Iceberg

 Iceberg [Dan or Nor, isberg, "ice mountain"], a piece of ice that has become detached from its parent GLACIER by a process known as calving. The glacier may be flowing into a FJORD, or may be an ice shelf (floating glacier of large dimensions extending beyond the coastline). Icebergs thus created may melt or disintegrate, giving rise to smaller pieces called "growlers" and "bergy bits". Icebergs calve from a parent glacier as a result of the manner in which the glacier enters the water or, subsequently, from tidal and wave action or possibly as a result of EARTHQUAKE shocks. Less commonly, icebergs may suddenly emerge from below water level and protrude as a result of forces acting on part of a glacier under the water.

Appearance

Most icebergs are white except along freshly calved ice cliffs, which tend to appear blue. Others may appear green, brown or black, or combinations of these colours. These icebergs have usually rolled over, exposing basal ice, or have emerged from below water level. The various colorations are caused by differences in density, air-bubble content and impurities. For example, black ice is of high density and bubble free; dark layers indicate the presence of rock materials derived from the base of the parent glacier. Occasionally, rocks may be found on the original upper surface of the iceberg. As the iceberg melts, these materials precipitate into marine or lake sediments.

Occurrence and Dimensions

Some icebergs are trapped in lakes; the vast majority occur in oceans. Flat-topped tabular bergs are sections of nearly flat ice shelves. In the Antarctic they are commonly several tens of kilometres square and several hundred metres thick. One of the largest tabular bergs ever viewed measured 160 km x 72 km and one of the longest measured 185 km. A typical height above waterline would be 35-45 m, implying a total ice thickness of 250-320 m, although much greater thicknesses are possible. In the Arctic Ocean, the term "ice island" is applied to pieces of floating shelf ice that form principally on the north coast of ELLESMERE ISLAND. These thin tabular icebergs are 20-60 m thick, often up to 100 km2 in area, and typically protrude 2-6 m above water.

Irregularly shaped icebergs are more typical of coastal Greenland and northern Canada. Many irregular bergs originate in Greenland fjords containing fast-flowing outlet glaciers coming from the inland ice sheet. Because these glaciers usually terminate below the snowline and because they are often extremely broken up, first in their journey to the sea and then by tidal and wave action, they can give rise to very irregular bergs of almost pure ice, their spires occasionally reaching 100 m above sea level.

Dynamics and Stability

The composition of antarctic tabular bergs gradually changes from snow on the top surface to ICE by about the waterline. This fact, combined with their tabular shape, makes them much more stable than typical arctic icebergs, which quickly tilt and finally roll on their voyage to destruction. Calving from an already tilted iceberg may shift the centre of gravity sufficiently to cause the iceberg to roll, posing a threat to ships.

In the long term, icebergs are driven principally by ocean currents, but other forces produced by wind shear and wave action, especially during storm conditions, may significantly influence the short-term motion of an iceberg. The Coriolis force also influences the drift path of icebergs. Melting below sea level takes place continuously; above sea level, intermittently, according to location.

Depending on the shape of the iceberg and its rock content, the volume of ice submerged compared to the total ice volume is in the ratio of the density of the ice to that of seawater, or about 0.88 in arctic and 0.85 in antarctic icebergs. The above-water shape of an irregular iceberg does not necessarily provide information about its underwater geometry; it may be discovered, however, through the use of airborne RADAR or side-scan SONAR from a ship. These studies and others (eg, towing tests and iceberg-stability investigations) are carried out at the Centre for Cold Ocean Resources Engineering (C-CORE), St John's, Nfld.

Because the ocean transmits wave energy, icebergs respond to wave action. Thus, in addition to drifting with ocean currents, icebergs are known to oscillate vertically and to roll with a periodic motion. Since icebergs possess natural periods of oscillation dependent on their density and their thickness, they may tune in to certain ocean waves.

Waves possessing the right period will tend to cause resonance in the iceberg, with a consequent increase in the size of oscillation. This behaviour changes continually with the changing shape and thickness of the iceberg. An ideal, rectangular block iceberg with a mean thickness of 200 m would have a natural period of oscillation of about 26 seconds, which is in the range of common wave-swell periods. The thinner arctic ice islands have a much lower natural period of oscillation and, having horizontal dimensions much greater than their thickness, tend to absorb ocean waves as filtered travelling waves, which induce flexing of the ice. As the ice island thins by melting, this process may lead to it fracturing and breaking into smaller pieces. Most Greenland bergs melt before reaching 40° N lat (roughly opposite Philadelphia, Pa), although occasionally some bergs reach almost 30° N. Satellite imagery has been used to track large bergs (see REMOTE SENSING).

Applications

T3 and other arctic ice islands on which aircraft can land have been used intermittently by the US and USSR as mobile research platforms for about the last 30 years. Since 1985 a Canadian station has been maintained on an ice island that calved from the Ward-Hunt Ice Shelf in 1983. Because many ice islands become trapped in Arctic Ocean current gyres, they survive for many years, melting and crumbling at the edges only slowly.

In 1977 and 1980 conferences were held to investigate the possibility of moving antarctic icebergs to places where water shortages are frequently acute, eg, Australia, California and Saudi Arabia. This controversial project, however, has not yet materialized.

Hazards

In the Northern Hemisphere icebergs present a threat to human activities. Off the coasts of Labrador and Newfoundland, current petroleum explorations necessitate the presence of drill platforms, which may be endangered by icebergs. In the vicinity of Valdez, Alaska, the Columbia Glacier is showing signs of disintegration in its terminal region. As a result, bergs will drift into Prince William Sound and pose a danger to oil tankers operating from the port of Valdez.

US Geological Survey glaciologists can post iceberg forecasts. Oil and gas operations in the Beaufort Sea and between the Queen Elizabeth Islands are threatened by collisions between even small ice islands and platforms and bottom structures (eg, PIPELINES). The threat to shipping in the North Atlantic is now minimal, as a result of the establishment of the International Ice Patrol after the sinking of the TITANIC (April 1912).