arth's polar regions are landscapes of ice and snow. But despite the seemingly mundane whiteness of it, on the open ocean there is a floating landscape of pack ice and mountainous icebergs as diverse as any landscape on the planet.
Icebergs are the juggernauts of the Arctic. Most of the bergs that prowl the shipping lanes of the North Atlantic originate from glaciers on the West coast of Greenland. Glacial ice forms out of compacted snow, so icebergs are made entirely of fresh water. As snow is squeezed under its own weight on the upper reaches of the glaciers, it begins to flow downhill to the sea. Tongues of ice flow out onto the surface of the water.
Gradually, wind and waves weaken the tongue of the glacier until it gives way. Great blocks of glacier break off in a process called "calving," and begin to drift southward in the current. Thousands of icebergs calve off of Greenland into the Baffin and Labrador seas every year. In the Antarctic, the ice breaks off in great sheets, hundreds of miles on a side.
Typically, one-fifth to one-seventh of an iceberg protrudes above the water. The submerged core is made of rock-hard, freshwater ice. The average age of that ice is thought to be about 5,000 years. But some may date back to the last ice age. In their glory, icebergs can tower hundreds of feet above the water.
In the summer, the iceberg armadas begin to crumble. Weakened by the sun, they sometimes collapse in a great, splashing mess, leaving a litter of "bergy bits" and "growlers."
Going with the Floe
Sea ice -- also called pack ice -- is a whole other beast. It forms from frozen seawater, as its name suggests, not compacted snow. Sea ice is dynamic: Like a plant or animal, it grows in stages, with different names for different stages. It changes with the seasons. And it is almost always on the move, impelled by currents and winds.
Sea ice forms when the temperature of the ocean surface falls below 28 degrees Fahrenheit. Unlike the calm surface of a lake, where ice forms in a gradually thickening sheet, the ocean surface is frequently stirred by waves. This leads to some interesting forms of ice. In turbulent water, tiny disc-shaped crystals of ice form into a substance called frazil. As they are stirred through the sea surface, the crystals give the water a greasy appearance, hence the name "grease ice." Another form that sea ice takes as it grows is shuga, composed of small chunks of ice that undulate on the surface of the water in a sheet. When the ice clumps together, it forms rounded sheets with upturned edges, called pancake ice. The pancakes damp down the waves somewhat, allowing pancakes to consolidate into larger pancakes. Eventually, the pancake ice freezes into floes, larger sheets of sea ice floating on the water's surface.
There are two types of pack ice: first-year ice and multi-year ice. First-year ice freezes in fall or early winter and melts in the summer. Typically, first-year ice is between a foot to six feet thick. Multi-year ice is simply pack ice that has survived at least one winter. A smaller fraction of the polar sea ice grows along shorelines and in enclosed bodies of water such as bays. This is called lastfast ice.
Sea ice is dynamic in another sense: It is alive. Far from being a frozen wasteland, floating pack ice is laced with creatures living in a complex food web.
Every winter, a floating rim of sea ice forms around the Antarctic with more than twice the surface area of China. Below the waterline the ice is wormholed with briny channels. Inside live algae, plant-like creatures that harness sunlight to make food. And they themselves are food to a range of larger creatures squirming under the ice -- free-swimming crustaceans called krill, for example. The krill, in turn, are food for fishes, whales, and other animals of the Southern Ocean. The algae crop may supply as much as one-quarter of the food available to animals in the winter, when a meal can be pretty hard to find.
-- By Daniel Pendick