New model predicts pumice drift patterns

by Mary Caperton Morton
Friday, August 8, 2014

In July 2012, the Havre volcano in the remote southwestern Pacific erupted, creating a raft of pumice that covered more than 400 square kilometers of ocean. Despite the raft’s massive size, the event went unreported for three weeks, until a passenger aboard a New Zealand-bound plane noticed the floating mass from above.

Now scientists are seeking a way to predict the drift patterns of such pumice rafts in order to protect shipping lanes.

“A raft the size of Havre is quite exceptional,” says Martin Jutzeler, a volcanologist at the National Oceanography Centre in Southampton in England and lead author of a new study detailing the occurrence, published in Nature Communications. In the past century, at least 17 sizeable pumice rafts at least several kilometers across have been reported all over the world; the massive eruption of Krakatoa in 1883 produced the largest raft on record, with sailors in the Indian Ocean reporting that they plowed through floating pumice for thousands of kilometers.

Boats that run into large, abrasive pumice rafts can sustain hull damage or damage to the engines if the pumice gets sucked into onboard cooling systems, Jutzeler says. The debris can also clog harbors and transport invasive organisms across thousands of kilometers of open water.

Because of the potential hazards, “we’re proposing to track pumice in a similar way to how the airline industry tracks volcanic ash in the atmosphere,” Jutzeler says.

Large pumice rafts can be monitored using satellites, but the debris is usually only dense enough to be visible for a couple of weeks or months after an eruption, even though it can float for years. “Satellites can only see big, thick rafts due to [the limited] resolution of the images,” he says. “Once the rafts begin to disperse, we rely on eyewitness accounts to help track the pattern of drift.”

To create a predictive model for the dispersal of pumice in the Pacific Ocean, Jutzeler and colleagues plugged satellite and eyewitness data collected for the Havre raft into an oceanographic modeling program that predicts wave and wind patterns.

“The tricky part is that each year, seasonal patterns can vary tremendously with changing El NiƱo cycles,” Jutzeler says. “Oceanic modeling in general is very good, but it still needs improvement.”

It’s also a challenge because pumice motions and trajectories are strongly controlled by two factors: waves and surface winds, “the combination of which makes modeling their movements difficult,” says Scott Bryan, a geoscientist at the Queensland University of Technology in Brisbane, Australia, who was not involved with the new study. “Winds are very seasonal and very regional so it ends up being quite a challenge to predict dispersal, especially over such huge areas.”

Past studies have tracked the Havre and other pumice rafts, but the new work is among the first to attempt to predict future drift patterns, Bryan notes. “It’s a noble goal to produce a forward predictive model, but it won’t be easy,” he says.

Jutzeler says that the new model is ready to be tested the next time a major subsurface eruption occurs, and that it ought to predict drift patterns in real time. “Now it’s just wait and see for the next event.”

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