What are Glaciers?
Glaciers are flowing masses of ice, created by years of snowfall and cold local temperatures. Approximately one tenth of the Earth is covered by glaciers. Glaciers are most numerous near the poles, covering most of Antarctica and Greenland and parts of Iceland, Canada, Russia, and Alaska; they also exist in mountainous regions on every continent except Australia. From the air, a glacier looks deceptively smooth and pliant; in reality, it is an abrasive mass that can reshape the Earth. The glaciers themselves are being reshaped by human activity. Recent measurements show that glaciers have been melting worldwide since the beginning of the Industrial Revolution in the mid-nineteenth century (when human beings first began to add large amounts of greenhouse gasses to the atmosphere). Water from melting glaciers is a significant input to rising sea levels worldwide, which threaten coastal ecosystems and the approximately 100 million people who live 3.28 ft (or about 1 m) or less above sea level.
How Glaciers are formed?
Snow that falls on low ground does not lie for long and soon melts away in the first warmth of early spring. On higher ground snow remains for a long time but even there it is usually all melted away by May. But there are places where even the summer sunshine cannot banish the snow. This is on mountains like the Alps at heights of more than 3,000 meters.
Glaciers are flowing masses of ice, created by years of snowfall and cold local temperatures. Approximately one tenth of the Earth is covered by glaciers. Glaciers are most numerous near the poles, covering most of Antarctica and Greenland and parts of Iceland, Canada, Russia, and Alaska; they also exist in mountainous regions on every continent except Australia. From the air, a glacier looks deceptively smooth and pliant; in reality, it is an abrasive mass that can reshape the Earth. The glaciers themselves are being reshaped by human activity. Recent measurements show that glaciers have been melting worldwide since the beginning of the Industrial Revolution in the mid-nineteenth century (when human beings first began to add large amounts of greenhouse gasses to the atmosphere). Water from melting glaciers is a significant input to rising sea levels worldwide, which threaten coastal ecosystems and the approximately 100 million people who live 3.28 ft (or about 1 m) or less above sea level.
How Glaciers are formed?
Snow that falls on low ground does not lie for long and soon melts away in the first warmth of early spring. On higher ground snow remains for a long time but even there it is usually all melted away by May. But there are places where even the summer sunshine cannot banish the snow. This is on mountains like the Alps at heights of more than 3,000 meters.
This height is known to geographers as the snowline or the limit of persistent snow. It varies according to the location on the globe: in the tropics, for example, the snowline is much higher at about 5,000 meters, and in the polar regions it is practically at sea-level.
If all the snow that falls on the Earth were to stay on the ground winter after winter, all the highest mountain-tops would be covered many times over. But snow only stays on valleys and hallowed-out mountainsides to form snow-fields.
When snow falls it is light and feathery. A piece of snow of this type measuring a cubic meter weights about 75 kilograms. But as the snow heaps up on the ground its weight causes the bottom layers to freeze into a hard glassy mass and the weight of a cubic meter rises to about 900 kilograms. The upper slopes of all the world’s mountain ranges are covered in these masses of snow. Once it finds an outlet this frozen snow begins to move slowly like a gigantic river of ice and a glacier is born.
If all the snow that falls on the Earth were to stay on the ground winter after winter, all the highest mountain-tops would be covered many times over. But snow only stays on valleys and hallowed-out mountainsides to form snow-fields.
When snow falls it is light and feathery. A piece of snow of this type measuring a cubic meter weights about 75 kilograms. But as the snow heaps up on the ground its weight causes the bottom layers to freeze into a hard glassy mass and the weight of a cubic meter rises to about 900 kilograms. The upper slopes of all the world’s mountain ranges are covered in these masses of snow. Once it finds an outlet this frozen snow begins to move slowly like a gigantic river of ice and a glacier is born.
Types of Glaciers?
