Assistant Professor Josef Dufek studied how pumice rocks travel through a volcano. The process affects the size of particles that are released during an eruption. The pictured rocks are from Mt. St. Helens in Washington. Credit: Josef Dufek

How
much ash will be injected into the atmosphere during Earth’s next
volcanic eruption? Recent eruptions have demonstrated our continued
vulnerability to ash dispersal, which can disrupt the aviation industry
and cause billions of dollars in economic loss. Scientists widely
believe that volcanic particle size is determined by the initial
fragmentation process, when bubbly magma deep in the volcano changes
into gas-particle flows.

But
new Georgia Tech research indicates a more dynamic process where the
amount and size of volcanic ash actually depend on what happens
afterward, as the particles race toward the surface. Their initial size
and source depth, as well as the collisions they endure within the
conduit, are the differences between palm-sized pumice that hit the
ground and dense ash plumes that jet into the atmosphere and can halt
aviation. The findings are published in the current edition of Nature
Geoscience.

Assistant
Professor Josef Dufek used lab experiments and computer simulations to
study particle break-up, known as granular disruption, in volcanic
eruptions. His team, which included the University of California,
Berkeley’s Michael Manga and Ameeta Patel, determined that shallow
(approximately 500 m below the surface) fragmentation levels likely
cause abundant, large pumice that are often seen in large volcanic
eruptions. If the fragmentation begins a few kilometers underground, the
volcano is more likely to emit fine-grained ash.

“The
longer these particles stay in the conduit, the more often they collide
with each other,” said Dufek, a faculty member in Georgia Tech’s School
of Earth and Atmospheric Sciences. “These high-energy collisions break
the volcanic particles into fractions of their original size. That’s why
deeper fragmentations produce small particles. Particles that begin
closer to the surface with less energy don’t have time for as many
collisions before they exit the volcano. They stay more intact, are
larger and often contained in pyroclastic flows.”

Assistant Professor Josef Dufek (right) and grad student Joe Estep on the slopes of Mt. St. Helens in Washington. Credit: Josef Dufek

The
team collected volcanic rock from California’s Medicine Lake volcanic
deposit for collision experiments. They also used glass spheres because,
like glass, pumice is heated and hardens before crystals are able to
form. Using a pumice gun that propels volcanic fragments using
compressed gases, Dufek and his team determined that particles must
collide at a minimum of 30 m per second to break into larger
pieces.

Using
numerical simulations, the researchers concluded that large pumice
particles (greater than fist size) will not likely remain intact unless
the fragmentation is very shallow. Abundant large pumice rocks in a
deposit provide an indication of the depth of fragmentation, which may
vary over the course of the eruption. Due to the depth and violent
nature of the process, scientists have had little record of the depth of
the fragmentation process, even though much of the eruptive dynamics
and subsequent hazards are determined in this process.

Dufek
and his team will next use the research to better understand the
dynamics of one of the most rare natural disasters: super volcanoes,
which produced the features in Yellowstone National Park.

“We
know very little about the eruption processes during super eruptions,”
said Dufek. “Indications of their fragmentation levels will provide
important clues to their eruptive dynamics, allowing us to study them in
new ways.”

Granular disruption during explosive volcanic eruptions

Source: Georgia Institute of Technology