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 can range from about 100 m (330 ft) to 1,000 m (3,300 ft).
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 icebergcalving, 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 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 formula for the denominators above is 1/((ρseawater-ρglacial_ice)/ρseawater), 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.