Modern Glaciation

At the present time, glacier ice covers as much as 10 per cent of the earth's land surface. We are now living in a glacial era that began in Antarctica about 40 million years ago. Though the latter stages have included many alternations between periods of full glaciation, when much of the northern hemisphers was covered by ice, and interglacial periods with much less ice, as the one at this present time. Scientists, for years, have been studying the phenomenon of glacial movement. The extent of their research ranges from different processes involved, to glacial speed, to the physical results of such massive ice retreats.

Permo-Carboniferous

Permo-Carboniferous was a period of great glaciation that occured about 250 million years ago. It is one of the many ice ages that has occurred on this earth's life time. This is also an era that has been used to submit proof that the continents were once a large land mass called Gondwana. Permo-Carboniferous rocks are widely distributed in Gondwana. The widespread distribution of Permo-Carboniferous glacial sediments in South America, Africa, Madagascar, Arabia, India, Antarctica and Australia was one of the major pieces of evidence for the theory of continentall drift. Glacial activity spanned virtually the whole of Carboniferous and Early Permian time (A.G. Smith 1997). Toward the end of the Carboniferous, and around 290 million years ago, Gondwanda hovered over the south polar regions, where glacial centers expanded across the continents, as evidenced by glacial deposits of tillites along with striations in ancient rocks. Thosse heavily grooved by the advancing glaciers showed lines of ice flow away from the equator and toward the poles, which is the opposite direction if the continents were situated where they are today. Overall, the southern continents drifted together over the South Pole, and massive ice sheets radiating outward from a central point crossed the present continentall boundaries. The Permo-Carboniferous ice sheet is so extensive that it can fit within a latitude circle of 50 degrees (A.G.Smith 1997).

Ice Flow

Ice flows by two main methods (commonly in combination): by internal deformation and by sliding on its bed (also known as basal sliding). A third mechanism- movement over a bed of deforming sediment - is also important at certain times.

As snow turns to ice its contituent crystals alter under the weight of material accumulation above them and under them. Gravity then induces spreading of the ice by creep under its own weight. This stress, from the weight, causes the ice to deform in a plastic manner. This deformation is a very slow process. The greatest deformation is near the bed and along the sides of the glacier. A typical flow pattern shows an initial rapid increase in velocity away from the margins, then a declining rate of increase towards the middle (Hambry and Alean 1992).

When a glacier slips over its bed, it is a process called basal sliding, which is the second component of glacier flow. Large quantities of summer meltwater reduce the friction between a glacier and its bed and cause faster flow.