Volcano’s “screams” may explain the strength of the eruptions

New Scientist : Over a two-week period beginning in March 2009, the Redoubt volcano in Alaska experienced a series of 20 eruptions that sent ash 15 km high. What was unusual is the eruptions were preceded by 0.5- to 1.5-magnitude tremors that created a noise that humans could have heard. In the last minute before the eruption, the tremors occurred at a rate of 30 per second. The seismic waves merged into a single signal with a pitch that entered the range of human hearing. Within the magma chamber it would have sounded like a scream, but at the surface of the cone, the sound was dampened to a hum. Eric Dunham of Stanford University and his colleagues analyzed the seismic data and localized the tremors’ source to a point 2 km below the crater near where the magma chamber fed into the volcanic conduit to the surface. Dunham and his team created a model of the activity, which showed that built-up pressure in the conduit may have been the root cause of the noise. They speculate that the pressure may have caused some of the magma to harden and partially block the conduit and that the rest of the magma pushed around the blockage, which created the rumbling. Alternatively, the pressure may have increased the thickness of the magma enough to cause it to flow less smoothly against the walls of the conduit. Either option could explain why the eruptions were so explosive. Regardless of the cause, it is the first known example of a volcano producing a “scream.”