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From ancient times to today, volcanoes have destroyed and threatened countless civilizations and cities. And even though we have some ways of monitoring and predicting their activity, we still don't have a surefire way to tell when a volcano will erupt.

And yet, nearly one in ten people lives within 100 kilometers of an active volcano. So, in search of more reliable ways to monitor volcanic activity, some scientists have started looking at volcanoes’ music. You might not think of volcanoes as especially musical, but they actually do generate sound.

It’s just infrasound, whose frequency is below the threshold of human hearing. These sound waves can come from different phenomena, like gas getting pushed out of vents or out through the surface of a lava lake. And that moving air acts kind of like air moving through a wind instrument: It makes different sounds based on the structure it’s passing through.

Just like a horn’s sound changes based on the shape and flare of the bell, a volcano’s sound can change depending on the shape and depth of its crater. For instance, a volcano with a deep crater might produce a deep sound, while a volcano with a shallow crater would produce a higher-pitched sound. Meanwhile, narrow craters also resonate for longer periods of time, while wide, dish-like craters might not even resonate at all.

And finally, the flare of the crater’s rim changes the sound’s timbre, which you can think of as the color of the sound. It’s like how a flute and oboe sound different even when they’re playing the same note. The same applies to volcanoes.

And all this information can be useful. Since these sounds are so tied to the volcano’s shape, they can change anytime the volcano changes shape. Which is not uncommon—especially when a volcano is getting ready to erupt.

It all begins when the pressure of magma starts mounting beneath the surface. This causes lots of little earthquakes around the volcano that can shift around rocks and other geological features. The pressure can also make crater walls change their shape, kind of like how a balloon’s shape changes as it inflates.

Eventually, those crater walls can even fracture. On top of that, if there’s a lava lake within the crater, the mounting pressure can cause it to rise. So, before a volcano erupts, the entire internal structure of the crater can be rearranged.

And all of this changes the volcano’s sound. One recent example of this happened in Chile in 2015. On March 3 of that year, Chile’s Villarrica volcano erupted.

It spewed lava and ash up to a kilometer up in the air for about 20 minutes—and then it all went quiet. As far as eruptions go, it wasn’t so remarkable. Some property was damaged, but no one died, and the volcano has been quiet since.

But the Villarrica eruption is important because it’s one of the few cases where a volcano’s sound was being monitored right before the eruption. And scientists actually observed a shift in Villarrica’s sound on March 1, 2015, two days before the eruption. For one, the frequency of the sounds increased, so they became slightly higher-pitched.

The sound waves also got broader, marking a change in the timbre of the sound. And finally, the volcano lost its resonance. Before March 1, Villarrica’s sound was reverberating in the crater, which is shaped like an upside-down funnel.

But after March 1, those reverberations stopped. That told researchers that something had changed—something inside the volcano was damping the sounds. And by the next day, new infrasound readings suggested that the lava lake, which was normally deep inside the funnel, had actually risen to nearly the rim of the crater.

The next day, it exploded. At the time, scientists didn’t realize that they had been listening to evidence of an eruption about to happen. They only put all the pieces together after the fact.

But now, scientists are looking at how they might be able to use data like this to predict eruptions in other, possibly more dangerous volcanoes. It’s tricky because there’s no “normal” volcano sound. Every volcano has its own sound, so the only way to tell if something is off is to detect a change.

And even the type of change is different from one volcano to the next. But by researching volcanoes for a long time, scientists can learn what different volcanoes sound like so they know a change when they hear one. No matter what, it’s unlikely that this technique will completely replace traditional methods of monitoring volcanoes, but it’s still exciting.

This is one more tool that we can use to protect ourselves against dangerous volcanic eruptions. And there’s something kind of beautiful about the fact that we can do that with volcano music. Thanks for watching this episode of SciShow!

And if you want to learn more about volcanoes, you might enjoy our episode about the biggest eruptions we’ve witnessed in our history. You can watch that one right after this! [♪ OUTRO].