Ice masses take on a variety of characteristics as they flow and retreat. Glaciers that pour down a valley from mountainous ground, for example, usually follow paths originally formed by rivers of snowmelt in the spring and summer. These glaciers, termed alpine or mountain glaciers, end either in valleys or in the ocean and tend to increase the sharpness and steepness of the mountains surrounding them by eroding them. They are thus, partially responsible for carving the high-relief mountain peaks of the Himalayas, Andes, and alpine regions of the Cascades and Northern Rocky Mountains.
Piedmont glaciers are large, gently sloping ice mounds. Piedmont glaciers are common in Alaska, Greenland, Iceland, and Antarctica.
Glaciers often form in small bowl-like valleys called cirques on the sides of mountains. Found in Norway, Iceland, Greenland, and Antarctica, glaciers within cirques usually do not move out of their basinlike areas.
The largest form of glacier is called an ice sheet or continental glacier, a huge ocean of ice that spreads slowly outward from its center. Ice sheets may cover millions of square miles and are so heavy that they cause the continental crust beneath to float lower on the Earth's mantle, like a heavy-laden barge. The largest ice sheet is found on Antarctica, where the ice is more than 2.5 mi (4 km) thick at its center, hiding entire mountain ranges (mapped using seismic waves and radar). The Antarctic ice sheet covers more than 5 million sq mi (12.9 million sq km), which exceeds the combined areas of the United States, Mexico, and Central America. It contains about 90% of all the world's ice and 70% of its fresh water. The Greenland ice sheet is 670,000 sq mi (1,735,000 sq km) in area, covering virtually the entire island. Smaller ice sheets are found in Iceland, northern Canada, and Alaska.
Glaciers' effects
While the Greenland and Antarctic ice sheets are enormous, they are only a fraction the size of the kilometers-deep ice sheets that have covered large portions of the Earth during extensive periods of glaciation, such as during an Ice Age. Geologists assume that glaciers have expanded to mammoth proportions at least six times over the past 960 million years, sweeping slowly down from the polar regions every 250 million years or so and persisting, usually, for 5–10 million years.
Most glaciers that exist today are remnants of the last glacial period, which lasted from 1.8 million to 11,000 years ago and which occurred in four periods of advance and retreat. At their maximum, the glaciers of this period covered 30–of the Earth's land surface, particularly in the Northern Hemisphere. As the glaciers advanced, they lowered sea levels by hundreds of feet, creating land bridges between continents. This is the most likely explanation for how humans reached North America from Asia—that is, glaciers probably crossed over via land that was exposed between Asia and Alaska.
As a glacier advances it grinds up the land beneath it, scooping up rocks and soil. These add to the glacier's weight and abrasive power; V-shaped valleys can be altered to U-shaped valleys, and mountains can go from peaky to rounded. As they melt, this burden of rock, gravel, and dirt is dropped in place. This material is termed glacial till. Glacial till, which accumulates preferentially along the leading edges of the advancing glacier, is deposited in huge mounds along glacier's edge when it ceases to advance and begins to melt, creating new hills, or moraines. Formerly placated areas are covered by 200 –1,200 ft (61–366 m) of till that was carried and dropped by glaciers. Chunks of ice buried in this till create large depressions that later become lakes called "kettle lakes." Glaciers also scour the land to great depths, creating larger lakes such as the Great Lakes of North America.
During the last ice age, much of the Earth's surface was depressed due to the weight of the glaciers. As the glaciers retreated, the crust began to rise. This crustal rebounding, as it is called, is still occurring at in parts of North America and Europe.
Glaciers advance relatively slowly, moving anywhere from a few centimeters per year to a few meters per day. When ice melts under the glacier as a result of pressure from above and friction with the ground, accumulated meltwater may act like a lubricant to increase the glacier's rate of flow; this sudden increase in speed is termed a surge.
