Glaciers & Ice Ages

Glaciers• Valley or Alpine glaciers

• Form in mountainous areas• Move downslope in valleys

• Ice sheets• Move outward from center

• Continental glaciers - large scale, “ice age” type. Presently found only in Greenland & Antarctica.

• Ice caps - similar to but smaller than continental glaciers. Found in Iceland & elsewhere.

• Piedmont - “foot of the mountain” glaciers.

The only present-day continental ice sheets are those covering Greenland and Antarctica. Their combined areas represent almost 10 percent of Earth’s land area. Greenland’s ice sheet occupies 1.7 million square kilometers, or about 80 percent of the island. The area of the Antarctic Ice Sheet is almost 14 million square kilometers. Ice shelves occupy an additional 1.4 million square kilometers adjacent to the Antarctic Ice Sheet.

Glacial movement• Slippage

• Lubricated by water & mud• Plastic flow

• Below about 50m, ice will flow due to pressure• Top 50m - zone of fracture - crevasses• Movement may be very slow to several

meters/day• Movement can be variable - “surging”

Ice movement and changes in the terminus at Rhone Glacier, Switzerland.

Greenland Glacier Photo by Dr. Sharon Johnson, University of California, Berkeley

• Zone of Accumulation• Where annual snowfall exceeds annual melting• Glaciers form only on land

• Zone of Wastage (or ablation)• Where rate of melting at least equals advance

of ice• Flow is always downslope (valley glaciers) or

outward from zone of accumulation (ice sheets).• Icebergs form where glaciers end at the sea.

The snowline separates the zone of accumulation and the zone of wastage. Above the snowline, more snow falls each winter than melts each summer. Below the snowline, the snow from the previous winter completely melts as does some of the underlying ice.

Whether the margin of a glacier advances, retreats, or remains stationary depends on the balance between accumulation and wastage (ablation). When a glacier moves across irregular terrain, crevasses form in the brittle portion.

Glacial Erosion• Glacial plucking

• fragments of bedrock are frozen into the sole of the glacier & plucked out

• Abrasion• Rock flour (loess)• Glacial striations• Glacial polish

Valley glacier erosional features

Photo by Dr. Sharon Johnson University of California,

Berkeley

Horn

Photo by Dr. Sharon Johnson University of California,

Berkeley

CirqueA bowl-shaped depression formed at the zone of accumulation by glacial erosion

AretePhoto by Dr. Sharon Johnson, University of California, Berkeley

U-shaped glaciated valley, Sierra Nevada, CAPhoto by Dr. Sharon Johnson, University of California, Berkeley

Yosemite Valley, CA - Glaciated valley with hanging valleys

Photo by Dr. Sharon Johnson, University of California, Berkeley

Glacial striations (grooves) in limestone bedrock, Upper Peninsula of Michigan

Photo © Jay Sinclair 1990

Roche moutonnee with striations, Marquette, Michigan

Photo © Jay Sinclair 1990

Photo © Jay Sinclair 1990

Photo © Jay Sinclair 1990

Glacial Deposition (drift)• Till - unsorted drift deposited directly by

melting glacier• Glacial erratics• Moraines

• End, terminal, & recessional• Ground moraines• Drumlins

• In valley glaciers:• Lateral • Medial

Moraine and outwash plain, Little Souix, Iowa

Photo © Jay Sinclair 2002

Banner Peak, CAGlaciated mountain with horn, cirques, U-shaped valley, and erratics

Photo by Dr. Sharon Johnson, University of California, Berkeley

Erratics in transport on glacier surfacePhoto by Dr. Sharon Johnson, University of California, Berkeley

Medial morainePhoto by Dr. Sharon Johnson, University of California, Berkeley

Photo by Dr. Sharon Johnson University of California,

Berkeley

Cirque & Moraines

Drumlins are streamlined hills formed of till. They form as a glacier advances over existing till.

Photo: T. Poulton, Natural Resources Canada

Glacial Deposition (drift)• Stratified drift (outwash) - sorted by

flowing water• Outwash plain - water flowing away from glacier• Kettles - blocks of ice surrounded by drift• Kames - water flowing down into glacier• Eskers - streams under glacier

Kame, Ladd, Illinois

Photo © Jay Sinclair 2002

Esker, Tawas, Michigan

Photo © Jay Sinclair 2002

Esker, Nova Scotia, Canada

Photo: Nova Scotia Department of Natural Resources

Ice Ages - Pleistocene Epoch• Several advances & retreats of ice over

last 2 million or so years, each cycle lasting on average about 100,000 years. Most notable advances in N. America:• Wisconsinan• Illinoian• Kansan• Nebraskan

Effects of Pleistocene Ice Ages

• Changed drainage of N. America to present pattern (Mississippi)

• Isostatic changes - crust was pushed down, now rising

• Pluvial lakes resulting from increased rainfall (Lake Bonneville)

• Changes in sea level - larger amount of the Earth’s water frozen, resulting in lower sea levels

Pluvial lakes of the Western United States. (After R. F. Flint, Glacial and Quaternary Geology, New York: John Wiley & Sons)

Major moraine systems of the Great Lakes region

Causes of Ice Ages• Plate tectonics - continents move toward

poles• Land masses must be present in order for

glaciers to form• Movement of land masses may disrupt ocean

currents which serve to moderate temperatures in polar regions

• Variations in Earth’s tilt/orbit• “Stretching” of orbit - 100,000 year cycle• Change of tilt - 41,000 year cycle• Precession or “wobble” - 26,000 year cycle

Causes of Ice Ages• Changes in sunspot cycle?

• Sunspots normally vary in number over an 11 year cycle.

• During the “Maunder Minimum” (ca. 1645 - 1715) there were almost no sunspots. This corresponded to the so-called “Little Ice Age” in Europe and North America, during which severe winters caused famine and hardship. This and other evidence suggests that sunspot activity increases the energy output of the sun. A decrease in sunspots, combined with other factors, might trigger an ice age.