Icebergs And Sea

Calving is a process in which large pieces of ice break off from the fronts of tidewater glaciers, ice shelves, or sea ice. Typically, the glacier cracks like an explosion, then a large chunk of ice splashes into the water, detaching from the glacier. Calving causes glaciers to retreat rapidly. Ice that has broken off an ice cap, polar sea, or calved off a glacier and is floating in open water is known as sea ice or, more commonly, icebergs. Presenting a serious hazard to ocean traffic and shipping lanes, icebergs have sunk numerous vessels, including the ill-fated RMS Titanic in 1912, killing 1,503. Icebergs float on the surface, but between 81 and 89 percent of the ice is submerged. The water level at which sea ice floats depends on the

Calving in Glacier Bay National Park, Alaska (Scott Kapich, Shutterstock, Inc.)

exact density of the ice, as determined by the total amount of air bubbles trapped in the ice and how much salt got trapped there during freezing.

Four main categories of sea ice may break off from larger glaciers, ice caps, or ice shelves to form many icebergs. The first comes from ice that formed on polar seas in the Arctic Ocean and around Antarctica. The ice that forms in these regions is typically about 10-15 feet (3-4 m) thick. Antarctica becomes completely surrounded by this sea ice every winter, and the Arctic Ocean is typically about 70 percent covered in the winter. During summer many passages open up in this sea ice, but during winter they reclose, forming pressure ridges of ice that may be 50-100 feet (up to tens of meters) high. Recent observations suggest that the sea ice in the Arctic Ocean is thinning dramatically and rapidly, and may soon disappear altogether. The icecap over the Arctic Ocean rotates clockwise, in response to the spinning of the Earth. This spinning is analogous to putting an ice cube in a glass, and slowly turning the glass. The ice cube will rotate more slowly than the glass, because it is decoupled from the edge of the glass. About one-third of the ice is removed every year by the East Greenland current. This ice then moves south and becomes icebergs, and thus hazards to shipping in the North Atlantic.

A second group of sea ice forms as pack ice in the Gulf of St. Lawrence, along the southeast coast of Canada; in the Bering, Beaufort, and Baltic Seas; in the Seas of Japan and Okhotsk; and around Antarctica. Pack ice builds up especially along the western sides of ocean basins, where cold currents are more common. Occasionally, during cold summers, pack ice persists throughout the summer.

Pack ice is hazardous when it becomes so extensive that it effectively blocks shipping lanes, or when leads (channels) into the ice open and close, forming pressure ridges that become too thick to penetrate with ice breakers. Ships attempting to navigate through pack ice have become crushed when leads close, and the ships are trapped. Pack ice has terminated or resulted in disaster for many expeditions to polar seas, most notably Sir John Franklin's expedition in 1845 in the Canadian arctic and Robert F. Scott's expedition from 1901 to 1904 to Antarctica. Pack ice also breaks up, forming many small icebergs, but because these are not as thick as icebergs of other origins, they do not present as significant a hazard to shipping.

Pack ice also presents hazards when it drifts into shore, usually during spring breakup. With significant winds pack ice can pile up on flat shorelines and accumulate in stacks up to 50 feet (15 m) high. The tremendous force of the ice is enough to crush shoreline wharves, docks, buildings, and boats. Pack ice blown ashore also commonly pushes up high piles of gravel and boulders that may be 35 feet (10.5 m) high in places. These ridges are common around many of the Canadian Arctic islands and mainland. Ice that forms initially attached to the shore presents another type of hazard. If it breaks free and moves away from shore, it may carry with it significant quantities of shore sediment, causing rapid erosion of beaches and shore environments.

Pack ice also forms on many high-latitude lakes, and the freeze-thaw cycle causes cracking of the lake ice. When lake water rises to fill the cracks, the ice cover on the lake expands and pushes over the shoreline, causing damage to any structures built along the shore. This is a common problem on many lakes in northern climates and leads to widespread damage to docks and other lakeside structures.

Icebergs derived from glaciers present the greatest danger to shipping. In the Northern Hemisphere most icebergs calve off glaciers in Greenland or Baffin Island, then move south through the Davis Strait into shipping lanes in the North Atlantic off Newfoundland. Some icebergs calve off glaciers adjacent to the Barents sea, and others come from glaciers in Alaska and British Columbia. In the Southern Hemisphere most icebergs come from Antarctica, though some come from Patagonia, the southern tip of South America.

Once in the ocean icebergs drift with ocean currents, but the Coriolis force deflects them to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. Most icebergs are about 100 feet to 300 feet (30.5-91.5 m) high, and up to about 2,000 feet (609.5 m) long. In March 2000 a huge iceberg broke off the Ross Ice Shelf in Antarctica. This berg was roughly the size of the state of Delaware, with an area of 4,500 square miles (11,655 km2) and rising 205 feet (62.5 m) out of the water. Icebergs in the Northern Hemisphere pose a greater threat to shipping, as those from Antarctica are too remote and rarely enter shipping lanes. Ship collisions with icebergs have resulted in numerous maritime disasters, especially in the North Atlantic on the rich fishing grounds of the Grand Banks off the coast of Newfoundland.

Satellites now track icebergs, and ships receive updated information about their positions to avoid disastrous collisions. Radio transmitters are placed on larger icebergs to monitor their locations more closely; many ships now carry more sophisticated radar and navigational equipment that helps track the positions of large icebergs and the ship, to avoid collision.

Icebergs also pose a serious threat to oil-drilling platforms and seafloor pipelines in high-latitude seas. Some precautions have been taken, such as building seawalls around near-shore platforms, but not enough planning has gone into preventing an iceberg from colliding with and damaging an oil platform or from one being dragged across the seafloor and rupturing a pipeline.

See also climate; climate change; ice ages; supercontinent cycles.

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