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Icebergs form when ice from large glaciers flows into the ocean and breaks into floating blocks. Most icebergs form in Alaska, Greenland, and Antarctica.
The answer depends on the context of the question. A block of ice may only take a second to crack off the side of a glacier and fall into the ocean, yet it might take hundreds to thousands of years for snow to accumulate within the glacier, and for the glacier to flow to the ocean and crack.
Ice within an iceberg is a sky-blue color because it only absorbs a small portion of red light that enters it; thus, it is tinted rather than perfectly clear.
Icebergs barely float because ice is just slightly less dense than water. About 90% of an iceberg's volume lies below the surface of the water, so its size can be very deceiving.
Icebergs come in all sizes. "Brash ice" ranges from the size of ice cubes to baseballs. "Bergy bits" refer to icebergs larger than baseballs and smaller than beach balls. Icebergs between 1 m to 3 m are called "growlers" because sailors often hear a growling sound as these icebergs bob in the water. Larger icebergs are simply called "bergs," and the largest of all are called "tabular bergs." These are plates of ice up to 1000 m thick; some are the size of a small state.
Also see "Icebergs compared to other sizes" on the NOAA Ocean Modeling Branch Web site.
The iceberg that sunk the Titanic is the most famous of all. Scientists speculate that this iceberg may have come from the Jacobshavn glacier in west Greenland, although there is no factual evidence to confirm this. That glacier is one of the largest on earth (about 15 km across) and drains most of the Greenland ice sheet. Jacobshavn is also the fastest glacier on Earth, moving over 7 km (4.5 miles) in a single year. All of that ice gradually enters the ocean and breaks up into small chunks each year, ranging from 10 m to 1 km across. In early spring of 1912, as with every spring, a section of this glacier may have broken apart into many medium-sized icebergs that drifted south along the Greenland coast and eventually out to the North Atlantic. On 14 April 1912, the Titanic struck an iceberg and sank. This tragic event led to the formation of the International Ice Patrol (IIP), which tracks iceberg movement in the north polar seas.
Extremely large icebergs form in Antarctica when ice breaks away from large flat plates called ice shelves. In recent decades scientists noted that the Ross Ice Shelf, the largest in Antarctica, was overdue for calving a sizeable iceberg. The ice shelf protruded into the ocean farther than any previously mapped extent. On 22 March 2001, a truly gigantic iceberg finally separated from the shelf, initially measuring 295 km long by 38 km wide -- slightly smaller than the state of Connecticut. It has since broken into several smaller icebergs while drifting near the front of the ice shelf. Several pieces drifted near McMurdo Station, the largest U.S. scientific station in Antarctica. Researchers were concerned that the icebergs would interfere with shipping lanes to McMurdo; however, they eventually became stuck against an island, preventing any further movement toward the shipping area.
Answers to the above questions were provided by Dr. Theodore Scambos at NSIDC. For more detailed answers to iceberg questions, see the International Ice Patrol's (IIP) Frequently Asked Questions page or contact the IIP directly.
The National Ice Center (NIC) assigns iceberg names based on the Antarctic quadrant where they were originally sighted. The quadrants have a counter-clockwise division as follows:
A = 0 to 90 degrees west longitude (Bellinghausen/Weddell Sea)
B = 90 to 180 degrees west longitude (Amundsen/Eastern Ross Sea)
C = 180 degrees west longitude to 90 degrees east longitude (Western Ross Sea/Wilkesland)
D = 90 to 0 degrees east longitude (Amery/Eastern Weddell Sea)
When an iceberg is first sighted, NIC documents its point of origin. NIC assigns the letter of the quadrant and a sequential number to the iceberg. For example, B-20 is the 20th iceberg found in Antarctica in Quadrant B. When a tracked iceberg splits into two or more fragments, each fragment is assigned the parent iceberg's designation plus an additional letter (for example, B10a).
(Answer by NIC)
Together NIC and the IIP track icebergs for vessels traveling the Arctic.
See NIC's icebreakers page.
See our list of iceberg imagery sites.
