Land Ice Ice of land origin, glacier, ice is composed initially of large accumulations of compacted snow that reach the sea as coastal glaciers and ice shelves. The leading edges of these glaciers break off (calve) and fall into the sea. This ice then drifts to sea as ICEBERGS.ICEBERGS. —Since 86% of the world’s glaciers occur in Antarctica, most icebergs originate around that continent. Most of the remainder of the world’s glaciers are located in Greenland. Greenland is the main source of icebergs in the Northern Hemisphere (about 90%). Nearly 70% of Greenland’s icebergs originate along the western coast near 68°N. Classification. —Icebergs are pinnacled (cone-shaped) or tabular (flat-topped and straight-sided). See figure 1-1-7. The structure, and to some extent the appearance, depends upon the ice that produces the berg. Pinnacled or irregular-shaped bergs come from glaciers that plow across uneven ground on their way to the tidewater, while the tabular bergs come from ice shelves that thrust directly out to sea. Pinnacled and irregular bergs are most prevalent in the Northern Hemisphere, while the tabular bergs are more prevalent in the Southern Hemisphere. Size and Depth. —Icebergs originating in Greenland ave rage 70 meters in height and 280 to 450 meters in length when first formed. The largest ones may exceed 120 meters in height and several miles in length. The tabular bergs of Antarctica average 30 to 40 meters in height, but their horizontal dimensions greatly surpass those bergs of the Northern Hemisphere. For example, one iceberg observed near Scott Island in 1956 measured 60 miles by 208 miles. The portion of an iceberg that is visible above the water is dependent upon the type of the berg and the density differences between the seawater and the ice. The type of berg (pinnacle or tabular) determines the height of the ice above the water. In the case of the tabular berg, the depth below the surface is about seven times the height above the water line. In the case of the pinnacle or irregular berg, the depth below the surface averages about five times that above the water line. With regard to density, seawater with a temperature of –1°C and a salinity of 35 ‰ produces a density condition that allows for nearly 90 percent of the ice to be submerged. Irregular icebergs often have rams (protrusions of ice beneath the surface). These rams can be a great hazard to vessels that might pass close by these type bergs. Drift. —While the general direction of the drift of icebergs over a long period of time is known, it may not be possible to predict the drift of an individual berg at a given place and time, for bergs lying close together have been observed to move in different directions. The reason for this is that icebergs move under the influence of the prevailing current at the iceberg’s submerged depth. This subsurface current often opposes the existing wind and sea or surface drift. BERGY BITS AND GROWLERS. —Like icebergs, bergy bits and growlers originate from glaciers and form when icebergs and other masses of land ice disintegrate. A bergy bit is a medium sized fragment of glacier ice and is about the size of a small cottage. A growler is a small frag-ment of ice about the size of a grand piano. It is usually of glacial origin, and generally greenish in color.
|
