ANN ARBOR—One moment, a stream of bubbly magma quietly oozes from a volcano. The next moment, it suddenly turns into a deadly pyroclastic flow, hurling rocks and spewing poisonous gases. Scientists have known that a process called fragmentation defines the lethal turning point, but they haven’t understood the process well enough to predict the critical moment when it will occur. Borrowing a theory from materials science, University of Michigan researcher Youxue Zhang believes he has solved the problem.

In a letter published in the Dec. 9 issue of Nature, Zhang, U-M professor of geological sciences, describes how he used “brittle failure theory” to understand fragmentation.

During fragmentation, he explains, multitudes of bubbles in the magma break up at the same time, releasing gas that was trapped inside. The bubble breakup can be viewed as a type of brittle failure, similar to what happens when glass shatters. Materials scientists will tell you that brittle failure occurs when stress on a material exceeds the material’s tensile strength.

That’s exactly what happens to bubbles in the magma, says Zhang. As the bubbles grow, stresses build. When the stress at the inner walls of the bubbles is greater than the strength of the magma, fragmentation occurs.

Using what they know about bubble growth in magma, Zhang and his students can calculate the likelihood of fragmentation for a given sample, based on the magma’s composition, initial water content and temperature. Eventually, Zhang hopes such calculations may help prevent tragedies like the one that occurred at Japan’s Mt. Unzen in 1991, when a pyroclastic flow killed 41 people, including three prominent volcanologists.