Smithsonian National Zoological Park l Friends of the National Zoo



A pressure ridge has forced fast ice up into wavelike shape.

NSF Polar Program

U.S. Antarctic Program

The National Zoo's Antarctica Expedition is sponsored by the National Science Foundation's Office of Polar Programs.

All photographs depicting Weddell seals were taken under NMFS Permit No. 763-1485-00 issued under the authority of the Marine Mammal Protection Act.




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About Ice

The Antarctic ice sheet covers an area about one-and-a-half times the size of the United States. In some places it is more than four kilometers thick. Antarctica is the driest continent—essentially, it is a desert. Its ice has formed through millions of years of snow accumulation.

The thickest part of the ice sheet is in Antarctica's interior. There, the sheet's sheer weight causes it to flow outward toward its edges where it gradually becomes thinner and eventually may break off in chunks we know as icebergs. That kind of ice originates on land and is called land ice.

Some of Antarctica's ice is sea ice—ice formed by the freezing of seawater. During the Antarctic winter, the ice sheet can nearly double in size largely due to the formation of sea ice.

A Weddell seal at a melt-pool filled with sludge ice.
The process goes like this: As temperatures get colder, seawater gradually thickens and starts to crystallize into lolly ice. If the water is relatively still, lolly ice further thickens into a state called sludge, and it can remain in this state as long as
temperatures permit. In the open ocean, patches of sludgy ice are called grease ice or lard ice because of its viscous, oily appearance.

As the temperature drops further, sludge coagulates into solid sheet ice or pancake ice—small, circular pads of ice with raised edges caused by frequent collisions with other ice chunks. Smaller chunks of sheet and pancake ice can join up and form floes of various shapes and sizes.

Floes can themselves join up to form the pack ice of the open ocean. Pack ice is free-floating sea ice and, because of its buoyancy, it is capable of considerable flexibility and movement. Some sea ice attaches to land, usually in protected bays and coves, where it becomes immobile—this is called fast ice. Because it is not buoyant, fast ice can be greatly deformed into fantastical shapes by the action of tides, waves, and the pressure of moving land ice.

During the pupping season, Weddell seals are found on the fast ice sometimes many miles away from open water. Because they have to find food in the water, Weddell seals congregate near fissures or weak spots in the ice that have openings to the water below. Powerful tides below the fast ice cause these fissures to form in a predictable way from year to year. Without this tidal effect, the fast ice sheet would freeze solid and Weddell seals would be excluded. Keeping these breathing/access holes open is a priority for the seals, which chew at the holes' edges with their canines and incisors to keep them open.

Did you know? Sea ice is melting fast due to global warming. Watch an animation.