Ice sheets and ice streams

Ice sheets are extremely large masses of ice covering most of Greenland and Antarctica. Areas within these ice sheets that move much more rapidly than surrounding areas are known as ice streams. The behavior of ice streams may play an important role in the stability of ice sheets, which could have a profound effect on sea levels.

Ancient Ice Sheets

Throughout Earth's history, periods of time have occurred in which large areas of land were covered with relatively permanent expanses of ice formed from snow. These periods, known as ice ages, sometimes lasted for millions of years. The ice sheets formed during the ice ages had a powerful effect on the physical appearance of the Earth's surface.

The earliest known ice age occurred during Precambrian times, more than 544 million years ago. The most recent and best-known series of ice ages occurred during the Pleistocene epoch, lasting from about 2.5 million to 10,000 years ago. During the last 730,000 years of this period, a series of eight climatic cycles, lasting roughly 100,000 years each, occurred. Each cycle alternated an ice age with a warmer period. Prior to 730,000 years ago, the climatic cycles were more frequent but of lesser magnitude.

During the most extensive Pleistocene ice ages, more than 45 million square kilometers of the Earth's land area was covered with ice sheets. Except Antarctica, most of this ice was located in the Northern Hemisphere. The largest ice sheet was the Laurentide ice sheet. At its greatest extent, this ice sheet reached from northern Canada to southern Illinois and from the Rocky Mountains to the Atlantic Ocean. The other major ice sheet in North America was the Cordilleran ice sheet, which reached from western Alaska to northern Washington.

In Europe, the Scandinavian ice sheet reached as far as Great Britain, northern and central Europe, and northern Russia. Between North America and Europe, the islands of Greenland and Iceland were also covered by ice sheets. Some evidence suggests that other major ice sheets may have existed. These include the Innuitian ice sheet north of Canada, the Barents ice sheet north of Russia, and the Kara ice sheet in the Baltic region.

The movement of ancient ice sheets profoundly affected the land left behind when the ice disappeared. The Laurentide and Scandinavian ice sheets carved numerous lakes in northern North America and Europe. The ice sheets left behind large sediment deposits in other areas, including much of the central United States, western Canada, and central Europe. These areas became flat or gently rolling regions with rich soil well suited for agriculture.

Modern Ice Sheets

The only two ice sheets still found on the Earth are located in Greenland and Antarctica. Masses of ice, similar to ice sheets but much smaller, are known as ice caps or ice fields. Unlike mountain glaciers, which move down sloping valleys in a particular direction, ice sheets and ice caps move outward in all directions.

The Greenland ice sheet in the mid-2020s measured about 1.73 million square kilometers (656,00 square miles), covering about 80 percent of the island's surface. The ice sheet is almost 2,400 kilometers long from north to south, with a maximum width of 1,100 kilometers from east to west at a latitude of 77 degrees north near its northern margin. The average altitude of the ice surface is 2,135 meters. The highest altitudes are found in two elongated domes or ridges. The southern dome reaches a height of almost 3,000 meters at a latitude of 64 degrees north, and the northern dome reaches a height of about 3,290 meters at a latitude of 72 degrees north.

About 8 percent of the freshwater ice on the Earth is found in the Greenland ice sheet. Although most of the surface on which the interior of the ice sheet rests is at about sea level, the margins of the ice sheet occur in more mountainous regions. This prevents the ice sheet from reaching the ocean in most areas. Along a portion of the northwest coast, however, the ice can reach the sea, creating numerous icebergs, which may endanger ships in the North Atlantic.

The Antarctic ice sheet, with an area of about 13.8 million square miles, covers nearly all the continent. Less than 2 percent of the surface of Antarctica is exposed through the ice in the form of mountain ranges or individual mountains known as nunataks. The ice sheet has an average thickness of about 2,000 meters and contains about 90 percent of the world's freshwater ice.

The western part of the Antarctic ice sheet, from a longitude of about 45 degrees west to a longitude of about 165 degrees east, contains numerous nunataks. It also contains two large ice shelves, regions where the ice sheet extends into the ocean. The Fichner-Ronne ice shelf has an area of about 390,000 square kilometers, and the Ross ice shelf has an area of about 496,000 square kilometers.

The eastern part of the Antarctic ice sheet is separated from the western part by the Transantarctic Mountains, which extend between the two ice shelves. Most of the eastern part consists of a single huge ice dome, reaching a peak thickness of more than 4,000 meters.

Movement of Ice Sheets

IIce sheets gain mass by accumulating snow, which is transformed into ice. Although a similar process may take only a few years in glaciers located in wet, relatively warm areas, snow is changed into ice much more slowly in ice sheets. This process may take several thousand years in some parts of Antarctica that are extremely cold and dry.

