An ice shelf is a thick, floating platform of ice that forms where a glacier or ice sheet flows down to a coastline and onto the ocean surface, typically in Antarctica or Greenland. The boundary between floating ice shelf and the grounded ice that feeds it is called the grounding line. When the grounding line retreats inland, water is added to the ocean and sea level rises.

In contrast, sea ice is formed on water, is much thinner, and forms throughout the Arctic Ocean. It also is found in the Southern Ocean around the continent of Antarctica.

Ice shelves flow by gravity-driven horizontal spreading on the ocean surface. That flow continually moves ice from the grounding line to seaward front of the shelf. The primary mechanism of mass loss from ice shelves is iceberg calving, in which a chunk of ice breaks off from the seaward front of the shelf. Typically, a shelf front will extend forward for years or decades between major calving events. Snow accumulation on the upper surface and melting from the lower surface are also important to the mass balance of an ice shelf.

The thickness of modern-day ice shelves ranges from about 100 to1000 meters. The density contrast between solid ice and liquid water means that only about 1/9 of the floating ice is above the ocean surface. The world's largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica.

Antarctic ice shelves
The Antarctic continent is mostly surrounded by ice formations. The coast composition is:

Recently, glaciologists have observed unexpected ice-shelf events, such as complete disintegration of some small ice shelves. For example, two sections of Antarctica's Larsen Ice Shelf broke apart into hundreds of unusually small fragments 100's of meters wide or less in 1995 and 2002. The breakup events are linked to climate change in the region, although there is not complete agreement about the most important causative process. The leading ideas involve enhanced ice fracturing due to surface meltwater and enhanced bottom melting due to warmer ocean water circulating under the floating ice.

The cold, fresh water produced by melting underneath the Ross and Flichner-Ronne ice shelves is a component of Antarctic Bottom Water.

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