

8989

Ice Dam

Breaks

9090

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!

9191

Giant Ripple Marks

• Aerial view of giant ripple marks

9292

Catastrophic Flood

• Flooding occurred about 13,000 to 15,000 years ago

9393

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

9494

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

9595

Palouse Falls, Washington

• Note the very large channel behind the falls

96

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