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GLACIERS AND GLACIATION

Importance of the Cryosphere

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Snow surface at Dome C Station, Antarctica.  Image taken by Stephen Hudson (GNU).

The cryosphere is part of the hydrosphere and includes glaciers and frozen sea ice.  It is located mostly at the north and south poles.

The cryosphere is an effective agent of erosion and deposition.  It can significantly modify landscapes.

The cryosphere impacts Earth’s climate by cooling water and air, and setting up oceanic and atmospheric circulation patterns.

The cryosphere impacts the biosphere because water cooled by polar ice has the largest concentration of dissolved oxygen and supports a diverse fauna (oxygen gas is most soluble in cold water).

 

Types of Glaciers

Glacier- A large, long-lasting mass of ice that forms on land and moves downhill under its own weight.

The two general categories of glaciers:

1.   Alpine/valley glaciers – glaciers formed in mountainous regions of high elevation (Example: Glacier National Park in Montana).

They are confined to a valley and flow downhill due to gravity.

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St. Mary Lake, Glacier National Park.  Public Domain Image taken by Ken Thomas.

2.   Continental/ice sheet glaciers– glaciers that exist over a large part of a continent (> 50,000 km2) in regions near the earth’s north or south poles (Examples: Antarctica, Greenland).

Antarctica has 85% of glacier ice currently on earth, and Greenland has 10%.

If Antarctica’s ice were to melt, the sea level would rise 70 m or 230 ft.

In Antarctica, two ice sheets (west and east) flow downward and outward from a central point: the Transantarctic Mountains. The East Antarctic Ice Sheet = 2700 m thick and the West Antarctic Ice Sheet = 4800 m thick.

 

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Public Domain Image (generated by NASA's Blue Marble data set).

 

Formation and Growth of Glaciers

Transition from snow to glacier ice: Powdery snow → Granules → Firn (somewhat compacted)→ Glacier ice.

Public Domain Image by Luis María Benítez.

B. Glacier ice moves downhill and is eventually lost (ablated) by melting, evaporation, or calving of icebergs.

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Icebergs calving off the coast of Greenland. Photo by Mila Zinkova (GNU).

Zone of accumulation- The upper part of a glacier where snowfall is collected.

Zone of ablation- The lower part of a glacier where ice is lost.

Equilibrium line- The boundary between these zones.  It  marks the highest point at which the glaciers snow cover is lost during the warm season.

The equilibrium line may migrate from year to year depending upon where there has been more accumulation or more ablation.

If there is more accumulation than ablation→ the equilibrium line moves down the glacier (positive budget).

If there is more ablation than accumulation→ the equilibrium line moves up the glacier (negative budget).

Terminus- The lower edge of a glacier (the position depends on the glacier’s budget).

The velocity of ice movement varies across the glacier with velocities being highest in the thick center and lowest at the thin edges (due to drag).

 

Movement of  Alpine Glaciers

Glacier speed varies from millimeters to tens of meters per day (depends on slope).

The base of a glacier slides over underlying rock on a thin layer of meltwater (basal sliding).

The thicker parts of glaciers (the centers) move faster than thinner parts of glaciers (the edges) which experience more frictional drag.

The lower part of glaciers flows plastically as individual ice grains move relative to each other (the zone of plastic flow).  Here the ice grains deform to accommodate movement.

The upper part of the glacier moves rigidly.  Here fractures called crevasses may develop here.

 

Erosion by Glaciers

Meltwater at the base of a glacier may work its way into cracks and freeze.  Pieces of bedrock may be worked loose and picked up by the moving glacier in a process is called plucking.

Image by Luis María Benítez (GNU).

Rocks carried along at the base of the glacier may grind and polish the bedrock (abrasion).  Large rocks may produce glacial striations in the bedrock as they move over it.

Erosional Features of Alpine Glaciation (Angular Topography):

Alpine Glaciation is characterized by the following types of erosional features:

Public Domain Image by Luis María Benítez.

