Further Reading

Gilbert, G. K. "Report on the Geology of the Henry Mountains-Reprint of 1880 Paper." Earth Science 31 (1976): 68-74.

glacier, glacial systems Any permanent body of ice (recrystallized snow) that shows evidence of gravitational movement is known as a glacier. Glaciers are an integral part of the cryosphere, that portion of the planet where temperatures are so low that water exists primarily in the frozen state. Most glaciers presently reside in the polar regions and at high altitudes. At several times in Earth's history, however, glaciers have advanced deeply into midlati-tudes, and the climate of the entire planet was different. some models suggest that at one time ice covered the entire surface of the Earth, a state referred to as the "snowball Earth." Glaciers are dynamic systems, always moving under the influence of gravity and changing drastically in response to changing global climate systems. Thus, changes in glaciers may reflect coming changes in the environment.

several types of glaciers exist. Mountain glaciers form in high elevations and are confined by surrounding topography, such as valleys. Specific types of mountain glaciers include cirque glaciers, valley glaciers, and fjord glaciers. Piedmont glaciers are fed by mountain glaciers but terminate on open slopes beyond the mountains. Some piedmont and valley glaciers flow into open water, bays, or fjords, and are known as tidewater glaciers. Ice caps form dome-shaped bodies of ice and snow over mountains and flow radially outward. Ice sheets are huge, continent-sized masses of ice that presently cover Greenland and Antarctica and are the largest glaciers on Earth, making up about 95 percent of all the glacier ice on the planet. If global warming were to continue to melt the ice sheets, sea level would rise by 230 feet (66 m). A polar ice sheet covers Antarctica. This ice sheet consists of two parts that meet along the Trans-antarctic Mountains. The Antarctic ice sheet contains shelves (thick glacial ice that floats on the sea), which form many icebergs by breaking and separating in a process called calving and which move northward into shipping lanes of the Southern Hemisphere.

Polar glaciers form where the mean average temperature lies below freezing, and these glaciers have little or no seasonal melting because the temperature does not increase sufficiently. Other glaciers, called temperate glaciers, have seasonal melting periods, when the temperature throughout the glacier may be at the pressure melting point (when the ice can melt at that pressure and both ice and water coexist). All glaciers form above snow line, the lower limit at which snow remains year-round, located at sea level in polar regions and at 5,000-6,000 feet (1,525-1,830 m) at the equator (Mount Kilamanjaro in Tanzania has glaciers, although these are melting rapidly).

Glaciers are sensitive indicators of climate change and global warming, shrinking in times of warming and expanding in times of cooling. Glaciers may be thought of as the "canaries in the coal mine" for climate change.

The Earth has experienced at least three major periods of long-term frigid climate and ice ages, interspersed with periods of warm climate. The earliest well-documented ice age is the period of the "snowball Earth" in the Late Proterozoic from about 710-650 million years ago, although there is evidence of several earlier glaciations. Beginning about 350 million years ago the Late Paleozoic saw another ice age lasting about 100 million years until 250 million years ago. The planet entered the present ice age about 55 million years ago. Varied underlying causes of these different glaciations include anomalies in the distribution of continents and oceans and associated currents, variations in the amount of incoming solar radiation, and changes in the atmospheric balance between the amount of incoming and outgoing solar radiation.

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The Basic Survival Guide

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