New research indicates volcanoes often make odd noises scientists call “screams” just before they erupt, a discovery that could lead to better predictions about when a volcano will blow its top.
The screams are harmonic tremors, and researchers at the University of Washington
believe they are caused when magma is forced through a narrow conduit at greater and greater pressure into the heart of the volcano.
The magma sticks to the rock surface of the conduit until there is enough pressure to move it again. Each of the movements causes a small earthquake, magnitude 0.5 to 1.5, and as the pressure builds, the frequency of the earthquakes increases until they “blend into a continuous harmonic tremor.”
The observations were made during the March 2009 eruption of Alaska’s Redoubt Volcano when harmonic tremors climbed to higher and higher frequencies and then stopped abruptly just before six of the eruptions, five of them coming in succession.
“Because there’s less time between each earthquake, there’s not enough time to build up enough pressure for a bigger one,” said Alicia Hotovec-Ellis, a University of Washington doctoral student in Earth and space sciences. “After the frequency glides up to a ridiculously high frequency, it pauses and then it explodes.”
The initial “noises” start at about one hertz and move up to 30 hertz just before pausing. Humans can hear sounds starting at about 20 hertz, and while scientists say someone lying on the ground directly above the magma conduit might be able to hear the higher frequency harmonic tremors, they don’t advise doing so as the last tremors can be closely followed by an explosion.
Upward-gliding tremors immediately before a volcanic explosion also have been documented at the Arenal Volcano in Costa Rica and Soufrière Hills volcano on the Caribbean island of Montserrat.
“Redoubt [volcano] is unique in that it is much clearer that that is what’s going on,” Hotovec-Ellis said. “I think the next step is understanding why the stresses are so high.”