1. U-shaped valley – The characteristic cross section profile of a valley carved by glacial erosion.

New Zealand's Franz Joseph glacier. Copyright, Matthew Cox

2. Hanging valley – A small valley that terminates abruptly high above a main valley.

Bridal Veil Falls in Yosemite National Park flowing from a hanging valley. (GNU Image)

3. Truncated spur – The triangular facet produced where the lower end of a ridge has been eroded by glacial ice.

4. Rock-basin lake – A lake occupying a depression caused by glacial erosion of bedrock.

5. Cirque – A steep-sided, amphitheater-like hollow carved into a mountain at the head of a glacial valley.

6. Horn – A sharp peak formed where cirques cut back into a mountain on several sides.

7. Arètes – A sharp ridge that separates adjacent glacial valleys.

Horn and arète in the Fairweather Range, Alaska. © Michael Collier

Erosional Features of Continental Glaciation (Rounded Topography):

The weight of the moving ice sheet produces rounded knobs and striated bedrock.  The orientation of the striations can be used to interpret the direction of movement of the ice sheet.

Image by Luis María Benítez (GNU).

Thick ice sheets may bury mountain ranges, rounding and streamlining them in the direction of ice movement.

 

Deposition by Glaciers

Glaciers pick up rock fragments from the underlying bedrock and from the valley walls.

Glaciers carry the debris without tumbling or sorting.  The deposited debris is called glacial till (angular, poorly sorted, and unlayered).  When till becomes lithified, it is called tillite.

Large boulders transported large distances by glaciers are called erratics.

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Yeager Rock, a 400 ton erratic on the Waterville Plateau, Washington.  Wikipedia Public Domain Image.

Moraine- An elongate deposit of glacial till.  There are several types:

Lateral moraine, Sierra Nevada, California. © Marli Miller, University of Oregon

1. Lateral moraine- Forms along the sides of a valley glacier as rocks fall from the steep cliffs of glacial valleys.

2. Medial moraine- Form where two tributary glaciers come together and adjacent lateral moraines join.

3. Recessional moraine- Deposits produced as glaciers recede during melting.

4. End moraine- A type of recessional moraine that forms at the terminus of a glacier during glacial recession as debris piles up long the front of the ice.  They tend to be crescent shaped.

5. Terminal moraine- Special type of end moraine that marks the farthest extent of the glacier.

Drumlin- A streamlined hill of till produced by continental glaciers (the origin is not entirely understood).  Drumlins point in the direction of ice movement.

Image by Luis María Benítez (Creative Commons Attribution ShareAlike 2.5).

Outwash Deposits- When glaciers melt, braided streams develop on the surface of land and carry away material known as outwash.

Eskers- Under the ice, meltwater moves in tunnels and carries sediments that can be deposited in sinuous ridges of sediment called eskers (well-sorted and cross-bedded).

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Esker at Sims Corner Eskers and Kames National Natural Landmark. Public Domain Image.

Kettle- A small depression in outwash formed when a block of glacial ice gets buried by sediment.  When the ice melts, a depression remain.  These may fill with water creating small lakes (glaciated Minnesota is the land of 10,000 lakes).

A kettle in a braided depost of Alaska's Bering Glacier, © Bruce Molnia, Terra Photographics.

Varves- Seasonal deposits of silt and clay that form alternating light and dark layers.

Each silt-clay set represents one year:

    -The silt is deposited during the warmer part of the year when more melting and sediment transport occurs.

    -The clay is deposited during the colder part of the year when the lake is frozen and the clay can settle out of suspension.

Varves can be used to determine how long a glacial lake lasted.

Varves of the Sanpoil River Valley, Washington. USGS Image.

 

 

Glaciation in the Past

At times in the past, colder climates resulted in more extensive glaciation than the present.

The last glacial period began ~2,000,000 years ago and ended ~10,000 years ago.

The North American ice sheet extended as far south as Ohio and Illinois.

Outwash till was deposited over the Midwestern U.S., providing good soil for farming.

The Great Lakes occupy basins that were formed by heavy lobes of ice.  These basins are undergoing isostatic crustal rebound.

Sea Level was ~ 130 m lower than it currently is today because sea water was locked up in large ice sheets at the poles.  Submerged stream channels and land mammal fossils on the continental shelf support lower sea level.

During the end of last glaciation, the climate was more humid than today.  In the western US, there was significant rainfall and numerous lakes (called pluvial lakes) once existed in Nevada, Utah, and California (Example: Utah's Great Salt Lake).

There is some evidence for glaciations occurring all the way back