DEFINITION OF ICEBERG

Calving refers to a process in which large pieces of

ice break off from the fronts of tidewater glaciers, ice shelves,

or sea ice. Typically, the glacier will crack with a loud noise

that sounds like an explosion, and then a large chunk of ice

will splash into the water, detaching from the glacier. Glaciers

may retreat rapidly by calving. Ice that has broken off an ice

cap or polar sea, or calved off a glacier and is floating in open

water is known as sea ice or, more commonly, as icebergs. Icebergs

present a serious hazard to ocean traffic and shipping

lanes and have sunk numerous vessels, including the famous

sinking of the Titanic in 1912, killing 1,503 people. Icebergs

from sea ice float on the surface, but between 81 and 89 percent

of the ice will be submerged. The exact level that sea ice

floats in the water depends on the exact density of the ice, as

determined by the total amount of air bubbles trapped in the

ice, and how much salt got trapped in the ice during freezing.

There are several main categories of sea ice that may

break up to form many icebergs. The first comes from ice

that formed on polar seas in the Arctic Ocean and around

Antarctica. The ice that forms in these regions is typically

about 10–15 feet (3–4 m) thick. Antarctica becomes completely

surrounded by this sea ice every winter, and the Arctic

Ocean is typically about 70 percent covered in the winter.

During summer, many passages open up in this sea ice, but

during the winter they re-close, forming pressure ridges of ice

that may be up to tens of meters high. Recent observations

suggest that the sea ice in the Arctic Ocean is thinning dramatically

and rapidly and may soon disappear altogether. The

icecap over the Arctic Ocean rotates clockwise, in response to

the spinning of the Earth. This spinning is analogous to

putting an ice cube in a glass and slowly turning the glass.

The ice cube will rotate more slowly than the glass, because it

is decoupled from the edge of the glass. About one-third of

the ice is removed every year by the East Greenland current.

This ice then moves south and becomes icebergs, forming a

hazard to shipping in the North Atlantic.

A second type of sea ice forms as pack ice in the Gulf of

St. Lawrence, along the southeast coast of Canada, in the

Bering, Beaufort, and Baltic Seas, in the Seas of Japan and

Okhotsk, and around Antarctica. Pack ice builds up especially

along the western sides of ocean basins, where cold currents

are more common on the west sides of the oceans.

Occasionally, during cold summers, pack ice may persist

throughout the summer.

Pack ice presents hazards when it gets so extensive that it

effectively blocks shipping lanes, or when leads (channels)

into the ice open and close, forming pressure ridges that

become too thick to penetrate with icebreakers. Ships

attempting to navigate through pack ice have become crushed

when leads close, and the ships are trapped. Pack ice has terminated

or resulted in disaster for many expeditions to polar

seas, most notably Franklin’s expedition in the Canadian Arctic

and Scott’s expedition to Antarctica. Pack ice also breaks

up, forming many small icebergs, but because these are not as

thick as icebergs of other origins they do not present as significant

a hazard to shipping.

Pack ice also presents hazards when it drifts to shore,

usually during spring breakup. With significant winds, pack

ice can pile up on flat shorelines and accumulate in stacks up

to 50 feet (15 m) high. The force of the ice is tremendous and

is enough to crush shoreline wharves, docks, buildings, and

boats. Pack ice that is blown ashore also commonly pushes

up high piles of gravel and boulders that may be 35 feet (10.5

m) high in places. These ridges are common around many of

the Canadian Arctic islands and mainland. Ice that forms initially

attached to the shore presents another type of hazard. If

it breaks free and moves away from shore, it may carry with

it significant quantities of shore sediment, causing rapid erosion

of beaches and shore environments.

Pack ice also forms on many high-latitude lakes, and the

freeze-thaw cycle causes cracking of the lake ice. When lakewater

rises to fill the cracks, the ice cover on the lake expands

and pushes over the shoreline, resulting in damage to any

structures built along the shore. This is a common problem

on many lakes in northern climates and leads to widespread

damage to docks and other lakeside structures.

Icebergs derived from glaciers present the greatest danger

to shipping. In the Northern Hemisphere most icebergs calve

off glaciers in Greenland or Baffin Island, then they move

south through the Davis Strait into shipping lanes in the

North Atlantic off Newfoundland. Some icebergs calve off

glaciers adjacent to the Barents Sea, and others come from

glaciers in Alaska and British Columbia. In the Southern

Hemisphere, most icebergs come from Antarctica, though

some come from Patagonia.

Once in the ocean icebergs drift with ocean currents, but

because of the Coriolis force they are deflected to the right in

the Northern Hemisphere, and to the left in the Southern

Hemisphere. Most icebergs are about 100 feet to 300 feet

(30.5–91.5 m) high, and up to about 2,000 feet (609.5 m) in

length. However, in March of 2000 a huge iceberg broke off

the Ross Ice Shelf in Antarctica, and this berg was roughly

the size of the state of Delaware. It had an area of 4,500

square miles (11,655 km2) and stuck 205 feet (62.5 m) out of

the water. Icebergs in the Northern Hemisphere pose a

greater threat to shipping, as those from Antarctica are too

remote and rarely enter shipping lanes. Ship collisions with

icebergs have resulted in numerous maritime disasters, especially

in the North Atlantic on the rich fishing grounds of the

Grand Banks off the coast of Newfoundland.

Icebergs are now tracked by satellite, and ships are

updated with their positions so they can avoid any collisions

that could prove fatal for the ships. Radio transmitters are

placed on larger icebergs to more closely monitor their locations,

and many ships now carry more sophisticated radar

and navigational equipment that help track the positions of

large icebergs and the ship, so that they avoid collision.

Icebergs also pose a serious threat to oil drilling platforms

and seafloor pipelines in high-latitude seas. Some precautions

have been taken, such as building seawalls around

nearshore platforms, but not enough planning has gone into

preventing an iceberg colliding with and damaging an oil

platform, or from one being dragged across the seafloor and

rupturing a pipeline.

See also GLACIER.

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