An ice shelf is a large floating platform of ice that forms where a
glacier
A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such a ...
or
ice sheet flows down to a
coast
The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...
line and onto the ocean surface. Ice shelves are only found in
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest co ...
,
Greenland
Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland is ...
,
Northern Canada, and the
Russian Arctic. The boundary between the floating ice shelf and the
anchor ice (resting on
bedrock
In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet.
Definition
Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...
) that feeds it is the grounding line. The thickness of ice shelves can range from about to .
In contrast,
sea ice is formed on water, is much thinner (typically less than ), and forms throughout the
Arctic Ocean
The Arctic Ocean is the smallest and shallowest of the world's five major oceans. It spans an area of approximately and is known as the coldest of all the oceans. The International Hydrographic Organization (IHO) recognizes it as an ocean, ...
. It is also found in the
Southern Ocean around the continent of
Antarctica
Antarctica () is Earth's southernmost and least-populated continent. Situated almost entirely south of the Antarctic Circle and surrounded by the Southern Ocean, it contains the geographic South Pole. Antarctica is the fifth-largest co ...
.
The movement of ice shelves is principally driven by
gravity
In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
-induced pressure from the grounded ice.
That flow continually moves ice from the grounding line to the seaward front of the shelf. In steady state, about half of Antarctica's ice shelf mass is lost to
basal melt
Ice sheet dynamics describe the motion within large bodies of ice, such those currently on Greenland and Antarctica. Ice motion is dominated by the movement of glaciers, whose gravity-driven activity is controlled by two main variable fact ...
and half is lost to
calving, but the relative importance of each process varies significantly between ice shelves. In recent decades, Antarctica's ice shelves have been out of balance, as they have lost more mass to basal melt and calving than has been replenished by the influx of new ice and snow.
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 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