WILLIAM WILCOCK had been monitoring the volcano for months as the tremors around its base grew more frequent. Such swarms of small quakes might, under normal circumstances, have stirred thoughts of alerting the authorities and perhaps warning residents nearby. But these weren’t normal circumstances. This volcano was 1.5 kilometres under the sea, and its nearest neighbours were a lot of tube worms and crabs.
Wilcock is a marine geophysicist at the University of Washington, Seattle, and part of an ambitious project to explore the 70 per cent of our planet’s surface that is invisible beneath the waves. Known as the , its aim is to wire up sections of the sea floor to an array of sensors that will continuously monitor in real time everything down there – from chemistry and currents to the ebb and flow of life. Wilcock’s volcano, Axial seamount some 500 kilometres off the US Pacific north-west coast, was laced with more cables than the average suburban neighbourhood.
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Volcanoes exemplify our ignorance of the deep ocean. We didn’t know underwater volcanoes encircled the planet until the 1950s, when cartographer Marie Tharp mapped the mid-Atlantic ridge. Using echo-sounding data from research cruises, she showed that this long rise in the Atlantic sea floor – extending, she would later find, almost from pole to pole – wasn’t one ridge, but two bisected by a valley (see “Land under”).
The true significance only became clear when Tharp and her colleague Bruce Heezen completed a map of the southern Atlantic. The ridges were fault lines where new ocean crust was emerging from the mantle and spreading, forcing the continents apart and powering volcanism. The discovery helped cement the still disputed theory of plate tectonics.
We now know there are a lot of volcanoes down there. “About 70 per cent of the volcanism on the planet occurs underwater,” says oceanographer , also at the University of Washington. We recently discovered one of the largest known volcanoes in the solar system hidden beneath the waves. At 300,000 square kilometres, Tamu Massif in the north Pacific has a footprint matching the gigantic Martian volcano Olympus Mons. Active areas peppered with volcanoes and hydrothermal vents continuously spew hot, nutrient-rich water into the ocean, altering its chemistry and currents and providing warmth and nourishment for microbes, large eyeless shrimp and bizarre tube worms on the otherwise cold and barren deep ocean floor.
Yet we know little about what makes undersea volcanoes tick. Only twice have autonomous submersibles caught fleeting glimpses of eruptions. Last year, of Columbia University in New York studied 25 years’ worth of recordings from deep sea hydrophones that were in effect listening in on the volcanoes. She found that volcanic activity peaked cyclically at times of the month and year when the combined pull of the sun and the moon was at its weakest. A longer cycling pattern was also evident in scars in the sea floor caused by ancient eruptions (). Tolstoy’s theory is that changes in pressure on the sea floor could be triggering eruptions.
“Hydrophones heard the lava exploding on contact with the seawater“
The frequency and magnitude of undersea eruptions matter because they produce greenhouse gases including carbon dioxide – and the more gas is pumped into the ocean from below, the less it can absorb from above, limiting the ocean’s dampening effect on global warming. “People have ignored sea floor volcanoes based on the idea that their influence is small,” says Tolstoy. “But that’s because they are assumed to be in a steady state, which they’re not.”
The aim with Axial seamount was finally to get solid data – provided the sensors were ready in time. The volcano’s increasingly frequent tremors, plus deformation measurements indicating a bulging magma chamber beneath, prompted the team to convene a meeting to discuss what they would do if it were to blow soon. At the time, most of the quakes were less than magnitude 1.5, barely noticeable on land. “I thought it might be a while before it erupted,” says Wilcock. “That proved to be wrong.”
Just two days after the meeting, on 24 April last year, the volcano experienced a “seismic crisis” associated with an eruption. Wilcock couldn’t properly measure how many earthquakes were happening, as they were probably overlapping each other. “You’re just counting the big ones,” he says. By his estimate there were 600 an hour. “In geologic time that’s a lot of activity happening very, very rapidly,” says Kelly. Hydrophones also caught a popping interpreted as steam or gas explosions triggered by hot lava interacting with the cold water. There were also, curiously, sounds possibly associated with ash-producing explosions familiar from volcanoes on land. “There were thousands of explosions recorded during the eruption of the lava flows and we are debating the source of those sounds,” says of Oregon State University, who is part of the team.
Pressure gauges showed that the sea floor dropped by 2.4 metres over 10 hours as lava streamed out. The lack of warning meant that although the seismometers and hydrophones were feeding in readings in real time, the rest of the sensors weren’t, so the observatory team faced an agonising wait to see if the instruments – and the precious data – had survived. When they arrived above the volcano that summer armed with a remotely operated submersible, they found the lava stream had flowed north and missed most of the instruments. The lava layer was over 100 metres thick, and hydrothermal fissures were producing a blizzard of stringy white bacterial “snowblower” material. In other areas, thick orange microbial mats covered the otherwise glassy volcanic basalts.
Many questions remain to be resolved about how these habitats are created and life comes to thrive there. Since last month, all the instruments on Axial seamount have been streaming data in real time, including a video feed of tube worms and crabs around a hydrothermal vent to the south of the site. For now the researchers are chewing over what they’ve learned from last year’s eruption and refining their models, but next time this volcano blows, they’ll be ready.
Read more in our ocean special: “High tech goes deep: A new age of ocean exploration”
This article appeared in print under the headline “Thar she blows!”
