Volcanic eruptions are awe-inspiring sights, but they can rapidly turn destructive and even deadly. However, it’s notoriously tough to predict the occurrence and severity of eruptions—forecasts are often based on simply looking for patterns in observables like ground shaking and deformation. Researchers have now found a link between a magma’s chemistry and the depth at which it resides underground. This discovery could help improve forecasts of volcanic eruptions, they suggested.
Eruptions are fueled by magma that accumulates beneath volcanoes. It’s important to know how deep magma is underground when modeling a volcanic eruption, said Daniel Rasmussen, a volcanologist at the Smithsonian Institution in Washington, D.C. But getting a handle on the physics that explains why magma lingers at a certain depth is perhaps even more critical, said Rasmussen. “We are starting to incorporate fundamental physics in eruption forecasts.”
More Than Just Rock
Contrary to popular belief, magma isn’t just molten rock—it also contains volatile substances like water and carbon dioxide. Previous theoretical research has suggested that the amount of water within magma might control the depth at which it resides. Rasmussen and his colleagues tested that hypothesis empirically using data collected from volcanoes around the world.
The researchers started by mining the published literature for geophysical estimates of the magma depth of various volcanoes. Rasmussen and his team amassed data—largely based on measurements of earthquakes and ground deformation—for 112 volcanoes. The estimates tended to cluster around roughly 5 kilometers. Curiously, that’s much deeper than the level of neutral buoyancy, the point at which the density of magma is equivalent to that of the surrounding rock. Something must be causing magmas to stall at greater depths, Rasmussen and his collaborators surmised.
Magma Time Capsules
Rasmussen and his colleagues next collected estimates of magmatic water content for 28 of the volcanoes with magma depth information. Getting a handle on magma’s water content is a tricky measurement to make, said Rasmussen, because water doesn’t stick around in magma when it erupts. Analyzing a piece of solidified lava therefore won’t reveal anything about the water content of the parent magma.
That’s because as magma ascends, it experiences lower pressures, and the water that’s dissolved within it tends to come out of solution—degas—and escape into the atmosphere, said Rasmussen. “When it’s erupted at the surface, all of the water has escaped to the atmosphere.”
For seven of the volcanoes in their sample, Rasmussen and his collaborators traveled to the Aleutian Islands to do fieldwork. (The team relied on literature data for the remaining 21 volcanoes.) Traveling via boat and helicopter, the researchers collected samples of ash from recent eruptions. Back in the laboratory, the team analyzed millimeter-scale crystals—mostly olivine—that had grown within the magma as it cooled. Sometimes such crystals preserve bits of magma, too, said Rasmussen. “As a crystal grows in the magma, it may grow irregularly, and when this happens, it can sometimes encase the magma it is growing from within it.”
These tiny inclusions of magma are little time capsules of the magma’s original chemistry, including its water content, said Rasmussen. “This is really our only way of measuring the water content of a magma.”
More Water, Deeper Magma
When the team compared magma depth and water content for their sample of 28 volcanoes, they found that the measurements were correlated: Magmas richer in water tended to be located at greater depths. Such a correlation makes sense, said Razvan Popa, a volcanologist at ETH Zürich in Switzerland who was not involved in the research, because water acts like a lubricant. “Dissolved water helps the melt flow more easily.” When water bubbles out of magma, the material left behind will tend to be more viscous and therefore less likely to ascend, said Popa. “The magma is more sluggish. It wants to stall.”
To investigate whether water content controls magma depth or vice versa, Rasmussen and his collaborators modeled the physical and chemical changes magma undergoes as it rises. They found that magmas tended to experience pronounced upticks in viscosity at depths that depended on the magma’s water content: Magmas richer in water tended to degas—and therefore stall—at greater depths. Water content accordingly controls magma depth, the team concluded. These results were published in March in Science.
This discovery sheds light on the physics of why magma lingers at different depths, Rasmussen and his colleagues proposed. And that’s something that can be incorporated into physics-based models to more accurately predict when an eruption might occur and how severe it might be, said Rasmussen. “The ultimate goal is to improve our eruption forecasts.”
—Katherine Kornei (@KatherineKornei), Science Writer