There are two major types of ice in the polar regions, sea ice and glacial ice, and they form through different methods. Sea ice forms in oceanic water when the ambient temperature is lowered to the freezing point of salt water. Glacial ice (including ice caps) forms through the simple accumulation of snow which becomes compressed by its own weight into solid ice. Sea ice formation is a seasonal phenomenon (although individual pieces of sea ice may last for several years), while glacial ice is generally a long-term structure lasting decades, centuries, or even millennia.
Sea water generally freezes at about -1.9° C (28.5° F), but this may vary somewhat depending upon its salinity (the higher the salt concentration-the lower the freezing temperature). It is interesting to note, however, that surface ice which forms slowly under calm conditions is not salty, because salt molecules in solution tend to concentrate in the remaining liquid as ice crystals form. Each Winter, the sea surface around Antarctica freezes, thereby forming a layer of ice 1 to 3 m (3 to 10 ft) thick and extending 100 to 200 km (60 to 120 mi) offshore. As the surface water cools down to the freezing temperature, ice crystals start to appear. If conditions are calm, the crystals join together, thicken, and form a fibrous structure called young ice.
Very often a slight swell occurs which causes the young ice to break apart into small sections which then continue to bump and grind against one another. This action forms roughly circular bits of thin ice with raised edges called pancake ice. As time progresses and air temperatures remain low, more crystals form and the pancakes eventually freeze together and create a solid layer several centimeters thick. Continued cold causes this layer to become fast to the shore, and it is then termed fast ice. Fast ice generally lasts throughout the Winter and perhaps well into the following Summer. Currents, storms, and tides tend to buckle and crack fast ice, producing open water areas and give access and breathing spots to penguins, petrels, seals, whales, etc. Fast ice breaks apart and forms ice floes during normal Summers, and the floes are moved around by currents until they pack up tightly together and cover large areas as pack ice. Areas of pack ice are often graded for the benefit of ship movements...1/10 ice coverage means 10% of the water’s surface is covered with ice and ships can move relatively freely, whereas 10/10 ice coverage means 100% of the surface is covered with ice and ship movement is severely restricted or impossible. Polynyas, or open water areas, form offshore when winds and/or currents disperse the pack ice.
Near the continent, the pack ice drifts in a westerly direction, but north of 65° S it drifts in an easterly direction. Pack ice areas are difficult to predict, but concentrations generally occur in the Weddell Sea, Ross Sea, and off the Pacific Ocean sector of Lesser Antarctica. The northern limit for the pack ice varies, but in Winter and early spring it may extend 800 km (500 mi) from the coast in some areas. The pack ice generally increases the area of the continent another 50% during this time of year by enclosing an area of approximately 19 million sq km (7.2 million sq mi).
In some areas, a cold spring season may allow fast ice to remain in place for more than one year. It is then called bay ice. Bay ice which persists for years is termed shelf ice, and may become very thick (perhaps measuring as much as 300 m, or 1,000 ft, in thickness). Shelf ice accumulates thickness from not only freezing sea water, but also from snow fall, and very often from glacial ice which moves to the coasts from higher areas on land. Occasionally, huge pieces of shelf ice break loose and drift away in the currents. These pieces of ice are called tabular bergs and can be enormous, perhaps hundreds of square kilometers in top surface area, and may drift for years before melting or becoming grounded. These are not to be confused with ice bergs, which break off from glaciers which reach the sea. In the Autumn of 1987, a huge tabular berg, more than 5,000 sq km (2,000 sq mi) in surface area, broke off from the Ross Ice shelf and began drifting in a north-northwesterly direction at a speed of 3 km (2 mi) per day. A data transponder buoy was dropped onto its surface in the Winter of 1988, and its movements were followed for several years by satellite photographs. Its convoluted path of drift helped scientists study the deep current patterns in the Ross Sea.
Sea ice prevents the ocean waters from warming the coasts significantly. It is important to note that islands within the limits of Winter pack ice (such as the South Shetlands, South Orkneys, etc.) compare closely with the continent in seasonal temperatures, soils types, flora, and fauna. Islands located outside the range of pack ice (such as South Georgia, Kerguelen, etc.) have continual open waters, milder Winters, longer growing periods, and much more diversified and advanced flora, and to a great extent, a different fauna.
