Ice Caps and Glaciers

Glaciers are masses of ice in motion that move slowly over the surface of the earth, under their own weight and through the action of gravity. For a glacier to form there must be sufficient precipitation, and it must be cold enough for the snow and resulting ice to remain throughout the entire year; that is a function of latitude and altitude.

The formation of a glacier is similar to the formation of a monomineralic metamorphic rock (a rock made of one essential mineral). Minerals are naturally occurring, crystalline, inorganic substances with a usually precise composition. Snowflakes clearly fit the definition: their composition is H2O, they form hexagonal crystals, and they are naturally occurring. Snowflakes accumulate as a fluffy mass of loose crystals, similar to the way in which sediments are deposited on the seafloor. Instead of having water trapped between the grains, however, snowflakes contain air.

As more snow falls to the surface the overlying snow presses down, forcing some of the air out and causing the points of the flakes to melt. The resulting water tends to migrate and refreeze toward the center of the flake, producing rounded grains called firn. These gran ular particles stick to each other as the melting and refreezing process continues, forming a mass similar to a sedimentary rock. With further snowfall and pressure more air is driven out, gradually transforming firn to a meta-morphic rock, glacial ice, a rock that has been formed by the addition of heat and pressure and contains interlocking crystals of ice. The ice has a blue color because all of the other colors are absorbed and only blue is reflected. This process of ice formation may take a few years or hundreds of years or even more, depending on the amount of melting and recrystallization and snow accumulation.

When the ice mass grows large enough in weight and thickness to flow down slope, it is called a glacier. Glaciers generally move slowly, perhaps 15 m a year, a speed that cannot be seen while standing in front of the ice front. However, some glaciers, especially those that are not frozen to the underlying rocks, can surge forward at 10 km a year, lubricated by water at their base.

Glaciers are classified by their size and where they occur. As the term describes, alpine glaciers, also known as valley or mountain glaciers, are found within mountain valleys. Where several alpine glaciers emerge from adjacent valleys onto a plain in front of the mountain, they coalesce forming a piedmont glacier like the Malaspina Glacier in Alaska, which is larger than Rhode Island. Large glaciers that cover parts of large land masses but are 50,000 square kilometers or less in size are called ice caps, while large ice masses are termed ice sheets and are usually a kilometer or more in thickness.

A glacier is not a rigid mass of ice. Glaciers behave more like warm wax, or thick honey or molasses, slowly moving at different rates within different levels of the ice mass. As a glacier travels down a valley its flow is retarded at the sides and bottom of the ice where it is in contact with bedrock. But within the glaciers the flow is not held back, and it flows more rapidly. Ice caps are so thick that the bedrock usually does not play an important role. They flow from the higher parts of the ice to their periphery, from the thicker part, to the thinner edge even on a flat surface. If you pour molasses, or almost any other liquid, note how it spreads away from the center. When you increase the pouring rate the speed of the flowing molasses will also increase. Ice caps behave in a similar way.

Many glaciers develop crevasses or cracks as they flow over an irregular bedrock configuration. Glaciers terminate where melting and evaporation, termed ablation, is equal to or greater than the forward motion of the ice: it has gone as far as it can under the current climatic conditions. Where glaciers enter the sea, they break off in pieces as a result of the up and down movements of the tide and the crevasse pattern. This calving process gives rise to the icebergs that float with the currents in the northern and southern seas and are a hazard to ships. The number and range of icebergs are determined by climate; during the Little Ice Age, 1540-1890, they were able to float farther away from polar areas than they do today.

The ice sheets on Antarctica and Green land contain about 75 percent of the freshwater on earth, an amount equivalent to about sixty years of worldwide precipitation. If the earth continues to heat up this resource will be lost, most of it mixing with seawater and becoming unavailable for human consumption. In addition, complete melting will cause sea levels to rise 80 m worldwide, drowning most coastal cities.

The Greenland ice sheet covers about 80 percent of Greenland's land mass and has an area of about 1,800,000 square kilometers, containing some 2,620,000 cubic kilometers of ice that is more than 3,200 m thick at the center of the island. This accounts for 8 percent of worldwide ice, and if all of it melted it would raise sea levels by about 6.5 m. The mass of ice flows from the center toward the perimeter of Greenland, where it encounters high mountains at its periphery, forcing the ice sheet to squeeze through narrow valleys to the sea, where they calve and produce most of the icebergs of the North Atlantic.

On the other hand, ice on Antarctica covers about 90 percent of the continent, about 13,586,000 square km, and in places it is more than 4,200 m thick. As a result there are 30,109,800 cubic kilometers of ice, a little more than 91 percent of the total. If it all melted, it would cause sea levels to rise by 73.44 m. Bedrock configuration mapping shows that the land below the ice is mountainous, and because of the weight of the ice, parts of West Antarctica may be 2.5 km deep. In a number of places the ice has flowed into the sea as a contiguous mass, forming an ice shelf that floats in the sea. The Ross Ice Shelf is about the size of the state of Texas and approximately 400 m thick.

The rest of the ice, valley glaciers, and ice caps cover 680,000 square km and have a volume of 180,000 cubic kilometers; if it were to melt it would cause sea level to rise about a half-

Broken Ice Shelf
The edge of the Ross Ice Shelf, Antarctica (broken up by a U.S. Coast Guard icebreaker during the Byrd expedition), 1947 (Library of Congress)

meter. Nevertheless, at this time about 80,000 glaciers have been inventoried, and some estimates indicate that there may be twice as many.

It has been suggested that rapid melting of glacial ice can cause the thermodynamic balance of ocean currents to be altered, resulting in a change of climate for adjacent continents. It has also been suggested that rapid melting of the Greenland ice cap would pour so much cold water into the ocean that the warm Gulf Stream would be altered, causing Europe to become much colder.

Glaciers are important tools for determining the nature of past and present climate change. By taking cores from glaciers and examining the trapped gases and particles, scientists determine the composition and climatic parameters as well as the geologic processes going on at the time. Numerous cores some thousands of feet long have been taken from Greenland, Antarctica, and many other glaciers from around the world. Some contain records as old as 500,000 years.

Antarctic glaciers are storehouses of meteorites. Thousands of meteorites have been discovered embedded in the glacial ice. Usually black, they stand out clearly in the white ice and are not easily lost among rocks strewn across the surface.

—Sidney Horenstein

See also: Glaciation; Pleistocene Epoch

Bibliography

Krabill, W., et al. 2000. "Greenland Ice Sheet High Elevation Balance and Peripheral Thinning." Science. 289 (5478): 428-430; Lythe, Mathew B., et al. 2001. "BEDMAP: A New Ice Thickness and Subglacial Topographic Model Of Antarctica." 2001. Journal Of Geophysical Research, B. 106 (6): 11335-11351; Plummer, Charles C., David McGeary, and Diane Carlson. 2002. Physical Geology, 9th ed. New York: McGraw-Hill; Press, Frank, and Raymond Siever. 2000. Understanding Earth, 3rd ed. New York: W. H. Freeman and Company; Winograd, Isaac J. 2001. "The Magnitude and Proximate Cause Of IceSheet Growth Since 35,000 Yr B.P." Quaternary Research. 56 (3): 299-307.

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