-Glaciers are large, slow-moving masses of land ice formed by the recrystallization of snow. For complex reasons, if the worldwide climate becomes cooler, and snow accumulates faster than it melts, the glaciers grow larger and cover more land.

-Ice ages (continental glaciation) have occurred many times during the geologic past (4 times within the last 2 million years):
1. Wisconsonian (180,000 to 12,000 years ago)
2. Illinoisian (550,000 to 390,000 years ago)
3. Kansan (1.4 million to 900,000 years ago)
4. Nebraskan (2 to 1.7 million years ago)

-There are 3 classifications of glaciers:
1. Alpine (valley) glaciers (mountainous terrain)
2. Piedmont glaciers (two alpine glaciers joined at base of mountain)
3. Continental glaciers (icecaps which cover much of a continent)

-10% of the Earth's surface today is covered by glaciers. Antarctica and Greenland are virtually covered by glaciers. At the height of past glacial ages, 32% of the Earth was covered by glacial ice. Most of Chicago is covered by glacial or glacial-related deposits .

-The glaciers contain the largest supply of freshwater in the world (more than the Great Lakes combined) - about 2.1 to 2.3% of the world's total water supply.

-If all the ice caps melted today, the worldwide sea level would rise 210 feet , flooding all coastal areas, where most of the world's people now live.

-The transformation of snow into ice in a glacier may be explained:
Snow and ice are both minerals (oh, you didn't know that?) - they are both naturally-occurring, solid, crystalline chemical compounds (formula: H2O). Recalling the hexagonal geometry of snowflakes, it is apparent that snow is "mostly air," and therefore, is less dense than ice. When a snow pile becomes thick, the great overlying weight causes the snow to collapse into a more dense intermediate form, called "firn" and then turn into ice.

-The average particle of ice remains in a glacier for about 10,000 to 100,000 years.

-Glacial ice sheets are so heavy that they cause the continent to sink slightly (somewhat similar to icebergs - large icebergs sit mostly below the water line). When the ice melts, the overlying weight decreases, causing the land to rise back up. This "balancing act" of the Earth's crust is called isostasy .

-Glaciation has worldwide impact, especially on climate, and the geographical distribution of plants and animals. These factors, over time, determine how life evolves on Earth. For example, modern humans (Homo sapiens sapiens ) appeared some time during the last ice age, 12,000 years ago. Typical ice age animals (big and hairy creatures), such as the wooly mammoth, sabertooth cats, giant Irish elk, etc. all became extinct. (If you ever wondered why the moose is the largest member of the deer family, it's because its ancestors lived during the ice age). Glaciers smooth out climatic extremes on the Earth by acting as energy buffers . Example: An increase in solar heat would warm the Earth and melt glacial ice, but much of this heat is absorbed in the process of changing from ice to water.

-Glaciers move, or flow , at a rate determined by gravity and the angle of slope. Under the tremendous weight of a mile-thick (5000+ feet) ice sheet, ice undergoes plastic deformation and behaves as a fluid.

-Glacial advance occurs when ice forms faster than it melts. Glacial retreat occurs when ice melts faster than it forms.

-Build-up of a glacier is gradual (taking about 100,000 years). Melting of a glacier is more abrupt (taking about 10,000 years).

-Glaciation is "not normal"; 90% of the geologic time has been free of glaciation . Our present climate is considered to be an inter-glacial period , and is cooler than the "normal" climate of the geological past, such as the time of the dinosaurs.

CAUSES OF GLACIATION: Extraterrestrial (cosmic) or earth-based?

Cosmic climate mechanisms

-Variations in the Sun's energy
-Supernova explosions
-Movement of the solar system through cosmic dust cloud
-Slight variations ("wobbles") in the earth's orbit or rotation

Earth-based climate mechanisms

-Greenhouse (warm) periods: extensive and prolonged volcanic activity produces excessive carbon dioxide , which traps more of the sun's heat, leading to warming of the climate, and the melting of the glacial icecaps.

-Icehouse (cool) periods: weathering of rocks and the water from melted glacial ice absorb carbon dioxide from air, acting as a carbon dioxide sink by making limestone (carbonate rock). Plants also remove carbon dioxide from the air.

Copyright © 1989 by William K. Tong