Earth & Geography

How Do Glaciers Move?

Here’s something we all take for granted: Liquid things flow like liquids, and solid things flow like, well… solid things don’t flow, right? Not so fast. Glaciers are cool, and not just cuz they’re made of ice. I’m here at Mendenhall Glacier, one of about 40 glaciers in the famed Juneau Icefield. Glaciers are cool, because the way that they move, even the very fact that they move seems to defy physics. A solid structure, that flows like liquid. How? THAT is a question of glacial proportions. [OPEN] To figure out how a glacier moves, first let’s go back to the beginning. <Show the big bang> Ahh, not that beginning.

The beginning of the glacier, and first you’ve got to get up there. I’m super excited, I’ve never been in a helicopter before and we’re gonna take this thing up to the top of a glacier. This ice is born in the ocean. Warm, moist air from the Pacific rises up coastal mountains, where it cools, condenses, and falls as snow and rain. 30 meters of snow, sometimes more, falls on this icefield every year. But you need more than a mountain of snow to make a glacier: you also need the right climate. Even in summer, temperatures in this region dip near freezing. What this means is the accumulation of snow in winter exceeds snowmelt in summer. So snow builds over time, with each layer landing on top of the one before it. Now, a cubic meter of fresh snow typically weighs 70 to 150 kilograms. That’s about as much as an adult human or two.

Most of that volume is air. But as it continues to pile up, collective forces on the fluffy stuff begin a transformation. *First, those pretty snowflake shapes are totally shattered into smaller crystals; about as big as grains of sugar. And as they get squished, the air pockets between them start to shrink. The snow gets denser. *After about two years, ground snow takes a new form, called “firn,” an intermediate state between snow and glacial ice. Firn is about two-thirds the density of water, and it can take decades to complete the transition into its final form. That final form is a big mass of dense, bubble-free ice. The ice in Mendenhall is flowing forward more than a half meter every day. Glaciers are often compared to “rivers of ice,” and that’s not wrong.

These giant solid structures behave with liquid-like tendencies. *I know what you’re thinking: “ice flows because it melts.” But glacier ice flows without melting. Mendenhall fits the definition of a solid: I can stand – even jump – on it, and it will hold its shape. Short term stress doesn’t affect that. But long-term stress, like bearing its own extraordinary weight, will cause it to deform and bend. What we’re told makes something a solid, rather than a liquid, is that its atoms and molecules are so tightly bonded, they can’t move past each other.

But this isn’t the case with glacial ice. Its water molecules are arranged in an orderly pattern, but under certain conditions they can still flow. Much of this has to do with the pressure melting point. As pressure increases, the melting point of ice decreases. When glacier ice stays close to – but just below – that point, it becomes malleable, much like how you can bend and deform solid metal when it’s heated near its melting point. *The deepest layers of a glacier are exposed to the most pressure. This is known as the “zone of plastic flow,” because the bonds between the ice crystals can be stretched rather than broken.

Here, the molecular bonds between ice crystals actually stretch and slide past each other, rather than break, like how a deck of cards deforms as cards slide past each other. <Have the word MAGIC come up behind joe for a few frames as he tosses the cards> When the bottom of the glacier needs to move around large obstacles, like boulders, even higher pressures on the uphill side cause the ice to melt, flow around the obstacle, and refreeze on the other side. <Could show a cross section of a boulder to show this, check out Figure 2) *As these processes repeat, the glacier creeps along, propelled by gravity, like a giant, gooey conveyor belt. *Up in the top of the glacier, things get a bit cranky. The upper 150 feet or so – “the zone of brittle flow” – doesn’t experience as much pressure. That makes it prone to cracking under stress, which is why you’ll find deep crevasses near the surface. Imagine a candy bar warping around a curved surface. The top has to stretch farther, faster, so it cracks, while the gooey bottom bends and stretches.

Some glacier movement comes from slipping on sediment or a thin layer of water, but most glaciers not at Earth’s poles move by this process of deforming. <let’s take a quiet moment here for some awesome mountain/Alaska footage, musical transition> But glaciers are a product of climate, and they change *with* the climate. Mendenhall Glacier is currently slinking along a 13-mile journey to its lowest point, Mendenhall Lake. The terminal edge of a glacier is one of the easiest places to see its movement in action, but it’s also where we can see how much things are changing. Glaciers never move backwards, and they are always melting.

But when mass melts away at the bottom faster than new mass is added up top, they can recede. It takes about 200 years for new ice on Mendenhall to move from the icefield to the lake. It’s a slow process, but warmer summers combined with less snow in winter are speeding things up. The glacier is melting faster than it’s growing – and it’s already retreated miles. Just a few decades ago, where I’m standing was covered in ice. How glaciers move is incredibly cool, but for that to continue, they have to stay that way. Stay curious. If you want to see more of Alaska’s incredible wild life, watch Wild Alaska Live, a special 3-night live event brought to you by PBS and BBC.

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