- Also called:
- continental glacier
ice sheet
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What is an ice sheet in geology?
In geology, an ice sheet is a glacier that extends in continuous sheets, moving outward in all directions, and covering a large landmass greater than 50,000 square kilometers (19,000 square miles).
When ice sheets extend from the coast over the ocean, they are called ice shelves. The Antarctic and Greenland ice sheets hold approximately 99% of the world's glacier ice. The advance of ice into temperate latitudes is a distinctive change seen during the Quaternary Period. Glacial erosion has sculpted deep alpine valleys and fjords.
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How are ice sheets different from glaciers?
Ice sheets and glaciers are both large masses of ice, but they differ in size and origin. Ice sheets are more extensive, exceeding 50,000 square kilometers, and they blanket large landmasses such as Antarctica and Greenland. Glaciers, on the other hand, are generally smaller and can be confined to valleys or mountainous regions.
Ice sheets extend in continuous sheets, moving outward in all directions, while glaciers have a more defined path of movement. Glaciers originate on land from the recrystallization of snow and solid precipitation. Sections of ice sheets that extend outward from the coast over the ocean are called ice shelves.
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Where on Earth are the major ice sheets found?
The major ice sheets on Earth are primarily located in Antarctica and Greenland. The Antarctic ice sheet is the largest, holding about 90% of the world's ice and 80% of its fresh water, with an average thickness of about 1,800 meters. The Greenland ice sheet is the second largest, containing 12% of the world's glacier ice. During the Pleistocene Epoch, large ice sheets also covered significant portions of North America and Europe.
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How do ice sheets form and grow?
Ice sheets form through the accumulation of snow over extended periods in areas where annual snowfall exceeds summer melting rates. This snow gradually transforms into crystalline ice as it gets buried under subsequent snowfalls. The process can be sped up by repeated cycles of melting and freezing. Over time, the density of the snowpack increases through evaporation and condensation, causing the tiny ice grains to fit more closely together.
When the density reaches approximately 830 to 840 kilograms per cubic meter, the spaces between grains seal off, rendering the material impermeable to fluids, marking the transition from firn to glacier ice. The growth and decay of ice sheets significantly influence local, regional, and global climate through interactions between the atmosphere and ocean.
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How do ice sheets change the Earth's surface over time?
Ice sheets dramatically reshape the Earth's surface through erosion and deposition. Glacial erosion carves deep valleys and fjords and creates features such as the Matterhorn. Ice sheets deposit sediments and debris, forming moraines that mark former ice sheet margins, as seen in Long Island, New York.
Drumlins, streamlined hills aligned with ice movement, are also formed from glacial till. Additionally, the weight of ice sheets causes the Earth's crust to subside, with the land rebounding after the ice melts, a process known as isostatic rebound. The melting of ice sheets also leads to rising sea levels, potentially submerging islands and coastal regions. Catastrophic floods from the drainage of glacial lakes can carve channels and leave unique geological features.
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Why are ice sheets important for understanding climate change?
Ice sheets are vital for understanding climate change because they hold records of past climate conditions. Scientists retrieve ice cores from glaciers and ice sheets in Greenland, Antarctica, and high mountains to study climate change over the last 100,000 years. The ice contains ancient snowfalls that have turned into crystalline glacier ice, with visible annual layers that can be counted.
The most important data recovered from ice cores are oxygen and hydrogen isotope ratios, which reveal the composition of the sea, air temperature, and cloud travel distance. Trapped air bubbles in the ice also provide measurements of greenhouse gases like carbon dioxide and methane, which are linked to climate. Additionally, ice cores can reveal ancient volcanic eruptions through the measurement of sulfate concentrations.
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ice sheet, any glacier that extends in continuous sheets, moving outward in all directions, whose area exceeds more than 50,000 square km (19,000 square miles). In general, such expanses of frozen water are called ice sheets if they are the size of Antarctica or Greenland and ice caps if they are smaller. During the 2020s ice sheets covered roughly 10 percent of Earthâs land area. Together, ice sheets and glaciers contain about 69 percent of the fresh water on Earth. The sections of ice sheets that extend outward from the coast and over the ocean are called ice shelves. For a full description of glaciers and the processes that affect them, see glacier.
Present-day ice sheets
Two great ice masses, the Greenland and Antarctic ice sheets, stand out in the world today and may be similar in many respects to the large ice sheets of the Pleistocene Epoch (about 2,588,000 to 11,700 years ago). About 99 percent of the worldâs glacier ice resides in these two ice masses, with 91 percent held in Antarctica alone.