Ice sheets lose mass through melting or when ice sheet pieces break into the sea. In the Greenland ice sheet, roughly half of the mass loss is caused by the melting of the surface of the ice, and roughly half is caused by the breaking off of icebergs. In the Antarctic ice sheet, very little surface melting takes place. Some melting of the bottoms of the ice shelves occurs, but the majority of mass loss is caused by the breaking off of icebergs, particularly from the ice shelves. In 2008, a piece of the Wilkins Ice Shelf measuring more than 400 square kilometers (160 square miles) broke off in the Antarctic Peninsula. In 2023 and 2024, large icebergs also broke off the Brunt Ice Shelf in Antarctica. 

In general, ice sheets move from central high points outward to the sea. This simple movement becomes more complex in areas where the underlying surface is very rugged. The flow of ice in Greenland is generally outward from the two ice domes. In eastern Antarctica, the flow of ice is generally outward from the single dome. The flow of ice in western Antarctica is more complicated but is generally toward the sea, particularly in the two ice shelves.

The speed of the movement of an ice sheet varies enormously. At the interior, the flow of ice may be only a few meters per year. As it moves outward, the rate increases to tens or hundreds of meters per year. Ice shelves move even more quickly. The outer edge of the Ross ice shelf moves at about 900 meters per year. By comparison, a typical mountain glacier moves at a speed ranging from about 50 to about 400 meters per year.

Ice Streams

Certain areas within ice sheets move at rates much faster than the surrounding ice. These regions are known as ice streams. Some ice streams may move as quickly as 1,000 meters (or 0.6 miles) per year. The largest glacier in the world in the early twenty-first century, Lambert Glacier, is an ice stream in Antarctica that moves around 1,200 meters (0.7 miles) annually. It is more than 2,500 (1.5 miles) meters thick and 400 kilometers (0.7 miles) long. Six ice streams flow into the Ross ice shelf, and two ice streams flow into the Fichner-Ronne ice shelf. Ice streams are also known to exist in other areas of Antarctica and Greenland.

Ice streams resemble glacial surges, in which mountain glaciers move forward much faster than usual. Glacial surges occur when mountain glaciers slide on a layer of mud made of wet sediment. This fact led scientists to speculate that an ice stream occurs when a part of an ice sheet slides on a similar layer of mud. Evidence for this hypothesis began to appear in the 1980s using a combination of satellite data, seismic data from controlled explosions on the surface of the ice, and data from holes drilled through the ice. Scientists confirmed that ice streams rest on a layer of wet, fine-grained sediment. This slippery mud reduces the friction beneath the ice, resulting in an ice stream. Surrounding areas generally rest on dry, solid bedrock, resulting in more friction and a slower rate of movement.

Some scientists believe that the wet sediment that allows the ice stream to move quickly can exist in areas only where the ice sheet rests on a layer of sedimentary rock. According to this hypothesis, the movement of the ice erodes the rock into sediment. At the same time, the ice traps heat from the Earth's interior. This heat causes the base of the ice to melt, wetting the sediment and allowing the ice to move more quickly. The faster movement results in more melting of ice, producing wetter mud. This cycle is repeated, allowing the ice to increase speed until an ice stream is formed.

This simple model of ice stream formation fails to explain some aspects of ice stream behavior. Some ice streams move on a relatively thin layer of sedimentary rock, while ice resting on a thicker layer of sedimentary rock nearby moves much more slowly. Other evidence suggests that the layer of sedimentary rock must be more than 100 meters thick to produce an ice stream. These observations continue to be studied in the twenty-first century.

Stability and Sea Level

The most important question facing scientists who study ice sheets and ice streams is whether the Antarctic ice sheet, which is two kilometers thick and covers 14 million square kilometers, is stable or whether large portions of it may disappear over a relatively short period of time. The melting of a significant part of the ice sheet, or the breaking off of a large portion of it into the ocean, would raise sea levels by an appreciable amount. In the most extreme case, the complete melting of the Antarctic ice sheet would raise the level of the ocean by about 60 meters, drastically altering the coastlines of the continents. The collapse into the ocean of the western part of the Antarctic ice sheet, believed to be less stable than the eastern part, would raise sea levels by about 5 meters, flooding many coastal cities.

Until fairly recently, the eastern part of the Antarctic ice sheet was generally believed to be very stable, having remained mostly unchanged for 15 million years. In the 1980s, fossils of microscopic sea organisms known as diatoms were discovered in the Transantarctic Mountains. This discovery implied that the ocean had extended into the interior of Antarctica 3 million years ago, suggesting that the ice sheet was much smaller at that time. However, scientists studying rock erosion and deposits of volcanic ash in Antarctica found evidence that the ice sheet had remained unchanged for at least 10 million years. Reconciling this contradictory evidence is a major concern of Antarctic researchers.

