#1206 How far can an iceberg float?

How far can an iceberg float? It depends on their original size, the season, and their location, but some icebergs can float for thousands of kilometers.

Not every piece of floating ice is an iceberg. An iceberg has to be 15m long at least to be considered an iceberg. They are formed when ice breaks off a glacier or an ice shelf and floats out to sea. The largest ever iceberg was found in 1956 and it was 335 km long and 97 km wide. If that were a country, it would be the 136^th largest country in the world, just above Belgium. An iceberg that size could last for years before it disappears and it would break up into many smaller icebergs before it completely melted away. The largest iceberg in recent history was called B-15. It was 295 km by 37 km and it broke off the Ross Ice Shelf of Antarctica in March 2000. It was about 800 m thick. It broke up into smaller icebergs in 2005. The US National Ice Center was still tracking one of these icebergs in 2023. That is how long they take to disappear. This kind of large flat iceberg is called a tabular iceberg. Incidentally, the word “iceberg” comes from Dutch word “ijsberg”, which means ice mountain.

When icebergs break off from an ice sheet or a glacier, they can float for a long way. Icebergs are obviously buoyant because they are made of ice, which is less dense than seawater. The ice weighs 920 kg/m3 and the seawater weighs 1,025 kg/m3. This means that the ice displaces a certain amount of seawater due to its volume, but that volume of displaced seawater weighs more than the iceberg itself and it pushes back on the iceberg. The weight of the seawater pushing back is more than the weight of the iceberg, so the iceberg floats. The difference in the density is only about 100 kg/m3, which is about one-tenth of the weight of the water, which is why about one-tenth of the iceberg is always above the water and the other nine-tenths is below.

Icebergs don’t actually float in the way that we imagine they do. We have all seen images of an iceberg where one small triangle is sticking out of the water and another larger triangle is under the water and pointing straight down. This isn’t actually how icebergs float because they are too unstable like that. The iceberg would actually roll over so that its longest side is horizontal to the surface of the sea. This is the same thing you would get if you dropped a log into a lake. When the log rose to the surface, it would float with its longest side horizontal to the surface of the water, not vertically. The only way you could get the log to float vertically, and the same with an iceberg, would to fasten a very heavy weight to one end of it.

Most icebergs last for about five years and they can float a long way in that time. Some icebergs will meet other ice sheets or coastlines and become frozen to the ice there. Some icebergs will get stuck in currents that keep them in the cold seas near the Antarctic and they won’t melt at all. However, most icebergs are gradually carried south by the winds and the currents in the sea. As they move south, the sea gradually warms up and they start to melt. When they melt they produce a trail of freshwater that travels behind them and is colder than the surrounding sea. If an iceberg is just left to melt in the water, it could last for several years, but strong winds and waves can break it up into smaller icebergs, called bergy bits. Also, as icebergs get smaller, their rate of melting gets faster. This is because the smaller the iceberg is, the larger the surface area to volume ratio they have. We looked at this when we looked at how small a mammal could be. Smaller animals have a larger surface area and small volume, so they lose heat rapidly because they have more surface area that is in contact with the air. This is the same with an iceberg. A smaller iceberg has more surface area in contact with the sea, which promotes more heat exchange, and it melts faster.

Floating icebergs can be dangerous to shipping. There is a place called Iceberg Alley, which is near the coast of Labrador in Canada. Icebergs float from the Arctic into the Atlantic Ocean and it is popular with tourists. These days, icebergs can be monitored with satellites and GPS, but sometimes it becomes necessary to move one. A boat, such as a tug boat, can easily move an iceberg. They are tremendously heavy, but pulling on them constantly with the force of a boat can very slowly move them to a different course. Some people have even talked about towing icebergs to places that don’t have enough water, but no one has actually done it yet. And this is what I learned today.

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