Chapter

TCEBERGS AND SEA ICE

lthough icebergs and sea ice often appear together, they are very different phenomena. Icebergs are floating masses of freshwater ice that have broken off from the seaward end of either a glacier or an ice shelf. They are found in the oceans surrounding Antarctica, in the seas of the Arctic and subarctic, in Arctic fjords, and in lakes fed by glaciers. In contrast, sea ice is essentially frozen seawater. It also occurs within the seas surrounding Antarctica and the Arctic Ocean, as well as its adjacent seas as far south as China and Japan.

Icebergs are magnificent structures composed of a freeboard section (height above the waterline) and an underwater section. Their size is greatest when they initially calve, and Arctic bergs tend to be much smaller than the largest Antarctic ones. After they calve from their parent glacier or ice shelf, icebergs may be released into the open ocean, where they slowly erode from wave action or melt from rising environmental temperatures.

The Origin of Antarctic Icebergs

Icebergs of the Antarctic calve from floating ice shelves and are a magnificent sight, forming huge, flat "tabular" structures. A typical newly calved iceberg of this type has a diameter

ICEBERGS

that ranges from several kilometres to tens of kilometres, a thickness of 200-400 metres (660-1,320 feet), and a freeboard, or the height of the "berg" above the waterline, of 30-50 metres (100-160 feet). The mass of a tabular iceberg is typically several billion tons. Floating ice shelves are a continuation of the flowing mass of ice that makes up the continental ice sheet. Floating ice shelves fringe about 30 percent of Antarctica's coastline. The transition area where floating ice meets ice that sits directly on bedrock is known as the grounding line.

Under the pressure of the ice flowing outward from the centre of the continent, the ice in these shelves moves seaward at 0.3-2.6 km (0.2-1.6 miles) per year. The exposed seaward front of the ice shelf experiences stresses from subshelf currents, tides, and ocean swell in the summer and moving pack ice during the winter. Since the shelf normally possesses cracks and crevasses, it will eventually fracture to yield freely floating icebergs. Some minor ice shelves generate large iceberg volumes because of their rapid velocity; the small Amery Ice Shelf, for instance, produces 31 cubic km (about 7 cubic miles) of icebergs per year as it drains about 12 percent of the east Antarctic Ice Sheet.

Iceberg calving may be caused by ocean wave action, contact with other icebergs, or the behaviour of melting water on the upper surface of the berg. With the use of tiltmeters (tools that can detect a change in the angle of the slope of an object), scientists monitoring iceberg-calving events have been able to link the breaking stress occurring near the ice front to long storm-generated swells originating tens of thousands of kilometres away. This bending stress is enhanced in the case of glacier tongues (long narrow floating ice shelves produced by fast-flowing glaciers that protrude far into the ocean). The swell causes the tongue to oscillate until it fractures. In addition, on a

Map showing the extent of collapse of the Larsen Ice Shelf The Larsen A Ice Shelf disintegrated in 1995, whereas the Larsen B Ice Shelf broke apart in 2002. Both events were caused by water from surface melting that ran down into crevasses, refroze, and wedged each shelf into pieces.

number of occasions, iceberg calving has been observed immediately after the collision of another iceberg with the ice front. Furthermore, the mass breakout of icebergs from the Larsen Ice Shelf between 1995 and 2002, though generally ascribed to global warming, is thought to have occurred because summer meltwater on the surface of the shelf filled nearby crevasses. As the liquid water refroze, it expanded and produced fractures at the bases of the crevasses. This phenomenon, known as frost wedging, caused the shelf to splinter in several places and brought about the disintegration of the shelf.

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