Most scientists believe that the western part of the Antarctic ice sheet is more likely to undergo a sudden loss of mass than the eastern part because it rests on bedrock, mostly far below sea level. This means that the ice shelves, floating at sea level, can move much more quickly than the ice sheet, which must move uphill. This effect could cause the border between the ice shelf and the ice sheet, known as the grounding line, to move inland, causing more ice to move into the ocean.

Whether the western part of the Antarctic ice sheet could be reduced in size quickly this way may depend on the behavior of ice streams. Some scientists believe that the starting point of an ice stream may be able to move inland over time, causing ice to move into the ocean at an increasing rate. Whether this is possible depends on whether the sediment on which the ice stream slides extends into the interior of the ice sheet.

Other scientists believe that ice streams may actually make the western part of the Antarctic ice sheet more stable than it would be without them. This may happen because the loss of ice from ice streams moving into the ocean could prevent the ice sheet from growing too large and collapsing into the sea under its own weight. In either case, a better understanding of ice streams is needed before scientists can predict the ice sheets' fate. In 2023, the United Nations warned that sea ice may decrease by one-quarter within the century.

For this reason, scientists continue to study ice sheets and ice streams and make critical discoveries. In 2023, scientists discovered an ancient river formed 34 million years ago beneath the East Antarctic ice sheet. This river was located using advanced technology, such as satellites and ice-penetrating radar. New research has also indicated that the Ross Ice Shelf, the largest ice shelf in Antarctica, moves about 6 to 8 centimeters once or twice daily, illuminating the interactions between ice sheets and ice shelves. Understanding ice sheets and ice streams provides insight into the Earth’s past and allows scientists to understand their role in global climate change. 

Principal Terms

glacier: a mass of ice formed from snow that persists from year to year

ice age: a period of time during which extensive ice sheets exist

ice cap: a glacier on a flat area of land

ice shelf: a portion of an ice sheet extending into the ocean

mountain glacier: a glacier in a sloping valley

Bibliography

Bailey, Ronald H. Glaciers. Time-Life Books, 1982.

Bennett, Matthew M., and Neil F. Glassner, editors. Glacial Geology: Ice Sheets and Landforms. 2d ed., Wiley-Blackwell, 2009.

De Angelis, Hernan, and Pedro Skvarca. “Glacier Surge After Ice Shelf Collapse.” Science, vol. 299, 2003, pp. 1560-1562, doi.org/10.1126/science.1077987. Accessed 15 Apr. 2023.

Evers, Jeannie, editor. "Ice Sheet." National Geographic Society, 10 Jan. 2023, education.nationalgeographic.org/resource/ice-sheet. Accessed 15 Apr. 2023.

“Giant Iceberg Breaks Away from Antarctic Ice Shelf.” European Space Agency, 25 Jan. 2023, www.esa.int/Applications/Observing‗the‗Earth/Copernicus/Giant‗iceberg‗breaks‗away‗from‗Antarctic‗ice‗shelf. Accessed 27 July 2024.

Hambrey, Michael, and Jurg Alean. Glaciers. 2d ed., Cambridge UP, 2004.

Hansen, Kathryn. “Antarctica's Brunt Ice Shelf Finally Breaks.” NASA Earth Observatory, 24 Jan. 2023, earthobservatory.nasa.gov/images/150880/antarcticas-brunt-ice-shelf-finally-breaks. Accessed 27 July 2024.

Hughes, Terence J. Ice Sheets. Oxford UP, 1998.

Jacobo, Julia. “Scientists Have Discovered an Ancient River Landscape Hidden Beneath the East Antarctic Ice Sheet.” ABC News, 24 Oct. 2023, abcnews.go.com/International/scientists-discovered-ancient-river-landscape-hidden-beneath-east/story?id=104221028. Accessed 27 July 2024.

Jiskoot, H., and D. T. Juhlin. “Surge of a Small East Greenland Glacier, 2001-2007, Suggests Svalbard-Type Surge Mechanism.” Journal of Glaciology, vol. 55, 2009, pp. 567-570.

Livermore, Beth. “Antarctic Meltdown.” Popular Science, vol. 250, Feb. 1997, pp. 38-43.

Radok, Uwe. “The Antarctic Ice.” Scientific American, vol. 253, Aug. 1985, pp. 98-105.

Robinson, Andrew. “Avalanches, Glaciers, and Ice Ages.” In Earthshock: Climate, Complexity, and the Forces of Nature. Thames and Hudson, 1993.

Walker, Gabrielle. “Great Rivers of Ice.” New Scientist, vol. 162, 1999, pp. 54-58, www.newscientist.com/article/mg16221825-000-great-rivers-of-ice. Accessed 15 Apr. 2023.