Glaciation, however, is much more complicated. When snow accumulates over a period of many years (that is, it doesn’t melt away after one season), the buildup creates a thick deposit in which the overlying mass tends to compress the lower snow layers into solid ice. During this, the individual snowflakes change into granules, which fuse into crystals of ice. Often, the air between the flakes becomes trapped, thereby creating air bubbles within the ice crystals. In polar areas, this produces huge and massive ice caps that can overwhelm and cover the entire landscape, including even mountains. Eventually, the ice mass thickens to the point where it begins to move due to a combination of gravity and the shape and slope of the ground surface. On steeper slopes this can occur when the thickness of the combined snow and ice reaches 15 m (50 feet) in depth. This is often referred to as glacial ice. Ifthe flowing ice is constrained by mountains, valley walls, or other land surface formations, it is known as a glacier.
Glacial ice is the world’s largest reservoir of fresh water, albeit in solid form. Nearly 99% of all glacial ice on Earth is contained within the huge ice sheets in the polar regions. In fact, this volume of ice is so large that if the ice sheets of both Greenland and Antarctica were to melt, it would cause sea levels to rise about 70 meters (230 ft). In addition to Antarctica, Greenland, Canada, Iceland, and Svalbard, there are also significant glaciers scattered around the world outside of polar regions, including Alaska and Chilean Patagonia. They are found on every continent, except Australia, and some even occur in tropical zones, but only on high mountains where low temperatures can endure. Many of these glaciers are so extensive, they accumulate and store H2O in what is known as the Accumulation Zone and release it as meltwater elsewhere as it flows downhill in the Ablation Zone. This can be a very important source of water for plants, animals when other sources are lacking.
Geological evidence indicates that throughout most of the Earth's history not only have the climatic zones between polar, temperate, and tropical sectors been much less distinct than they are today, but the polar regions were free of permanent ice. The poles have always received their quota of the sun's radiant energy at a low angle, and therefore have always been cooler than the equatorial region. Why, then, is it so different today?
Permanent ice probably began forming in Antarctica as early as Miocene times, perhaps 20 million years ago. The effects of the formation of the Antarctic Ice Cap were felt throughout the world, but it is interesting to note that the ice age in the northern hemisphere began much later. Permanent ice began forming in North America, Greenland, and Iceland between two and three million years ago and appeared in central Europe and Asia soon afterwards. The ice sheets have fluctuated considerably in the northern hemisphere over the last million years, but the Antarctic Ice Cap has remained relatively stable.
The northern ice sheets began retreating about 20,000 years ago, and have since uncovered vast areas of land which are now covered with tundra and taiga. Antarctica, on the other hand, is still locked in its ice age. However, we have observed some dramatic changes in various ice shelves and fast ice zones in the peninsular region over the past couple decades. Much of the Antarctic continent is surrounded by a ridge of moraine between 100 and 300 km (60 to 180 mi) offshore and in waters up to 500 m (1,650 ft) in depth, which demonstrates a former edge of the ice mantle. The massive bulk of the ice cap probably would have protected it from changes caused by minor climatic fluctuations, but changes in world sea levels generated by glaciations in the northern hemisphere expanded the coastline of Antarctica considerably, allowing its ice mantle to enlarge respectively.
The Antarctic Ice Cap contains about 85% of the world's ice, or about 80% of all the fresh water on Earth. It is estimated to weigh about 24,500 million billion kg (27 million billion tons), and each year it receives 1,800 trillion kg (2 trillion tons) of new snow and ice. However, it loses about the same amount of ice that it gains each year to melting, evaporation, snow blown out to sea, and of course, huge tabular bergs that break away from ice shelves and ice bergs which break off glaciers and drift away to sea. The enormous weight of the ice cap has forced the underlying continent to sink approximately 1,000 m (3,300 ft) into the Earth's crust.
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