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All the Volcanoes We Can’t See

by Erik Olsen  |  March 12, 2018

“This whole story starts with plate tectonics,” says Cornel de Ronde, a co-Principal Investigator of the Brothers volcano expedition. Dr. de Ronde is referring to an idea that has really been understood only for the last five decades or so, that the Earth’s crust is made of numerous moving plates of rigid rock floating on top of plastic-like magma.

Plate tectonics is the reason we are here in New Zealand, exploring the southern section of the Kermadec-Tonga arc, a 2,500-kilometer (1,550-mile) chain of at least 40 undersea volcanoes that host some of the most active hydrothermal systems on the planet.

Most people know about the volcanoes of Hawaii and Washington state, home of Mt. St. Helens. But those are just the volcanoes we can see. About 85 percent of the world’s volcanic activity that we know of occurs on the seafloor, far beyond our sight. Using deep submergence vehicles like Jason, we can study these volcanoes in much greater detail than before, and it’s opening our eyes to wonders we’ve never seen before.

Dr. de Ronde has been coming to Brothers volcano for two decades, and, “Each time we come we find something new,” he says.

Before coming here, most of the hydrothermal vents that were studied were found in regions where the Earth’s tectonic plates are spreading apart, like the East Pacific Rise or the Mid-Atlantic Ridge. What’s different about the Kermadec-Tonga arc, however, is that these volcanoes are part of a subduction zone, where plates are colliding and one is being drawn down beneath the other. At some point the subducting plate begins to melt and the magma produced makes its way up through cracks and fissures to form volcanoes on the seafloor, like Brothers.

seafloor separate


Plate Movements


Volcanoes found along subduction zones, like the ones we are studying also have very interesting and very diverse types of hydrothermal vents. At some, almost always near the summit, seawater travels through cracks in the seafloor and becomes heated by the underlying magma chamber or the hot rocks in the seafloor. This hot water is then able to dissolve metals present in the volcanic rock, such as copper and gold. Eventually, this mineral-rich fluid rises back through the crust and is discharged on to the seafloor. When the fluid comes in contact with the cold seawater, it can no longer hold those metals in solution and they precipitate as metal sulfides, making the fluid look like dense, black smoke. The metals in that “smoke” form towering chimneys, some of them 20 meters (66 feet) high.

Other, less showy forms of hydrothermal vents that we’ll visit, are called seeps because the fluid flows from the seafloor more slowly. The seeps at Brothers are surprisingly acidic—much closer to battery acid than seawater—and are very gassy, possibly due to interactions with the magma deep below the seafloor.

Scientists know the chimneys and seeps are down there on Brothers, but they don’t know what is going on beneath the seafloor to form them. That’s why a bigger team is coming back in May of this year to drill into part of the hydrothermal system—to understand the distribution of metals and minerals inside part of the volcano.

To Dr. de Ronde, it’s all very exciting. He’s one of the world’s authorities on the undersea geology of this region, having been here fourteen times over the last 20 years to conduct mapping, sampling and numerous other studies all up and down the arc. He’s trying to better understand how seafloor hydrothermal systems are connected to the way submarine volcanoes work.

And it’s not just about metals. What happens here is of immense importance for understanding the way our planet’s crust was formed and is eventually destroyed, the chemistry of the ocean and its impact on global warming, and, quite possibly, the origin of life.

“So you have it all here,” says Dr. de Ronde. “A great natural laboratory in which to come and study processes that occur along a plate boundary and the effects of that subduction on our planet.”