During the earth’s history, there have been many changes in the pattern of climatic regions. The most recent change came about as a result of a fall in temperature over the whole of the earth’s surface, so that the area occupied by ice and snow was greatly increased. This period began some 700 000 years ago and is known as the Ice Age. At the height of the Ice Age, temperature conditions were only about 5°C lower than they are today. This meant, however, that large areas of the northern hemisphere (and to a lesser extent the southern hemisphere) received more of their precipitation in the form of snow. This accumulated and became ice, which in turn moved down slopes and valleys in the form of glaciers and ice sheets. During the last 20 000 years, conditions have become gradually warmer and at present the area of the world occupied by ice is only about one-third of its former extent. Today, only two major ice caps are still present, in Greenland and Antarctica. The former covers an area of 1 872 000 sq km (720 000 square miles) while the latter is more than 13 million sq km (5 million square miles). They are made up of compact sheets of ice, hardened and crystallized to a depth of over a kilometre. In Marie Byrd Land, Antarctica, the ice cap was measured and found to be more than 4200 metres (14 000 feet) thick. Under such a colossal weight, the land sinks gradually.
The permanent snow line represents the level at which the loss of snow due to melting in the summer fails to remove the snow accumulated during the winter. At the equator, the snow line is between 4200 and 4800 metres (14 -16 000 feet). The further north we go, the lower the snow line becomes. In the Polar regions, it is at sea-level.
When snow falls, it is extremely light and powdery, and has a crystal form. It occupies three or four times the volume of an equivalent amount of water. As this snow slowly melts, the crystal form is lost and the snow becomes more compacted with fewer air spaces. At this stage, it is known as firn or neve. The weight of more snow on top of this may compact it into ice.
Under the force of gravity, the ice may move downslope and become channeled into already existing valleys. Ice moving in this way is known as a glacier-‘river of ice.’ The speed of movement varies; some glaciers move only centimetres a year, others several hundred metres a week. When a glacier leaves the upland area and enters a lowland area, the ice spreads outwards and several glaciers may coalesce to form an ice sheet. Ice sheets move very much slower than glaciers. When the ice sheets reach right down to the sea, they often extend outwards into the polar waters and float as ice shelves. They terminate in precipitous cliffs, when they break into individual blocks. These are called icebergs. While afloat in the sea, icebergs assume a tabular or irregular shape and only one-ninth of the mass is visible above the surface. They diminish in size when approaching warmer waters and are eventually melted, dropping the rock debris that was frozen inside them on the sea bed.
The rate of movement of the glacier is greatest in the middle where there is little obstruction. The sides and the bottom are held back by friction with the valley side spurs and the valley floor. If a row of stakes is planted across a glacier in a straight line, they will eventually take a curved shape down the valley showing that the glacier moves faster at the centre than the sides.
In the Alps, the average rate of flow is about one metre a day; in Greenland, it may be more than 15 metres, but in Antarctica, where there is little heat to melt the ice, glaciers move only a few centimetres a day. The Aletsch Glacier in the Bernese Oberland of Switzerland is 16 km (10 miles) long, affording some spectacular sights to Alpine tourists. Though, it is the longest glacier in Europe, it is short compared with those of Alaska and the Himalayas, which measure more than five times that length.
At the foot of mountain ranges, several glaciers may converge to form an extensive ice-mass called a piedmont glacier. The best-known piedmont glacier is the Malaspina Glacier of Alaska, which is 104 km (65 miles) long and 40 km (25 miles) wide, covering an area of more than 4200sq km (1600 square miles).
Combined glaciers of such dimension are now rare and in most continents only valley or Alpine glaciers are seen.