Amazing Facts

Pack Ice

Ice packs develop from the accretion of small
ice crystals. To begin
with the crystals coalesce into small thin platelets, known as frazil
ice. As the ice thickens, the frazil ice platelets freeze together into
a continuous skin of ice which, in a rough sea, may break up into
individual discs a half to a meter across. The plates collide with each
other, which often causes their edges to turn up and form what is known
as pancake ice.

As the sea freezes still further, the ice
thickens and the pancake
ice joins together to form an ice floe which may be from 10m to as much
as 8km across. The seawater underneath the ice is insu­lated by it
against the cold air, so the ice floe thickens only slowly, reaching a
thickness of about two meters in the Arctic and three meters in the
Antarctic at the end of the winter of the first year. In the summer the
Sun melts the surface of the ice floe, which forms pools of fresh
water. The ice melts through completely in places, but if the floe
survives the summer, an even thicker floe is formed when the water

The wind can change smooth ice floes into
bumpy packs of ice by
moving them together, so that they crush and deform. In addition, it
can cause the ice to crack along lines of weakness. The movement of ice
packs can also cause cracks in the ice which open into long, narrow
channels or into patches of open water, called polynyas. The water in
polynyas usually freezes quickly, forming a thin sheet of ice which may
be crushed if the sides of the polynya start to close together.

The Arctic Ocean has a large area of permanent
pack ice which is
even more extensive in winter, when it reaches Alaska, Russia,
north­ern Canada and Greenland. Ordinary ships cannot cross the Arctic
because of this ice, and even ice-breakers cannot force their way
through the thicker areas. But submarines can travel below the ice
pack, using sonar to detect the thickness of the ice above. The
Antarctic has a smaller area of pack ice which consists of large
floating ice shelves that join onto the landmass.

Related Tags: Ice  Sea  Ocean  
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