When an undersea volcano erupted off the coast of Oregon in May 2015, Scott Nooner wasn’t surprised—he was excited.

Nooner and his colleague William Chadwick had predicted that the half-mile-high Axial Seamount would erupt within 15 months, a forecast based on years of watching the volcano inflate and deflate.

Because Axial Seamount’s lava chamber is concealed by only a relatively thin layer of Earth’s crust, it acts as a window into volcanic processes that are often quite difficult to observe on land, and it could bolster the ability to predict eruptions more than days or weeks ahead of time.

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“By studying a volcano with a very shallow and well-imaged magma chamber and a simple crustal structure, we can learn things about how volcanoes work that is difficult in settings where the magma chamber is deep and the crust much more complicated,” says the University of Washington’s William Wilcock.

“Axial Seamount is actually a relatively simple volcano, and it has what appears to be relatively repeatable behavior,” adds Nooner, a geophysicist at the University of North Carolina, Wilmington, speaking today at the American Geophysical Union’s annual meeting in San Francisco.

“We can understand these simple systems first, and then move to more complex volcanoes and apply similar ideas.”

What’s more, a sunken network of sensors installed around the volcano relayed information about the eruption in real time. It was the first time scientists had gotten a good look at a deep-sea eruption in action, and now those data—reported in three papers in the journals Science and Geophysical Research Letters—are telling the story of what happened 4,600 feet beneath the sea.

An Uneasy Sleep

Roughly 300 miles off the coast of Oregon, Axial Seamount is a somewhat feisty giant. It’s the youngest of the volcanoes that straddle the Juan de Fuca ridge, where two tectonic plates go their separate ways and fresh magma can bubble up from inside the planet.

Before the 2015 eruption, Axial Seamount had last spilled lava over the seafloor in 2011—an eruption that scientists discovered completely by accident. Unlike volcanoes on land, submarine volcanoes don’t often broadcast their eruptions in the form of towering ash clouds, fountains of fire, or disrupted flights. It took a serendipitous visit from remotely operated rovers to reveal the recent lava flow.

When Axial Seamount’s 2011 eruption ended, scientists assumed the volcano would fall into a peaceful slumber, as it had before. But almost immediately, there were signs that the volcano was restless.

“Something definitely changed within the volcano’s magmatic system,” Nooner says.

Measurements from a small array of sensors indicated that the seafloor was slowly rising, suggesting that the magma chamber from which the volcano drew its fiery reserves was refilling. As that chamber filled, it jostled the seafloor and set off hundreds of earthquakes each day.

View Images A short-period seismometer on a three-legged triangular base gets lowered onto the seabed to become part of the underwater sensor network. Photograph by University of Washington

Over time, those temblors increased in frequency. They also rose and fell with the tides, spiking in number when the tide was low.

Based on how quickly the swelling magma deformed the seafloor, Nooner and Chadwick could calculate when the rising pressure might overwhelm the thin crust separating fire from water.

Using measurements gathered between 1998 and 2011, they initially estimated that Axial Seamount would break within a few years. But then, the volcano began inhaling faster and faster, and in September 2014, the pair redid their calculations and predicted an eruption sometime in 2015.

By the time April 2015 rolled around, more than 2,000 earthquakes were rumbling around the volcano each day.

On April 24, the center of the volcano’s crater dropped. A large crack had probably opened up somewhere below, offering the swelling magma a way out. A sustained tremor rumbled, and between 500 and 600 earthquakes shook the volcano in an hour.

As if on cue, the seafloor heaved, and Axial Seamount erupted early that morning. A new crack funneled lava northward, eventually sending it over the northeastern edge of the curiously rectangular caldera. Then the crack jumped west and grew to at least nine miles long, fueling thousands of small explosions along the way.

In all, more than 37,000 explosions occurred as lava met seawater, curled around it, and rapidly heated the water into steam.

Around the end of May, Axial Seamount fell silent. More than 79 billion gallons of lava had left its magma reservoir. The chamber deflated, lava stopped seeping, and within a month, only 20 earthquakes rumbled each day.

High Tech in the Deep Sea

Fortunately, scientists had installed an elaborate volcano-monitoring network on Axial Seamount just a few months earlier, making the submarine mountain one of the world’s most wired volcanoes. Now, seven seismometers ring the crater, and various hydrophones and pressure sensors monitor changes in the seafloor’s depth.

View Images A ship deploys a broadband seismometer and low-frequency hydrophone as part of the Axial Seamount observation network. Photograph by University of Washington

“The 2015 eruption at Axial Seamount is the best monitored submarine eruption so far,” writes the University of Iceland’s Freysteinn Sigmundsson in a commentary accompanying the publications.

The whole array is connected to facilities on land via a fiber-optic cable, which also delivers electricity and Internet to the bottom of the ocean. When the volcano started shuddering, scientists eagerly monitored its vital signs remotely—in real time.

“This is a great laboratory to study volcanic processes,” says David Clague of the Monterey Bay Aquarium Research Institute. “The volcano is evolving with time and continues to evolve now.”

A product of the Ocean Observatories Initiative, the network is providing crucial information about how undersea volcanoes work—a worthy task, given that roughly 80 percent of Earth’s volcanic activity is submerged.

In addition to creating islands and helping the seafloor spread, undersea volcanoes are considered some of the best environments for deep marine life to thrive, and not just on this planet, but potentially on others. For instance, tantalizing signs of hydrothermal activity have been spotted inside the watery depths of Saturn’s moon Enceladus.

The thickest flows produced by Axial Seamount are covered in a mat of hydrothermal deposits and microbes, and megaplumes produced during the eruption contained a lot of microbes that appear to have been flushed out of the subsurface.

“They basically provide a window into the subsurface biosphere,” Wilcock says.

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Rapid Response

According to the papers based on the new data, it appears as though Axial Seamount’s magma chamber filled up until the pressure became overwhelming and all hell broke loose.

“It looks like an excellent study with clear correlations between melt inflation and subsequent eruptions,” says Bob White of the University of Cambridge.

View Images Lava flows from the 2015 eruption contain microbes that appear to have been flushed out of the ocean subsurface. Photograph by University of Washington

That might seem like an obvious mechanism for producing eruptions, but such things haven’t been observed before at undersea volcanoes.

The wealth of data also revealed some surprises. For one, volcanoes don’t normally draw another fiery breath as rapidly as Axial Seamount did. Secondly, the explosive nature of the 2015 eruption hadn’t been observed before, and the volcano produced lava with a high concentration of crystals, something that differs significantly from its previous eruptions. That’s probably because it drew upon a different magma reservoir than before—a chamber in the north, rather than the south.

“Most of the geologists working on Axial expected the lava flow to go south,” says Wilcock, who has closely examined the behavior of the faults surrounding the volcanic caldera.

But at least now we know that such eruptions are not impossible to predict. Already, Axial Seamount’s magma chamber is refilling, causing the volcano’s surface to rise by about 50 centimeters a year.

“If the volcano continues at that rate, we expect an eruption sometime in about three years or so,” Nooner says. He and the others are keeping an eye on it and are ready to swoop in and do some science when the volcano blows.