SCIENCE

On 27 August 1883, the Earth let out a noise louder than any it has made since. It was heard 3,000 miles away “like the distant roar of heavy guns.” Think, for a moment, just how crazy this is. If you’re in Boston and someone tells you that they heard a sound coming from New York City, you’re probably going to give them a funny look. But Boston is a mere 200 miles from New York. What we’re talking about here is like being in Boston and clearly hearing a noise coming from Dublin, Ireland. (Nautilus)

What caused such a roar? Use our resources to find out.

(Editor’s note: Yes, we should be calling the still-active volcano Krakatau, but for historic consistency’s sake, we’re going to stick with Krakatoa for our purposes here.) 

Discussion Questions

• Read our super-short article on the 1883 Krakatoa eruption. Sound waves were not the only waves produced by the eruption. What other kind of waves were associated with the event?
  • Water waves. The Krakatoa eruption produced enormous, deadly tsunamis.
  • Seismic waves. The Krakatoa eruption was preceded by earthquakes, whose seismic waves could be felt kilometers away.
  • Light waves. OK, scientists are still skeptical about light as a wave, but it definitely has wave-like properties, and people saw Krakatoa blow itself up. Some even lived to tell about it.

• According to the Nautilus article, the 1883 eruption of Krakatoa was the loudest sound in recorded history. But, powerful as Krakatoa was, it was far from the most powerful volcano in history. Why do you think the eruption of the Yellowstone Supervolcano—much more explosive than Krakatoa—didn’t make more noise?
  • It probably did. The key words in the first statement is recorded history. People weren’t measuring barometric pressure 2.2 million years ago, when Yellowstone went boom. (Don’t worry, it will boom again. Read more about it here.)

• Take a look at the remarkable video above, which records a volcanic eruption and its resulting sound. Why can we (and videographers on the boat) see the eruption before we hear it?
  • Light is faster than sound. (In fact, light is faster than just about anything in the known universe.) It’s why we see lightning before we hear thunder.

• Watch the video at about 10-15 seconds. We see a bright “halo” quickly moving above the volcano. What exactly are we seeing here?
  • What we’re seeing is a shock wave creating clouds by forcing molecules of water vapor together as the shock wave passes through the air. According to the Nautilus article, “When the volcano erupts, it produces a sudden spike in air pressure; you can actually watch as it moves through the air, condensing water vapor into clouds as it travels.”

• In 1883, meteorologists in Europe, Asia, and Australia used barometers to track the blips in air pressure associated with the eruption at Krakatoa. Do you think these were blips of unusually high air pressure or low air pressure? What instruments do you think volcanologists and other scientists use to record volcanic sounds these days? Take a look at this page from Oregon State for some help.
  • The 1883 barometers were definitely recording blips of high atmospheric pressure, as the eruption’s shock wave shoved air molecules closer together (increasing their density and pressure).
  • According to Oregon State, scientists use microphones and hydrophones (underwater microphones) to record volcanic sounds. These instruments can record sounds audible to human ears as well as infrasound data—sound waves with a frequency so low that people can’t hear them. Of course, scientists and amateur meteorologists around the world still rely on barometers to measure air pressure. Read more about barometers here—better yet, make your own.

TEACHERS’ TOOLKIT

Nautilus: The Sound So Loud That It Circled the Earth Four Times

This Day in Geographic History: Krakatoa Erupts

University of Oregon: Volcano Sounds

NG encyclopedia: barometer