Active volcanoes often send out signals advertising that they are awake: small earthquakes and venting gases usually aren't good news. But often, the messages aren't clear. Now, researchers have another tool to help predict when a volcano may blow. A new study shows that chemical patterns in volcanic crystals match up with patterns in volcanic earthquake and gas recordings—giving scientists a chance to save thousands of lives before it's too late.
Tiny volcanic crystals, often just 50 to 100 micrometers across, float suspended in magma, the ultrahot mix of molten rock and dissolved gases that can rise beneath volcanoes. Many of these crystals have concentric bands that look like tree rings. Events like a new pulse of hot magma into the chamber—like that which can precede an eruption—can cause elements inside a crystal, such as iron and magnesium, to migrate toward the crystal's core or toward its edges, creating new rings. Once a minor eruption preceding the main event spews a crystal-bearing magma above ground, it all solidifies and locks in the record of the volcano's past, which geologists studying the threat from the volcano can collect and interpret.
Kate Saunders, a volcanologist at the University of Bristol in the United Kingdom, wondered whether the crystals could also be used to predict the future. She and colleagues studied a mineral crystal called orthopyroxene from Mount St. Helens in Washington. The active volcano erupted catastrophically on 18 May 1980, and continued to produce smaller eruptions through October of 1986. The team characterized the chemical patterns of 98 crystals collected over the course of the eruptions and compared those patterns with records of earthquakes and gas release that other researchers collected during this same time period.
Crystals start to build up a year prior to eruptions and peak just before an explosion happens, the team reports online today in Science. Crystals with magnesium-rich rims and iron-rich cores, which signify heating from the intrusion of new magma, were associated with deep earthquakes. A spike in these magnesium-rimmed crystals also occurred just prior to the massive 18 May eruption, indicating that pulses of new magma preceded the blast. Crystals with iron-rich rims and magnesium-rich cores, which form when the magma is degassing and cooling, corresponded with peaks in sulfur dioxide gas release. These crystals peaked prior to later eruptions at Mount St. Helens.
Knowing how the chemical fingerprints of crystals link up with other recorded signals will help scientists read a volcano's past to better interpret its future warning signs, says Saunders. "We can tell if we expect to see new pulses of magma, or if we expect the magma to sit there and degas, and we can start to work out what signs we should be looking for in the monitoring data" to predict eruptions.
"It's a neat package, the fact that they can work backwards with these crystals to nail down the timing of magmatic [activity]," says Carl Thornber, a volcanologist with the United States Geological Survey at the Cascades Volcano Observatory in Vancouver, Washington. "It gives us much more solid information to interpret what's going on down below, and how to interpret all the measurements we're making."
"The technique can be transferred to any volcano," Saunders says. Scientists can't monitor the moving and shaking of every volcano worldwide, but they can often collect the products of lesser eruptions, such as these deep-formed crystals, to understand the volcano's behavior. They can also watch for spikes in the types of crystals that might foretell an explosion. "If we look at these erupted products, we can build up a picture of what's happened and what we expect to happen if the volcano suddenly reawakes."