Lee Dye: Why Some Glaciers Get Dirty

A woman stormed into the mayor's office a while back to demand why Alaska's capital city of Juneau didn't clean up its premier tourist attraction, the spectacular Mendenhall Glacier on the outskirts of town.

The glacier, fed by a 500-square-mile ice field just over the mountains from Juneau, is littered with dirt, marring the view for tourists like herself, the woman complained.

That old yarn makes the rounds up here every year as locals yak it up over the lack of glacial sophistication among the tourists who flock to this region to gaze on some of nature's grand treasures, including the Mendenhall, which is beating a hasty retreat back into the mountains these days like nearly all Alaskan glaciers.

But the tourist had a point that has puzzled experts for years. Why should some glaciers, which form from pristine snow that falls in the mountains, far away from urban populations, be so dirty. Indeed, some glaciers are so filthy that they look like black smudges across the snow rather than the deep blue ice characteristic of glaciers.

And it's not just that the glaciers pick up a little dirt from the fringes as they move back and forth, carving deep "U"-shaped valleys. Dirt, rocks, and all sorts of debris can be found deep inside the ice, far from the edges, frozen in a place where common sense says it shouldn't be.

Like the lady tourist, scientists have also wondered why that should be the case. Now, they think they've figured it out.

Growing Bottoms Up

Experts from several universities, including Lehigh, Michigan State, Buffalo and Penn State, have been examining glaciers in Alaska and Iceland for years now, trying to figure out how the glaciers pick up and transport silt and other debris. The research is funded by the National Science Foundation and the Cold Regions Research and Engineering Laboratory in Hanover, N.H.

Of course, these scientists aren't all that interested in the esthetics that bugged the tourist in Juneau. They are trying to get a better grasp on how glaciers carve those deep valleys and literally move mountains.

That's one of the most dynamic of all mountain-building processes, and to do their job, glaciers must push around thousands of tons of material as they gradually reshape the land.

But that doesn't explain why some of the dirt winds up in the middle of the ice, and is exposed scores of years later as the ice above it melts under the summer sun.

In an article in the recent international journal, Nature, the scientists reveal evidence that glaciers don't owe all their growth to snowfall. They also grow from the bottom up in a process that geologist Grahame Larson of Michigan State compares to opening a can of soda pop that has been in a freezer a bit too long.

Suddenly released from the pressure of the can, the super-cooled liquid soda forms ice on the rim of the can. A similar process occurs in glaciers, the scientists say, and they have given it a name, "glaciohydraulic supercooling."

Why Blue Ice Is Blue

Edward B. Evenson, professor of earth and environmental sciences at Lehigh, says the process begins when the warmer days of summer cause the surface of the glacier to begin to melt. The warmer water penetrates down through the ice to the bottom of the glacier, where it becomes trapped beneath the immense weight of the ice and becomes "supercooled."

Eventually the water freezes again and becomes attached to the bottom of the glacier, along with whatever dirt and silt it can pick up. The ice and the dirt later separate, forming alternating layers of clean and dirty ice. That process continues all summer long, Evenson says.

So in the winter a glacier picks up new layers of ice on its surface as snow falls in the higher elevations. And in the summer, as it moves down the valley toward the sea, melting somewhat along the way, it picks up new layers of ice and dirt as it grows from the bottom up.

Ultimately, some of those dirty layers will be exposed as the ice melts, much to the consternation of tourists who expect glaciers to be perfectly blue, not black. Incidentally, glaciers are blue because the ice is compacted so tightly that little air is left between the ice crystals, and the glacier thus reflects more blue light than any other color.

These giant pieces of ice are truly among the planet's most spectacular and dynamic structures, but their days may be numbered. Nearly all glaciers in Alaska are retreating, as they have been for thousands of years since the end of the last Ice Age, but these days they are galloping at full throttle.

Los Angeles Ice

Just a couple of decades ago a new visitor's center was built overlooking the Portage Glacier near Anchorage, Alaska's largest city. But within a few years the glacier had retreated so far that it could no longer be seen from the center. Now, to see it you have to get in a boat.

That's pretty remarkable when you think that it hasn't been all that long since glaciers extended as far south as Los Angeles. No kidding.

Lewis Owen of the University of California, Riverside, reveals evidence in the current issue of Geology that shows that a glacier clung to the slopes of San Gorgonio Mountain, just 75 miles east of Los Angeles, just 12,000 years ago. Owen thinks it might have even been there as recently 5,000 years ago, but not everybody agrees with that date.

Too bad it's gone. Southern California, now sweltering in the heat and facing chronic water shortages, could use a big chunk of ice these days.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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