Glaciers - A clue to the Earth's past and future
While the effects of glaciers—scouring, till deposits, and rebound—can tell us where they have been in the past. Scientists continue to debate the reasons why ice ages occur, but the consensus view is that several factors interact to produce them: (1) placement by continental drift of large land masses near the poles, on which glaciers can form; (2) uplift of continental plates by plate-tectonic forces, with subsequent changes in global circulations of air and water; (3) reductions in the amount of carbon dioxide in the atmosphere, with diminished greenhouse effect; and (4) long-term oscillations in the shape of the Earth's orbit and the tilt of the its poles.
Present-day glaciers are providing clues to recent and future changes in climate. Satellite radar and aircraft-mounted laser altimetry systems have recently been used to measure contemporary glaciers with great accuracy; the data show that many glaciers are retreating, reflecting an overall global warming trend. The glaciers in the Alps in Europe have lost an estimated one-third to one-half of their ice in the last century, while Alaskan glaciers losing ice thickness at an average rate of about 6 ft (2 m) per year, retreating at rates of 2 mi (3.2 km) in 20 years. By glacial standards, this is a hasty retreat. The U.S. National Academy of Sciences has predicted that, if global temperatures rise from 1.5–5°F (0.75–2.5°C) over the next century as a result of the greenhouse effect, significant portions of the Earth's ice cover could melt. This would result in flooding of every continent's coastlines. Indeed, sea level is already rising. Global average sea level has been rising at about .12 in (3 mm) per year for the last decade, and this rate is expected to accelerate. Alaskan glaciers—which contain for about 13% of the world's glacier area but whose melting accounts for about half of observed sea-level rise—have been thinning twice on average as fast over the last five years as during the preceding 40. In Peru, glacial melting is occurring at exponentially increasing speed; the present rate is 33 times the rate between 1963 and 1978. There is little doubt that global climate change caused by human agricultural and industrial activity is contributing strongly to these effects; data from Antarctic ice cores have shown a direct correlation between warming and cooling trends and the amount of the two major greenhouse gases, carbon dioxide and methane, in the atmosphere. These same cores show significant increases in both gases in the past 200 years. Today, thanks to human activity, atmospheric carbon dioxide is at its highest level in at least 420,000 years.
Glaciers may offer clues about the possibility of life on other planets. In Switzerland, bacteria have been found living under the ice sheets. If microbes can thrive in the dark, cold environment under glaciers, the vast ice sheets that blanket Jupiter's moon Europa and which underlie the soil of Mars may have their own microscopic residents.
KEY TERMS
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Alpine or mountain glaciers
—Glaciers that form at high elevations in mountain regions and flow downhill through valleys originally created by rivers.
Cirques
—Small basinlike depressions in the sides of a mountain that provide sites for circular glaciers to form.
Glacial till
—Rocks, soil and other sediments transported by a glacier then deposited along its line of farthest advance.
Ice age
—An extended period of time in the Earth's history when average annual temperatures were significantly lower than at other times, and polar ice sheets extended to lower latitudes.
Ice sheet
—The largest form of glacier and the slowest moving, covering large expanses of a continent.
Iceberg
—A large piece of floating ice that has broken off a glacier, ice sheet, or ice shelf.
Kettle lakes
—Bowl-shaped lakes created by large boulders or ice blocks, which formed depressions in the Earth's surface.
Meltwater
—Melted ice in the glacier's bottom layer, caused by heat that develops as a result of friction with the Earth's surface.
Moraines
—Large deposits of glacial till that form hills.
Piedmont glacier
—Large, gently sloping glaciers found at the feet of mountains and fed by alpine glaciers.
Surging
—A sudden increase in a glacier's movement as a result of meltwater beneath the glacier that decreases friction.
Resources
Books
Bender, Lionel. Glacier: The Story of the Earth. New York: Franklin Watts, 1988.
Walker, Sally M. Glaciers: Ice on the Move. Minneapolis: Carolrhoda Books, 1990.
Periodicals
Bradley, Ray. "1000 Years of Climate Change." Science. 5470 (May 26, 2000): 1353–1355.
Meier, Mark F., and Mark B. Dyurgerov. "How Alaska Affects the World." Science. 5580 (July 19, 2002): 350–351.
Other
