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. Ice shelves are only found in Antarctica, Greenland, Canada and the Russian Arctic. The boundary between the floating ice shelf and the anchor ice (resting on bedrock) that feeds it is called the grounding line. The thickness of ice shelves ranges from about 100 to 1000 meters.

Edge of the Ross Ice Shelf

In contrast, sea ice is formed on water, is much thinner (typically less than 3m), and forms throughout the Southern Ocean around the continent of Antarctica.

Ice shelves are principally driven by gravity-driven pressure from the grounded ice. That flow continually moves ice from the grounding line to the seaward front of the shelf. The primary mechanism of mass loss from ice shelves was thought to have been iceberg calving, in which a chunk of ice breaks off from the seaward front of the shelf. A study by NASA and university researchers - published in the 14 June 2013 issue of Science - found however that ocean waters melting the undersides of Antarctic ice shelves are responsible for most of the continent's ice shelf mass loss.

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. Ice may also accrete onto the underside of the shelf.

The density contrast between glacial ice and liquid water means that 1/9 up to 1/6 of the floating ice is above the ocean surface - depending on how much air at which pressure is contained in the bubbles within the glacial ice, stemming from compressed snow. The height of the shelf above the sea can be even larger, if there is a lot of less dense firn and snow above the glacier ice.

The world's largest ice shelves are the Ross Ice Shelf and the Filchner-Ronne Ice Shelf in Antarctica.

Antarctic ice shelves

A total of 74 percent of the Antarctic coastline has ice shelves attached. Their aggregate area is over 1,550,000 km2. The thinning and loss of mass of pacific shelves is modulated by global water temperatures. In El Niño state, the new snow thickens the shelf, but warm water melts the belly of the shelf more and its mass regardless of added snow declines. A 2018 paper found that a local thinning leads to accelerated ice flow hundreds of kilometers far away.

Ice shelves of Westarctica

Rates of basal melt of Antarctic ice shelves

Westarctica has eight major ice shelves surrounding its coastal regions. From east to west, they are: