Icebergs

KEY TERMS

Resources

An iceberg is a large mass of free-floating ice that has broken away from a glacier. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica, and a few icebergs come from smaller Alaskan glaciers. Snow produces the glaciers and ice sheets so, ultimately, icebergs originate from snow. In contrast, sea ice is produced by freezing saltwater. The word iceberg has its roots in Nordic and German words meaning, literally, ice mountain.

Icebergs in the southern hemisphere are monitored by the United States National Ice Center, founded in 1995, which assigns a letter and serial number identifier (for example, iceberg C18) to each large iceberg measuring more than 6 mi (10 km) along one or more sides. The letter identifiers delineate the longitude of origin. A is assigned to icebergs originating between longitudes 0° and 90°W, B is assigned to icebergs originating between 90° and 180°, C is assigned to icebergs originating between 180° and 270°, and D is assigned to icebergs originating between 270° and 0°. Icebergs in the North Atlantic are monitored by the International Ice Patrol (ICP), created in 1914 in response to the sinking of the H.M.S. Titanic and operated by the U.S. Coast Guard. Because information about icebergs in the North Atlantic is critical for safe navigation and commerce, the ICP is funded by Belgium, Canada, Denmark, Finland, France, Germany, Greece, Italy, Japan, The Netherlands, Norway, Panama, Poland, Spain, Sweden, the United Kingdom, and the

United States. Two other organizations, the Canadian Ice Services and the Danish Meteorological Institute, monitor icebergs near Canada and Greenland.

Icebergs were originally tracked using reports from ships, and then aircraft, but are currently monitored using remote sensing satellites. Satellites using synthetic aperture radar and infrared imagery are particularly effective at tracking icebergs across oceans that are often shrouded in fog and clouds.

Icebergs are formed by a process called calving, in which large pieces of ice break off of a glacier. Icebergs consist of freshwater ice, soil and rock debris, and trapped bubbles of air. The combination of ice and air bubbles causes sunlight shining on the icebergs to refract, coloring the ice spectacular shades of blue, green, and white. Color may also indicate age. Blue icebergs are old. Green icebergs contain algae and are young. Icebergs come in a variety of shapes and sizes, some long and flat, others towering and massive.

An iceberg floats because it is lighter and less dense than salty seawater, but only a small part of the iceberg is visible above the surface of the sea. Typically, about 7/8 of an iceberg is below sea level, so they drift with ocean currents rather than wind. Scientists who study icebergs classify true icebergs as pieces of ice that are greater than 16 ft (5 m) above sea level and wider than 98 ft (30 m) at the water line. Smaller pieces of floating ice are called bergy bits (3.3-16 ft or 1-5 m tall and 33-98 ft or 10-30 m wide) or growlers (less than 3.3 ft or 1 m tall and less than 33 ft or 10 m wide). The largest icebergs can be taller than 230 ft (70 m) and wider than 738 ft (225 m). Chunks of ice more massive than this are called ice islands. Ice islands are much more common in the southern hemisphere, where they break off the Antarctic ice sheets, than in the northern hemisphere.

Icebergs can also be classified by their shape. Flat icebergs are called tabular. Icebergs that are tall and flat are called blocky. Domed icebergs are rounded with gentle slopes. Drydock icebergs have been eroded by waves so that they are somewhat U-shaped. Perhaps the most spectacular are the pinnacle icebergs, which resemble mountain tops, with one or more central peaks.

The life span of an iceberg depends on its size, but is typically about two years for icebergs in the Northern Hemisphere. Because they are larger, icebergs from Antarctica may last longer. Chief among the destructive forces that work against icebergs are wave action and heat. Wave action can break icebergs into smaller pieces and can cause icebergs to collide and fracture. Relatively warm air and water temperature gradually melt the ice. Because icebergs float, they drift with water currents towards the equator into warmer water. Icebergs may drift as far as 8.5 mi (14 km) per day. Most icebergs have completely melted by the time they reach about 40 degrees latitude (north or south). There have been rare occasions when icebergs have drifted as far south as Bermuda (32 degrees north latitude), which is located about 900 mi (1,400 km) east of Charleston, South Carolina. In the Atlantic Ocean, they have also been found as far east as the Azores Islands in the Atlantic Ocean, off the coast of Spain.

The largest recorded iceberg, denoted B15, calved from the Ross Ice Shelf of Antartica in May 2000. It was approximately 180 mi (290 km) long and 25 mi (40 km) wide (larger than the state of Delaware), but in 2002 disintegrated into smaller icebergs. The largest of those fragments, iceberg B15A, became the world’s largest iceberg. B15A persisted until October 2005, when imagery from the Envisat satellite, operated by the European Space Agency, showed that it disintegrated near Cape Adare, Antarctica.

The R.M.S. Titanic struck an iceberg and sank on April 14, 1912, when the great ship was on her maiden voyage. More than 1,500 people died in that disaster, which occurred near Newfoundland, Canada. As a result of the tragedy, the International Ice Patrol was formed to protect shipping interests in the North Atlantic. Counts of icebergs drifting into the North Atlantic shipping lanes vary from year to year, with little predictability. During some years, no icebergs drift into the lanes; other years are marked by hundreds or more—as many as 1,572 have been counted in a single year. Many ships now carry their own radar equipment to detect icebergs. In 1959, a Danish ship equipped with radar struck an iceberg and sank, resulting in 95 deaths.

Modern iceberg research continues to focus on improved methods for tracking and monitoring icebergs, and on learning more about iceberg deterioration. In 1995, a large iceberg broke free from the Larsen ice shelf in Antarctica. This iceberg was 48 mi (77 km) long, 23 mi (37 km) wide, and 600 ft (183 m) thick, approximately the size of the country of Luxembourg, and isolated James Ross Island (one of Antarctica’s islands) for the first time in recorded history. The unusually large iceberg was monitored by to ensure it did not put ships at peril. According to some scientists, this highly unusual event could be

KEY TERMS

Calving —The process in which huge chunks of ice or icebergs break off from ice shelves and sheets or glaciers to form icebergs.

Ice island —A thick slab of floating ice occupying an area as large as 180 sq mi (460 sq km).

Ice sheet —Glacial ice covering at least 19,500 sq mi (50,000 sq km) of land and obscuring the landscape below it.

Ice shelf —That section of an ice sheet that extends into the sea a considerable distance and which may be partially afloat.

Sea ice —Ice that forms from the freezing of saltwater; as the saltwater freezes, it ejects salt, so sea ice is fresh, not salty. Sea ice forms in relatively thin layers, usually no more than 3-7 ft (1-2 m) thick, but it can cover vast areas of the ocean surface at high latitudes.

evidence of global warming. Surges in the calving of icebergs, known as Heinrich events, are also known to be caused by irregular motions of Earth around the sun that cause ocean waters of varying temperatures and salinity to change their circulation patterns. These cycles were common during the last glacial period, and glacial debris was carried by iceberg armadas to locations like Florida and the coast of Chile. Scientists have captured icebergs for study, including crushing pieces to measure their strength. During World War II (1939-1945), plans were made to make floating airfields from flat-topped bergs (but this never got past the planning stage). Some people have proposed towing icebergs to drought-stricken regions of the world to solve water shortage problems; however, the cost and potential environmental impact of such an undertaking have so far discouraged any such attempts.

Resources

BOOKS

Gosnell, M. Ice: The Nature, the History, and the Uses of an Astonishing Substance. New York: Knopf, 2005.

Hambrey, M. and J. Alean. Glaciers. Cambridge, United Kingdom: Cambridge University Press, 2004.

Tarbuck, E.J., F.K. Lutgens, and D. Tasa. Earth: An Introduction to Physical Geology. Upper Saddle River, NJ: Prentice Hall, 2004.

Elaine Martin

Icebergs

Introduction

An iceberg is a large piece of ice that has broken, or calved, from the front of a glacier into a body of water. Icebergs form mostly during the spring and summer, when warmer weather increases the rate of calving at the boundaries of ice sheets and smaller outlying glaciers. Because the density of ice is slightly less than that of sea water, icebergs float with approximately 90% of their mass found below the ocean surface. Due to increases in global temperatures, the increased calving of icebergs from large glaciers and ice shelves may eventually have considerable impact on sea level and thus on coastal ecosystems.

Historical Background and Scientific Foundations

After an iceberg sank the ocean liner RMS Titanic in 1912, iceberg tracking began in earnest with the formation of the

International Ice Patrol (IIP) in 1914. This group, with the financial support of thirteen member nations including the United States, began following icebergs with ships and later with airborne radar as they moved along “Iceberg Alley” from west Greenland down the Labrador current. Because Iceberg Alley follows the Grand Banks shipping lanes located southeast of Newfoundland, Canada, the monitoring of large bergs by the IIP is imperative to the safety of cargo, fishing, whaling, and cruise vessels in the North Atlantic.

The true pioneers of iceberg science, however, were the Soviet researchers who set up 31 ice camps on sea ice floes or glacial bergs in the Arctic Ocean from 1937 until the collapse of the Soviet Union in 1991. The data collected in these camps included long-term measurements of arctic weather and studies of how sea-ice forms. The Americans followed suit in 1952 by establishing Fletcher's Ice Island, a research site on a large, drifting ice-shelf berg used until it migrated into the Atlantic in the 1970s. These Soviet and American research camps allowed scientists to collect hundreds of thousands of daily measurements, including a chronological atlas of arctic sea ice distribution. Since the loss of the drift stations, satellites and automated buoys have been used for data collection. However, modern technology has been unable to replace the daily, on-site measurements made by researchers living on the ice itself.

Even with the extensive data collected by arctic researchers over six decades, much about icebergs remains unknown, including how currents, winds, and tides can affect their movements. It is difficult to track the movements of icebergs of a kilometer square or less, which comprise half the total mass of Antarctic bergs. As these small icebergs are prone to capsizing and are filled with crevasses, placing global positioning trackers on them has proved to be a daunting task.

Icebergs have been helpful in the study of certain ocean currents. It has been assumed that the deep keels and low profiles of icebergs cause them to reflect primarily the movement of subsurface currents rather than of winds. The estimated 1,500 cubic km of icebergs breaking off Antarctic glaciers yearly do appear to follow the counter-clockwise motion of the Antarctic coastal current. Nevertheless, satellites show that many icebergs spin out of the coastal current into seas farther north, possibly carried on regional gyres. Fragments originally close together may then move in opposite directions, finally ending up hundreds of kilometers apart.

Icebergs are categorized by the IIP according to their size and shape. The smallest size category, growlers, are less than 3 ft (1 m) in height and 16 ft (5 m) in length. This name comes from the growling sound sailors hear as these icebergs bob in the water. Size classes then increase from “bergy bits” to “small,” “medium,” “large,” and finally “very large” icebergs, the latter measuring greater than 246 ft (75 m) in height and 699 ft (213 m) in length.

WORDS TO KNOW

CALVING, ICEBERGS: Separation of an iceberg from a glacier at the point where the glacier flows into the sea. When an iceberg calves, it breaks loose and floats free in the water: this immediately raises sea level slightly, with no further change as the iceberg melts.

GLACIAL BERGS: Icebergs that have formed by breaking off the edge of a glacier that is flowing to the sea.

GLACIER: A multi-year surplus accumulation of snowfall in excess of snowmelt on land and resulting in a mass of ice at least 0.04 mi² (0.1 km²) in area that shows some evidence of movement in response to gravity. A glacier may terminate on land or in water. Glacier ice is the largest reservoir of freshwater on Earth and is second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia.

ICE FLOES: Plates of floating sea ice, that is, ice formed by freezing of the top layer of the ocean; distinct from icebergs, which are thicker and are produced by the breakup of glaciers flowing into the sea. Any floating plate of ice wider than 6 mi (10 km) is termed an ice field, not an ice floe.

PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.

TABULAR BERG: Flat-topped (i.e., like a table, hence “tabular”) iceberg, usually formed by breaking off from the edge of a floating ice shelf.

TIDEWATER GLACIER: Glacier that flows into the ocean, as opposed to melting on dry land. Tidewater glaciers in Greenland and the West Antarctica Peninsula have significantly speeded up their flow to the sea in recent years, contributing to accelerated sea-level rise.

In addition to this categorization based on size, icebergs are classified according to their shapes. The two main categories are tabular (an iceberg with steep sides and a flat top with a length-to-height ratio of greater than 5:1) and non-tabular (all icebergs that do not exhibit the tabular shape). These non-tabular bergs are then further subdivided into dome (an iceberg with a rounded top), pinnacle (an iceberg with one or more spires), wedge (an iceberg having a steep vertical side on one end and sloping on the other), dry-dock (an iceberg that has eroded so that a slot or channel is formed), and blocky (an iceberg with a flat top and steep vertical sides).

The largest iceberg ever reliably measured was B–15, a tabular berg that calved off the Ross Ice Shelf in Antarctica in March, 2000. At 4,250 square mi (11,000 square km), an area larger than the state of Rhode Island, B–15 subsequently broke into smaller bergs, some still large enough to support their own weather systems of high winds, blowing snow, and fog.

Impacts and Issues

Interest in icebergs recently intensified with record increases in the calving of bergs from continental ice shelves in outlying parts of Antarctica and tidewater glaciers in the Northern Hemisphere. While the calving of B–15 in Antarctica was preceded by years of cracking and is considered to be part of a long-time, natural calving process in the Ross Ice Shelf, in 1995—without prior warning—approximately 500 square mi (1,300 square km) of the Larsen A Ice Shelf fell apart into thousands of icebergs over the course of just a few days.

The Larsen A Ice Shelf is situated on the Antarctic Peninsula, where average temperatures over the past 55 years have risen more than 4.5°F (2.5°C). Scientists were unsure whether this breakup was caused by global climate change, but an even more dramatic ice-shelf breakup occurred in 2002, when the Larsen B ice shelf, 1,254 square mi (3,250 square km), broke apart in only 35 days. In 2006, scientists with the British Antarctic Survey reported that the Larsen B breakup was definitely due to human-caused global warming, not natural processes. Other scientists have reported evidence that global warming is accelerating the movement of glaciers to the sea in both Antarctica and Greenland. The glaciers dump their mass into the oceans in the form of icebergs, raising sea level.

As global temperatures have risen, the rates of snowfall, ice melting, and glacier flow have risen as well. These opposing processes tend to offset each other on a regional scale, but data show that the Antarctica and Greenland ice sheets are both losing mass overall. Scientists estimate that these two ice sheets are presently losing 125 billion metric tons of ice per year. The current rate of ice loss from these sources increases sea level rise over background levels by only 1.4 in (0.35 mm) per year; however, ice loss has accelerated over the last decade and will likely continue to accelerate over the course of the twenty-first century. Scientists are concerned as to how the resultant sea level rise may impact human populations living in coastal areas.

The loss of large icebergs from ice shelves in the Antarctic due to climate warming can also have wide-ranging impacts on coastal ecosystems. When the B–15 iceberg restricted the northwestward drift of pack ice in the southwestern Ross Sea off Antarctica, large areas of what would normally be open sea were covered with ice. Thus, one of the most biologically productive regions in the Antarctic saw a decrease in phytoplankton productivity of more than 40%. This had cascading effects on those organisms, such as krill, that depend on phytoplankton growth, and on higher-level predators such as the Adélie penguins that feed on krill.

See Also Antarctica: Melting; Glacier; Heinrich Events; Melting; Polar Ice.

BIBLIOGRAPHY

Periodicals

Krajick, Kevin. “Tracking Icebergs for Clues to Climate Change.” Science 292 (2001): 2244–2245.

Marshall, Gareth J., et al. “The Impact of a Changing Southern Hemisphere Annular Mode on Antarctic Peninsula Summer Temperatures.” Journal of Climate 19 (2006): 5388–5404.

Shepherd, Andrew, and Duncan Wingham. “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets.” Science 315 (2007): 1529–1532.

Smith, H. Jesse. “Climate Science: The Iceberg Cometh.” Science 296 (2002): 619.

Wuethrich, Bernice. “Lack of Icebergs Another Sign of Global Warming?” Science 285 (1999): 37.

Web Sites

“International Ice Patrol (IIP): Frequently Asked Questions.” International Ice Patrol. <http://www.uscg.mil/lantarea/iip/FAQ/FAQ_Icebergs.shtml> (accessed October 19, 2007).

“Questions and Answers about Icebergs.” National Snow and Ice Data Center. <http://nsidc.org/icebergs/questions.html> (accessed October 16, 2007).

Michele Chapman

Icebergs

An iceberg is a large mass of free-floating ice that has broken away from a glacier. Beautiful and dangerous, icebergs wander over the ocean surface until they melt. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica . A few icebergs originate from smaller Alaskan glaciers. Snow produces the glaciers and ice sheets so, ultimately, icebergs originate from snow. In contrast, "sea ice" originates from freezing saltwater . When fragments break off of a glacier, icebergs are formed in a process called calving. Icebergs consist of freshwater ice, pieces of debris, and trapped bubbles of air. The combination of ice and air bubbles causes sunlight shining on the icebergs to refract, coloring the ice spectacular shades of blue, green, and white. Color may also indicate age; blue icebergs are old, and green ones contain algae and are young. Icebergs come in a variety of shapes and sizes, some long and flat, others towering and massive.

An iceberg floats because it is lighter and less dense than salty seawater, but only a small part of the iceberg is visible above the surface of the sea. Typically, about 80-90% of an iceberg is below sea level , so they drift with ocean currents rather than wind . Scientists who study icebergs classify true icebergs as pieces of ice which are greater than 16 ft (5 m) above sea level and wider than 98 ft (30 m) at the water line. Of course, icebergs may be much larger. Smaller pieces of floating ice are called "bergy bits" (3.3-16 ft or 1-5 m tall and 33-98 ft or 10-30 m wide) or "growlers" (less than 3.3 ft or 1 m tall and less than 33 ft or 10 m wide). The largest icebergs can be taller than 230 ft (70 m) and wider than 738 ft (225 m). Chunks of ice more massive than this are called ice islands. Ice islands are much more common in the Southern Hemisphere, where they break off the Antarctic ice sheets.

Because of the unusual forms they may take, icebergs are also classified by their shape. Flat icebergs are called tabular. Icebergs which are tall and flat are called blocky. Domed icebergs are shaped like a turtle shell, rounded, with gentle slopes. Drydock icebergs have been eroded by waves so that they are somewhat U-shaped. Perhaps the most spectacular are the pinnacle icebergs, which resemble mountain tops, with one or more central peaks reaching skyward.

The life span of an iceberg depends on its size but is typically about two years for icebergs in the Northern Hemisphere. Because they are larger, icebergs from Antarctica may last for several more years. Chief among the destructive forces that work against icebergs are wave action and heat . Wave action can break icebergs into smaller pieces and can cause icebergs to knock into each other and fracture. Relatively warm air and water temperature gradually melt the ice. Because icebergs float, they drift with water currents towards the equator into warmer water. Icebergs may drift as far as 8.5 mi (14 km) per day. Most icebergs have completely melted by the time they reach about 40 degrees latitude (north or south). There have been rare occasions when icebergs have drifted as far south as Bermuda (32 degrees north latitude), which is located about 900 mi (1,400 km) east of Charleston, South Carolina. In the Atlantic Ocean, they have also been found as far east as the Azores, islands in the Atlantic Ocean off the coast of Spain.

An iceberg struck and sank the R.M.S. Titanic on April 14, 1912, when the great ship was on her maiden voyage; more than 1,500 people lost their lives in that disaster, which occurred near Newfoundland, Canada. As a result of the tragedy, the Coast Guard began monitoring icebergs to protect shipping interests in the North Atlantic sea lanes. Counts of icebergs drifting into the North Atlantic shipping lanes vary from year to year, with little predictability. During some years, no icebergs drift into the lanes; other years are marked by hundreds or more—as many as 1,572 have been counted in a single year. Many ships now carry their own radar equipment to detect icebergs. In 1959, a Danish ship equipped with radar struck an iceberg and sank, resulting in 95 deaths. Some ships even rely on infrared sensors from airplanes and satellites. Sonar is also used to locate icebergs.

Modern iceberg research continues to focus on improving methods of tracking and monitoring icebergs, and on learning more about iceberg deterioration. In 1995, a huge iceberg broke free from the Larsen ice shelf in Antarctica. This iceberg was 48 mi (77 km) long, 23 mi (37 km) wide, and 600 ft (183 m) thick. The monster was approximately the size of the country of Luxembourg and isolated James Ross Island (one of Antarctica's islands) for the first time in recorded history. The megaberg was monitored by airplanes and satellites to make sure it did not put ships at peril. According to some scientists, this highly unusual event could be evidence of global warming . Surges in the calving of icebergs known as Heinrich events are also known to be caused by irregular motions of Earth around the Sun that cause ocean waters of varying temperatures and salinity to change their circulation patterns. These cycles were common during the last glacial period, and glacial debris was carried by "iceberg armadas" to locations like Florida and the coast of Chile. Scientists have "captured" icebergs for study including crushing to measure their strength. During World War II, plans were made to make floating airfields from flat-topped bergs (but this never got past the planning stage). Some people have proposed towing icebergs to drought-stricken regions of the world to solve water shortage problems; however, the cost and potential environmental impact of such an undertaking have so far discouraged any such attempts.

Resources

books

Colbeck, S. C. Dynamics of Snow and Ice Masses. New York: Academic Press, 1980.

Lewis, E.O., B.W. Currie, and S. Haykin. Detection and Classification of Ice. Letchworth, England: Research Studies Press, 1987.

Sharp, R. P. Living Ice: Understanding Glaciers and Glaciation. Cambridge: Cambridge University Press, 1988.

periodicals

Ballard, R. D. "A Long Last Look at the Titanic." National Geographic (December 1986): 698-727.

Dane, M. "Icehunters." Popular Mechanics (October 1993): 76-79.

Monastersky, R. "Satellite Radar Keeps Tabs on Glacial Flow." Science News (December 1993): 373.

Nicklin, F. "Beneath Arctic Ice: Life at the Edge." National Geographic (July 1991): 2-31.

Raney, R. K. "Probing Ice Sheets with Imaging Radar." Science 262 (1993):1521-1522.

Steger, W. "Six across Antarctica: Into the Teeth of the Ice." National Geographic (November 1990): 67-95.

Vogt, P. R., and K. Crane. "Megabergs Left Scars in Arctic." Science News (August 1994): 127.

Elaine Martin

KEY TERMS

Calving

—The process in which huge chunks of ice or icebergs break off from ice shelves and sheets or glaciers to form icebergs.

Ice island

—A thick slab of floating ice occupying an area as large as 180 sq mi (460 sq km).

Ice sheet

—Glacial ice covering at least 19,500 sq mi (50,000 sq km) of land and obscuring the landscape below it.

Ice shelf

—That section of an ice sheet that extends into the sea a considerable distance and which may be partially afloat.

Sea ice

—Ice that forms from the freezing of salt water; as the saltwater freezes, it ejects salt, so sea ice is fresh, not salty. Sea ice forms in relatively thin layers, usually no more than 3–7 ft (1–2 m) thick, but it can cover vast areas of the ocean surface at high latitudes.

Icebergs

An iceberg is a large mass of free-floating ice that has broken away from a glacier. Beautiful and dangerous, icebergs wander over the ocean surface until they melt. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica . A few icebergs originate from smaller Alaskan glaciers. Snow produces the glaciers and ice sheets so, ultimately, icebergs originate from snow. In contrast, "sea ice" originates from freezing salt water . When fragments break off of a glacier, icebergs are formed in a process called calving.

Icebergs consist of freshwater ice, pieces of debris, and trapped bubbles of air. The combination of ice and air bubbles causes sunlight shining on the icebergs to refract, coloring the ice spectacular shades of blue, green, and white. Color may also indicate age; blue icebergs are old, and green ones contain algae and are young. Icebergs come in a variety of shapes and sizes, some long and flat, others towering and massive.

An iceberg floats because it is lighter and less dense than salty seawater, but only a small part of the iceberg is visible above the surface of the sea. Typically, about 80–90% of an iceberg is below sea level, so they drift with ocean currents rather than wind . Scientists who study icebergs classify true icebergs as pieces of ice that are greater than 16 ft (5 m) above sea level and wider than 98 ft (30 m) at the water line. Of course, icebergs may be much larger. Smaller pieces of floating ice are called "bergy bits" (3.3–16 ft or 1–5 m tall and 33–98 ft or 10–30 m wide) or "growlers" (less than 3.3 ft or 1 m tall and less than 33 ft or 10 m wide). The largest icebergs can be taller than 230 ft (70 m) and wider than 738 ft (225 m). Chunks of ice more massive than this are called ice islands. Ice islands are much more common in the Southern Hemisphere, where they break off the Antarctic ice sheets.

Because of the unusual forms they may take, icebergs are also classified by their shape. Flat icebergs are called tabular. Icebergs that are tall and flat are called blocky. Domed icebergs are shaped like a turtle shell, rounded, with gentle slopes. Drydock icebergs have been eroded by waves so that they are somewhat U-shaped. Perhaps the most spectacular are the pinnacle icebergs, which resemble mountain tops, with one or more central peaks reaching skyward.

The life span of an iceberg depends on its size but is typically about two years for icebergs in the Northern Hemisphere. Because they are larger, icebergs from Antarctica may last for several more years. Chief among the destructive forces that work against icebergs are wave action and heat. Wave action can break icebergs into smaller pieces and can cause icebergs to knock into each other and fracture. Relatively warm air and water temperature gradually melt the ice. Because icebergs float, they drift with water currents towards the equator into warmer water. Icebergs may drift as far as 8.5 mi (14 km) per day. Most icebergs have completely melted by the time they reach about 40 degrees latitude (north or south). There have been rare occasions when icebergs have drifted as far south as Bermuda (32 degrees north latitude), which is located about 900 mi (1,400 km) east of Charleston, South Carolina. In the Atlantic Ocean, they have also been found as far east as the Azores, islands in the Atlantic Ocean off the coast of Spain.

One of the best-known icebergs is the one that struck and sank the RMS Titanic on April 14, 1912, when the ship was on her maiden voyage. More than 1,500 people lost their lives in that disaster, which occurred near Newfoundland, Canada. As a result of the tragedy, the Coast Guard began monitoring icebergs to protect shipping interests in the North Atlantic sea lanes. Counts of icebergs drifting into the North Atlantic shipping lanes vary from year to year, with little predictability. During some years, no icebergs drift into the lanes; other years are marked by hundreds or more—as many as 1,572 have been counted in a single year. Many ships now carry their own radar equipment to detect icebergs. As recently as 1959, a Danish ship equipped with radar struck an iceberg and sank, resulting in 95 deaths. Some ships even rely on infrared sensors from airplanes and satellites. Sonar is also used to locate icebergs.

Modern iceberg research continues to focus on improving methods of tracking and monitoring icebergs, and on learning more about iceberg deterioration. In 1995, a huge iceberg broke free from the Larsen ice shelf in Antarctica. This iceberg was 48 mi (77 km) long, 23 mi (37 km) wide, and 600 ft (183 m) thick. The iceberg was approximately the size of the country of Luxembourg and isolated James Ross Island (one of Antarctica's islands) for the first time in recorded history. The megaberg was monitored by airplanes and satellites to make sure it didn't put ships at peril. According to some scientists, this highly unusual event could be evidence of global warming . Surges in the calving of icebergs known as Heinrich events are also known to be caused by irregular motions of Earth around the Sun that cause ocean waters of varying temperatures and salinity to change their circulation patterns. These cycles were common during the last glacial period, and glacial debris was carried by "iceberg armadas" to locations like Florida and the coast of Chile. Scientists have "captured" icebergs for study including crushing to measure their strength. It has been proposed to tow icebergs to drought-stricken regions of the world to solve water shortage problems; however, the cost and potential environmental impact of such an undertaking have so far discouraged any such attempts.

See also Glaciation; Ocean circulation and currents

Iceberg

An iceberg is a large mass of free-floating ice that has broken away from a glacier. (Glaciers are flowing masses of ice, created by years of snowfall and cold temperatures.) Beautiful and dangerous, icebergs are carried about the ocean surface until they melt. Most icebergs come from the glaciers of Greenland or from the massive ice sheets of Antarctica.

The process of icebergs breaking off of a glacier is called calving. Icebergs consist of freshwater ice, pieces of debris, and trapped bubbles of air. The combination of ice and air bubbles causes sunlight shining on the icebergs to color the ice spectacular shades of blue, green, and white. Icebergs come in a variety of unusual shapes and sizes, some long and flat, others towering and massive.

An iceberg floats because it is lighter and less dense than the salty seawater, but only a small part of the iceberg is visible above sea level. Typically, about 80 to 90 percent of an iceberg is below sea level. Scientists who study icebergs classify true icebergs as pieces of ice that are higher than 16 feet (5 meters) above sea level and wider than 98 feet (30 meters) at the water line. The largest icebergs can be taller than 230 feet (70 meters) and wider than 738 feet (225 meters). Chunks of ice more massive than this are called ice islands. Ice islands are much more common in the Southern Hemisphere, where they break off from the Antarctic ice sheets.

The life span of an iceberg depends on its size, but is typically about two years in the Northern Hemisphere. Because they are larger, icebergs from Antarctica may last for several more years. The main destructive forces that work against icebergs are wave action and heat. Wave action can break icebergs into smaller pieces. It can also force icebergs to knock into each other, which can fracture them. Relatively warm air and water temperature gradually melt icebergs.

Words to Know

Calving: Process of iceberg formation in which a glacier flows into the sea and large chunks of glacial ice break free due to stress, pressure, or the forces of waves and tides.

Ice island: Thick slab of floating ice occupying an area as large as 180 square miles (460 square kilometers).

Ice sheet: Glacial ice that covers at least 19,500 square miles (50,000 square kilometers) of land and that flows in all directions, covering and obscuring the landscape below it.

Ice shelf: Section of an ice sheet that extends into the sea a considerable distance and that may be partially afloat.

Since icebergs float, they drift with water currents toward the warmer waters near the equator. Icebergs may drift as far as 8.5 miles (14 kilometers) per day. Most icebergs have completely melted by the time they reach about 40 degrees latitude, north and south. There have been rare occasions when icebergs have drifted as far south as the island of Bermuda in the Caribbean, and as far east as the Azores, islands in the Atlantic Ocean off the coast of Spain.

Loss of the Titanic to an iceberg

One of the best-known icebergs is the one that struck and sank the ocean liner Titanic on her maiden voyage in the spring of 1912. More than 1,500 people lost their lives in that disaster, which occurred near Newfoundland, Canada. As a result of the tragedy, 16 nations agreed to monitor icebergs to protect shipping interests in the North Atlantic sea lanes. Counts of icebergs drifting into the North Atlantic shipping lanes vary from year to year. Some years no icebergs drift into the lanes; other years are marked by hundreds or more. Many ships now carry their own radar equipment to detect icebergs. Some ships even rely on infrared sensors from airplanes and satellites. Sonar is also used to locate icebergs.

Modern iceberg research continues to focus on improving methods of tracking and monitoring icebergs and on learning more about iceberg deterioration. In 1995, a huge iceberg broke free from the Larsen Ice Shelf in Antarctica. Between the fall of 1998 and the spring of 1999, 662 square

miles (1,714 square kilometers) of area from the Larsen Ice Shelf calved away. In the fall of 2000, an iceberg measuring 30 by 11.5 miles (48 by 18.5 kilometers) calved from the Ross Ice Shelf in Antarctica. According to some scientists, these highly unusual events could be evidence of global warming.

Some people have proposed towing icebergs to regions of the world that suffer from drought. However, the cost and potential environmental impact of such a project have discouraged any such attempts.

[See also Glaciers ]