SUPPLEMENTAL LECTURE MATERIALS...

Sea Level Changes

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Glaciers

A glacier is any body of flowing ice that has been formed on land by compaction and re-crystallization of ice

Two types

Alpine Glaciers

Areas that have been eroded by alpine glaciers generally have sharp, angular landforms. The glaciers also widen, deepen, and straighten their valleys. Alpine glaciers are small, usually covering about 100 mi2 or less. They are found in high elevations where snow remains most of the year.

Three types of alpine glaciers

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Growth of Glaciers

Glaciers are composed of ice that originally fell as snow. At this point, it is about 80% air and 20% ice. Gradually, the snow transitions to glacier ice through a combination of compaction and re-crystallization of the ice.

The term for partially compacted snow is firn or Neve. Layers of firn become glacier ice after density continues to increase. Glacier ice is about 10% air.

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To change snow to glacial ice takes different amounts of time depending on climate.

Zones of Glaciers - Glacier Mass Balance

Ablation (wastage) - lower part of valley glacier: zone where glacier loses mass thru melting, evaporation, and sublimation. Ablation > snowfall in this region.

Accumulation - upper part of valley glacier: the zone of perennial snow cover - region where snow is added to a glacier. Snowfall > ablation in this region.

Equilibrium line (firn line) - separates the two zones - elevation to which the snow recedes during the ablation season

* if accumulation > ablation, firn line moves down = positive budget and the glacier advances

* if ablation > accumulation, firn line moves up = negative budget and the glacier retreats

* if ablation = accumulation, firn line doesn't move or has minor movements = balanced budget or dynamic equilibrium. End of the glacier does not move.

**Even if the end of the glacier is not moving (or retreating) it is important to know that at all times, glacial ice is always flowing! Note in the illustration below the position of the rock over time. All materials are constantly transported towards the end of the glacier - think of the glacier like a giant conveyor belt.

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Alpine Glacial Landforms and Features

A. Bergschrund: crack (crevasse) at the head of the glacier next to the mountain and separates flowing ice from stagnant ice or the rock itself. If water gets into the bergschrund and freezes, plucking may occur and the size of the cirque will increase.

B. Crevasses: cracks in the ice surface. Occur as the ice is flowing over rough terrain.

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A bergschrund is seen near the top of the glacier in Alaska. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

 

Hikers walking along crevasses on the surface of British Columbia's Llewellyn Glacier, the largest glacier draining the Juneau Icefield to the east. These crevasses near the end of the glacier may be as much as 4.5 meters wide at the surface. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

 

Glacier Movement

Beneath 100 ft: due to the weight and pressure, ice crystals becomes layered and the ice moves like a plastic. This is called internal plastic deformation and occurs in the zone of plastic flow. The ice bends and does not crack so there are no crevasses at depth.

Towards the surface: the ice is brittle and the ice cracks. This is the zone of brittle flow and the cracks are caused crevasses.

Glacial Erosion

Two methods of glacial erosion...

Glacial Plucking: glacier lifts and incorporates rock and soil into the glacial ice by plucking or quarrying. Plucking encouraged by weathering: water freezing in cracks, expanding and breaking rock pieces loose. Plucking produces a rough, jagged surface.

Glacial Abrasion: rock fragments carried with the ice act as cutting tools scraping and grinding against rocks and wearing them away. Produce grooves and scratches called striations. These striations indicate flow direction. Abrasion generally smooths the surface over which the glacier is moving.

A feature in the landscape is produced by a combination of plucking and abrasion. It is called a roche moutonee and is rounded on the upstream side (abrasion) and rough from plucking on the downstream side. A roche moutonee is an erosional landform. The picture on the right is Lembert Dome found in Tuolumne Meadows, Yosemite N.P.

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Other Erosional Landforms Associated with Alpine Glaciation

Erosional landforms are found in the accumulation zone of the glacier.

1. Arete: formed when 2 cirque glaciers on opposite sides of mountain cut away the mountains, leaving a sharp ridge between the two cirques.

2. Horn: when 3+ cirques intersect, you get a horn. Best example: the Matterhorn in Switzerland

3. Col: pass formed where two cirques have intersected to produce a low saddle between high peaks.

4. Glacial Trough: a huge U-shaped valley carved by a glacier

5. Fjord: a glacial trough fills with sea water. (remember, rivers cut V-shaped valleys)

6. Tarn: a lake in a cirque

7. Hanging Valley: higher trough of a tributary glacier that feeds into the main glacial valley. A large drop-off is located at the confluence.

