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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 and Canada. The boundary between the floating ice shelf and the grounded (resting on bedrock) ice that feeds it is called the grounding line. The thickness of ice shelves ranges from about 100 to 1000 meters. In contrast, sea ice is formed on water, is much thinner (typically less than 3m), 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-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 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.〔http://www.nasa.gov/home/hqnews/2013/jun/HQ_13-183_Melting_Ice_Shelves.html〕 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, which is denser than normal 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. The term captured ice shelf has been used for the ice over a subglacial lake, such as Lake Vostok. ==Canadian ice shelves== 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. ==Antarctic ice shelves== ''See also:'' ''List of Antarctic ice shelves'' A total of 44 percent of the Antarctic coastline has ice shelves attached. Their aggregate area is 1,541,700 km2.〔()〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Ice shelf」の詳細全文を読む スポンサード リンク
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