PENN STATE (US) — A 3-D model of the Mount St. Helens volcanic eruption more than 30 years ago is expected to help seismologists map potential flows at blast-dangerous volcanoes worldwide.
“We took on the modeling of enormously complicated pyroclastic density currents, notably the classic, notorious May 1980 lateral blast that destroyed 500 square kilometers of forested terrain at Mount St. Helens,” says Barry Voight, professor emeritus of geology and geological engineering at Penn State.
Mount St. Helens in Washington state erupted catastrophically on May 18, 1980, creating a low-angle lateral blast with astonishing energy and particle content. The blast lasted less than five minutes, but caused severe damage over 230 square miles, killed 57 people, and destroyed 250 homes and 47 bridges.
The damage was not caused by lava flows, but by a fast moving current of superheated gas that carried with it a heavy load of debris.
“Volcanic lateral blasts are among the most spectacular and devastating of natural phenomena, but their dynamics are still poorly understood,” researchers write in a new study, published in the journal Geology.
The model was created using the parameters of the Mount St. Helens blast including equations to determine mass, momentum, and the heat energy of the gas, along with the size, density, specific heat, and thermal conductivity of the solid particles.
“We integrated a wide range of geophysical and geochemical data to develop rigorous initial and boundary conditions for hydrodynamics calculations that reproduced, to an amazing degree, the observed dynamics of the blast envelope,” Voight says.
The model closely matched the complicated boundaries of the region of devastation and observed results on the ground. For example, in the model, the areas of ground where pressures imply that trees would be blown down fit the actual locations of destroyed forests.
“The calculations provided much insight into internal dynamics of the blast explosion cloud that could not be observed directly,” says Voight.
A combination of gravity and the shape of the terrain are the most important factors controlling where the blast travels and where it causes damage. Pyroclastic blasts are blocked by mountains and channeled down river ravines and canyons.
Previous models of the Mount St. Helens blast considered it to be dominated by a supersonic expanding jet of gas that originated at the volcanic vent, but the new research suggests that apart from an initial burst that impacted a region less than 3.6 miles from the vent, the blast current was gravity driven.
As the distance from the vent increased, the blast current weakened because of the energy lost while trying to go over obstacles. Spreading in all directions caused a slowing of the flow and that particle sedimentation removed energy from the flow, Voight says.
“Our present results demonstrate that, where detailed geological constraints are available and thanks to the availability of modern supercomputers, 3-D transient and multiphase flow models can fairly accurately reproduce the main large-scale features of blast scenarios.”
The National Science Foundation and the European Commission supported this research.
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