Why Are So Many Volcanoes Underwater?

Volcanoes are well known for being, you know, fiery and hot. And yet, they can be found dotting the floor of the famously frigid deep ocean – a place where their existence seems to defy intuition. Why do they appear there? How do they survive? And what happens when they erupt?

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Turns out, some of those questions are easier to answer than others.

Why are most volcanoes underwater?

Think of all the volcanoes on Earth. There are, what? A thousand? One thousand five hundred? That’s about right. Quite a lot, is the point. Now multiply that number by four. That’s how many more volcanoes are thought to be under the oceans – but why?

Your first thought might be that it’s not all that surprising. After all, more than 70 percent of the Earth is covered in water – statistically, then, it makes sense that a similarly high proportion of volcanoes would be found in these same areas. It’s a good argument – but it’s wrong.

The problem is that volcanoes aren’t spread out evenly across the planet. In fact, they usually turn up in very predictable places: along tectonic fault lines. Three out of every four of them can be found along the Pacific Ring of Fire, for example, with 10 percent of the world’s volcanic activity being found in Japan alone.

The question, therefore, is less “why are so many volcanoes underwater”, and more “why are so many tectonic plate edges underwater” – and that question has a few answers. Firstly, subduction – the process in which one tectonic plate slides underneath another as they collide, and the movement responsible for much of the volcanism in the Pacific Ring of Fire – requires the presence of water to soften up the mantle enough to accept the new addition. That doesn’t account for every tectonic rift, but it does basically rule out the possibility of a lot of them occurring in the middle of a continent.

Secondly, and probably more importantly, we have the Wilson cycle: an admittedly idealized, but nevertheless illustrative explanation of how supercontinents are created and broken up by tectonic activity. Essentially, it says that as two plates move apart, they necessarily create a big basin that becomes a sea – even if they were originally connected in the middle of a big body of land. 

Basically, then, most tectonic plates meet under the water because, well, it’s very hard not to break apart two massive bits of continent and not end up with an ocean in between. And where there’s tectonic plates moving about, usually, you get volcanoes – even if they happen to be thousands of kilometers under the sea.

What happens when an underwater volcano erupts?

Like the mighty blobfish, a volcano can look very different at the ocean floor compared to on land. Those at the surface are, at least stereotypically, like a big, fiery, explosive mountain – see Mounts Etna or Rainier – or else the same but less steep, like those in Hawai'i or Iceland.

Of course, that’s not every volcano at the surface – there are plenty of weird flat ones out there, or even ones that have gone inverted and are covered in lakes. But it’s that explosive, red-hot lava aspect which seems to define them – that, we can’t get away from.

But how can that hold up underwater? Not just underwater, in fact, but at the very bottom of the ocean, where temperatures often top out at about 4°C (39°F)? What, you might ask, would an underwater volcano eruption actually look like?

Well, it’s a good question. “For the most part, we don't know” how underwater volcanoes work, admits the Smithsonian Institute’s Ocean center. “Scientists are in the dark when it comes to understanding underwater volcanoes because the eruptions are cloaked from view by thousands of feet of water.”

Of course, we have some clues. When the West Mata Volcano, whose base is around 3 kilometers (9,842 feet) underneath the Pacific Ocean near Fiji, erupts, we get to witness “a bright flash of hot magma that is blown up into the water before settling back to the seafloor,” the Institute says. “The explosion throws ash and rock into the water, and molten lava glows below.”

Many underwater volcanoes aren’t so dramatic. We sometimes see bubbles or mini-“eruptions” at the ocean’s surface when one blows – “think of the bubbles in a coke bottle spurting out when a shaken bottle is opened and the pressure is released all at once,” the Smithsonian explains – but “underwater the magma still faces the crushing pressure of tons and tons of ocean water once it reaches the seafloor.”

That means that the lava solidifies into very different shapes than the ones we see on land. With so much water pushing and cooling it down, it simply can’t explode out everywhere like it does in the air, and instead flash-cools into volcanic glass or bulbous pillow lava. Sometimes, even that is impossible: below depths of around 2,200 meters (7,200 feet), the pressure is too great for water to boil; instead, “when water hits hot magma at 800 degrees Celsius [1,472 degrees Fahrenheit] it vaporizes in an instant,” the Smithsonian explains. 

“Its rapid expansion into steam can be strong enough to break the lava apart,” it says. “On the flip side, when magma comes in contact with water the temperature change is so dramatic that the magma instantly solidifies in a process called quenching.”

An explosion of life

Like their on-land counterparts, underwater volcanoes can be devastating when they erupt – and bounteous once the dust (or magma) settles.

“Most of the seafloor is relatively plain,” Deborah Kelley, a professor in the University of Washington School of Oceanography and director of the Regional Cabled Array, explained earlier this year. “But when you get to the vent fields, you realize that the volcano is an oasis of life.”

By why risk your neck by settling down next to a potential explosion site? Well, if you’re a deep-sea critter, the area around a volcano is like an all-you-can-eat buffet located in a spa: the cracks in the seafloor caused by volcanic activity create hydrothermal vents; they release warm, mineral-rich water full of chemicals that feed a host of bacteria, and they in turn attract species from increasing links along the food chain.

“What we’ve found is that, even though the chemistry is similar at the different vents, the microbial life can be very different,” said Kelley. “They’re like little islands with distinctive communities.”

This is more than just a niche little set of data points for geologists and oceanographers to study. Underwater volcanoes are so good at stirring up biodiversity like this that some experts think we might have them to thank for all life on Earth. 

“Underwater volcanoes during Snowball Earth played a crucial role” in the transformation of our world from frozen ice-planet to the flourishing Blue Marble we know it as today, said Tom Gernon, Associate Professor in Earth Science at the University of Southampton, in 2016. 

“As [lava] deposits piled up on the sea floor, chemical reactions between the rock and the largely ice-covered ocean led to massive amounts of calcium, magnesium, silica and, perhaps most significantly, phosphorus being released,” Gernon explained. “Our calculations show that this chemical build-up is sufficient to explain the thick cap carbonate deposits.”

“The results help explain how our planet got oxygen in its atmosphere and oceans – enabling life to evolve from single-celled organisms into animals,” Gernon said. “The result as Snowball Earth ended was one of the most extraordinary icehouse to greenhouse transitions the planet has ever experienced.”

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