Formation of Sea Ice
Only the top layer of water needs to cool to the freezing point. Convection of the surface layer involves the top 100 – 150 m, down to the pycnocline of increased density.
In calm water, the first sea ice to form on the surface is a skim of separate crystals which initially are in the form of tiny discs, floating flat on the surface and of diameter less than 2–3 mm. Each disc has its c-axis vertical and grows outwards laterally. At a certain point such a disc shape becomes unstable, and the growing isolated crystals take on a hexagonal, stellar form, with long fragile arms stretching out over the surface. These crystals also have their c-axis vertical. The dendritic arms are very fragile, and soon break off, leaving a mixture of discs and arm fragments. With any kind of turbulence in the water, these fragments break up further into random-shaped small crystals which form a suspension of increasing density in the surface water, an ice type called frazil or grease ice. In quiet conditions the frazil crystals soon freeze together to form a continuous thin sheet of young ice; in its early stages, when it is still transparent, it is called nilas. When only a few centimeters thick this is transparent (dark nilas) but as the ice grows thicker the nilas takes on a grey and finally a white appearance. Once nilas has formed, a quite different growth process occurs, in which water molecules freeze on to the bottom of the existing ice sheet, a process called congelation growth. This growth process yields first-year ice, which in a single season may reach a thickness of 1.5–2 m.
In rough water, fresh sea ice is formed by the cooling of the ocean as heat is lost into the atmosphere. The uppermost layer of the ocean is supercooled to slightly below the freezing point, at which time tiny ice platelets, known as frazil ice, form. As more frazil ice forms, the ice forms a mushy surface layer, known as grease ice. Frazil ice formation may also be started by snowfall, rather than supercooling.
Slush is a floating mass formed initially from snow and water. Shuga is formed in agitated conditions by accumulation of slush or grease ice into spongy pieces several inches in size.
Waves and wind then act to compress these ice particles into larger plates, of several meters in diameter, called pancake ice. These float on the ocean surface, and collide with one another, forming upturned edges. In time, the pancake ice plates may themselves be rafted over one another or frozen together into a more solid ice cover, known as consolidated ice pancake ice. Such ice has a very rough appearance on top and bottom.
Sea ice is largely fresh, since a lot of the ocean salt is expelled during ice formation. The resulting ice is riddled with minute brine-filled channels and consequently is still salty (about 1% salt, compared with about 3.5% salt in the ocean). The expelled brine has an important bearing on the ocean circulation.
If sufficient snow falls on sea ice to depress the freeboard below sea level, sea water will flow in and a layer of ice will form of mixed snow/sea water. This is particularly common around Antarctica.
Russian scientist Vladimir Vize (1886–1954) devoted his life to study the Arctic ice pack and developed the Scientific Prediction of Ice Conditions Theory, for which he was widely acclaimed in academic circles. He applied this theory in the field in the Kara Sea, which led to the discovery of Vize Island.
Read more about this topic: Sea Ice
Famous quotes containing the words formation of, formation, sea and/or ice:
“I want you to consider this distinction as you go forward in life. Being male is not enough; being a man is a right to be earned and an honor to be cherished. I cannot tell you how to earn that right or deserve that honor. . . but I can tell you that the formation of your manhood must be a conscious act governed by the highest vision of the man you want to be.”
—Kent Nerburn (20th century)
“... the mass migrations now habitual in our nation are disastrous to the family and to the formation of individual character. It is impossible to create a stable society if something like a third of our people are constantly moving about. We cannot grow fine human beings, any more than we can grow fine trees, if they are constantly torn up by the roots and transplanted ...”
—Agnes E. Meyer (1887–1970)
“Let men say we be men of good government, being governed, as the sea is, by our noble and chaste mistress the moon, under whose countenance we steal.”
—William Shakespeare (1564–1616)
“Every incident connected with the breaking up of the rivers and ponds and the settling of the weather is particularly interesting to us who live in a climate of so great extremes. When the warmer days come, they who dwell near the river hear the ice crack at night with a startling whoop as loud as artillery, as if its icy fetters were rent from end to end, and within a few days see it rapidly going out. So the alligator comes out of the mud with quakings of the earth.”
—Henry David Thoreau (1817–1862)