Ice shelf

An ice shelf is a thick suspended 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 can range from about 100 m (330 ft) to 1,000 m (3,300 ft). An ice shelf is a thick suspended 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 can range from about 100 m (330 ft) to 1,000 m (3,300 ft). In contrast, sea ice is formed on water, is much thinner (typically less than 3 m (9.8 ft)), and forms throughout the Arctic Ocean. It also is found in the Southern Ocean around the continent of Antarctica. Ice shelves are principally driven by gravity-induced 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 June 14, 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 at least 1/9 of the floating ice is above the ocean surface, depending on how much pressurized air is contained in the bubbles within the glacial ice, stemming from compressed snow. The formula for the denominators above is 1 / ( ρ s e a w a t e r − ρ g l a c i a l i c e ) / ρ s e a w a t e r { extstyle 1/( ho _{seawater}- ho _{glacialice})/ ho _{seawater}} , density of cold seawater divided by kg/m3 is about 1.028 and that of glacial ice from about 0.85 to well below 0.92, the limit for very cold ice without bubbles. 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. The term captured ice shelf has been used for the ice over a subglacial lake, such as Lake Vostok. All Canadian ice shelves are attached to Ellesmere Island and lie north of 82°N. Ice shelves that are still in existence are the Alfred Ernest Ice Shelf, Milne Ice Shelf, Ward Hunt Ice Shelf and Smith Ice Shelf. The M'Clintock Ice Shelf broke up from 1963 to 1966; the Ayles Ice Shelf broke up in 2005; and the Markham Ice Shelf broke up in 2008. A large portion of the Antarctic coastline has ice shelves attached. Their aggregate area is over 1,550,000 km2.