Geosciences Grad Student Tackles Explosive Eruptions in Chile

By Brianne Phillips

Holding up a slide of incredibly thinly sliced magma from the Llaima Volcano, Boise State graduate student Aaron Marshall points to the narrow, branching network of bubbles surrounded by crystals. Marshall collected the sample in Chile during spring semester, and it may hold the key to explaining the volcano’s uncharacteristically explosive eruption about 16,000 years ago.

Photo of volcano

“I think the gas bubble geometry is being controlled by the nucleation of all of these tiny little crystals,” said Marshall, who is working on his doctorate in geoscience. “The rapid crystallization of these tiny crystals stiffens the magma as it rises through the crust, preventing gas from escaping. As the magma continues to rise, pressure builds up that eventually causes the magma to fragment apart as though it were solid. Those are the two main factors you’d need for an explosive eruption – trapped gas and overpressure.”

The magma of Llaima is mafic, meaning that it has a low silica content and should therefore have low viscosity. Normally, gases are able to easily escape from this magma, resulting in lava flows or mildly explosive eruptions. However, when Llaima erupted, its incredible explosiveness rivaled that of the 1980 Mount St. Helens eruption, defying expectation.

Marshall ventured to Chile for an entire month of research and data collection on Llaima in January. The fruitful field season enabled Marshall to collect frozen magma samples from deposits around the volcano, and charcoal – the charred remains of trees that were destroyed during the eruption by hot gasses and ash. Radiocarbon dating these remains can provide an accurate timeline for when the eruption occurred.

Photo of Aaron Marshall
Aaron Marshall Explaining Magma

Processing the magma to understand its composition and why gases were trapped is highly specialized, but discovering what caused the shift in the style of explosion is essential to informing hazard forecasting and resiliency for people living near active mafic volcanoes.

“At Lawrence Berkeley National Laboratory, I use a method called X-ray computed microtomography. A particle accelerator fires x-rays at a sample, which allows us to construct 3D images of the crystal and bubble networks within the rock,” explained Marshall. “I will use the 3D structure to model gas flow through the sample. This will tell me if the gas could escape easily through a network of vesicles, or if the lack of a vesicle network prevents gas flow. If the gas does not flow through the sample, we will know that the explosive nature of the eruption was driven by trapped gas.”

Prior to leaving for Chile, Marshall expressed a few concerns with some of the finer details of the trip.

“I don’t speak very much Spanish at all,” said Marshall. “And the truck I had to rent is a manual; I’ve never driven a manual.”

He had to practice the new skill before his trip with the help of Professor Mark Schmitz.

Marshall has been working on Llaima for over two years, and credits Department of Geosciences Associate Professor Brittany Brand, Mark Schmitz and Assistant Professor Dorsey Wanless with challenging him to become the geologist he is today. Prior to coming to Boise State, Marshall was a hard hydrogeologist; volcanology was a completely new challenge.

“I started grad school with a seven or eight day field trip to Oregon to do field work for an advanced field methods class that was all volcanology,” said Marshall. “Brittany Brand told me ‘You’re going to have to take this class and I know that you’ve never done a single bit of volcanology in your whole life; don’t worry, you’ll get through it.’”

Marshall’s research currently is supported by grants from the National Geographic, the National Science Foundation Division of Earth Sciences and a Department of Geosciences Burnham grant.