How Icebergs Work

An iceberg is a massive chunk of ice floating in the ocean­. It isn't sea ice or pack ice, whi­ch forms as the result of ocean water freezing. An iceberg was once part of a glacier, but it broke off, fell into the ocean and floated away. Icebergs are made of freshwater. They can have very irregular shapes, like mountains of ice, or they can be flat with steep sides, like a plateau of ice.

Since icebergs are made completely out of ice, let's take a minute to talk about ice. It's the solid phase of water, reached at a temperature of 32 degrees Fahrenheit for pure freshwater. Saltwater has a lower freezing temperature -- essentially, any molecules that aren't water molecules get in the way of the water molecules when they try to "line up" and form ice. Saltwater also has a greater density (amount of mass for a given volume) than freshwater because those extra molecules are heavier than plain water molecules. Ice is unique among solids in that it is the only solid that's less dense than the liquid phase of the same material. When water freezes, the molecules fit themselves into a crystalline shape. The particular structure of ice leaves lots of extra space between the molecules, reducing the density of ice. This is why icebergs float, and it will allow us to figure out how much of an iceberg lies unseen below the surface, which we'll get to shortly.

A glacier is a massive slab of ice and snow that can be hundreds of miles long and thousands of feet thick. Glaciers form when the amount of snow that melts in the summer is less than the amount of snow that falls in winter, resulting in a gradu­al buildup of snow and ice. Glaciers "flow" downhill at a very slow rate under the force of their own weight, eventually reaching the ocean. At the coast, the ice extends out beyond the land, floating on the water and forming an ice shelf. The weight of the ice combines with tidal motions raising and lowering the shelf to create fissures in the ice. Eventually, a chunk of ice will separate from the glacier, a process known as calving. This can be incredibly spectacular, with massive ice slabs splashing down into the ocean creating enormous waves. Other times, the newly formed iceberg simply floats away.

­Glacial ice (and therefore icebergs) is made from snowfall, which is freshwater. That's why icebergs are made from freshwater, not saltwater.

The life of an iceberg begins thousands or even tens of thousands of years before it reaches the ocean. Glaciers build layer upon layer of ice; centuries of this compression along with infinitesimal movement toward the ocean create a particularly dense form of ice. It may appear blue instead of white because most of the air bubbles have been squeezed out of it.

Once it calves, an iceberg will usually live for three to six years. This period can be drastically shortened if the iceberg floats into warmer waters. During the course of its life, waves batter the iceberg and smash it into land or other icebergs. Frequent thaw/melt cycles (both seasonal and daily) open huge crevasses within the iceberg. A complex internal structure develops. Some icebergs simply melt away to nothing.

Some bergs don't go out so quietly. An expedition sponsored by National Geographic encountered the cataclysmic end of one large iceberg, reached during the peak of the nightly freeze cycle at a point when the destabilized internal structure could no longer support the iceberg's weight. Dr. Gregory Stone, a member of the expedition, described the incident in his book, "Ice Island":

A small percentage of icebergs never stray into warmer waters and may float in the icy Arctic for 50 years or more.

There are six official size classifications for icebergs. The smallest ones are called growlers, and they're a little smaller than your car. The next larger size is a bergy bit (yes, that's the official classification), which can be about the size of a small house. The remaining four size categories are less colorful: small, medium, large and very large. Just how big is a very large iceberg? Technically, any iceberg more than 240 feet high and 670 feet long is in the "very large" category.

The true upper limit is amazing. An iceberg broke off of the Ross Ice Shelf in Antarctica in March 2000. Called Iceberg B-15, it was almost half a mile thick and its area was about 4,500 square miles -- roughly the size of Connecticut. It was the largest iceberg ever found by the U.S. National Ice Center since the center began tracking them about 25 years ago [source: Stone]. Considering that small time frame, we can assume that there must have been even more massive icebergs in the unrecorded past. Icebergs this large are sometimes referred to as ice islands. Arctic icebergs are generally smaller than Antarctic icebergs because the Arctic doesn't have the vast, uninterrupted spans of open ocean that Antarctica has. These expanses of ocean allow the formation of huge ice sheets.

Icebergs are also classified by shape. Tabular icebergs are flat sheets of floating ice. They're more common, and form at much larger sizes, in Antarctica. Non-tabular icebergs can take a variety of shapes, from tall spires to cube-like or completely irregular. The shapes change considerably as the iceberg melts. Sometimes they form arches or even canyons, with two separate "tips" connected underwater.

