Scientists Are Racing to Understand the Fury of Tonga’s Volcano

On December 20, Hunga Tonga-Hunga Ha'apai—an underwater volcano in the South Pacific topped with a diminutive and uninhabited island—awoke from a seven-year slumber. The volcano spluttered and crackled, creating a large plume of ash. Ten thousand miles away, in England, Simon Proud, a satellite data researcher at the University of Oxford, began to monitor the twitching volcano using an array of satellites.

As 2021 ticked into 2022, what had appeared to be the beginnings of an almighty eruption seemingly calmed down. Then, early in the morning on January 14 local Tongan time, a 12-mile-high plume of ash pierced the sky. The volcano became increasingly turbulent, and hundreds of lightning discharges shot out of the maelstrom every second, bombarding the land and ocean. And one day later, in the late afternoon of January 15, satellites captured a cataclysm in action.

Back in England, when Proud woke up that day and checked his computer, he saw a tower of ash unlike anything he, or anyone else, had ever seen. Satellites had captured images of a huge column of ash that billowed out 22 miles above the island into a shadowy, tempestuous canopy 160 miles long. Rising from the canopy’s heart was a thin, transient spike of volcanic debris reaching an altitude of 34 miles—about five times the height of a cruising passenger jet. “What the heck is this?” Proud recalls thinking. “I looked at the data, and I thought, this is so far outside anything I’ve seen before. It’s just unreal.”

Jaws dropped across the world. The explosion that produced the ash cloud, one estimated to be equivalent to 10 million tons of TNT, unleashed 25,000 times more energy than the lethal blast in the Lebanese capital Beirut in August 2020. The Tonga eruption is easily one of the largest explosions this century. And it didn’t stop there.

“Then there was the shockwave,” says Mike Cassidy, a volcanologist at the University of Oxford. It emanated from the volcanic blast at 600 mph and caused pressure spikes on the other side of the planet. “No one’s ever seen that before.” Within 20 minutes of the explosion, four-foot tsunami waves cascaded over Tongatapu, the archipelagic Kingdom of Tonga’s main island. By the time minor tsunami waves hit Japan and the western shorelines of the Americas, ash had already smothered multiple Tongan islands, killing off agriculture, polluting water supplies, disrupting electrical infrastructure, and cutting off roads and runways. The submarine communication cable connecting the archipelago to the rest of the world was damaged, severing the nation’s international phone and internet services. It likely won’t be repaired for several weeks.

Volcanologists couldn’t believe what they were witnessing. No matter which metric you picked, this was an astonishing, terrible eruption. And as suddenly as the volcanic violence dwindled, a global detective story began. What series of geologic events created such a devastating eruption? And what research needs to be done to crack the case?

The general mechanisms of volcanic eruptions are broadly known. But the catastrophic explosion on January 15 needs a more thorough examination and, ultimately, a novel explanation. When Hunga Tonga-Hunga Ha'apai erupted, Shane Cronin, a volcanologist at the University of Auckland in New Zealand, had the same reaction as everyone else, volcanologist or not: holy shit.

“But there was actually a ‘holy shit’ moment on December 30,” he says. On that day, a decently tall plume emerged from the volcano. “That put me on notice because it was very violent.” Then came another skyscraping plume just prior to the main event. Both featured relatively little volcanic material, but did contain a lot of gas. And gloopy magma filled with gas is bad news. Very much like an agitated fizzy drink trapped in a bottle, if you suddenly remove the cap, that gas expands and blasts the drink out of the top with remarkable momentum. In other words, these two volcanic belches indicated that the magma reservoir had a lot of trapped gas, portending the epic blast yet to come. “In 20/20 hindsight, those were a big warning to us,” says Cronin.

The two once-conjoined but now segregated islands of Hunga Tonga and Hunga Ha'apai are the small surface expression of a far larger, 12-mile-long cauldron-shaped volcano (known as a caldera) beneath the waves. And it’s long been known that that titan contains a lot of gassy magma. Cronin is the coauthor of a recent study that looked into the caldera’s volcanic past. It found that its magma reservoir takes many centuries to refill, and last weekend’s major paroxysm takes place roughly once a millennium, the result of the violent and sudden emptying of most of that cache of molten rock.

