Sea ice is simply frozen ocean water. It forms, grows, and melts in the ocean. In contrast, icebergs, glaciers, ice sheets, and ice shelves all originate on land. In the Southern Hemisphere, sea ice only develops around Antarctica, occurring as far north as 55 degrees south latitude. Every winter the continent of Antarctica doubles in size due to sea ice.
Sea ice is only found in remote polar oceans. On average, sea ice covers about 25 million square kilometers (9,652,553 square miles) of the earth, or about two-and-a-half times the area of Canada.
Sea ice is categorized by annual thickness of first-year ice or multi-year ice. First-year ice can be up to 1 meter (3ft) thick, multi-year ice is typically 2 to 4 meters (6.6 to 13.1 ft) thick. Any ice over 3 meters is strong enough to support large vehicles like airplanes.
Unlike fresh water, when sea water reaches a freezing point the salt causes the density of the water to increase. As a result, sea ice forms very slowly because the salt water sinks away from the cold surface before it cools enough to freeze. Additionally, the freezing temperature of salt water is lower than fresh water; ocean temperatures must reach -1.8 degrees Celsius (28.8 degrees Fahrenheit) to freeze. Generally, the top 100 to 150 meters (300 to 450 feet) of water must be cooled to the freezing temperature before sea ice can form.
Unlike a glacier, which generally flows in one direction, an ice field flows outward in all directions from the center. If an ice field covers more than 50,000 square kilometers (20,000 square miles), it is defined as an ice sheet. Although ice sheets covered much of the Northern Hemisphere during a series of Pleistocene Ice Ages, the Earth now has just two major ice sheets, one on Greenland and one on Antarctica.
Like a glacier, an ice sheet forms through the accumulation of snowfall, when annual snowfall exceeds annual snowmelt. Over thousands of years, the layers of snow build up, forming a flowing sheet of ice thousands of feet thick and tens to thousands of miles across. As the ice thickens, the increasing height of snow and ice causes the ice sheet to deform and begin to flow.
The Antarctic ice sheet is the largest single mass of ice on Earth. It covers an area of almost 14 million km2 (3.3 million mi2) and contains 30 million km3 of ice (7.2 million mi2).
An ice shelf is a thick slab of ice, attached to a coastline and extending out over the ocean as a seaward extension of the grounded ice sheet. Ice shelves range in thickness from about 50 to 600 meters, and some shelves persist for thousands of years. The icebergs that break of from ice shelves are called Tabular icebergs and are characterized by their flat, rectangular shape, contrasting the irregular shape of glacial icebergs.
Antarctica has 15 major ice shelf areas, and 10 of the largest appear in this map (left). The Wilkins Ice Shelf is an example of a composite ice shelf comprised of both glacier-fed ice and fast ice thickened by snowfall. The others are glacier-fed, but ice formed from direct snowfall accumulation is a significant part of all permanent ice shelves.
The world’s largest recorded iceberg, B15, was calved from the Ross iceshelf in 2000.
A glacier is a persistent body of dense ice that is constantly moving under its own weight. Glaciers begin to form when snow remains in the same area year-round, where enough snow accumulates to transform into ice. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize, forming grains similar in size and shape to grains of sugar. Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features.
Icebergs are pieces of ice that formed on land and float in an ocean or lake. Icebergs come in all shapes and sizes, from ice-cube-sized chunks to ice islands the size of a small country. The term “iceberg” refers to chunks of ice larger than 5 meters (16 feet) across. Icebergs form when chunks of ice calve, or break off, from glaciers, ice shelves, or a larger iceberg.
Icebergs that break from ice sheets have a distinctive flat, rectangular shape. The size ratio is typically 1:5 height to width. The worlds largest recorded iceberg in the world was tabular iceberg called B15, it measured around 295 kilometres (183 mi) long and 37 kilometres (23 mi) wide, with a surface area of 11,000 square kilometres (4,200 sq mi)—larger than the whole island of Jamaica. Calved from the Ross Ice Shelf of Antarctica in March 2000, Iceberg B-15 broke up into smaller icebergs, named B15-A through Z.
Tabular Ice bergs