Iceberg

I

Introduction

Iceberg, mass of freshwater ice that is calved, or broken off, from a glacier or an ice shelf (a huge slab of permanent ice that floats on water near the edges of polar land masses) and that floats in the ocean or in a lake. Ice floats because it is less dense than water. A typical iceberg shows only about one-fifth of its total mass above the water; the other four-fifths is submerged. Icebergs can be large. The largest iceberg ever sighted was 335 km (208 mi) long and 97 km (60 mi) wide, about the size of Belgium. It was sighted in November 1956 by the crew of a United States Coast Guard icebreaker in the Ross Sea, off Antarctica. Icebergs pose a hazard to shipping and to offshore activities, such as offshore oil drilling, in polar and subpolar waters.

Icebergs can have many different forms, depending on their origin and age. They are usually classified as tabular (resembling a flat tabletop), rounded, or irregular. From massive tabular icebergs calved from ice shelves to small irregular bergs that have been weathered and scoured by wind and waves, they present spectacular sights in the polar and subpolar seas.

Icebergs were known to early mariners and explorers and to sealers and whalers who hunted their prey in Arctic and Antarctic waters. Beginning in the 20th century, icebergs have been used as stable platforms for scientific stations. Some people have suggested towing icebergs to places where water is scarce and melting them there, but this idea has not yet been implemented.

II

Origin, Size, and Classification

As ice in coastal glaciers slides toward sea level, it eventually breaks off and falls into the ocean in various-sized pieces. The smallest pieces, which are football-size to table-size chunks less than 2 m (7 ft) in diameter, are called brash ice. Larger pieces that float low in the water are called growlers because waves washing over them produce a growling sound. Bergy bits are ice pieces between 2 and 5 m (7 and 16 ft) in diameter. All larger bodies of ice are called icebergs.

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The largest icebergs come from ice shelves, which are the outer extensions of large icecaps and ice sheets, such as those located in Antarctica. (Icecaps are dome-shaped ice formations that are anchored on land and extend outward from an interior location, while ice sheets are flat extensions of icecaps and may cover both land and water.) Glaciers feeding into an icecap or ice sheet push the ice shelf out to sea. Ice shelves float on the ocean and can be 200 to 300 m (650 to 980 ft) thick. The pieces that eventually break off from ice shelves can form icebergs hundreds of kilometers long and tens of kilometers wide. These icebergs may weigh billions or even trillions of tons. Icebergs that break off from ice shelves are called tabular icebergs because their flat upper surface resembles a flat tabletop.

The icebergs that come from glaciers that flow to the sea are generally smaller than those from ice shelves. When the ice reaches the sea, pieces break off and fall into the water. The icebergs then drift away from the shore. Glaciers in Greenland and Alaska produce many such icebergs every year.

Icebergs are driven away from the shore by winds and ocean currents and slowly break up and melt. The smallest pieces disappear first, and the large tabular icebergs last the longest. On average, an iceberg lasts about four years, but many icebergs last much longer. Tabular icebergs often have spectacular caves in their sides, carved out by wave action. As an iceberg shrinks, it may tilt or roll over so that the smooth underside or lines etched out by the waves come into view. Icebergs transformed in this way often have fantastic shapes.

Each of the three major types of icebergs (tabular, rounded, and irregular) has numerous subcategories. Tabular icebergs show no signs of rollover and may be horizontal (totally flat), uneven, domed, tilted, or blocky. Rounded icebergs have been smoothed by the water and have rolled over. Irregular icebergs have angular or irregular features. Their shapes include pinnacled, pyramidal, drydock, castellated, jagged, blocky, roofed, and rounded. Drydock icebergs have a low sunken area in the middle that is often awash. The skylines of castellated icebergs resemble the battlements on a castle.

III

Distribution

Icebergs occur mostly in Antarctic and Arctic waters. The Arctic has several sources of icebergs, the icecap of Greenland being the major source. Large glaciers, such as the Jakobshavn Glacier on the west coast of Greenland, produce many icebergs, including, most likely, the iceberg that sank the British ocean liner Titanic in 1912. As a result of the Titanic disaster, several governments set up the International Ice Patrol, the ships and aircraft of which keep an eye on these Arctic icebergs, which may be seen as far south as the Grand Banks (latitude 40º-50º north) and on occasion down to latitude 30º north. The International Ice Patrol has sometimes towed smaller icebergs out of the way of shipping or of an offshore oil-drilling platform.