Rare though it may have been, this explosion still came from a volcanic bomb, and bombs need triggers—but what kind? Cronin and his colleagues have an idea: Over time, fluids dissolved in the magma, like water, began to bubble out as gases, upping the pressure on the rocky cap above. The volcano inflated, causing cracks to appear in its cap. Eventually, the seawater above infiltrated those fissures and encountered the magma. That’s when all hell broke loose.

This water, rapidly heated, was vaporized into a gas. If this happened miles below the sea surface, the intense weight of the ocean would suppress the expansion of the gas into the surrounding magma. But being just a few hundred feet below the waves, the water blasted the magma out of the way like a superpowered pneumatic pump, fragmenting the molten rock into millions of pieces. “And boom,” says Cronin. “Away we go.”

That first explosion clears the way for more magma to meet seawater, which creates more explosions that lets even more magma meet seawater, all while the vast reservoir of molten rock dramatically depressurizes and rushes into the sea. “That is going to result in a very violent chain reaction,” says Sam Mitchell, a volcanologist at the University of Bristol. “There is water to spare, heat to spare, and magma to spare.” And in a heartbeat you create a 10-megaton explosion.

That’s the hypothesis, anyway. To confirm this requires chemistry. If scientists can collect ash that was produced both prior to and during the paroxysm, the different chemical and textural features of both sets of particles will reveal the explosion’s trigger. If the ash is extremely fine, plentiful, and exhibits tiny fractures, for example, it almost certainly came from magma angrily interacting with seawater.

Chemistry will also reveal what turned the magma into a pressurized bomb in the first place. The prevalence of a certain type of microscopic volcanic crystals would reveal the magma sat just below the surface for many years, slowly degassing and pressurizing. But the presence of a specific coating on these crystals would indicate that a recent injection of magma came in from below, adding a critical amount of heat, gas, and pressure to the reservoir. The blast’s ginormous ash plume, too, will provide scientists with vital clues. But it took a few days to properly calculate its dimensions.

Plumes happily rise into the troposphere, the lowest layer of the atmosphere and the bit that contains most of the world’s weather. Temperature drops with altitude, so based on how cold the plume is, you can roughly measure how high it goes, says Proud. “In this case, when it’s blasted all the way through into the stratosphere, things get a little bit more challenging,” he adds. The stratosphere warms with altitude, so using temperature in this rarified air produces erroneous plume heights.

Instead, Proud and his colleagues used multiple satellites to visually calculate its height. And after marking the plume’s canopy at 22 miles, with a central spike at 34 miles—a spike reaching an even higher atmospheric layer, the mesosphere—Proud only had one way to describe it: “It’s totally nuts,” he says.

The staggeringly high-altitude plume gives an indication of how explosive the eruption was, says Cassidy, which in turn will help explain the mechanisms that led to such a major blast. “It must have been a really explosive event,” says Proud. The erupted ash, he adds, must have been going up close to the speed of sound in order to get that high. But working out what caused the blast is just one half of the puzzle. The other is the tsunami’s trigger, and although it is tempting to simply blame the blast, its origin story is not quite as clear cut.

Submarine volcanoes that quickly build unstable islands above water through eruptions are prone to generating dangerous tsunamis. Prior to this month’s disaster, the most recent lethal volcanic tsunami was the 2018 eruption of Indonesia’s Anak Krakatau, which killed hundreds of people. And whether a tsunami is caused by a meteor impact, an earthquake, or a volcano, the number one rule remains unchanged: You need to move a large mass of something into water. But there are various ways a volcano can achieve this: an underwater explosion, the collapse of the volcano’s flank (as happened with Anak Krakatau), the collapse of the entire volcano, or vast amounts of volcanic debris from the eruption plume tumbling into the sea.

