The Great Ice Age
from the book, "Our Fascinating Earth"
The Great Ice Age
Glaciers have moved across the face of the earth many times in the geologic past, always sculpting the land and leaving behind numerous lakes and swamps. The oldest known glacial epoch occurred nearly two billion years ago. In southern Canada, extending east to west about 1,000 miles, is a series of deposits of glacial origin. This earliest Canadian ice sheet would have been at least 1,000 miles long, 1,000 miles wide, and thousands of feet thick. New evidence indicates that the ice flowed into the northern part of the United States, at least into the area of Michigan.
Glacial deposits found in South Africa, central India, and western Australia are thought to be of an ago equivalent to those in Canada. If so, the Ice Age of two billion years ago must have been quite extensive and probably lasted for many millions of years. Since then the earth has undergone many periods of refrigeration.
Within the last billion years or so the earth has experienced at least six major phases of refrigeration that apparently occurred at intervals of about 150 million years. Each may have lasted as long as 50 million years. The cause of these periodic ice ages is a deep enigma of earth history. Scientists have advance many theories ranging from changing ocean currents to sunspot cycles. No single theory is sound, and doubtless many factors are involved. One fact that seems certain, however, is that the earth is still in a glacial age.
The term ice age can be confusing since it generally refers to a period when parts of the earth are undergoing glaciation lasting one or two million years. These epochs are usually marked by a series of glacial advance and retreats (interglacials). But when geologists refer to a period of global cooling that can last for many millions of years, they are talking about an ice era. The fifty—million—year epochs mentioned earlier are classified as ice eras. Understandably then, several ice ages can and do occur within an era.
The most recent ice era began about sixty—five million years ago and seems to coincide with the great Cretaceous extinction. Mounting evidence indicates that the earth was struck by a colossal comet or asteroid. Debris ejected into the upper atmosphere, along with global forest fires sending vast quantities of soot into the lower atmosphere, blocked the sun, reduced surface temperature, and produced acid rain. Along with killing off the remaining dinosaurs and three—fourths of the known plant and animal species, the impact may have slightly changed the earth's orbit. This would explain why the planet has remained generally cool and shows no signs of returning to the warm Mesozoic climate of the dinosaurs.
The ice era was slow in starting, but approximately fifty—five million years ago glaciers began to form in Antarctica. The ice, grew, shrank, grew again, and gradually expanded until it coalesced into the domeshaped ice sheet that, by twenty million years ago, covered the entire Antarctic continent as it still does. It wasn't until about twelve million years ago that glaciers began to spread in the mountains of Alaska. Greenland's glaciers are relatively new within this ice era, because this land was not covered by ice until three million years ago.
Now the stage was set for the great Ice Age, a time that geologists call the Pleistocene. Nearly two million years ago a series of ice advances began, at times covering over one—fourth of the earth's land surface with great sheets of ice thousands of feet thick. During this last epoch of refrigeration the ice advanced and retreated by melting at least four times. Evidence appears to indicate that each succeeding glacial advance was more severe than the previous one. The most severe began about 50,000 years ago and ended about 10,000 years ago.
During the times of glacial retreat (interglacials) worldwide climates were, on the average, much warmer than they are at present. Usually the interglacial phase lasted many thousands of years. Ever since the climax of the last advance, the ice has been in a stage of retreating, and world climates, although fluctuating, are slowing warming. Scientists consider the earth still to be in a glacial stage because one—tenth of the globe's surface is still covered by glacial ice.
Greenland and Antarctica are capped by five million cubic miles of ice, and valley glaciers are common in the mountains of the world. However, well—kept records clearly show that the last hundred years have seen a marked worldwide retreat of ice. Swiss resorts built during the early 1900s to offer scenic views of glaciers now have no ice in sight! If this glacial retreat continues, and all the ice melts, sea levels would rise 200 to 300 feet, flooding many of the world's major cities. New York and Boston would then be visited only by scuba divers.
Perhaps the retreat is only temporary, and thousands of years in the future the earth will cool and undergo another period of refrigeration. Huge mountains of ice will then re—form and advance on the land, engulfing whatever civilization stands in their path. Or perhaps the Ice Age is really ending; only time and submerged coastal cities will tell.