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
Horn Photo by Dr. Sharon Johnson University of California, Berkeley
Cirque A bowl-shaped depression formed at the zone of accumulation by glacial erosion Photo by Dr. Sharon Johnson University of California, Berkeley
Arete Photo by Dr. Sharon Johnson, University of California, Berkeley
U-shaped glaciated valley, Sierra Nevada, CA Photo 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
Photo Jay Sinclair 1990 Roche moutonnee with striations, Marquette, Michigan
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, CA Glaciated mountain with horn, cirques, U-shaped valley, and erratics Photo by Dr. Sharon Johnson, University of California, Berkeley
Erratics in transport on glacier surface Photo by Dr. Sharon Johnson, University of California, Berkeley
Medial moraine Photo by Dr. Sharon Johnson, University of California, Berkeley
Cirque & Moraines Photo by Dr. Sharon Johnson University of California, Berkeley
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.