Glaciers

Late in the Pleistocene Epoch, some 30.000 or 40.000 years ago, nearly half of North America, all of northern Europe, Greenland and Antarctica and much of northern Asia were covered by great blankets of snow and ice called continental glaciers. At the same time valley glaciers in all the high mountain regions of the earth were much larger than the present ones, and thousands were in existence where none are now. It is estimated that more than one - fifth of the whole land surface, about 12.000.000 square miles, was covered with ice during this time.

Much has been written on the length of time represented by the Pleistocene Epoch but since many of the factors are indeterminate, no accurate statement can be made. Estimating the time has elapsed since the continental glaciers entirely disappeared from Europe and North America is also impossible. Several methods have been used for determining the length of the postglacial time both in Europe and North America but most of them are unreliable.

Now 5.000.000 square miles of Antarctica and 600.000 square miles of Greenland are covered with glacial ice. In addition, there are hundreds of valley glaciers in the high mountains of North America, the Alps, the Caucasus, the Andes and the Himalayas. Nearly all present glaciers are the remnants of the much greater ones of Pleistocene times. Our studies of present glaciers help us in understanding the Pleistocene glaciation which occurred so recently that it is in a large measure responsible for the topography of several million square miles of the earth’s surface.

Three conditions are necessary for the formation of a glacier: abundant snowfall; second, cool or cold temperatures; and third, a sufficiently low rate of summer melting and evaporation, so that snow fields endure and increase in size through a long period of years. Snow field may accumulate on plains, plateaus or mountains. Wherever the conditions are favourable, the snow field grows in depth and in surface area from year to year. The transformation of snow to glacial ice occurs chiefly in the snow fields. As it falls through the air, snow consists of delicate, thin, tabular, hexagonal crystals. After having lain on the ground for some time and having been covered by later falls, the snow gradually changes to granular ice which is called névé. This change is brought about by the partial melting of the snow crystals due to the weight of the overlying load. The water from the melting snow trickles down and almost immediately freezes, thus making grains of ice. A thick snow bank formed by the successive snowfalls of only one winter will have ice at the bottom, thoroughly granular snow in the centre and slightly altered snow at the top. After many years of accumulation the ice at the bottom of the snow field becomes very thick and, at last, is ready to move.

Дата добавления: 2015-01-03