Icebergs are made of fresh water and float around in sea water, small ones weigh hundreds of tonnes, typical ones weigh 100,000+ tonnes and the biggest ones billions of tonnes. They can be very beautiful and very dangerous.
Were do icebergs come from?
Icebergs start off life as snow falling on land to form glaciers or ice sheets.They form mostly in the Arctic and Antarctic where they build up over many hundreds or thousands of years flowing downhill towards the sea under their own weight as frozen rivers.
Eventually they reach the sea and pushed by the ice behind them will begin to float on the surface of the sea if the angle they meet it at is shallow. If the angle is steeper and the ice more broken up after the passage over uneven bed rock, they break off in smaller pieces. When ice bergs break off from their glacier or ice sheet, they are said to be "calved". This can happen many times a day in the summer months for fast-moving glaciers.
If the glacier reaches the sea at a shallow angle through a narrow space an ice-tongue may be formed which can stretch for several miles. Eventually the floating ice sheet or tongue forms cracks and pieces break off.
This is the way the largest ice bergs form called tabular bergs as they are flat topped like a table, they are sometimes called ice islands as they are so large and can be long lived, 5 years or more. They are not always enormous however, many variables determine how regularly pieces fall off, it can be years between larger ice bergs being calved from a particular ice-shelf.
Erebus glacier tongue, the large smooth area is the frozen surface
of the sea, the glacier tongue has been pushed out by the Erebus
glacier. To the left at the base of the tongue are smaller ice
bergs and bergy pieces that have broken from a part of the glacier
meeting the sea at a steeper angle. Area shown approx. 20 km
x 20 km.
A small iceberg is calved from a small Antarctic glacier, this produced two large pieces about 10m x 5m x 5m and a lot of smaller icy rubble.
An enormous iceberg being calved from the Ross Ice Shelf in 2000, the berg named B15 was around 295km long and 37km wide, an area of 11,000 square kilometers, larger than many small countries with over 3 trillion tonnes of ice.
How big are icebergs?There is no real definition of what signifies the lower end of the ice berg size scale but anecdotally it is around 5m, anything smaller than this is called a "bergy bit" or "brash ice" if it is icy rubble.
Antarctic icebergs can be of both of glacial origin or ice shelf origin, the tabular ice shelf ones are the biggest by a long way, a dimension of 10 km is not unusual for tabular bergs. The largest iceberg ever recorded was sighted in 1956 by the icebreaker USS Glacier, it was at the time 335 x 97 kilometers, at 31,000 square kilometers nearly three times as large as B15 above. This was at a time before satellite observations and measurements were taken, so it may have originally been even bigger. Tabular bergs can be 40m high out of the water giving a total thickness of around 320m or over 1000 feet.
Arctic icebergs are produced from glaciers rather than ice shelves and so are not going to reach the same sizes, though can still be enormous regularly reaching 70m (230ft) above the water with the largest recorded being 168m (550ft) above sea level though of an irregular shape. Antarctic glaciers also produce enormous non-tabular icebergs.
An approx. 45m (150 feet) high irregularly shaped Antarctic iceberg.
Part of tabular iceberg or ice island B15 in the Ross sea. This iceberg was studied for several years after it was calved in 2000, partly by erecting weather and GPS tracking instruments on it. It is thought that icebergs of such a size are produced once every 50-100 years.
How big is the "Tip of the iceberg"?
Everybody knows that most of an iceberg lies under the water, but most don't know that the amount beneath the surface varies from about 50% to 99%.
The cause of the variation is largely in the amount of air that is trapped in the ice so affecting its buoyancy. An average iceberg will be about 1/5th to 1/7th above the surface. Very low lying pieces of ice in the water are known as "growlers". These tend to be made of very clear ice with little or no trapped air which makes them appear dark against the sea. They present a particular hazard to shipping with little visible above water and the dark colour making them especially difficult to see.
Some icebergs carry rocks and stones of all sizes down to gravel and fine powder so appearing dark grey or black in parts like the one in the picture above. The debris accumulated on the ice when it was part of a glacier becoming frozen into the ice itself. As the berg melts into the sea, this debris is spread around which can result in a fertilizing effect if it is rich in minerals.
A small iceberg as seen from underneath
How can people stand next to icebergs?Ice-bergs drift around the Southern Ocean carried by the currents and blown by the winds. In the winter the sea-ice freezes around them and effectively glues them in place until the spring when the ice breaks up and they can begin to move again.
During this frozen-in time, it is possible to travel out across the sea-ice and walk right up to the bergs. It can be quite magical standing next to an ice-berg, especially when the sun is shining and glistening off the ice. The sun can also penetrate the ice and be reflected off inner surfaces giving a whole variety of effects and colours from white through a range of vivid blues, quite an unreal experience. Less welcome is the fact that the sun warms up the ice making it expand slowly causing a whole range of eerie sounds, creaks and groans with the occasional crack and bang. The knowledge that bergs can tip up even in the winter months makes it a less than comfortable place to want to stand for too long. The surrounding sea-ice also moves up and down the side of the berg with the tide adding to the array of noises.
