In the Southern Hemisphere, the Antarctic ice sheet overflows its land support to form shelves of ice on the sea; huge pieces, as much as 200 km. across, break off to form icebergs. In Northern Hemisphere icebergs are generally not over 150 metres across. However most icebergs are from some 20 glaciers on the west coast of Greenland. When glaciers, drifting from mountains and valleys, reach the sea, they got broken into big pieces by sea waves and tides. These big pieces of ice are called icebergs. However there are some glaciers which do not break even after floating for long distances in the sea and appear like mountains of ice.
The sizes of the different icebergs vary. The smallest iceberg measures 5 to 6 metres in length and breadth but the bigger ones might be hundreds of metres long. Some icebergs with lengths and breadths of half a kilometre have been seen floating in the oceans. They float in sea water because ice is lighter than water. Their one-tenth part remains above water and the remaining under water. For example, if a 50 metre high iceberg is seen above water, then 450 metres of it will be under water. These icebergs contain huge quantities of snow. Some of them may contain 200,000,000 tons of ice. As an iceberg floats, some of the ice melts and pieces break off. Eventually, it completely disappears.
Because of their huge weight and volume, the icebergs do not move in the sea on their own. They are pushed ahead by the sea waves. They are very dangerous for ships. A collision with them can cause a ship wreck. Though they can be detected with the help of modern instruments, yet mishaps do take place occasionally. On 14th April 1912, a ship named Titanic was broken to pieces when it collided with an iceberg. This ship was sailing to New York; 1513 passengers, who were aboard this ship lost their lives.
A similar accident took place on January 30, 1959 when the ship Hans Hedtoft collided with an iceberg in southern Greenland and was wrecked. Now the United States and other countries have formed an International Ice Patrol. The patrol uses ships, planes and radars to locate icebergs.