Glaciers
Glaciers
Clues to Earth’s Past and Future
Glaciers are flowing masses of ice created by years of snowfall and locally cold temperatures. Approximately one tenth of the Earth is covered by glaciers, which are most numerous near the poles. Glaciers cover most of Antarctica and Greenland and parts of Iceland, Canada, Russia, and Alaska. They also exist in mountains on every continent except Australia. 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 Form
Glaciers are created in areas where the air temperature where snow can accumulate without melting from year to year. Snowflakes may partially melt
when they come into contact with the ground. As the air temperature drops further, the partially melted snow refreezes, turning into ice. The resulting mixture of snow and ice is compacted as additional layers of snow accumulate on top. Eighty percent of fresh snow is air; as the weight of fresh upper layers of snow and ice increases, air is pressed out of the lower layers. Over time, snow accumulates and the slab of ice grows steadily thicker (if the glacier is in a growth phase). Eventually, the layer of ice begins to flow slowly downhill under the influence of gravity and it is considered a glacier.
Types of Glaciers
Glaciers take on a variety of characteristics as they flow and retreat. Glaciers that flow 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 steepness of the surrounding mountains surrounding by erosion. Glaciers are partially responsible for carving the high-relief mountain peaks of the Himalayas, Andes, and alpine regions of the Cascade Mountains and northern Rocky Mountains.
Piedmont glaciers are large, gently sloping ice mounds. Piedmont glaciers are common in Alaska, Greenland, Iceland, and Antarctica.
Alpine glaciers commonly 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 type of glacier is the ice sheet or continental glacier, which spreads slowly outward from its center. Ice sheets may cover millions of square miles and are heavy enough to bend Earth’s crust. The largest existing ice sheet is in Antarctica, where the ice is more than 2.5 mi (4 km) thick at its center and covers entire mountain ranges (the buried mountains were 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.
Effects of Glaciers
The Greenland and Antarctic ice sheets are only a fraction the size of the ice sheets that have covered large portions of Earth during previous periods of glaciation popularly known as ice ages. Geologists have found evidence of at least six major ice ages during the past 960 million years, moving 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 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. Archeological discoveries suggest that humans first entered North America from Asia by crossing a land bridge across the present day Bering Sea during the last ice age.
As a glacier advances it erodes soil and rock that are incorporated into the ice and add to the glacier’s weight and abrasive power. As they melt, this burden of rock, gravel, and dirt is left in place to form a deposit known as glacial till. Glacial till, which accumulates preferentially along the leading edges of the advancing glacier, is deposited in mounds along glacier’s edge when it ceases to advance and begins to melt, creating new hills or ridges known as moraines. Some previously glaciated areas are covered by 200– 1,200 ft (61–366 m) of till. Chunks of ice buried in till melt to form large depressions known as kettles, which can fill with water and become kettle lakes. Glaciers can also scour the land to great depths, creating larger lakes such as the Great Lakes of North America.
During the last ice age, parts of Earth’s lithosphere was bent downward under the weight of the glaciers. As the glaciers retreated and the weight was removed, the surface began to readjust to its previous height. Crustal rebound, as it is called, is still occurring at in parts of North America and Europe. The rate at which rebound has occurred since the last ice age has allowed geologists to estimate the viscosity, or resistance to flow, of Earth’s asthenosphere.
Glaciers advance relatively slowly, moving anywhere from a few inches(centimeters) per year to a few feet (meters) per day. When ice melts under the glacier as a result of pressure from above and friction with
KEY TERMS
Alpine or mountain glaciers —Glaciers that form at high elevations in mountain regions and flow downhill through valleys originally created by rivers.
Cirques —Bowl-shaped 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 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 Earth’s surface.
Meltwater —Melted ice in the glacier’s bottom layer, caused by heat that develops as a result of friction with 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.
the ground, accumulated meltwater may reduce stress along the base of the ice and increase the glacier’s rate of flow; this sudden increase in speed is termed a surge
Clues to Earth’s Past and Future
Scientists continue to debate details of the reasons why ice ages occur, but the consensus view is that several factors interact to produce global ice ages: (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 Earth’s orbit and the tilt of the its poles.
Present-day glaciers provide clues to recent and future climate change. 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 rapid 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, significant portions of Earth’s ice cover could melt and cause widespread flooding of coastal areas. 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 if warming continues. Alaskan glaciers—which contain for about 13% of the world’s glacier area, but the melting of which accounts for about half of observed sea-level rise—have been thinning on average twice as fast over the last five years as during the preceding 40. In Peru, glacial melting is occurring at increasing rates; 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 the retreat of glaciers around the world. 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, in large part as the result of 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 on Earth, the vast ice sheets that blanket Jupiter’s moon Europa and which underlie the soil of Mars may have their own microscopic residents.
See also Ice age refuges.
Resources
BOOKS
Hambrey, M. and J. Alean. Glaciers. 2nd Ed. Cambridge: Cambridge University Press, 2004.
Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, New Jersey: Prentice Hall, 2002.
Sally Cole-Misch