Chapter 18
Chapter 18: Amazing Ice: Glaciers and Ice Ages
Feature Articles
The Rest of the Story: The Discovery of Pleistocene Mammals
by Stephen Marshak
As early as the 1600s, scholars were arguing over the meaning of enormous bones found in sedimentary deposits in Europe. Most thought the bones were relicts of giant humans that had been drowned in Noah’s flood. But in 1796, the French anatomist Georges Cuvier used his detailed knowledge of anatomy to show that these giant bones instead resembled those of elephants but were not the bones of any elephant species then alive. Since no one had ever seen these elephants and it seemed unlikely that they could be hiding anywhere, Cuvier reasoned that they must be extinct. The concept seemed outrageous to people of Cuvier’s time, who could think of no reason for extinction to occur.
In the early 1800s, explorers discovered still more bones of extinct mammals: cave bears, giant lions, hyenas, and saber-toothed cats. Construction workers even unearthed hippopotamus bones in glacial drift beneath London. The final documentation of Pleistocene mammals came in the early twentieth century, when several Russian expeditions found wooly mammoths that had been frozen intact in the Siberian ice. Much of the mammoth’s skin, hair, and flesh remained, and their 30,000-year-old meat was still edible (as far as the sled dogs were concerned).
The Human Angle: A Speedy Glacier Teaches Geologists a Lesson
by Elizabeth Lane Mason
The Columbia Glacier in Alaska’s Prince William Sound is retreating at incredible speeds. During the past two years, the glacier has increased its rate of retreat to approximately 1km a year. This is particularly impressive because the Columbia Glacier is flowing downhill at rates of up to 34 meters a day, one of the fastest flow rates in the world.
For 2,000 years, the Columbia Glacier slowly advanced from 10,000 feet of elevation in the Chugach Mountains to its terminus in Prince William Sound. When it reached the sound, it stopped advancing and remained static for almost 200 years while the rate of ablation matched the rate of sublimation. During this time, the glacier built up a terminal moraine that lent it further stability. In 1982, the balance began to tip in favor of sublimation and the glacier began to retreat.
Once the glacier stepped back from the moraine, it was left floating in deep water. At its terminus, it is 1,000 feet deep and 3 miles wide. A floating mass of ice this size is extremely unstable and the glacier’s calving rate increased. As the rate of retreat increased, both flow velocity and extensional stretching of the glacier also increased.
Scientists studying the glacier are learning a lot about poorly understood tidewater glacial systems. The high flow velocity of tidewater glaciers is accomplished by gravitational pull and basal sliding. Flow velocity increases in the downhill direction so that the glacier is moving fastest at its toe. This causes the glacier to stretch which in turn causes it to thin. Thinning reduces the overburden pressure on the base of the glacier. For landlocked glaciers, a lessening of overburden pressure would result in a reduction of pressure melting at the base, slowing the advance. Because the base of the Columbia Glacier is below sea level, basal water pressure remains constant and contact between the base of the glacier and the underlying bed is reduced.
The result is increased sliding which leads to increased thinning which in turn reduces overburden pressure which leads to increased sliding. The Columbia Glacier appears to be caught in a vicious cycle that may explain why tidewater glacial retreat is a seemingly irreversible process.
Since the mid-1970’s, the Columbia Glacier has been the subject of a lot of research, partly because it offers a unique opportunity for scientists to study the dynamics of tidewater glacial retreat. But the glacier has also been scrutinized because of its threat to more than 600 oil tankers that pass through Prince William Sound each year. In 1989, the Exxon Valdez changed its course to avoid a chunk of ice from the Columbia Glacier and ran into an underwater reef, spilling 11 million gallons of oil. In the past two years, the calving rate has increased dramatically and during August and September, geologists estimate that the glacier dumps 1,000 tons of ice into Prince William Sound each second posing an ever increasing danger.
REFERENCES
Pfeffer, W.T., J. Cohn, M. Meier. 2000. “Alaskan glacier beats a dramatic retreat”. EOS Transactions 81, no. 48: 577.