The Ward Hunt Ice Shelf, on the northern coast of Ellesmere Island in Canada’s high Arctic, produces tabular icebergs. One of the icebergs from this shelf was 50 m (160 ft) thick and covered an area of 90 sq km (35 sq mi). Between 1952 and 1978 it was used as a manned scientific research station that included huts, a power plant, and a runway for wheeled aircraft. Discovered by U.S. Air Force Colonel Joseph Fletcher, the iceberg was named T-3 or Fletchers Ice Island. It moved around the Arctic Ocean for many years, eventually exiting through the Fram Strait, between Greenland and Svalbard, and moving around the southern tip of Greenland to disintegrate and melt in Davis Strait.

Antarctica produces most of the world’s icebergs. Glaciers and ice shelves occur along the coast of the entire continent, with ice shelves existing along 30 percent of the coastline. The ice shelves include the three largest in the world: the Ross, the Filchner, and the Ronne ice shelves. Antarctic icebergs initially move westward with the coastal current before entering the eastward Antarctic Circumpolar Current system, which carries them north where they melt in warmer waters. The northernmost sighting of an Antarctic iceberg was made in 1894 at latitude 27º south. Antarctic icebergs generally travel only as far north as latitude 35º south in the Atlantic and Indian oceans and latitude 45º south in the Pacific Ocean. At any time, tens of thousands of large icebergs may be in the Southern Ocean, as the southernmost portions of the Atlantic, Indian, and Pacific oceans are sometimes called. A survey made in 1965 counted 30,000 icebergs in an area of 4,400 sq km (1,700 sq mi). Near the coast of Antarctica many icebergs are stranded in shallow water, and ships trying to reach coastal research stations often must travel through spectacular iceberg alleys.

IV

Current Research

Current iceberg research centers on satellite observations of icebergs, the use of icebergs to study past climate changes, and studies of using icebergs for drinking and irrigation water.

Satellites have proven a boon for detecting and tracking icebergs. Several satellites use a special type of radar that can track icebergs day and night and even through clouds. Using such satellites, a group of Norwegian scientists tracked a very large iceberg, about 100 by 50 km (60 by 30 mi) in size, over a period of ten years. This iceberg followed the Weddell Sea coast of Antarctica for two years, got stuck in shallow waters for five years, and then bumped into the Larsen Ice Shelf, breaking off another iceberg 90 by 35 km (55 by 22 mi) in size. Both icebergs disintegrated later in the Southern Ocean. The drift tracks of icebergs often provide useful information on ocean currents.

Satellites have been used to measure changes in the size of ice shelves. In the Antarctic Peninsula area, ice shelves are retreating rapidly, most likely because of a warmer climate in the region. Since the 1960s these ice shelves have lost about 10,000 sq km (4,000 sq mi) by calving off large icebergs. This process may occur quite rapidly. For example, within four months in 1995 the Larsen Ice Shelf lost 1,640 sq km (630 sq mi), of which over half was lost in just two days by the shedding of numerous smaller icebergs. Some of the smaller ice shelves in the region may soon disappear completely.

Icebergs have been a useful research tool in dating past events. Icebergs carry rocks and other debris from land. When the icebergs melt, they drop the debris on the ocean floor. This debris has been used to identify periods when large groups of icebergs existed as a result of the collapse of large ice sheets. Rock debris found on the floor of the Atlantic Ocean has been dated and has been found to coincide with the onset of major climatic changes. In many instances these climatic changes began with an abrupt warming.

In view of shortages of water for drinking and irrigation in many parts of the world, towing of icebergs to locations where water is needed most has been seriously considered. This process would be slow, as it would entail maneuvering an iceberg into favorable ocean currents with the aid of powerful tugboats and landing it on shores where it could be quarried or melted. Towing an iceberg 1 km (0.6 mi) in length to Australia would provide water more cheaply than obtaining water by desalinating ocean water. However, the many technical difficulties involved have not been satisfactorily solved so far.

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