Post from a Scientist: Hearing the Ice

Imagine a valley filled with ice, from wall to wall, miles wide.  This is a photograph that I took of the Taku Glacier in Alaska when I first saw it, from an airplane:

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What’s underneath all that ice?  How deep does the ice go?  Are there streams underneath?  Rocks? Mud?  This is what scientists want to know about glaciers–we need to know how much ice they have and what they are traveling over in order to know how fast they will melt and cause sea level rise.  But we can’t just dig up a glacier to see what’s underneath–the Taku Glacier (pictured above) is 1000 feet thick!  The Taku Glacier once floated on the ocean, where the warmer water melted it rapidly, and the tides pulled off icebergs.  The glacier shrank because of this, until it was protected from the ocean tides by a thick layer of mud.  But the mud is washing away, so someday the glacier will be in the open ocean again.  So we wanted to see how much mud is left.

We cannot see what is underneath the ice, but we can hear it.  If you had a metal thermos and you wanted to know how much liquid was left inside, you could tap on the metal to give you an idea about whether it was mostly full or mostly empty.  For a thick glacier, you need to tap hard to listen to the echoes.  Last spring, when there was still snow on the ground, we took a helicopter to the Taku Glacier and tapped hard on it–with explosives!  (They weren’t big enough to hurt the glacier.) We spent ten days on the ice surface doing our survey.  The view from camp looked like this…

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…just a white expanse of 1000-foot-thick ice, hiding everything underneath.  But by recording the echoes under our feet, we learned that there is still a layer of mud down there, dozens of feet thick.  We’ll tap again in 2016 to see how much mud will have been lost in two years.

- Jenna, University of Alaska Fairbanks, USA

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Now trekking an Alaskan glacier!

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2 Responses to Post from a Scientist: Hearing the Ice

  1. Jonathan says:

    So wasn’t the explosion really loud? how could you hear the glacier, too?

    • Jenna says:

      To our ears, the explosions were actually pretty quiet because we buried our explosives 15 feet under the ice. The echoes coming off the bed were even quieter (we actually used instruments called geophones to ‘hear’ the echoes, because they are too quiet for people to hear). So, yes, the explosions covered up the sound of the echoes, but fortunately echo-sounds last longer than the direct explosion sounds (echoes have to travel farther) so only part of the echo was covered up. We were also able to filter out the direct explosion sounds using a computer, because the explosions themselves are lower-pitched than the echoes.
      Hope that answers your question! Thanks for asking.

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