8. Paternoster Lakes: a sequence of small lakes found in the shallow depressions carved out of the glacial trough

9. Cirque: the bowl-like depression that is carved by a glacier

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South view of the terminus of Cleve Creek Glacier (name printed in red), AK, plus several retreating unnamed, small valley glaciers. Notice the aretes running perpendicular to the Cleve Creek Glacier (yellow arrows). Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

 

Juneau icefield and horn in the mountains of Alaska. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

 

 

Near Skagway, Alaska displays the typical features of alpine glaciation. Notice the cols along the arete. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

 

 

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Fjord in SE Alaska. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Marli Miller, University of Oregon

 

Several retreating unnamed, small valley glaciers and several recently deglacierized cirques (orange arrows) and ridges north of the terminus of Tonsina Glacier, north-central Chugach Mountains, Alaska. Note the fresh moraine deposits and the tarn lakes. Also note the excellent examples of aretes (yellow arrows). Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Bruce Molnia, Terra Photographics

Bridal Veil Falls issues forth from a classic example of a hanging valley into Yosemite Valley here in Yosemite National Park. Main glacial valley is in the middle (pink area) and the hanging valley (tributary glacial valley) is outlined in yellow. Image Copyright © Lynn Newman

Depositional Features Associated with Alpine Glaciation

 A. All glacial deposits are included in the term drift (both sorted and unsorted)

  1. Till: unsorted drift laid down by the ice
  2. Glaciofluvial deposits: meltwater deposits (sorted)

B. Moraines: Usually linear deposits of debris (till) carried by the glacier. Occur along margins of the ice. Appear as small hills or ridges after glacier has melted away. Material is unsorted.

Types of Alpine Moraines

If the moraine marks the farthest extent of the ice = terminal moraine

if the moraine marks a halt in retreat (a stable, equilibrium period) = recessional moraine

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C. Meltwater filled with sediment is called glacial outwash. Often resembles an alluvial fan and is called a valley train. Material is sorted.

Continental Glaciers

Continental glaciers cover at least 50,000 km2 and are unconfined by topography. Areas eroded by continental glaciers are smoother and rounded.

A. During the Pleistocene (last ice age = 1.65 million years ago), there were two additional major continental ice sheets: the Laurentide Ice Sheet (N. America) and the Scandinavian Ice Sheet (N. Europe).

B. During the Pleistocene, there were four major advances and retreats. Advances are cooler periods called glacials and retreats are warmer times, called interglacials. The ice did not disappear during the entire period, it just advanced and retreated a bit. During the Last Ice Age, glaciers covered 30% of the Earth's surface and lobes of the Laurentide Ice Sheet advanced as far south as the Ohio and Missouri Rivers.

C. Two continental glaciers exist today: Greenland (3000m or 10,000' thick) and Antarctica (4000m or 13,000' thick)

D. What happens to the land when it is under that much ice? Answer.

E. Isostatic depression: land is pushed down by the weight of the ice, sometimes below sea level. Example: Hudson Bay

F. As the ice retreats, land starts to rise which is called isostatic rebound. Sweden, Canada and eastern Siberia are currently rebounding at about 2 cm / year.

G. Ice shelf: a large plate of moving ice that has slid out on top of the ocean. Can be 100s of feet thick, e.g., Ross Ice Shelf, Antarctica

H. Sea Ice: frozen sea water

I. Iceberg: formed by pieces of a glacier breaking off (calving). Only find icebergs around Greenland and Antarctica and a few small glaciers around the oceans.

J. Meltwater stream: a river created by melting ice. Meltwater streams carry tons of sediment and tend to be braided streams.

Continental Glacier Landforms and Features

A. Moraines: continental glaciers are so big that they don't have sides. Therefore, there are no lateral or medial moraines. They do have terminal and recessional moraines (again, unsorted materials).

B. Continental end moraines show the shape of the ice sheet that created them. They often appear as huge curved paths called "lobes". When two lobes of a continental glacier join, they form an "interlobate" moraine. The interlobate moraine is only found with continental glaciers.

C. After a glacier is gone, the moraine appears as a belt of low hills and ridges

D. Outwash plains: extensive, smooth plains covered with sorted deposits carried by meltwater. Can cover 100s of square miles.

E. Outwash plains can be marked by small holes and knobby hills. The hills are called "kames" and the holes are called "kettles". Depressions fill with water to become "kettle lakes". An area with these features is called Kame and Kettle Topography.

A kame is a conical hill of sorted glaciofluvial deposits. Sediment deposition in ice pits and crevasses.

F. Drumlin: a streamlined hill composed of molded glacial drift (unsorted). Shape indicates the direction of ice flow where the broad, steep side faces the direction the ice came from.

G. Esker: a narrow, winding ridge composed of glaciofluvial gravels. Leaves a winding ridge of rock (sorted).

H. Glacial Erratics: boulders that differ from the local bedrock. Rocks brought and dumped by a glacier and can indicate the direction of ice flow.

I. Deranged drainage: numerous lakes, swamps, low relief, extensive exposure of striated and polished rock with little to no soil. Landscape called Ice-scoured plains.

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Above: Kame, from southcentral Wisconsin.

Above: Drumlins in Lake Sinissippi, an artificial lake near Hustisford, WI. View is toward the northwest. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Louis Maher

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Above: Esker in Blue Lake, 10 miles northeast of Elk River, MN.

View is to the northeast. Image Courtesy United States Geological Survey; Image source: Earth Science World Image Bank http://www.earthscienceworld.org/images Copyright © Louis Maher

Above: Glacial erratic.

  


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