17 July: Icebergs

On this date in 1586 John Davis, English explorer, described an iceberg in his journal. 10 things you might not know about icebergs.

1. You probably know that 90% of an iceberg is underwater, hence the saying, "tip of the iceberg". What you may not know is why. It's to do with the relative densities of ice and salt Water. Ice is 90% as dense as water, so 90% of an iceberg is under the surface. In comparison, wood is half as dense as water, so wood floats with half its volume under water.
2. The word iceberg comes from the Dutch term ijsberg which means “ice mountain”.
3. Icebergs come in all shapes and sizes, but officially, an iceberg is a chunk of floating ice larger than 5 meters (16 feet) across. The largest ones may be the size of a small country. Smaller chunks have different names - "bergy bits" which are icebergs about the size of a house, and "growlers" which are about the size of a grand piano. Bergy bits and growlers are actually more dangerous to shipping because they don't show up on radar.
4. 93% of the world's icebergs can be found in the Antarctic. They are also commonly found in the North Atlantic. The ones in the Antarctic tend to be bigger. In 1987 an iceberg, with an area of 6,350 sq. km, calved from the Ross Ice Shelf. It weighed about 1.4 trillion tonnes and could have provided everyone in the world with 240 tonnes of pure drinking water. The largest iceberg recorded in the Northern Hemisphere was encountered in 1882 near Baffin Island. It was 13 km long, 6 km wide, and was about 20 m above water. It weighed over 9 billion tonnes – enough for everyone in the world to drink a litre of water a day for more than 4 years.
5. Where do icebergs come from? They are the edges of glaciers which break off, or "calve" into the ocean and drift away. How many new icebergs are formed in a year will vary but to give you a rough idea, about 40,000 medium to large icebergs calve from Greenland glaciers every year.
6. As glaciers are formed on land by the gradual build up of Snow, the ice in an iceberg, and any air trapped in bubbles inside it, will be about 10,000-15,000 years old. Unlike the sea around it, iceberg ice isn't salty, and it would be perfectly safe to break up and put in a Gin and Tonic. In fact, in Newfoundland, iceberg ice is collected and sold as bottled water. However, climbing onto an iceberg to get the ice isn't safe at all, so don't do it.
7. Icebergs have blueish streaks sometimes because meltwater has collected in crevasses on the glacier and frozen, making pure ice, with no bubbles. This reflects Blue light. The bulk of the iceberg is compacted snow, which will have tiny bubbles in it. This reflects all light so the iceberg appears white. Rarely, an iceberg may have a Brown or Black streak, caused by dust from volcanic eruptions falling on a glacier and becoming trapped. In general, though, the winds carrying volcanic dust don't mix with Arctic air masses.
8. The temperature inside an iceberg is between -15 and -20 degrees Celsius. The speed at which they travel varies with the drifts and currents of the sea and is usually around 0.7 km/h but can be as high as 3.6 km/h. When they reach warmer climates, it is attacked by both the warmer air and the warmer water. Any cracks will widen and pieces will break off. As an iceberg melts, it releases nutrients into the water around it, which, biologists have found, leads to an increase in plankton and other sea life.
9. If an iceberg melts quickly, it makes a sound like soda water fizzing, which is called the "Bergie Seltzer". This is due to the air being released from many tiny bubbles.
10. The most famous iceberg ever is, of course, the one that hit the Titanic. After the Titanic disaster, several nations got together to form the International Ice Patrol to warn ships of icebergs in the North Atlantic. Today they use aircraft and radar to track them, but only icebergs larger than 500 square meters (5,400 square feet).