Although they form in far northern or southern areas, icebergs can float thousands of miles. An iceberg from the arctic floated as far south as Bermuda [source: Bryant]. The typical range for icebergs is much narrower. Antarctic icebergs are mostly trapped in the circumpolar current, never giving them a chance to float north. However, they have been known to interrupt shipping lanes between Australia, South America and South Africa [source: Bryant].

While any glacier can form icebergs, the vast majority of all northern icebergs calve from 20 or so glaciers in or near Greenland, mostly on the western side [source: International Ice Patrol]. The Labrador Current carries a small percentage of these bergs into the North Atlantic, where they can interfere with shipping. Arctic glaciers calve up to 15,000 icebergs annually, although an average of just under 500 ever float south of the 48th parallel [source: International Ice Patrol]. This number can fluctuate extensively, as iceberg formation is affected by distant weather patterns, climate conditions and long-term "calving cycles" that we don't fully understand [source: Bryant].

­Ocean currents carry icebergs because most of the iceberg's mass is below water. In fact, icebergs often float into strong winds when the current is moving in the opposite direction.

Initial appearances suggest that icebergs are sterile blocks of ice floating in frigid, lifeless waters. In fact, icebergs generate their own mobile ecosystems.

Even in the coldest seas, icebergs are always melting at least a little bit. This melting has a major impact on the ocean around an iceberg. The freshwater from the berg creates a pool of freshwater that can extend a nautical mile away from the iceberg [source: Stone]. This water is colder than the surrounding seawater; the temperature variation creates thermal currents in the vicinity of the iceberg.

Life thrives on and around an iceberg. Young icefish hide in small iceholes to avoid predators, while a variety of invertebrates like jellyfish and siphonophores congregate in the area. Many of them come to feed on krill, tiny shrimplike creatures. Snow petrels nest on the icebergs and feed on the sea life nearby.

Icebergs can have a negative impact on ocean and animal life as well. When massive Antarctic bergs get stuck in certain areas, they block the migration path of Emperor penguins. The penguins need to get to the ocean to feed -- with too many icebergs in the way, they have to walk much farther [source: Space.com]. In areas where oceanic currents bring many icebergs, the ocean floor is often blasted clear of all life. The huge ice chunks extend deep under water, scraping and gouging the sea floor. Over the course of many years, these impacts render the sea floor almost completely lifeless.­

There is not, nor is there ever likely to be, an iceberg as famous as the nameless chunk of Arctic ice that gouged the side of the RMS Titanic in April 1912. Built to be "unsinkable," the ship struck an iceberg on her maiden voyage. Rather than striking head-on (which the ship likely would have survived), the ship grazed the iceberg below the waterline, creating a long gash that allowed water into multiple compartments. As the ship slowly filled with water, passengers fled on lifeboats. But there weren't enough lifeboats for all aboard, and the strange, slow pace at which the disaster unfolded led many lifeboats to be launched partly full, because passengers didn't think the situation was that serious [source: Eaton]. The result: 1,500 people lost their lives in the icy North Atlantic waters a few hundred miles south by southeast of St. John's, Newfoundland (accounts vary on the exact number of casualties).

The loss of the Titanic would have far-reaching effects. The area known as Iceberg Alley lies off the coast of Newfoundland, almost precisely where the Titanic went down. Fourteen passenger liners sank there between 1882 and 1890 [source: Bryant]. In the wake of the Titanic disaster, an international agreement led to the formation of the International Ice Patrol. Administered by the U.S. Coast Guard, the Ice Patrol keeps a close watch over Iceberg Alley, warning shipping traffic away from the "limit of all known ice," the area where the patrol believes ice presents a serious danger. They report not a single loss of life or property outside the limit in all the years they have been active [source: International Ice Patrol].

The International Ice Patrol sends out flights of C-130 Hercules aircraft to find icebergs. They also collect ice-sighting data from ships in the area. All the information is fed into a computer that uses tracking models and oceanic current information to estimate where the known icebergs will float to and when they'll get there. They provide this information via the Internet and radio to all nearby ships. In addition, advanced radar systems on each ship can spot larger bergs miles away, even in fog or storms. While the problems caused by icebergs have dropped dramatically since the early 20th century, the risk will never disappear completely.

The Coast Guard has experimented with different ways to track icebergs, including spraying them with brightly colored dyes or planting radio transmitters on them. They've even tried different ways to destroy icebergs, mostly by dropping bombs on them [source: International Ice Patrol].

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