Greenland
The Greenland Ice Sheet, which covers some 80 percent of Greenland, is massive compared with other glaciers in the world except that of Antarctica. A single large ice sheet extends over about 1,700,000 square km (656,000 square miles), while other isolated glaciers and small ice caps totaling between 76,000 and 100,000 square km (about 29,000 and 39,000 square miles) occur around the periphery. The ice sheet extends from about latitudes 60° N to 80° N, and it is almost 2,200 km (1,380 miles) long in a north-south direction, with its greatest width being approximately 1,100 km (680 miles) at a latitude of 77° N, near its northern margin. The mean elevation of the ice surface is 2,135 meters (7,000 feet).
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The bedrock surface is near sea level over most of the interior of Greenland, but mountains occur around the periphery. Thus, this ice sheet, in contrast to the Antarctic Ice Sheet, is confined along most of its margin. The ice surface reaches its greatest elevation on two north-south elongated domes, or ridges. The southern dome reaches almost 3,000 meters (about 9,800 feet) at latitudes 63°â65° N; the northern dome reaches about 3,290 meters (10,790 feet) about latitude 72° N. The island hosts some 2,900,000 cubic kilometers (696,000 cubic miles) of ice, which would raise global sea levels by 7.4 meters (24 feet) if melted completely.
The unconfined ice sheet does not reach the sea along a broad front anywhere in Greenland, and no large ice shelves occur. The ice margin just reaches the sea, however, in a region of irregular topography in the area of Melville Bay southeast of Thule. Large outlet glaciers, which are restricted tongues of the ice sheet, move through bordering valleys around the periphery of Greenland to calve off into the ocean, producing the numerous icebergs that sometimes cross North Atlantic shipping lanes. The best known of these is the Jakobshavn Glacier, which, at its terminus, typically flows at speeds of 20â22 meters (about 66â72 feet) per day, which can increase during the summer months to as much as 46 meters (150 feet) per day.
Antarctica
Antarctica, in contrast, hosts the worldâs largest present-day ice sheet. The continent is about 14,200,000 square km (5,500,000 square miles) in size, and thick ice covers about 98 percent of its bedrock. Mountain ranges and isolated nunataks (a term derived from Greenlandâs Inuit language, used for individual mountains surrounded by ice) locally protrude through the ice. The ice shelves are extensive in area. The ice sheet, with its associated ice shelves, covers an area of 13,829,000 square km (5,340,000 square miles); exposed rock areas total less than 200,000 square km (77,200 square miles).
The mean thickness of the ice is about 1,800 meters (5,900 feet), and the volume of the ice sheet contains about 29,000,000 cubic kilometers (7,000,000 cubic miles) of ice, representing about 90 percent of the worldâs ice and 80 percent of its fresh water. The land surface beneath the ice is below sea level in many places, but this surface is depressed because of the weight of the ice. If the ice sheet were melted, global sea levels would probably rise about 45â60 meters (150â200 feet) and uplift of the land surface would eventually leave only a few deep troughs and basins below sea level. Because of the thick ice cover, Antarctica has by far the highest mean elevation of the continents.
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Pleistocene ice sheets
Researchers note that during periods in Earthâs pastâsuch as the Cryogenian Period (about 720,000,000 to 635,000,000 years ago) and Carboniferous Period (about 358,900,000 to 298,900,000 years ago)âgargantuan ice sheets covered much of the planetâs surface; however, the most recent period of significant glaciation occurred during the Pleistocene Epoch. During this interval, Greenland and Antarctica were covered in ice, but so were large parts of North America and Europe. The largest ice sheets in North America were the Cordilleran Ice Sheet (which was similar to the present-day Greenland Ice Sheet in size, extending south from Alaska to northern Washington and east from the Pacific Ocean to the Rocky Mountains) and the Laurentide Ice Sheet (which rivaled the present-day Antarctic Ice Sheet in size, extending more than 13,000,000 square km [5,000,000 square miles] across Canada and the northern United States).
Extensive parts of Eurasia were also covered by ice domes and large glaciers during the Pleistocene, but the largest contiguous mass of ice was contained within the Scandinavian Ice Sheet (also called the Fennoscandian Ice Sheet). At its maximum extent, the Scandinavian Ice Sheet nearly reached latitude 48° N. It is estimated to have covered about 6,600,000 square km (2,500,000 square miles) and attained a thickness of up to 3,000 meters (9,800 feet).