Sea ice types
Sea ice is any form of ice found at sea which has originated from the freezing of sea water. It can be broadly described as new ice, young ice, first-year ice and old ice. These categories broadly reflect the age of the ice and include different forms and thicknesses of ice at various stages of development.
Even in winter only a small fraction of sea ice close to the Antarctic continent occurs as a continuous and uniform sheet. This ‘land fast’ ice is pinned to the coast and does not move. The majority of the ice occurs in a wide band around the continent and is referred to as ‘pack ice’. This is a region of highly variable ice conditions, including broken pieces (floes) with a range of sizes, ages and thicknesses, present in varying concentrations.
The pack is highly mobile, moving with the wind and currents, with its characteristics constantly changing. There is frequently some open water (leads) between the floes and it is common to see ice at various stages of development present in the same area. This is the result of the dynamic nature of the pack, with the thickness of floes increasing through ‘rafting’ and ‘ridging’ (see definitions below) as they interact, and new open water areas constantly being created, allowing new ice to form.
Some of the different forms of Antarctic sea ice are described below. These definitions come from the Antarctic Sea Ice Process and Climate website, which has a comprehensive description of sea ice – its formation, characteristics, importance in climate and marine ecosystems, and images.
The Woods Hole Oceanographic Institution website has useful information about Arctic sea ice.
Accumulations of floating ice made up of fragments not more than 2 m across; the wreckage of other forms of ice.
Brash is common between colliding floes or in regions where pressure ridges have collapsed.
Sea ice which forms and remains fast along the coast, where it is attached to the shore, to an ice wall, to an ice front, between shoals or grounded icebergs. Vertical fluctuations may be observed during changes of sea-level. Fast ice may be formed in situ from sea water or by freezing of pack ice of any age to the shore, and it may extend a few metres or several hundred kilometres from the coast. Fast ice may be more than one year old and may then be prefixed with the appropriate age category (old, second-year, or multi-year).
First year ice
Sea ice of not more than one winter's growth, developing from young ice; thickness (typically) 0.3–2 m. May be subdivided into thin first-year ice/white ice, medium first-year ice and thick first-year ice.
Fine spicules or plates of ice, suspended in water.
Frazil ice formation represents the first stage of sea ice growth. The frazil crystals are usually suspended in the top few centimetres of the surface layer of the ocean and give the water an oily appearance. In the open ocean the crystals may form, or be stirred to a depth of several metres by wave-induced turbulence.
A later stage of freezing than frazil ice when the crystals have coagulated to form a soupy layer on the surface. Grease ice reflects little light, giving the surface a matt appearance.
Grease ice behaves in a viscous fluid-like manner, and does not form distinct ice floes.
Young ice 10–15 cm thick. Less elastic than nilas and breaks on swell. Usually rafts under pressure.
Young ice 15–30 cm thick. Under pressure more likely to ridge than to raft
Old ice up to 3 metres or more thick which has survived at least two summers’ melt. Hummocks (hillocks of broken ice that have been forced up by pressure) even smoother than in second-year ice, and the ice is almost salt-free. Colour, where bare, is usually blue. Melt pattern consists of large interconnecting irregular puddles and a well-developed drainage system.
A general term for recently formed ice which includes frazil ice, grease ice, slush and shuga.
A thin elastic crust of ice, easily bending on waves and swell and under pressure, thrusting in a pattern of interlocking ‘fingers’ (finger rafting). Has a matt surface and is up to 10 cm in thickness. May be subdivided into dark nilas and light nilas.
Dark nilas is less than 5 cm thick and very dark in colour. Light nilas is 5–10 cm thick and reflects proportionately more light than dark nilas, depending on its thickness.
Predominantly circular pieces of ice from 0.3–3 m in diameter, and up to 10 cm in thickness (unrafted), with raised rims due to the pieces striking against one another. It may be formed on a slight swell from grease ice, shuga or slush or as the result of the breaking of ice rind, nilas or, under severe conditions of swell or waves, of grey ice.
Pressure process whereby one piece of ice overrides another. Most common in new and young ice.
The pressure process by which sea ice is forced into ridges.
A ridge is a line or wall of broken ice forced up by pressure. May be fresh or weathered. The submerged volume of broken ice under a ridge, forced downwards by pressure, is termed an ice keel.
Old ice which has survived only one summer’s melt. Because it is thicker and less dense than first-year ice, it stands higher out of the water. In contrast to multi-year ice, summer melting produces a regular pattern of puddles. Bare patches and puddles are usually greenish-blue.
An accumulation of spongy white lumps, a few centimetres across; they are formed from grease ice or slush and sometimes from anchor ice rising to the surface.
Snow which is saturated and mixed with water on land or ice surfaces, or as a viscous floating mass in water after heavy snowfall.
Snow ice forms by refreezing flooded snow, creating an ice layer that bonds firmly to the top surface of a floe. Ice formed by this process and makes a significant contribution to the total mass of Antarctic sea ice.
Ice in the transition stage between nilas and first-year ice, 10–30 cm in thickness. May be subdivided into grey ice and grey-white ice.