We usually like to think of glaciers as slow, lumbering masses of ice moving almost imperceptibly down from high mountain hideaways. “Moving at a glacial pace” is a phrase woven into everyday language, commonly applied people or events who just can’t seem to get going.
But these creeping cities of ice are alive with energy — seismic energy. An icequake unleashed as a half-mile thick glacier calves into the sea can be equivalent to a magnitude 5.0 earthquake.
Below is a video of the Jakobshavn glacier in Greenland, taken in 2008, in which a chunk of glacier calves into the sea. As it flips it grinds against the sea floor, causing an ice quake (which is the sound you hear):
Scientists have known about internal icequakes for years, too — small seismic signals presumably let loose when cracks in the ice slide like fault lines, or crevasses open up. But it turns out there is a third flavor of icequake: water flowing under high pressure can also be powerful enough to cause seismic jitters.
It’s a strange thought, but scientists make the analogy to lava flowing beneath a volcano — liquid rock is under such pressure that it rumbles through Earth as it flows, giving off a distinct signature. Monitoring stations look for these “lava-quakes” for signs a volcano may be about to erupt.
And so it is with water in glaciers. Writing in the journal Geology, researchers at the University of Alaska Fairbanks argue that cracking ice and rushing water make two different seismic noises. By distinguishing between them they can tell whether a glacier is full of water, breaking up, or both.
Why does this matter? You guessed it — climate change. Like a doctor using a stethoscope to monitor a patient’s heartbeat, these glacier docs are keeping tabs on the health of the world’s ice.
Each spring, seismic activity increases in a glacier amid slew of cracking ice and flowing melt water. That’s normal. But if glaciers’ vital signs are changing over time, the best way we’re going to know about is by listening closely.
Video: Jason Amundson