The main types of ice on Earth are sea ice, which forms when the ocean surface freezes; glacier ice, which forms on land as snow is buried and squeezed into ice over years to millennia; ice shelves, the floating edges of ice sheets; icebergs, which break off glaciers and ice shelves; and ground ice in permafrost, soil that stays frozen for two years or more. Almost all of it is the same substance at the molecular level, the hexagonal crystal called ice Ih, while about twenty other crystal forms exist mainly under laboratory pressures. What separates the geographic types is not chemistry but how and where the ice formed: from salt water or fresh, on land or at sea, from falling snow or from water in the ground.
So this guide sorts ice by how it forms. It starts with the crystal, then follows each formation route to a named place where it can be seen at work, from the Ross Sea to the Siberian interior.
Pauling’s 1935 hexagon: what happens when water freezes
At normal pressure, liquid water cooled below 0 °C locks into a lattice of hexagonal rings. In 1933 the British physicists John Desmond Bernal and Ralph Fowler set out the “ice rules”: each oxygen atom is bonded strongly to two hydrogen atoms and weakly, by hydrogen bonds, to two more on neighbouring molecules. In 1935 the American chemist Linus Pauling proposed the accepted crystal structure of ordinary ice and used the rules to explain why ice keeps some disorder even close to absolute zero.
The geometry has a consequence that shapes the whole planet. Because the hydrogen bonds hold the molecules at angles close to 109.5°, the lattice is open and roomy. Ice at 0 °C has a density of about 0.917 g/cm³, against almost exactly 1.00 for liquid water, so water expands by about 9% when it freezes. Ice therefore floats. Lakes and seas freeze from the top down, leaving liquid water and living things beneath; and freezing water in cracks can split rock, the root of freeze–thaw weathering.
Melting also takes a great deal of energy. Turning ice at 0 °C into water at 0 °C absorbs as much heat as warming the same mass of water by 80 °C, which is why large bodies of ice change temperature slowly and buffer the climate around them.
Tammann, Bridgman and the ices you will rarely meet
Squeeze ice hard enough and the hexagonal lattice gives way to denser arrangements. The German chemist Gustav Tammann described ice II and ice III in 1900, compressing ice at around −70 to −80 °C under about 2,000 atmospheres. The American physicist Percy Bridgman extended the work from 1912, mapping the boundaries between the high-pressure forms and giving them the Roman-numeral names still used today. Around twenty crystalline phases have now been identified.
Almost none occurs naturally at Earth’s surface, with one striking exception. In 2018 ice VII was found trapped as tiny inclusions inside natural diamonds, where the rigid diamond lattice had preserved the pressure of the deep mantle after the stones reached the surface. The International Mineralogical Association then recognised ice VII as a distinct mineral. For everything below, though, the ice is plain ice Ih; the differences come from where and how it formed.
Frazil to old ice: sea ice in the Ross Sea and the Arctic
Seawater does not freeze at 0 °C. Its dissolved salts lower the freezing point to about −2 °C at typical ocean salinity. When the surface reaches it, the first ice appears as tiny discs less than 3 mm across that grow star-shaped arms and break apart into a soupy suspension of crystals called frazil, or grease ice. In calm water the crystals freeze together into a thin, elastic crust called nilas, up to 10 cm thick, that bends over swells without breaking. From then on, the sheet thickens as water freezes onto its underside.
Waves change the sequence. In rough water the slush is jostled into round plates 30 cm to 3 metres across with raised rims where they bump together: pancake ice, here crowding the Ross Sea off Antarctica.
Sea ice is classified by age. Young ice is 10 to 30 cm thick; first-year ice, which grows through one winter and melts the next summer, reaches 0.3 to 2 metres; old ice that has survived at least one summer is usually 2 to 4 metres thick. Old, multiyear ice is far more common in the Arctic, where much of the pack is hemmed in by land, than in the Antarctic, where floes drift north into warmer water and melt. Sea ice covers about 7% of Earth’s surface and 12% of the oceans. In the Arctic it shrinks to a minimum in September and peaks in March or sometimes February; in the Antarctic the cycle is reversed, with the minimum in February and the maximum in September or October.