It's quite a humbling experience standing close to an ice berg the size of an office block, particularly when you consider that the bit beneath the thin sheet of ice you're standing on is the size of 8 or 9 office blocks.
The whole scale of Antarctica is really quite awesome, such moments and places serve to remind us that we are really quite insignificant to the motions of the planet. These motions that took place before we "discovered" them, continue oblivious to our presence and will still continue when we are no longer around to see them.
Why are ice bergs so varied in shape, size, colour and pattern?
Many icebergs start off life as a very irregular shape, though not all of them, tabular bergs start flat.
As they age they undergo many changes. Melting is the most obvious change, this can be the part above the water when the temperature goes above freezing point, or less obviously the part below the water when the temperature of the sea water goes above freezing point. Waves and currents erode the edges and underside unevenly causing internal stresses that can result in fractures along complex fault lines. The result is that the iceberg doesn't simply melt like some giant ice-cube, long before that it will change its center of balance and tip up, it may crack along one of those fault lines and split into two or more pieces. This will happen many times and when it does so it exposes different parts of the berg to the air or sea. All this erosion and tipping and cracking results in icebergs that vary enormously in shape.
Eventually the berg erodes into pieces small enough to melt away into the sea. If a berg is caught in currents around the frigid polar seas, it can take years, if they get carried away by currents and winds into seas that are just a little warmer, the process speeds up enormously.
The freshwater that icebergs are made of fell as snow, it accumulated over many hundreds or thousands of years into glaciers and ice-caps. Often when the snow fell it was very light and contained a lot of air between the flakes and ice crystals. Sometimes when the ice fell it was somewhat warmer and the snow was wetter and heavier, this meant less air was trapped.
When more air is trapped a layer of what will become whiter ice is laid down, when less air is trapped, a layer of what will become bluer or darker ice is laid down. Add to this the effect of the weight of deep burial and longer times of compression of glacial ice before it reaches the sea to become icebergs and we get ice bergs formed in a range of blue shades through to white, often striated showing when snow fell in larger or smaller quantities and warmer or colder conditions. These layers will have originally been horizontal or nearly so.
Other lines through the berg to the horizontal striations are caused by cracks forming as the ice flowed over the land, sometimes cracks opening and closing, sometimes an open crack will fill with spring and summer melt water which then freezes again to give a deeper blue vein.
Are icebergs safe?
The vast majority of the time icebergs just sit there doing their own thing which consists mainly of bobbing about in the sea or, or maybe resting on the sea bed melting slowly.
Every now and then however they tip up or split into pieces, even small icebergs weigh in the region of hundreds of tonnes and many of them being hundreds of thousands or millions or more. They always do tip up and crack at some point. When they do so, you don't want to be anywhere near them, in the winter months they can destroy many square miles of sea-ice around them with the resulting waves. There are stories of people having passed a snoozing berg one day on strong stable sea-ice and then the next day seeing the same berg in the middle of several square miles of fractured ice like the difference between a completed jigsaw puzzle and a pile of jigsaw pieces.
Icebergs can be very tall and ice is very heavy, imagine all that falling from 100m or more like the giant hammer of Thor's bigger, stronger and angrier icy cousin. Having said that I've never heard of anyone actually being hurt in such an event - a combination of the rarity of it happening, few people being in a dangerous place and the alertness and speed of people getting away if in a boat.
This is the lesser known danger of icebergs, there is also of course their better known antisocial habit of sinking ships full of people. The most famous iceberg in history is simply known as "The iceberg that sank the Titanic". A fairly typical irregularly shaped Arctic iceberg and one of the larger ones from eye witness accounts, it calved most probably from the west coast of Greenland at Ilulisat and was swept away by currents to be hit by the then largest ship in the world on its maiden voyage with the loss of 1,500 lives.
These days ships have radar and can spot bergs as large as the one that the Titanic hit on the 14th of April 1912 from a long way away. Larger icebergs are tracked by satellites and the information is available to shipping. These methods however are only good for very large icebergs, entering a sea with many icebergs in a ship with radar results in he screen being covered in reflections from ice, far too many to be of much use if you have to navigate through or near. In seas with a lot of ice, there is no substitute for manual observation, having someone on the bridge of the ship watching out for ice and avoiding it.
Surprisingly perhaps, it is smaller pieces that can be the most dangerous as they are less easy to see. The most dangerous of all are those "growlers", very dense, clear or dark in colour which float low in the water making. They can go straight through the hull of a small boat travelling at speed or cause fatal damage to larger ships.
November 2007 the tourist cruise ship M/V Explorer was holed and subsequently sank by an iceberg in the Bransfield Strait, Antarctica, the time taken from incurring the damage to sinking was about 15 hours, the ship sank in around 1,500m (4,920 feet) of water. Fortunately, by luck rather than design, there was no loss of life and all passengers and crew were rescued.