Answer: In the sound it makes when erupting. Or rather, the infrasound—the low-frequency rumble just below the range of human hearing.
Robin Matoza, a graduate student at the Scripps Institution of Oceanography in La Jolla, California, and his colleagues used infrasound arrays to record eruptions at two volcanoes—Mount St. Helens in Washington and Tungurahua in Ecuador. Then they sped up the recordings to several hundred times normal speed (they probably played Beatles records backwards as teenagers, too) and discovered that the resulting audio spectrum looks very similar to the pattern produced by a jet or rocket engine in full roar ( see the spectrum and listen to the volcano audio here).
No surprise, really. The physics of the two systems are similar—hot gas being expelled through a narrow opening at high velocity. A volcano like Mount St. Helens may spew out millions of tons of ash, steam, and other material, and the opening might be hundreds of yards wide, but essentially an eruption is a lot like a gigantic jet blast. Researchers studying jet engine noise have found that factors like exhaust velocity, temperature, and large-scale turbulence in the flow determine the character of the sound, and the same may be true for a volcano's roar. Matoza and his colleagues suspect that the amount and type of ash expelled during an eruption plays an important role in shaping the infrasound spectrum.
The research, funded by the National Science Foundation and due to be published soon in the Journal of Geophysical Research, is driven by more than just curiosity. It may help volcanologists predict the amount of ash fall from future eruptions based on the infrasound spectrum. And, in a nice example of payback to the aviation world, such predictions would help warn airlines about volcanic dust that can choke jet engines in flight.