Page 200 - (DK) Ocean - The Definitive Visual Guide

198 POLAR OCEANS

Sea Ice TESTING THE ICE

Pancake ice, consisting of ice platelets,
SEA ICE IS SEAWATER THAT HAS FROZEN at the ocean can be up to 4 in (10 cm) thick. Waves
and wind have caused these platelets
surface and floats on the liquid seawater underneath. to collide, hence their curled-up edges.
It includes pack ice—ice that is not attached to the
shoreline and drifts with wind and currents—and
fast ice, which is frozen to a coast. Sea-ice formation
and melting influences the large-scale circulation
of water in the oceans. It has important stabilizing
effects on the world’s climate, since it helps control
the movement of heat energy between the polar
oceans and atmosphere. Sea ice strongly reflects solar
radiation, so in summer it reduces heating of the
polar oceans. In winter, it acts as an insulator,
reducing heat loss. Today, scientists are concerned
about shrinking sea ice in the Arctic because
of its possible effects on climate and wildlife.

Formation
Seawater starts to freeze when it reaches a temperature of
28.8˚F (-1.8˚C), slightly cooler than the freezing point
of fresh water. Sea-ice formation starts with the appearance
of tiny needlelike ice crystals (frazil ice) in the water.
Salt in seawater cannot be incorporated into ice,
and the crystals expel salt. The developing sea ice
gradually turns into a thick slush and then, under
typical wave conditions, into a mosaic of ice
platelets called pancake ice. Subsequently, it
consolidates into a thick, solid sheet, through
processes such as “rafting” (in which the ice
fractures and one piece overrides another)
and “ridging” (where lines of broken ice
are forced up by pressure). Where ridging
occurs, each ridge has a corresponding
structure, a keel, that forms on the
underside of the ice. Newly formed,
compacted sheet ice is called first-year
ice and may be up to 12 in (30 cm) thick.
It continues to thicken through the
winter. Any ice that remains through to
the next winter is called multi-year ice.

HOW ICE FORMS
The stages of sea ice formation vary according
to whether the sea surface is calm or affected
by waves. A typical sequence in an area of
moderate wave action is shown below.
OCEAN ENVIRONMENTS GREASE ICE PANCAKE ICE FIRST-YEAR ICE MULTI-YEAR ICE

The ice pancakes congeal, consolidate, and
Further thickening, for a year or more,
Fine ice spicules, called frazils, appear in the Wave action causes the grease ice to break
water. These coagulate into a viscous soup
thicken through processes such as rafting and
produces multi-year ice. This has a rough
into slushy balls of ice, called shuga. These
of ice crystals, called grease ice.
surface and may be several yards thick.
clump into platter shapes called pancakes.
ridging to form a continuous sheet of ice.