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 A mysterious undersea volcano snapped by a submersible

Copyright ROV Team GEOMAR

Surprise undersea volcano could offer unique window into Earth’s interior

In 2015, a German research team sent a submersible to the bottom of the Pacific Ocean. West of Peru, the camera-mounted robot explored a vast expanse of sea floor, 4 kilometers (more than 2 miles) deep, known for its extreme flatness. “It’s very dark,” recalls Antje Boetius, a biologist at the Alfred Wegener Institute. “Then you switch on the lights of the robot and see a new landscape that no one has ever seen before.”

One feature in particular took Boetius by surprise: a small mountain rising 300 meters from the seabed, its steep slopes covered in lava shaped like pillows, and anomalous dark veins that are likely to be magma deposits. The team had discovered an undersea volcano, or seamount, on an abyssal plain—a geologically inactive region that shouldn’t have any.

In a new study, the scientists suggest the seamount could represent a completely new type of seafloor volcanism, fueled by a hidden, shallow reservoir of magma. Evidence of isolated volcanism such as this could be a unique window into Earth’s interior, says Adam Soule, an oceanographer at the University of Rhode Island, Narragansett Bay, and director of the Ocean Exploration Cooperative Institute, who was not involved with the study.

Most of Earth’s 1500 active volcanoes are found underwater. The vast majority of these form at the boundaries of tectonic plates: either seafloor spreading centers, where freshly erupted lava drives plates apart, or at subduction zones, where plates dive beneath one another, leading to melting of the mantle beneath that can fuel eruptions.

Seafloor volcanoes can also occur above mantle hot spots, fixed plumes of magma rising from deep within Earth. As our planet’s plates drift over them, the plumes lead to chains of volcanoes, like the Hawaiian islands.

However, none of these existing theories explains the new seamount, which is nowhere near any known mantle plumes and lies hundreds of kilometers from the boundaries of the Nazca plate on which it sits, says Colin Devey, a volcanologist at the Geomar Helmholtz Centre for Ocean Research Kiel.

After using the submersible to explore the volcano for more than 4 hours, Devey, Boetius, and colleagues mapped the site by sending sonar beams from their ship and listening for the echoes. The team found a lack of ocean sediment covering the lava, as well as a curious lack of animals such as starfish, which usually colonize the hard surfaces of seamounts quickly, they report in Marine Geology. The scientists also observed that some of the lava had broken through the slow-growing mineral crust that covered the seamount. Both observations suggest the volcano erupted recently, perhaps sometime in the past few thousand years, destroying the biodiversity that had been thriving there.

Still, Kenneth Rubin, a volcanologist at the University of Hawaii, Manoa, who was not involved in the work, says it’s “premature” to declare this a young volcano. He says the lack of sediment could be explained by ocean-bottom currents scouring it away. Another submersible should be sent down, he says, to observe the site again and collect specimens to date them.

Devey’s team also investigated what might possibly be fueling a volcano in the middle of nowhere. By using data from previous studies that used seismic techniques to image Earth’s upper mantle, the researchers found an unusually high concentration of magma at about 70 kilometers below this part of the Nazca plate. Seafloor spreading centers often contain magma reservoirs at such relatively shallow depths, but Devey says it was surprising to find magma at this depth under an abyssal plain.

The chemistry of the magma here could be different, Devey speculates, allowing it to melt at a lower temperature. Alternatively, he suggests current theories may be wrong about how permeable abyssal plain plates are to even small amounts of magma, and that these expanses could be dotted with many more young volcanoes.

If this is indeed volcanism, the traditional theories are unlikely to apply and it must, in fact, have been formed by one of these lesser known mechanisms, Rubin says.

Even if the seamount’s origin remains something of a mystery, the discovery shows how the oceans’ abyssal plains, largely unmapped, are fertile places for discovery, Soule adds. “I’m not really surprised that discoveries are made in this part of the ocean, just because we haven’t looked at it all that much.”

Devey intends on doing more dives with submersibles within the next few years to create a detailed map of the region and to collect samples. If he’s lucky, he says, he might even discover another new seamount. “It’s like finding a needle in a haystack on the first attempt,” he says. “But I wonder how many needles are actually down there.”