Icebergs
Icebergs are thick masses of ice floating in the ocean. They form when large chunks of ice break off a glacier or an ice shelf and float free in the sea. This is called calving. Small calving events are a regular occurrence but occasionally an extreme calving event takes place.
Many new icebergs were formed in 2002, with the break up of the Larsen B Ice Shelf (iceberg B-15). Other notable extreme events include the Larsen C Ice Shelf (iceberg A-68) in 2017 and the Amery Ice Shelf (iceberg D-28) in 2019.
Icebergs in Antarctica
Dr Mike Williams from NIWA explains where the icebergs from Antarctica come from and how they were formed.
Icebergs can vary in size, but the largest-known iceberg, B-15, was 295 km long and 38 km wide when it broke off the Ross ice shelf in 2001. This is about the same size as Jamaica –10,600 km².
B15 in McMurdo Sound
Dr Mike Williams explains what effect the iceberg B15 had on the penguin colony nearby when it blocked the inlet to McMurdo Sound.
This sounds huge – but you wouldn’t have been able to see the whole thing as around 90% of an iceberg is actually under water.
Tip of the iceberg
The tip of an iceberg which is visible above the waterline is usually only about 1/7 or 1/8 of the mass of an iceberg. Seawater is slightly denser than ice and this means that the iceberg floats with most of its mass below the surface. In comparison a cork, which has a density of about 20% that of water, floats close to the water surface. Ice has about 90% of the density of water but the density of seawater is also influenced by the temperature and salinity.
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The reason why ice floats and is lighter than water is that a certain mass of ice occupies more space than the same mass of water. This is related to the characteristics of hydrogen bonds.
Floating icebergs
Icebergs are thick masses of ice floating in the ocean. The reason why ice floats and is lighter than water is that a certain mass of ice occupies more space than the same mass of water. This is related to the characteristics of hydrogen bonds. This iceberg is floating off the Otago coastline.
In 2006, icebergs were sighted off New Zealand’s Otago coastline. This was a rare event – Earth Sciences New Zealand (formerly NIWA) reports the previous sighting from the mainland was near Dunedin in 1931! Icebergs that approach New Zealand’s sub-Antarctic islands are usually pushed eastward by the Antarctic Circumpolar Current, and out into the Pacific Ocean.
Monitoring and naming icebergs
The U.S. National Ice Center (USNIC) has been using remote sensing to track icebergs in the southern hemisphere since 1978. The Center originally monitored icebergs over 1900 m along at least one axis but has expanded its criteria to include icebergs of around 70 square km or greater.
Naming icebergs
Find out how icebergs are named and that icebergs can have siblings as well.
USNIC assigns names to the icebergs to assist with tracking. The naming convention splits Antarctica into four quadrants – labelled A, B, C, D in an anticlockwise direction. Each iceberg is named for the quadrant where it originates, along with a sequential number and sequential letters if the iceberg itself calves. For example, B-15 is the 15th iceberg tracked from Quadrant B. B-15 calved numerous times – reaching the end of the alphabet. B-15Z also broke into large pieces. By 2021, B-15AB became the last piece of B-15 to remain on the USNIC list. As of 2025, B-15AB still sits off the tip of the Antarctic Peninsula.
Related content
Dr Mike Williams, whose videos are in this article, is a featured scientist in our Icy ecosystems content.
Icebergs feature in the International Polar Year’s 8-week voyage to the Ross Sea.
Learn more about glaciers in Antarctica. When glaciers flow over the sea, large pieces of ice can break off and produce icebergs.
In Aotearoa New Zealand, we’re more likely to see calving from freshwater glaciers.
Activity idea
In the activity Melting glacial ice students investigate the effect that contact with water has on melting ice, which is one of the reasons why icebergs disappear with time.
Useful links
The varied sights and sounds of Antarctic icebergs begin to reveal themselves in this Radio NZ podcast with NIWA scientist and Deep South National Science Challenge director, Dr Mike Williams: Voice of the iceberg 2.
Visit the U.S. National Ice Center for up-to-date news on icebergs around the world.
