Volcanic Rush

High-speed videos capture rocks flying at unheard-of rates

The fiery fountains of erupting volcanoes seem tailor-made for the Discovery Channel. But scientists, too, are interested in capturing footage of these natural spectacles, especially for what it can reveal about how superheated gas and rock blast out at up to supersonic speeds.

Lab-made volcanic eruptions (shown) support field data showing rocks flying out of a volcano at breakneck speeds. Courtesy of M.A. Alatorre-Ibargüengoitia

EXPLOSIVE VIEWS Lugging sophisticated cameras to volcanic eruptions has paid off big for scientists. 1. A team has tracked the velocity, size and angle of spewed particles at Stromboli in Italy. 2. Single particles are tracked manually in a video from Yasur in Vanuatu. 3. A thermal camera captures volcanic bombs leaving Fuego in Guatemala. Courtesy of J. Taddeucci: (1) M. Moroni/DICEA/Univ. la Sapienza; (2&3) Istituto Nazionale di Geofisica e Vulcanologia

New high-speed videos from Italy’s Mount Stromboli show that its continual eruptions can belch stuff out more than twice as fast as scientists had thought. This surprising finding is bolstered by laboratory experiments that grind up rock and eject it at high pressure, in a sort of tabletop eruption. “We think we’re getting close to what’s going on in the throat and gut in a volcano,” says Donald Dingwell, a volcanologist at Ludwig Maximilians University Munich whose team has done much of the lab work.

The research suggests new ways to think about natural hazards, such as how far away people should stay from an eruption and how tiny ash fragments can be lofted kilometers high — potentially shutting down airspace, as the Icelandic volcano Eyjafjallajökull did in 2010.

Scientists have long listened to the heartbeat of many of the planet’s most awesome volcanoes, from Mount Etna in Sicily to Kilauea in Hawaii. On the surface, seismometers measure tiny quakes that could signal magma starting to rise from deep in the ground. Overhead, satellites watch for the landscape rising or deflating like a giant geological breath, another possible indicator of an imminent eruption. And plenty of cameras have shot gorgeous imagery of fire sprays, oozing lava and other volcanic wonders.

But researchers don’t yet understand the physics of what happens in the volcano’s throat at the moment when magma spurts through a vent, shattering into tiny fragments and larger, more deadly “bombs” that hurtle into the air. By definition an explosion happens quickly, making it hard to study. So recently Jacopo Taddeucci of the National Institute of Geophysics and Volcanology in Rome decided to lug a high-speed camera to an eruption to take a look.

Such sensitive cameras, built to photo­graph high-speed processes in more tame environments, weren’t meant for the toxic gases and ash falls atop an active volcano. At first, Taddeucci says, “I was so worried about destroying the camera or breaking it.” But eventually he and his colleagues developed a lightweight, rugged version that they hauled first to Stromboli, off the coast of Sicily. They set up shop several hundred meters from the active vent and started filming the rocks zooming in all directions. “When you’re there, you’re not as scared for the camera as for yourself,” Taddeucci says.

Projectile paths

By videotaping explosions at up to 1,000 frames per second, dozens of times faster than the film speed for a Hollywood movie, Taddeucci could trace how quickly particles flew from Stromboli’s throat. The ash bits whizzed at up to 405 meters per second, more than twice that ever measured before. He captured particles being lofted upward by convective air currents. He saw the crater floor seething restlessly just before it exploded. He could even watch shock waves coming out, one after another, as gas pockets gave way and let out fresh material, his group wrote in January in Geophysical Research Letters. No one had ever before seen these explosive moments in such detail.

Along with the high-speed camera, the scientists also set up a thermal camera to measure the temperature of particles and a microphone to record the booms. Together, the new data offer one of the first glimpses into volcanic processes that have been almost entirely unknown to science until now. “This really opens a new range of perspectives on explosive volcanism,” Taddeucci says.

Bruce Houghton, a volcanologist at the University of Hawaii at Manoa, agrees; he is trying to get money to pay for a similarly sophisticated camera setup at the Big Island’s Kilauea. “It is really a spectacular advance,” Houghton says.

Experiments in Dingwell’s lab show the same kind of explosive moments going on. After collecting and grinding up rocks from real-world eruptions, the researchers put the rock powder into one high-pressure tube meant to simulate the insides of a volcano and then let the material rapidly decompress into a second tube. The team has a front-row seat for this explosion. “This time we’re watching and deciding when and how,” Dingwell said in February in Vancouver at a meeting of the American Association for the Advancement of Science.

A high-speed camera captures the size and speed of accelerating particles. The lab videos show shock waves, like those Taddeucci sees at Stromboli, driving pulses of material from the eruption. Careful measurements show that the pulses are dictated by the size of particles being destroyed as gas pockets blow out. “That’s why it’s interesting that our lab is observing what Jacopo is also seeing,” says Miguel Alatorre-Ibargüengoitia, a researcher in the lab. “We can really replicate what’s going on.”

In the lab experiments, Alatorre-Ibargüengoitia knows exactly what pressure the rocks exploded under, data he can use to calculate the depth from which the simulated magma erupted. “In nature, we don’t know any of these parameters,” he says. The work shows, for instance, that particles shooting out of the eruption drop off in speed proportionally to the depth from which they were ejected. That information, in turn, can help pin down how much magma is spewing out of an eruption, along with how much gas it contains and at what pressures. Alatorre-Ibargüengoitia and colleagues described the work last year in Earth and Planetary Science Letters.

The team is also working to see what factors control how far large rocks get ejected from volcanoes. By measuring the pressure needed to throw a lab rock of a certain size a certain distance, the scientists can better calculate the immense forces that fuel eruptions such as those at Mexico’s Popocatépetl. Alatorre-Ibargüengoitia has measured rocks traveling up to 400 meters per second, the same sort of speeds recorded at Stromboli for much smaller particles. These larger “volcanic bombs” can land as far as five kilometers from the actual eruption vent.

Safety first

Eventually, the scientists hope to see their lab work and occasional videotaping translate into better monitoring of live volcanoes. “There’s still an important gap between experiments and the real world,” says Alatorre-Ibargüengoitia. “We are trying to close this gap.”

For instance, colleagues at the University of Hamburg have been tracking the speed of stuff flying out of volcanoes using Doppler radar, finding that some standard monitoring techniques don’t always accurately reflect the size and energy of an explosion. (Not that better information always results in safer conditions; at Etna’s current eruption, tourists have regularly brushed past warning signs and continued dangerously close to the summit to take pictures.)

For their part, Taddeucci and his colleagues have already taken their high-speed camera to three other volcanoes, in Guatemala and in Vanuatu in the South Pacific, and seen similarly fast ejections there. Next, Taddeucci wants to clock some of the biggest and baddest things in volcanology: pyroclastic flows, or massive avalanches of gas and rock that catapult down the sides of mountains.

Such flows have taken the lives of volcanologists, including experienced photographers Maurice and Katia Krafft in 1991. “You have to find a suitable volcano that you can look at without dying,” says Taddeucci.

Alexandra Witze is a contributing correspondent for Science News. Based in Boulder, Colo., Witze specializes in earth, planetary and astronomical sciences.

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