16-Dec-2009 23:15 GMT
Like an angry dog, a volcano growls before it bites, shaking the ground and getting noisy before erupting. This gives scientists a chance to study the tumult beneath a volcano and may help them improve the accuracy of eruption forecasts.
Emily Brodsky presented recent findings on pre-eruption earthquakes today at the fall meeting of the American Geophysical Union in San Francisco. She is an associate professor of Earth and planetary sciences at the University of California, Santa Cruz.
Each volcano has its own personality, Brodsky says. Some rumble all the time. Others stop and start. Some rumble and erupt the same day, while others take months. Some never do erupt. Brodsky is trying to find the rules behind these different personalities.
"Volcanoes almost always make some noise before they erupt, but they don't erupt every time they make a noise," she said. "One of the big challenges of a volcano observatory is how to handle all the false alarms."
Brodsky and Luigi Passarelli, a visiting graduate student from the University of Bologna, have compiled data on the length of pre-eruption earthquakes, the time between eruptions, and the silica content of the lava from 54 volcanic eruptions over a 60-year period.
They found that the length of a volcano's run-up - the time between the start of the earthquakes and an eruption - increases the longer a volcano has been dormant or "in-repose". What's more, the underlying magma is more viscous or gummy in volcanoes with long run-up and repose times.
Scientists can use these relationships to estimate how soon a rumbling volcano might erupt. A volcano with frequent eruptions over time, for instance, provides little warning before it blows. The findings can also help scientists decide how long they should stay on alert after a volcano starts rumbling.
"You can say, 'My volcano is acting up today, so I'd better issue an alert and keep that alert open for 100 days or 10 days, based on what I think the chemistry of the system is,' " Brodsky said.
Volcano observers are well-versed in the peculiarities of their systems and often issue alerts to match, according to Brodsky. But this study is the first to take those observations and stretch them across all volcanoes, she said.
The underlying physics all lead back to magma. When the pressure in a chamber builds high enough, the magma pushes its way to the volcano's mouth and erupts. The speed of this ascent depends on how viscous the magma is. This depends in turn on the amount of silica in the magma. The less silica, the runnier the magma. The runnier the magma, the quicker the volcanic chamber fills and the quicker it will spew, says Brodsky.
The path from chamber to surface isn't easy for magma as it forces its way up through the crust. The jostling of subsurface rock causes pre-eruption tremors, which change in length and severity depending on how freely the magma can move.
"If the magma's very sticky, then it takes a long time both to recharge the chamber and to push its way to the surface," Brodsky said. "It extends the length of precursory activity."
Thick magma is the culprit behind the world's most explosive eruptions, she explains. This type of magma traps gas and builds pressure like a keg. Mount St. Helens is an example of a volcano fed by viscous magma.
Brodsky and Passarelli have charted the dynamics of magma flow using a simple analytical model of fluids moving through channels. The next step, Brodsky said, is to test the accuracy of their predictions on future eruptions.
But volcanoes are very messy systems, with wildly varying structures and mineral ingredients, Brodsky says. Observatories will likely have to tweak their predictions based on the unique characteristics of each system.