Researchers are beginning to understand why an underwater volcano in Tonga erupted so explosively – and what happened next. Evidence collected by two groups suggests that when the center of the volcano collapsed, it spewed out a huge amount of magma which reacted violently with the water, fueling several large explosions and hundreds of much smaller explosions.
The Hunga Tonga–Hunga Haʻapai volcano erupted on January 15, 2022, producing the largest atmospheric explosion in recorded history. It sent shock waves around the world and a plume of ash into the upper atmosphere.
In May, Shane Cronin, a volcanologist at the University of Auckland, New Zealand, led a group that navigated above the volcano’s caldera, the central depression that forms when a volcano erupts, and used sonar to map its structure. They found that the four kilometer wide caldera had dropped from less than 200 meters below sea level to over 850 meters.
“The volcano produced this huge new caldera,” Cronin explains. He estimates that some 6.5 cubic kilometers of rock were thrown, roughly equivalent to a sphere as wide as the Golden Gate Bridge in San Francisco, California. “It was an incredible find,” says Taaniela Kula, Tonga’s deputy secretary for lands and natural resources in Nuku’alofa and research collaborator. “It creates a better picture of the mechanism of the volcano.” The work was presented at a meeting of the European Geosciences Union (EGU) in Vienna on May 26.
The reason for the large explosion was likely the interaction between large amounts of magma and water at the start of the eruption, Cronin says. “You have water at 20 degrees and you have magma at 1,110 degrees coming into direct contact,” he says. Such a large temperature difference meant that when the water was forced into contact with the magma by the eruption, it exploded. Each interaction pushed water deeper into the edges of the magma, Cronin explains, increasing the surface area of contact and causing further explosions in a chain reaction.
The initial caldera depth was also just shallow enough for water pressure not to suppress the explosion, but deep enough for magma to be supplied with huge amounts of water to power the interactions, resulting in several large explosions and hundreds of much smaller explosions every minute. Eyewitnesses from the day of the eruption reported “crackles and noise like artillery fire” up to 90 kilometers from the eruption, Cronin said. “These are not sounds I’ve heard before from erupting volcanoes,” he says.
Ash grains recovered from Tonga after the eruption also suggest that there was a violent interaction between magma and water. When the seawater came into contact with the magma, it produced shock waves powerful enough to fracture the grains, said Joali Paredes-Mariño, a geological engineer at the University of Auckland, in a work presented at the EGU.
A separate expedition by a team from New Zealand’s National Institute for Water and Atmospheric Research (NIWA) in Auckland visited the volcano in April, but did not cross the caldera. They sampled ash from the seabed around the volcano, which showed the eruption was likely followed by dramatic pyroclastic flows, hot streams of ash and lava that rained down the submerged sides of the caldera. The rushing hot ash turned the surrounding seabed into a white desert that “wiped out everything,” says trip leader Kevin Mackay, a marine geologist at NIWA.
These flows spread underwater for thousands of square kilometers from the eruption, ripping seabed cables – including those providing Tonga’s access to the internet, which have yet to be fully restored. – and fueling the tsunamis that swept over nearby islands, reaching up to 18 meters in height. At the bottom of the sea, nothing seems to have survived, although samples are still being analyzed to determine the extent of the damage. “We don’t even think bacteria live there,” Mackay says. “That’s how toxic we think the sediment is.”
Samples collected by the NIWA team are used to study potential impacts on oxygen levels and ocean acidification, says NIWA biogeochemist Sarah Seabrook.
However, not everything was decimated. Satellite data showed a large bloom of phytoplankton in the ocean after the eruption, which fed on nutrients released by the blast, Seabrook says. And on nearby hills that rose above the seabed just 15 kilometers from the eruption, life was thriving, Mackay says. “We expected life to be universally destroyed.”
water vapor plume
Other research presented to the EGU by Philippe Heinrich at the Alternative Energies and Atomic Energy Commission near Paris showed that the pressure wave from the eruption produced a tsunami all the way to the French Mediterranean coast, at 17,000 kilometers, with several centimeters of altitude. recorded increase. Luis Millán of NASA’s Jet Propulsion Laboratory in Pasadena, California, also discovered that the eruption sent a plume of water vapor that reached a height of 53 kilometers, well into the stratosphere. This plume, which has now circled the globe, has increased the water vapor content of the stratosphere by 146 teragrams (146 trillion grams), or 10%, and is likely to remain in the atmosphere for at least a year. “We’ve never seen anything like this before in the entire satellite era,” says Millán.
Some research suggests that there were clues of what was to come. Thomas Walter of the German Geoscience Research Center in Potsdam said seismological readings indicate possible partial collapse of the caldera wall in the hours before the event. “That’s a very weak index,” he said. “But it may indicate that we have a collapse first and then the explosion.”
Cronin agrees there may have been some warnings. Satellite imagery showed that part of the volcano’s protruding northern rim was falling into the sea the day before the eruption. “This could have indicated the early stages of caldera collapse,” he says. It could be a crucial tool for predicting future underwater eruptions. “If we missed the big clue that this big one was coming, then obviously that’s a lesson we’re going to learn,” Cronin says.