For those of us who don’t study them every day, volcanoes tend to catch our attention when they cause commotion that we can see. We know them for their clouds of ash, their scorching lava and the porous rocks their eruptions leave behind.
“We work to understand the life cycles of volcanoes, starting from their origins,” he said. That means tracing molten rock, or magma, as it forms deep in the Earth’s interior and migrates upward toward the surface. “Then we can ask how, when and why they erupt.”
Rasmussen’s latest research marks a step toward deciphering that life cycle. In a new study published in the journal Science, he and his colleagues set their sights on the bodies of magma that collect beneath active volcanoes, lingering in the Earth’s crust before surging to the surface. Their finding — that magma tends to pause its journey deeper in the crust if it contains more water — could help scientists anticipate future eruptions.
“We really want to better forecast the eruptions that are happening every day,” Rasmussen said. “One way we can do that is to understand the conditions of the magma that’s being stored before it erupts.”
Rasmussen and his team focused on arc volcanoes, which form in places where one of Earth’s tectonic plates slides beneath another.
“Volcanoes form in different settings,” Rasmussen said. “But this is the most common kind of volcano that we see at the surface.”
As the plates grind past each other, the heat and pressure can churn the surrounding rock into a molten mass. That’s how magma forms deep underground — and it doesn’t stay put for long.
“Think of a balloon at a birthday party that you fill with helium,” Rasmussen said. “The helium is less dense than the air, so the balloon rises. The same thing happens with magma.”
The molten rock, less dense than its solid surroundings, starts to rise toward the surface. The end of that journey is eruption, sending the magma out through openings in the crust along with volcanic gases and, depending on the type of eruption, billows of ash.
But the magma doesn’t usually make it all the way up to the surface in one go.
“At some stage during the ascent, the magma will commonly stall and cool off to form a magma body,” before it continues its landward journey, Rasmussen said. “There are a lot of questions about where this happens and why, and that was the focus of our study.”
It’s long been assumed that magma pauses in the crust when it reaches what’s known as neutral buoyancy: the point where the magma’s density is equal to the density of the surrounding rock and it neither sinks nor rises, like a SCUBA diver floating in the ocean.
“But that isn’t really well-supported by evidence,” Rasmussen said.
So he and his colleagues decided to investigate whether it is in fact neutral buoyancy, or something else, that determines how deep in the crust these magma bodies form. To do that, they needed to visit some volcanoes.
Summits and stormy seas
The Aleutian Islands, which run for over a thousand miles from the Alaska Peninsula to Russia’s Kamchatka Peninsula, are home to over 50 volcanoes, most of which are active. Formed where the Pacific Plate slides underneath the North American Plate, this string of islands is one of the planet’s major earthquake- and eruption-prone areas.
Its volcanoes are also some of the least studied in the world. “It’s a hard place to do work,” Rasmussen said. “It’s very remote, and the weather conditions are notoriously rough.” And that’s from someone who’s done fieldwork in Antarctica.
The team spent two summers among the islands, scaling volcanoes and gathering samples of the rocks that formed when magma erupted and cooled. They traveled and slept on a boat, which brought its own challenges.
“The seas are so rough out there, and it’s not just that the swell is big,” Rasmussen said. “You also get what are called confused seas, when the swell comes at you from different directions and the rocking of the boat is unpredictable.” More than a few of the passengers got seasick.
And there were other surprises on land. “We were working in this one area for a while,” Rasmussen said. “I was off in front of the group with my nose in some outcrop of rocks.” Suddenly, one of his colleagues caught his attention. There, maybe 20 yards away, stood a large brown bear and her cub. They decided they’d collected enough samples that day.
What rocks reveal
Once they finished their fieldwork, the team combined their data from the Aleutian Islands with information about rock samples that scientists had collected from other volcanoes. They also compiled observations, made when scientists measure ground motions or waves traveling through the Earth, about the makeup of the crust at those sites. That left them with two large sets of data: the compositions of volcanic rocks from around the world and the depth at which magma is stored beneath each of those volcanoes.
As the researchers analyzed these datasets, they noticed something striking. When they plotted the amount of water in each rock sample against the depth of the magma at that site, a clear relationship emerged. Rocks containing more water, which they argue must have formed from magma containing more water, corresponded to deeper magma bodies.
“We said, ‘Okay, this is really interesting. We have a correlation between water and depth, but why?’” Rasmussen recalled.
To untangle that question, the researchers modeled the underground movement of magma at several different volcanoes. They noticed that most of the magma bodies actually didn’t seem to form where they would achieve neutral buoyancy. Instead, magma appeared to stall in the crust right as its viscosity, or how thick it is, started to increase.
That suggested a potential explanation for the water-depth relationship.
“When magma ascends, the water in it starts to form gas bubbles,” Rasmussen said. “That makes the magma much stickier, or more viscous, and it gets kind of gummed up.” The magma begins to turn sluggish and collect in magma bodies.
“Magma with more water will begin to form gas bubbles at greater depth,” he said. “It becomes gummed up deeper and forms its reservoir deeper.” Water content, the team had found, could well play a major role in the pit stops that magma takes on its way to the surface.
Focus on forecasting
Understanding why magma travels the way that it does is crucial when it comes to anticipating eruptions. After all, many of these magma bodies will continue their journeys eventually, propelled to the surface following an earthquake or landslide, or when more magma worms its way into the reservoir.
“There’s been a big shift recently in the way we’re forecasting eruptions,” Rasmussen said. “We’re moving into an era where models are physics-based, kind of like the ones used in weather forecasting.”
To produce reliable forecasts, those models require loads of data about volcanoes and the magma underneath them. That means scientists need to understand the magma’s temperature, pressure, storage depth, gas bubbles and more, as well as how all those variables relate.
That’s where this study comes in. “Our results help us understand the conditions of the magma storage area,” Rasmussen said.
He hopes this and future research will enhance eruption forecasting models.
“Every day, there are between 40 and 50 volcanoes that erupt, putting hundreds of millions of people that live in their vicinity, as well as people traveling by air, at risk,” he said. “Our work increases our understanding of magma’s properties, so we can build physical models that describe whether it’s going to erupt and when.”
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