How do icebergs form?

Icebergs are blocks of fresh-water ice that break off from glaciers and float out to sea. Glaciers are formed in polar regions where snowfall lasts for centuries, or even millennia, without entirely melting, and is eventually compressed into ice.

In the North Atlantic, most icebergs originate from the tidewater glaciers of Western Greenland. Compressed snow becomes firn, a granular snow, transformed eventually by pressure into a dense ice. The weight of the icecap builds, causing the ice to flow as much as 60 feet a day through openings in the coastal mountains.

Rising and falling tides cause slabs of ice to break off and form moving "rivers of ice." Here's a picture of a glacier releasing icebergs into the sea. Usually, about 7/8 of an iceberg is hidden below the waterline, hence the expression "tip of the iceberg." Most arctic icebergs melt before they ever reach the Atlantic Ocean.


Here are some other notable iceberg factoids:

• The southernmost iceberg ever reported in the Northern Hemisphere was spotted at latitude 30�, 150 miles from Bermuda, in 1926.
  
• Icebergs are categorized by size: Growlers, the smallest, are less than 3' (1m) high and less than 16' (5m) long. The next size up is called a bergy bit. An iceberg over 240' (75m) high and 670' (213m) long is considered very large.
  
• All icebergs can be dangerous to ships -- massive icebergs can cause great damage, but smooth, submerged ones are perilous because they are hard to detect and avoid.
  
• The largest icebergs on Earth are found in Antarctica, where one the size of Rhode Island was spotted.