The incredible shrinking glacier Ice: North America's biggest glacier has been behaving oddly as it shrinks, sending off chunks of ice.

Sun Journal

February 10, 1997|By Doug Birch | Doug Birch,SPECIAL TO THE SUN

Geologic change is usually measured in millimeters and takes place over millenniums. But Alaska's Bering Glacier, the largest on the North American continent, is fracturing, retreating and sometimes surging at record speeds.

Its skittishness may be a sign that the planet is in for a rough ride.

In the past few years, the Bering -- roughly the size of Delaware -- has triggered mammoth floods, carved away hillsides, bulldozed bird colonies and calved icebergs as large as small islands. The cause of all this instability is simple: This enormous glacier is shrinking.

The Bering Glacier is 140 miles long, an average of about 10 miles wide and a half-mile deep.

Beginning at an altitude of 18,000 feet in the St. Elias Mountains, the glacier covers about 2,000 square miles and contains 1,000 cubic miles of ice. But during this century, the Bering has lost about 3 percent of its surface size, and about 600 feet in thickness near its terminus.

Most of the world's subpolar glaciers have been dwindling since about the turn of the century. And some climatologists and geologists think the thawing is another important signal that Earth's climate is becoming warmer.

Off and on since 1974, Bruce F. Molnia of the U.S. Geological Survey has been flying a stretch of the Alaskan coast to study the restless Bering.

Molnia and his colleagues have found evidence that about 5,000 years ago, the Bering was much smaller, with its terminus many miles from the Pacific Ocean.

Then, more snow began to fall on the ice field in winter than melted in the summer, causing the glacier to inch down fjords until it reached the coastline and its maximum size in about 1100 A.D.

For the next 800 years, the glacier slumbered, neither advancing nor retreating. Then in 1900, something happened. The summer thaw began to claim more of the behemoth's annual coat of snow. The 2,000-square-mile block of ice began to shrink, and gradually lost 600 feet of thickness in its pancake-shaped lower end.

When growing, a glacier moves slowly but inexorably -- advancing as its tremendous weight forces the ice crystals to deform, causing the seemingly rigid ice to flow like a block of extremely dense syrup.

When shrinking, it becomes far more active. It can gush rivers of water, splinter into icebergs at its terminus, retreat up the path it scours through the terrain and then slide back down in what is called a surge.

In April 1993, the Bering began what was probably its fourth or fifth surge in this century, advancing an average of a half-mile a month.

For two years it slid along, at one point creeping at a rate of more than 300 feet per day. (While a giant tortoise walks about 100 times faster, this is lightning speed for a block of ice larger than the Great Salt Lake.)

White thunder -- the sound of the ice breaking up -- echoed across the landscape. Water roared off the glacier in a single stream, carrying far more water than Niagara Falls. "It's a deafening roar," Molnia says. "There were these explosive cannon shots where big blocks of ice were being thrown off the glacier."

"Imagine a wall of ice [nearly] stretching from Baltimore to New York City, 10 miles wide and a half a mile thick," he says. Now imagine it moving a third of a city block a day as pieces of its terminus break up in very slow motion.

"It was one of the more spectacular glaciological events of this century," he says.

A surge occurs when meltwater becomes trapped under the glacier and in hidden pools and lakes. As the water rises, it presses against the ice above. At a critical point, it overcomes friction and gravity takes over, causing the glacier to hydroplane along its own base. In the Bering's case, water collected under its enormous belly to an average depth of about 3 1/2 feet before the glacier began to slip and slide.

A surging glacier can do serious damage. During its recent rampages, the Bering bulldozed through one of the largest rookeries in the central gulf region of Alaska, on islands in Vitus Lake. It pushed over the eggs of thousands of birds, including a population of threatened Dusky Canada geese. A burst ice dam flooded the land to the west, washing out a valley that teemed with moose and other wildlife.

Humans, though, weren't threatened; the nearest town is about 80 miles away. The only potential threats to safety are the huge icebergs that calve off the face of the glacier. For now, they are about as dangerous as rubber ducks in a bathtub, because they are trapped in Vitus Lake.

The lake is separated from the Pacific by a ring of gravel and rocks dumped by the advancing glacier and breached at one point by a relatively shallow river. Only smaller icebergs can escape via the river into the Pacific, where they melt quickly in the warmer ocean water.

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