Glaciers: Defying Physics While Moving Down Mountains

Candice Gaukel Andrews April 2, 2019 0
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Glaciers, such as the 13-mile-long Mendenhall Glacier in Alaska’s Juneau Ice Field, are often compared to rivers of ice—and that’s not far off the mark.

I count glaciers among my favorite things. They come in beautiful shades of blue, they’re cold and they’re ephemeral.

But how can I describe something as old and massive as a glacier as short-lived and momentary? Because you really won’t see any one glacier twice in its same state. Glaciers constantly change their shapes and positions, as they dance down mountains.

In truth, even the very fact that glaciers move at all seems to defy physics.

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As a glacier forms, air bubbles in the snow are squeezed into the crystal structure of the ice. Thus, dense glacial ice has no air bubbles, but it contains trapped air nevertheless.

From birth to bubble-free

Most of the world’s glaciers are located near the poles, but glaciers exist on all of the world’s continents, even Africa. While Australia itself doesn’t have glaciers, it is considered part of Oceania, which includes several Pacific island chains and the large islands of New Zealand and Papua New Guinea. Both of these nations have glaciers.

Very specific climatic conditions are necessary in order for a glacier to be born. The accumulation of snow in winter has to exceed the snowmelt in summer. That allows layers of snow to pile up year after year. Such conditions typically prevail in polar and high-alpine regions.

A cubic meter (35 cubic feet) of snow weighs 155 to 330 pounds, or about the same as one or two adult humans. However, most of the volume in that cubic meter of snow—about 90 to 95 percent—is air. As it piles up, the snow’s collective force initiates a transformation. Snowflake shapes are shattered into smaller grains as they’re pressed and compacted together, shrinking the air pockets between them. The snow becomes denser.

After about two years, this ground snow transitions into a new form called firn. Firn is an intermediate composition between snow and glacier ice. It has about two-thirds the density of water. It can take many more for years for firn to change into its final state: dense, air-bubble-free ice.

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When a glacier moves rapidly around a rock outcrop or over a steep area in the bedrock, pressures build up in the ice, creating crevasses, or cracks, on the glacier’s surface.

Propelled by pressure

You might think that a glacier moves because the ice underneath it melts and becomes liquid, flowing the whole glacier downhill. But a glacier can move without melting. Long-term stress—such as bearing its own, enormous weight—can bend and deform the ice.

It works like this: as pressure increases, the melting point of ice decreases. And when glacier ice stays close to but just below that point, it becomes malleable.

The deepest layers of a glacier are subjected to the most pressure. Those layers are known as the “zone of plastic flow” because it’s here that the molecular bonds between the ice crystals actually stretch and slide past each other rather than break.

When the bottom of the glacier needs to move around large obstacles, such as boulders, even higher pressures on the uphill side cause the ice to melt, flow around the obstacle and refreeze on the other side. As these processes continue, the glacier creeps forward, propelled by gravity.

In the upper 150 feet or so of the glacier, however, known as the “zone of brittle flow,” the ice isn’t under as much pressure. So, when it’s stressed, it’s prone to crack, which forms crevasses.

Some glacier movement does come from slipping on sediment or on a thin layer of water, but most glaciers not at the Earth’s poles comes from the process of deforming.

The terminal edge of a glacier is one of the easiest places to see its movement in action. ©Joseph, flickr

Tracking transients

Glaciers are the products of climate, so they change with the climate, as you’ll see in the video below. In it, you’ll visit Mendenhall Glacier, one of about 40 in the Juneau Ice Field in Alaska. The ice of Mendenhall Glacier is flowing forward more than a foot and a half every day along a 13-mile journey to a low point, Mendenhall Lake.

Glaciers never move backwards, and they’re always melting. But when mass melts away at the bottom faster than new mass is added at the top, they can recede.

At the Mendenhall Glacier, it takes about 200 years for new ice to move from the ice field down to the lake. That’s a slow process. But warmer summers combined with less snowfall in winter means that things are speeding up. The glacier is retreating faster than it’s growing.

So, glaciers are transient. They flow; they move. And though their lives may be measured in centuries rather than years, they won’t last.

And that makes them all the more precious to me.

Here’s to finding your true places and natural habitats,

Candy

 

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