When the next big volcano blows, will we get a buoyant plume of sky-choking ash or a ground-scorching pyroclastic flow peppered with palm-sized chunks of pumice? It turns out that chunks of rock racing up the throat a volcano make all the difference in how it will spew. 

Both types of eruption clearly have their downsides:

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Most volcanologists assume that this fundamental difference in particle size—fine-grained ash or fist-sized rock—is determined early on, when bubbly magma deep in the volcano first changes into a rising stream of gas and bits of rock during a process known as fragmentation.

But a new study published online June 22 in Nature Geoscience reveals that the final particle size actually depends more on what happens next: how often those particles collide with each other as they race to the surface. In other words, it’s less about how the magma fragments and more about how far those fragments have to travel.

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Josef Dufek of Georgia Tech University used high-energy experiments and computer simulations to study particle break-up, known as granular disruption. His team, which included Michael Manga and Ameeta Patel of the University of California, Berkeley, determined that a volcano is more likely to emit fine-grained ash if fragmentation begins a few kilometers underground.

“The longer these particles stay in the conduit, the more often they collide with each other,” Dufek said in a press release. “These high-energy collisions break the volcanic particles into fractions of their original size.”

On the other hand, “particles that begin closer to the surface with less energy don’t have time for as many collisions before they exit the volcano,” he said. Only if magma fragmentation starts shallow—say, within 500 meters deep—will the larger chunks of pumice remain intact, the study suggests.

Knowing that eruptions dominated by pyroclastic flows originate relatively close to the surface could help scientists predict the behavior of restless volcanoes in the future.

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Photo: Soufriere Hills volcano at night (via Richard Roscoe/Photovolcanica.com)