The hidden power of submarine volcanoes

An ash plume that surpassed the stratosphere. An explosion you could hear from thousands of miles away. Massive waves reaching all the way to Peru. These were just a few of the freak events that followed the massive Hunga Tonga-Hunga Ha’apai volcanic eruption on January 15.

Preliminary estimates suggest that Hunga Tonga’s blast was the most explosive volcanic eruption in over 30 years, says David Clague, a volcanologist who recently retired from the Monterey Bay Aquarium Research Institute after several decades of research. The most recent eruption to surpass Hunga Tonga’s explosivity came from the Philippines’ Mount Pinatubo in 1991, Clague says. (More recent estimates suggest Hunga Tonga may have been even more powerful than Pinatubo.)

But, unlike Mount Pinatubo, Hunga Tonga is a submarine volcano. Its central crater lies about 500 feet below the surface of the Pacific Ocean — around 2,000 miles east of Australia.

Submarine volcanoes are common, but underwater eruptions as powerful as Hunga Tonga’s are incredibly rare. According to Clague, the world likely just witnessed “the largest submarine eruption since Krakatau” — the famously destructive eruption in the Indonesian archipelago’s Sunda Strait in 1883.

Hunga Tonga has given us all a grim reminder that underwater volcanoes can be just as powerful as the above-ground variety that typically dominates our attention. So, let’s consider the causes and impacts of these intense oceanic eruptions.

Submarine volcanoes can be formed in a few different ways, says Philipp Brandl, a volcanologist based out of Germany’s GEOMAR Helmholtz Centre for Ocean Research Kiel. Like most land volcanoes, Hunga Tonga was formed from interactions between tectonic plates, Brandl says. As one plate in the Pacific Ocean slid under another — in a process known as “subduction” — water was added to Earth’s hot upper mantle, and pockets of magma formed as a result. With enough pressure, some of that magma burst to the surface. This is how the Hunga Tonga volcano was created, Brandl explains.

When submarine volcanoes erupt, they tend to release the same substances as land volcanoes: lava, ash and a variety of gases. But the way these eruptions take place can be very different. For one thing, you’d probably expect the water covering a submarine volcano to suppress the intensity of its eruption. This is often the case, says Brandl: “If you’re talking about eruptions in deep water — let’s say more than 500 meters of water depth — then the water pressure, in most cases, prevents really major explosive eruptions.”

But the reverse is true for submarine volcanoes in shallow water, like Hunga Tonga. Volcanoes like these have only a thin layer of water on top of them, and this water can actually make eruptions even more explosive.

As super-hot lava hits ocean water, Clague explains, it can turn that water into steam almost immediately. This leads to near-instantaneous expansion: Steam takes up over 1,000 times the space of liquid water. When seawater expands into steam, it breaks the erupting lava into smaller chunks, Clague says. These chunks then hit other areas of liquid water, turning them into steam, and so on. “These types of reactions are a major concern in the design and operation of nuclear reactors,” Clague points out. Likewise, this runaway effect is one of the central reasons that the Hunga Tonga eruption was so intense.

As Hunga Tonga erupted, it also released a massive cloud of ash, reaching about 300 miles in diameter at its maximum extent. According to Clague, ash plumes like these can spell disaster for sea life. As ash settles back down following a submarine eruption, much of it sinks to the bottom of the ocean, coating the seafloor. “Animals that are attached to the bottom [of the sea] are probably most at risk of getting buried and being unable to move through the sediment to get back on the surface,” Clague says. Thus, sponges and corals living close to the volcano — where the ash density is greatest — will almost certainly die after a major eruption. More mobile creatures can burrow or swim through the fallen ash, but may still find themselves in trouble if their food sources are now buried, Clague notes.

Ash is certainly an issue for human life as well. As photographs from Tonga show, many of the Polynesian nation’s islands were coated in ash, rendering Tonga’s airport unusable for several days and contaminating the drinking water for thousands of inhabitants.

But, in terms of human wellbeing, Hunga Tonga’s ash was not nearly as damaging as the tsunamis it caused. Tonga’s government confirmed that three people died as a result of the tsunamis and dozens of houses were destroyed across the archipelago. These tsunamis were so intense that they caused massive waves to cross the entire Pacific Ocean, reaching the coast of Peru over 6,000 miles away. The waves even triggered an oil spill and drowned two people at a beach.

Tsunamis aren’t common following submarine eruptions, but they’re not unheard of, especially if the eruption is intense enough, says volcanologist Shane Cronin with the University of Auckland in New Zealand. Tsunamis are more likely when underwater volcanoes explode out of their side rather than out of their top in what’s known as a “flank collapse,” Cronin says. Scientists are still researching if this is what happened with Hunga Tonga — but the massive, vertical ash plume suggests that a vertical eruption is more likely, Cronin notes.

Now, you may have also heard that underwater eruptions can help form new islands. That’s true — but only sometimes. Smaller-scale eruptions of Hunga Tonga in 2014 and 2015 released enough magma to join together two islands on the rim of the volcano, Cronin says. But the major eruption on January 15 was more destructive than it was creative. It blasted away the land connecting those two Tongan islands, separating them once again, and even stripping away parts of the older islands in the process.

Hunga Tonga probably isn’t done erupting, Cronin adds; the volcano has been active for years and is likely to remain that way for some time. “It’s highly likely that continued eruptions will happen,” says Cronin. “But the conditions that generated this very special, large explosion — I would be surprised if we could repeat those.”