Shockwaves, too, can generate tsunamis. Not long after the January 15 blast, tsunami waves weren’t just detected around the shorelines of the Pacific, but elsewhere in the world, including the Caribbean Sea. Such waves couldn’t have been caused by the movement of the volcano’s rock, as continental barriers would have blocked them. Instead, it appears that the shockwave—which, at the time of writing, has travelled around the planet three times—didn’t just stay airborne. It interacted with distant seas, causing them to bob up and down, triggering small tsunamis thousands of miles from the explosion’s source.

This is a phenomenon known as a meteotsunami. Although previously detected underneath potent storm systems, this may be the first time a volcano has been detected causing one in a different ocean basin altogether. But although it may have played a small role, scientists are not currently eyeing the shockwave, but the redecoration of the volcano itself, as the prime suspect behind the severe Tongan tsunami.

But how, precisely, was the tsunami caused? If it was a flank collapse, the rocky debris underwater would fan out in a single direction away from the now-felled sector of the volcano. If the entire volcano collapsed in on itself after its magmatic foundations were rapidly evacuated from its vent, then you might expect a ring of debris radiating out around its perimeter, with perhaps more wreckage in one direction if the collapse was asymmetrical. And an underwater explosion, depending on whether it was directed or more widespread across the volcano, could be represented by either of these two debris patterns.

The only way to find out, says Mitchell, is to look. Blasting acoustic waves from boats down to the volcano, perhaps by using small explosives or pneumatic air guns, and subsequently receiving their reflections, can tell scientists about the dimensions and properties of the rocks below. This allows them to make a map of the volcano post-eruption, and comparing it to pre-eruption maps can reveal how the volcano has changed shape or if it’s blown a new hole in its side. Robotic diving vehicles, those controlled remotely by a pilot or fully autonomous submersibles requiring no human input, could also be deployed to scour the seafloor.

In addition to this submarine sleuthing, the buoys and coastal gauges that measured the tsunami wave heights and arrival times across the Pacific Ocean will be crucial. After this data is collected, it can be plugged into computer models to try and recreate the tsunami. If a simulated tsunami is found to match with the pattern of underwater debris, then researchers can confidently reconstruct the volcanic event that caused the real deal.

Preliminary satellite data gives some early hints. “I don’t think a huge caldera drop, maybe, is the answer,” says Cronin. The two islands of Hunga Tonga and Hunga Ha'apai don’t appear to have sunken much post-eruption, suggesting the volcano didn’t fully collapse. There is also plenty of volcanic debris on the seafloor created by similarly explosive but far more ancient events to last weekend’s blast. That implies that, even though there was a massive explosion above water, a significant amount of that blast may have transpired underwater. If so, it could have propelled vast amounts of volcanic debris—comparable to the volume jettisoned skyward—into the sea, triggering a tsunami. But until this fieldwork is conducted, a conclusion is beyond reach. For the time being, “there’s a lot of scratching of heads,” says Cronin.

From the blast to the tsunami, the science behind this singular eruption is rife with unanswered questions. In fact, right now, there are only two certainties: The first is that this has been a tragedy for Tonga, but future lives will be saved if this eruption’s deadly features can be decoded; the second is that Tonga, a small nation now scarred by this eruption, cannot achieve this scientific goal alone.

The kingdom’s own volcanologists, including those at the Tongan Geological Services, have been monitoring nearby volcanoes, which they know better than anyone. But the agency has very little funding, says Mitchell. “They can’t go out and do huge bathymetric surveys and deploy ocean-bottom seismometers,” he adds. Scientists from around the world, then, must come together to crack the case of Hunga Tonga-Hunga Ha'apai. “If we help, it needs to be in tandem with them, not in lieu of them.” And what helps protect Tonga also protects millions of others around the world. Earth is dotted with similarly gigantic volcanos that will, one day, unleash similarly devastating eruptions. And, when that happens, knowledge gained from the Tonga eruption could prove crucial in giving early warning when another planet-shaking blast is about to occur.