Snow to firn to glacier ice: the 420,000-year core at Vostok
Glacier ice forms by a completely different route, from snow on land. Where more snow falls each winter than melts each summer, it piles up and is buried. Under the weight of later snow, the flakes lose their delicate shapes, round off into granular névé and then compact into firn, a dense, sugary material with a density of about 0.35 to 0.9 g/cm³. The name is Swiss German for “last year’s”. Over years to centuries, deeper firn is squeezed until the air between grains is sealed off into bubbles, and it becomes glacier ice. When it is thick enough, it begins to flow under its own weight; on steep slopes that can happen with as little as 15 metres of ice.
On the scale of continents this produces ice sheets. The Greenland ice sheet covers about 1.71 million km², roughly 80% of the island, with an average thickness of 1,673 metres. In Antarctica the process runs so slowly, in such cold, that the ice preserves a layered record of past climate. At Vostok Station, at 3,488 metres on the East Antarctic Ice Sheet, where the lowest reliably measured natural temperature on Earth, −89.2 °C, was recorded on 21 July 1983, Soviet and French teams drilled a core to 3,623 metres in 1996. Its layers record about 420,000 years of past climate and cover four previous glacial periods; drilling stopped there over worries about contaminating Lake Vostok, some 4,000 metres below the surface. The Greenland ice sheet guide looks at the Northern Hemisphere’s equivalent.
The Ross Ice Shelf: an ice sheet afloat
Where an ice sheet flows off the land into the sea, it does not stop at the coast. It floats out as an ice shelf, still attached to the glaciers feeding it from behind. Unlike sea ice, an ice shelf is freshwater glacier ice, often hundreds of metres thick.
The Ross Ice Shelf is the largest in Antarctica, about 500,809 km² as measured in 2013, roughly the size of France and about 800 km across. Its seaward edge is a nearly vertical cliff more than 600 km long, standing 15 to 50 metres above the water, with about 90% of the floating ice hidden below. Sir James Clark Ross came upon it on 28 January 1841 aboard Erebus and Terror; sailors called it “The Barrier” because it blocked any passage south. For later expeditions, beginning with Robert Falcon Scott’s Discovery Expedition of 1901–1904, it became the starting area for attempts on the South Pole.
Icebergs: Ilulissat’s calving glacier and the drifting A23a
Icebergs are the pieces that break, or calve, off the front of glaciers and ice shelves. To count as an iceberg, a piece of freshwater ice must be more than 15 metres long; smaller fragments are bergy bits and growlers. Because ice (about 920 kg/m³) is only slightly less dense than seawater (about 1,025 kg/m³), only about a tenth of an iceberg shows above the surface.
Greenland’s icebergs are often irregularly shaped. Many come from Jakobshavn Glacier, known in Greenlandic as Sermeq Kujalleq, which drains about 6.5% of the Greenland ice sheet, produces around 10% of Greenland’s icebergs and sheds some 35 billion tonnes of ice a year into the Ilulissat Icefjord, a UNESCO World Heritage Site since 2004. Some of its icebergs are so tall they run aground in the fjord for years. Antarctic ice shelves instead release flat-topped tabular bergs. One, A23a, calved from the Filchner–Ronne Ice Shelf in 1986, sat grounded on the seabed for decades, began drifting in 2020 and, at about 3,500 km², was the largest iceberg in the world in January 2025. It grounded off South Georgia that March, drifted again, and was breaking up north of the island by September 2025; by July 2026 it had disintegrated or was close to it. The record holders are listed in the largest icebergs ever recorded.
Ground ice and permafrost: the Batagaika crater in Yakutia
The last type forms underground. Permafrost is soil, rock or sediment that stays below 0 °C for two years or more. Above it sits an “active layer” that thaws each summer and refreezes each winter. Permafrost underlies about 11% of the world’s land surface and 15% of the exposed land in the Northern Hemisphere, around 18 million km² across Alaska, Canada, Greenland, Siberia and the Tibetan Plateau. The deepest reaches more than 1,500 metres down, and the oldest has been frozen for around 700,000 years.
Ground ice only shows itself when it melts. The Batagaika crater in Russia’s Sakha Republic, about 660 km north-northeast of Yakutsk, is the largest known permafrost crater in the world: a gash about a kilometre long and up to 100 metres deep in the East Siberian taiga. The ground began to sink in the 1960s, after the surrounding forest was cleared and the permafrost beneath started to thaw; flooding has enlarged it since, and it is still growing as its walls thaw and slump, the headwall retreating by more than 10 metres a year in the 2010s. Palaeontologists have found Ice Age fossils in the mud around its rim. Thawing permafrost also releases carbon stored for millennia, which is why the explainer on permafrost treats it as one of the climate’s key feedbacks.