Castles and Tablets of Ice

Pinnacle Iceberg (Photo courtesy of K. Bruce Lane Photography, )

During the peak of the last ice age, one-third of the Earth's land surface was covered by thick sheets of ice. Their high albedo reflected a great deal of sunlight out into space, which cooled Earth and allowed the ice sheets to grow. (See our Ice Ages web page.) Ice sheets give birth to icebergs. This process is known as calving. Most bergs are calved from ice sheets off the western coast of Greenland and Antarctica. Icebergs are found in both the Arctic and Antarctic regions. The bergs from these two areas differ, however, in form and size.

So what criteria must a chunk of ice meet to officially be called an iceberg? By definition, icebergs are at least seventeen feet proud of the water and fifty feet long. Anything smaller is called a growler or bergy bits. One of the biggest Greenland bergs ever reported by the Coast Guard was 550 feet above the sea. Icebergs in the Arctic regions are formed from mountain glaciers fed by the Greenland ice sheets and are high and narrow, with above-water shapes resembling towers; these are called castle bergs. Large tabular icebergs are found at the ice shelves of Antarctica. One large tabular Antarctic iceberg in 1987 was reported to be 100 miles long, 25 miles wide, and 750 feet thick.

Castle or pinnacle bergs (like the one pictured above) are found only in the Arctic regions. Tabular bergs (like the one pictured below) are found in the Antarctic regions.

Tabular Iceberg (Photo courtesy Earth and Space Research,
Tip of the Iceberg, (Photo courtesy of K. Bruce Lane Photography, )

Have you ever heard the phrase, "it was just the tip of the iceberg?" It's a phrase or metaphor that figuratively means that what you see is not all there is. This is literally true of an iceberg. The higher the iceberg rises above the ocean surface, the deeper its base projects into the water. The exact depth of immersion depends on the difference in density between the ice and the seawater, but it will be three to nine times the iceberg's height (WOW!) above the water line.