Eruptions Not Always Imminent From Slow Magma
August 20, 2013

Magma Can Lie Buried In The Earth’s Crust For Much Longer Than Previously Believed

April Flowers for - Your Universe Online

A new modeling study from the University of Washington reveals reservoirs of silica-rich magma of the sort that causes the most explosive volcanic eruptions can persist for hundreds of thousands of years in the Earth’s upper crust without triggering an eruption.

In other words, regions such as Yellowstone National Park, known to have experienced a massive volcanic eruption in the past, could have a large pool of magma festering beneath it – and still not be close to erupting as it did 600,000 years ago.

“You might expect to see a stewing magma chamber for a long period of time and it doesn’t necessarily mean an eruption is imminent,” said Sarah Gelman, a UW doctoral student in Earth and space sciences.

Recent research models have predicted reservoirs of molten rock, or silica-rich magma, form on and survive for geologically short time scales – basically tens of thousands of years – in the cold upper crust before solidifying. These models also suggest that to reach a large enough volume and pressure to cause an eruption, the magma had to be injected into the Earth’s crust at a high rate.

Gelman and her collaborators - Francisco Gutiérrez, a former UW doctoral student now with Universidad de Chile in Santiago, and Olivier Bachmann, a former UW faculty member now with the Swiss Federal Institute of Technology in Zurich – took these models further by incorporating changes in the crystallization behavior of silica-rich magma in the upper crust and temperature-dependent heat conductivity. The findings, published in the journal Geology, showed the magma could accumulate more slowly and remain molten for a much longer period than the models previously suggested.

Although the relationship between them is unclear, there are two types of magma identified by scientists. Plutonic magma never erupts. Instead, it freezes in the Earth’s crust and becomes a craggy granite formation like those commonly seen in Yosemite National Park. The second type is volcanic magma, associated with eruptions, whether continuous “oozing” types of eruption such as Hawaii’s Kilauea Volcano or more explosive eruptions such as Mount Pinatubo in the Philippines or Mount St. Helens in Washington state.

It has been suggested plutonic formations are what remain in the crust after volcanic material is ejected during major eruptions. According to Gelman, it is possible magma chambers in the crust could consist of a core of partially molten material feeding volcanoes surrounded by more crystalline regions that ultimately turn into plutonic rock. She said it is also possible the two rock types to develop independently

The new model suggests reservoirs of molten magma in the crust persist for far longer than some scientists believe. Scientists judge how the magma has been affected by being in the crust using silica content. Magma begins to crystallize as it is forced up a column from lower in the Earth to the crust. As the magma moves higher, the crystals start to drop out, leaving the remaining molten rock with higher silica content.

“These time scales are in the hundreds of thousands, even up to a million, years and these chambers can sit there for that long,” Gelman said.

Gelman added even if the magma begins to solidify before it erupts, it is a long process. More crystals form as the magma cools, giving the rock a mushy consistency. The magma is still molten and capable of erupting, however the mushy rock will behave differently than magma that is much hotter and has fewer crystals.

For volcanic “arcs” found near subduction zones where one tectonic plate is diving beneath another, the implications of these findings are significant. Arcs are found around the globe, in places like the Andes Mountains of South America and the Cascades Range of the Pacific Northwest.

To detect magma pools beneath these arcs, researchers have developed different techniques but they have still been unable to determine how long the reservoirs have been there. The explosive potential increases as the volcanic magma becomes more silica-rich with time.

“If you see melt in an area, it’s important to know how long that melt has been around to determine whether there is eruptive potential or not,” Gelman said. “If you image it today, does that mean it could not have been there 300,000 years ago? Previous models have said it couldn’t have been. Our model says it could. That doesn’t mean it was there, but it could have been there.”