Snow and Ice
The Ice CapAs large as Antarctica is, when the surrounding sea freezes over, the ice cover doubles its area -- extending the continent to approximately 30 million square miles (14,200,000 square kilometers). Even during summer, almost the entire continent is covered by ice with an average thickness of almost a mile (1.5 km)! The sea-ice reaches its maximum extent by September, but continues to increase in thickness until October when it can be over 5 feet (1.5 m) thick.
This icecap contains over 7 million cubic miles (30 million cubic km ) of ice -- about 90 per cent of all ice existing in the world and 68 per cent of the world's fresh water. The weight of the Antarctic ice is so great that in many areas it actually pushes the land below sea-level. This process of the earth's crust being deformed is known as isostasy. Without its ice cover Antarctica would eventually rise up another 1500 feet (450 m) above sea-level.
Sea-IceSea-ice build-up is the most extensive seasonal process in the world's oceans, and the area covered by ice during this seasonal change from summer to winter is greater than the whole area of the Antarctic continent itself.
There are distinct stages in the transition from sea water to sea-ice. Because of its salt content, sea water usually begins to freeze at -28°F (-1.8°C). First, crystals form on the surface of the brine creating an oily sheen known as grease-ice. This further evolves into a slush known as frazil-ice
The sea-ice gradually thickens as more and more water from below freezes and as snow falls from above. Nilas is the name for the next stage, a thin elastic crust of ice up to 4 inches (10 cm ) thick which easily bends when influenced by swell and waves. Ocean swell and waves may cause the grease-ice to break apart and refreeze several times before forming a solid sheet. In this process, distinctive discs of ice with turned up edges form, the result of being bumped together.
While sea-ice is extensive around the continent, at its peak it by no means forms a continuous mass. It is broken up by large shifting areas of open water, known as polynyas. These may measure up to 60 miles (100 km) across.
Sea-ice tends to grow quickly at first, reaching half its maximum thickness within a month. The freezing over of the sea usually begins in late March (autumn). The break-up of the sea-ice in spring (October to January) happens even faster, possibly due to the presence of pigmented algae which absorbs more solar radiation than ice. Sometimes the sea ice builds up over successive seasons; when this occurs, the salt is gradually leached from the ice into the surrounding ocean to the point where the water from melted sea-ice is quite drinkable. Melt water from old sea ice and icebergs was the main way early Antarctic ships were able to replenish their water supplies.
Our knowledge of sea-ice today has been increased enormously due to the
information relayed by polar orbiting satellites. This information can be
translated into charts showing pack-ice distribution, a boon to researchers
and navigators. The thickness and extent of pack-ice and fast-ice are the
main reasons that ships, even ice-strengthened vessels, cannot reach
Antarctic research stations for much of the year. Most ship-based scientific
research cruises also have to be limited to the summer months, which is why
we know so little about the science of sea-ice. Return
IcebergsThe Antarctic icecap moves slowly but constantly, flowing under its own massive weight towards the coast. In places the ice protrudes into the surrounding waters. The effects of waves, swell, currents and tides soon bend and twist the floating ice tongues and shelves, causing chunks of all sizes to break off in a process known as "calving". The separation is often a locally cataclysmic event, creating a boiling sea in the vicinity and a wave. Long before the Antarctica continent can be seen, vestiges of its glaciers and iceshelves in the form of massive icebergs loom on the horizon.
Icebergs come in all shapes and sizes . As they melt and break up, the small pieces that they shed are given distinctive names such as "growler", "bergy bits" and "brash", depending on their size. Icebergs themselves can also be categorised into tabular, irregular or rounded icebergs and their shape is usually an indication of their age. Antarctica as a rule has much larger icebergs than the Arctic. A large Antarctic iceberg may weigh 400 million ton, tower ten stories above the surface of the water and contain enough fresh water to supply a city of three million people for a year. Some icebergs breaking off from the Antarctic shelf can be a much as 2,500 sq. kilometres in area and more than 300 meters thick.
Each year an enormous number of icebergs calve from Antarctica. While the resulting icebergs do eventually melt, many last for years and even decades if they become grounded. In 1963 one monster iceberg measuring 70 miles by 47 miles (110 km by 75 km) was observed by satellite. It lasted until 1970. One of the longest icebergs ever recorded, designated a B9, broke away from the Ross Ice Shelf in October 1987. The size of the state of Delaware when it first calved, it measured 86 x 22 nautical miles (160 x 40 km). The largest icebergs originate from the iceshelves. The Ross Iceshelf, a vast table of floating ice as large as France, is gradually being pushed away from the Antarctic landmass.
Most icebergs are less than 180 yards (200 m) across at the waterline. The biggest icebergs, which account for only 4 per cent by number, are 51 per cent of the entire volume of ice. These giants are tabular icebergs which calve from icesheets and are often 650 to 1000 feet (200 to 300 m) thick. The paths of icebergs larger than 10 km across can be plotted by satellite images. Icebergs may mainly drift under the influence of currents and their depth draught (the portion below the waterline) may cause them to run aground.
Icebergs that drift northwards melt quickly in the warmer seas, but close to the Antarctic coast where the sea-water temperature measures only +1°C, they can last for years. While melting does not play a big role in the disintegration of the larger icebergs, it is the main reason why small icebergs eventually disappear. The amazing shapes that irregular and rounded icebergs often acquire is the result of melting under water, so that when the icebergs roll upside down the sculptured forms become visible. This rotation can take place gradually, up to a point. Then when the iceberg becomes top-heavy and unstable, it turns over in a matter of seconds, often breaking into smaller pieces in the process. Such an event can be very dramatic, creating waves and a roar that can be heard for miles.
Up-turned, rounded bergs can be very smooth and often display a visible notch indicating the old waterline where waves used to wash against and erode it. Sometimes weird shapes, towers or tunnels are formed by wave action beneath the surface and when the berg rotates they become visible. Some icebergs have bands or strips of jade coloured ice.
Typically, icebergs do not survive at latitudes lower than the northern limit of Antarctic sea-ice, near 48°S. Towing icebergs to dry southern Australian cities such as Perth and Adelaide has been proposed as a way of relieving these cities' freshwater shortages during summer. It is believed, however, that such a journey would take many months, in which time half the iceberg would have melted. The means by which the resulting fresh water could be harvested also presents a problem because the icebergs have such a deep draught that they could not be towed closer than about 20 miles (30 kms) to the coast. Environmentalists have also expressed concern about the possible effects on the local climates that melting icebergs could have. Return
|Photography ©Jonathan Chester, Extreme Images||© 1995 Terraquest. All Rights Reserved.|