A kid throwing a temper tantrum and a volcanic eruption have a lot in common. Both are preceded by a low pitched whine that immediately informs either a geologist or a parent that something is amiss.
That sound gets higher and higher in pitch as the minutes fly by. There's that brief period of calmness, lasting only seconds until they blow and make people wish they evacuated the premises.
A new study published online in the journal Nature Geoscience examined this pattern of screaming, calmness, and ultimately explosion at the Redoubt Volcano in Alaska when it erupted back in 2009. A volcano normally makes sounds outside the range of human hearing, but this particular eruption produced something audible.
Alicia Hotovec-Ellis, a volcanic seismology doctoral student at the University of Washington and one of the authors on the study, says that it's not so much a "scream" as it is a steady drone approximately 30 hertz in frequency. "It's a very soft and very low hum, right at the limit of your perception," she told ABC News.
Unlike the lung power and vocal chords that give a child's scream its energy, the volcano's scream comes from its harmonic tremor. According to Diana Roman, a volcanologist at the Carnegie Institution of Washington, you can think of the tremor as a series of really weak earthquakes. "A single earthquake lasts only for a few seconds, but tremors can last minutes," she told ABC News.
The tremor itself can barely be felt by humans, registering between 0.5-1.5 on the Richter Scale. Roman said that despite the tremors increasing in frequency the closer the volcano is to erupting, the average person still wouldn't be able to feel it. "Even if you were standing on the volcano, [the tremor] is only detectable by seismometers," she said.
At Redoubt, the time between when the tremor becomes audible and when the volcano erupts is very short. "You have maybe a minute before the eruption," said Hotovec-Ellis. Right after the tremor hits its highest frequency, the volcano goes abruptly silent before exploding about 30 seconds later.
Though the volcanoes in Alaska are active, they aren't much of a hazard to people because of how remote the area is. Roman said that they're actually more of a threat to aircraft. "During a previous eruption, a 747 plane ingested some of the volcano's ash and lost its engines," she said. "It didn't crash, but it was a near-miss disaster." She adds that observing these weak tremors in hours to days in advance can help pilots, giving them extra time to change their flight paths away from the volcano.
Harmonic tremors have been observed in other volcanoes around the world. "The earthquakes [leading up to the eruption] are like run of the mill earthquakes you see in California, nothing special or exotic," said Hotovic-Ellis.
However, she admits that something strange is going on at the Alaskan Volcano. "Normally, it only goes up to 5 hertz, not 30 Hz. That's kind of what makes Redoubt special."