When Alaska’s Mount Redoubt volcano rumbled to life this past spring, images of the plume of ash rising from it probably revived terrifying memories among 240 people who survived its last eruption in 1989.
They’d been passengers on KLM flight 867, a Boeing 747 bound for Anchorage. Ten hours after the volcano erupted, the plane flew through an ordinary-looking cloud. Except it wasn’t a cloud. It was ash from the Redoubt eruption.
The plane lost all communications, radar, electronic cockpit displays—and, within the span of one minute, all four engines. It plunged almost 15,000 feet before the crew managed to restart three of the engines. The plane landed safely in Anchorage, having sustained $80 million worth of damage. “The whole aircraft looks like it was sand-blasted,” said an FAA spokesperson at the scene.
Rick Conrey, a technician in Washington State University’s GeoAnalytical Lab, says volcanic ash isn’t soft and floaty like the ash made by burning paper or wood. Volcanic ash particles are tiny rocks, sharp enough to scratch airplane metal and fine enough to get into all but the most tightly-sealed compartments. If volcanic ash gets wet, it conducts electricity, and if it gets into a working jet engine, it melts into a gooey gob that mucks up the compressor blades. It’s also very hard to avoid. Once the plume thins out, pilots can’t tell whether a smudgy patch up ahead is ash or a harmless cloud. Even radar can’t tell them apart. Ash rides the wind for days or weeks and can damage planes more than 3,000 miles from the volcano that produced it.
Ever since the 1989 incident, says Conrey, the Federal Aviation Administration has sent WSU’s GeoAnalytical Lab test samples from Mount Redoubt and other volcanoes all over the world to try to gauge how potentially destructive the volcanoes are.
Conrey uses the lab’s x-ray fluorescence spectrometer to determine the composition of the magma spewing out of a volcano. It may be mostly basaltic, mostly rhyolitic (high in silica), or a mixture. The differences between them determine how dangerous the eruption is going to be.
Basaltic magma flows easily. Gases from deep underground are able to rise through it and escape into the air, like bubbles rising to the top of a pot of boiling water. The magma might spurt a few hundred feet into the air, but on the whole it behaves more like a river than a geyser. Conrey says that’s why a volcano like Hawaii’s Kilauea, which has been erupting continuously since 1983, isn’t especially dangerous unless you are directly in its path—and it doesn’t produce much of an ash plume.
By contrast, rhyolitic magma does not flow easily. It explodes. Rhyolitic magma is thick and gooey. Gases get trapped within it, like bubbles caught in honey or shampoo. As the magma nears earth’s surface and the pressure on it drops, the gases in it burst out with incredible force.
“The more you have of silica, the more viscous it is and the more gas it will hold and get more explosive,” says Conrey. Mount St. Helens, which hurled ash 60,000 feet into the atmosphere, wasn’t even on the high end of the silica spectrum; had its magma contained more silica, it would have erupted with even greater force.
Analyzing samples from past eruptions tells geologists a lot about a volcano’s history, but samples of fresh material are also necessary whenever a new eruption begins, because the kind of magma a volcano produced before doesn’t always predict what kind it will produce the next time. In fact, different kinds of magma can emerge from a single eruption. “It’s a lot more complex than we thought just 30 years ago,” says Conrey. “People didn’t realize it was that complicated because it was so terribly difficult to get the answers.”
Back then, learning the composition of a magma required a couple of weeks of lab work. Now Conrey can prepare a sample in half a day and the x-ray analyzer can deliver a preliminary finding in 10 minutes and a complete evaluation in about an hour. The lab procedure is straightforward. Conrey powders the sample and mixes it with flux, a lightweight compound that enables the rock powder to melt at a lower temperature than it would otherwise. Most powdered rock won’t melt until between 1300 and 2000 degrees Celsius, and at such high temperatures, some of the rock vaporizes rather than melts. “You want to use a low enough temperature to keep everything in the pot,” says Conrey.
He then chills the molten mixture to make it solidify into a disk of glass about the size of a Thin Mint Girl Scout cookie. The spectrometer analyzes the glass and determines what elements it contains, and in what amounts. If it’s high in silica, the USGS and FAA will alert the aviation community to the looming danger.