Geoscience Reference
In-Depth Information
Pack-ice also forms in inland lakes where winter
freezing takes place. Pack-ice generally extends more
southward in the northern hemisphere along the
western sides of oceans because of prevailing
southward-moving cold currents at these locations. In
particularly severe winters, pack-ice can extend
towards the equator beyond 50° latitude in both the
southern and northern hemispheres. If the following
summers are cold, or the ice build-up has been too
severe, then it is possible for large areas of pack-ice not
to melt seasonally.
Excessive pack-ice between Greenland and Iceland
terminated the Greenland Viking colony because it
prevented contact with Europe after the middle of the
fourteenth century. Except for the early 1970s, pack-
ice limits in the Arctic have retreated poleward in the
twentieth century, concomitant with global warming.
Pack-ice becomes a hazard only when it drifts into
shore, or when it becomes so thick that navigable leads
through it have a serious chance of closing and forming
pressure ridges. Whaling and exploration ships have
been crushed in this way. In the summer of 1985-1986
and 2001-2002, the Australian Antarctic research
program was severely hampered by thick pack-ice that
closed in, stranding supply vessels and crushing and
sinking a ship. In the Canadian Arctic, thick pack-ice
has historically posed a threat to shipping and increased
the cost of re-supplying northern settlements. Pack-ice
terminated numerous journeys to the Canadian Arctic
in the nineteenth century, the most famous of which
was the loss of the Franklin expedition.
The third type of sea-ice consists of shore-fast ice.
This ice grows from shore and anchors to the seabed.
It effectively protects a shoreline from wave attack
during winter storms but, upon melting, can remove
significant beach sediment if it floats out to sea. This
type of ice poses a unique hazard at the shoreline and
will be discussed later.
The fourth category of sea-ice consists of icebergs.
Icebergs form a menace to shipping as exemplified by
that most notorious disaster, the sinking in April 1912
of the Titanic with the loss of 1503 passengers and
crew. Icebergs in the northern hemisphere originate in
the north Atlantic, as ice calves from glaciers entering
the ocean along the west coast of Greenland and the
east coast of Baffin Island. Icebergs coming from
the east coast of Greenland usually drift south,
round the southern tip of Greenland, and join this con-
centration of ice in the Davis Strait. Along the southern
half of the west coast of Greenland, icebergs collect in
fjords and discharge into the Strait at fortnightly inter-
vals on high tides. Along the northern half of the
Greenland coast, glaciers protrude for several kilo-
meters into the sea and icebergs calve directly into Davis
Strait. About 90 per cent of all icebergs in the northern
hemisphere collect as swarms in Davis Strait during
severe periods of pack-ice, and move south into the
Labrador current as the pack-ice breaks up in spring.
These icebergs then pass into the Atlantic Ocean, off
Newfoundland, with widely varying numbers from year
to year. Fewer than 10 per cent of icebergs originate in
the Barents Sea from glaciers on the islands of Spitsber-
gen, Zemlya Frantsa-Iosifa (Franz Josef Land), Novaya
Zemlya, and Severnaya Zemlya.
Whereas winds determine the direction that pack-
ice moves, ocean currents control the movement of
icebergs. Because of Coriolis force, ice drifts to the
right of the wind in the northern hemisphere and to
the left in the southern hemisphere. Icebergs in the
north Atlantic usually have heights of tens of meters,
although some are up to 80 m in height and over 500 m
in length. The most southerly recorded sighting in the
northern hemisphere was near Bermuda at 30.33°N.
In the Antarctic, icebergs usually originate as calved
ice from the Ross and Filchner iceshelves. Here,
glaciers feed into a shallow sea forming an ice bank 35
and 90 m above and below sea level, respectively. The
production of icebergs in the Antarctic is prolific,
probably numbering 50 000 at any one time. Icebergs
breaking off the Ross Ice Shelf may reach lengths of
100 km. The largest yet measured was 334 km by
96 km with a height above water of 30 m. This one
tabular ice block had a volume that exceeded twice
the annual ice discharge from the entire Antarctic
continent. In 2000, an iceberg - named B15 and
measuring 295 km by 37 km - broke off the Ross Ice
Shelf. The number of observed icebergs has increased
fivefold; however, technological advances in iceberg
observation account for most of this increase. In
addition, the shelves are undergoing the end of a major
calving cycle that recurs every 50-100 years. Tabular
icebergs have been found as far north as 40°S, with the
most northerly recorded sighting being 26.05°S,
350 km from the Tropic of Capricorn. This is unusual
because the Antarctic circumpolar current between
50° and 65°S forms a warm barrier to northward ice
movement. About 100 smaller tabular icebergs, up to
30 km 2
in size, also exist in the Arctic Ocean, drifting
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