Researchers are beginning to understand why the eruption of an underwater volcano off Tonga was so explosive — 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 that reacted violently with the water, fueling several large explosions and hundreds of much smaller explosions.
The Hunga Tonga-Hunga Ha’apai volcano erupted on January 15, 2022, producing the largest atmospheric explosion in recorded history. It sent shock waves around the world and a cloud of ash into the upper atmosphere.
In May, Shane Cronin, a volcanologist at the University of Auckland in New Zealand, led a group that navigated 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 more than 850 meters.
“The volcano produced this huge new caldera,” says Cronin. He estimates that about 6.5 cubic kilometers of rock were thrown away, roughly the equivalent of a sphere as wide as the Golden Gate Bridge in San Francisco, California. “It was an incredible find,” says Taaniela Kula, Tonga’s Assistant Secretary for Land and Natural Resources in Nuku’alofa and a research contributor. “It creates a better picture of the volcano’s mechanism.” The work was presented at a meeting of the European Geosciences Union (EGU) in Vienna on 26 May.
The reason for this big explosion was likely the interaction between large amounts of magma and water when the eruption began, says Cronin. “You have water at 20 degrees and you have magma at 1,110 degrees coming into direct contact,” he says. Such a huge temperature difference meant that when the water was forced into contact with the magma by the eruption, it exploded. Each interaction pushed the water further into the magma’s edges, says Cronin, increasing the surface area of contact and generating more explosions in a chain reaction.
The initial depth of the caldera was also shallow enough that the water pressure would not suppress the explosion, but deep enough that the magma was fed massive amounts of water to fuel the interactions, resulting in several large explosions and hundreds of explosions. much smaller every minute. Eyewitness accounts from the day of the eruption reported “cracks and noises like artillery fire” up to 90 kilometers from the eruption, says Cronin. “These are not sounds I’ve heard from volcanoes erupting before,” he says.
Grains of ash recovered from Tonga after the eruption also suggest that there was a violent interaction between magma and water. When seawater came into contact with 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 paper presented at EGU.
A separate expedition by a team from the National Institute of Water and Atmospheric Research of New Zealand (NIWA) in Auckland traveled to the volcano in April but did not pass through the caldera. They collected ash from the seafloor around the volcano, which showed that the eruption was likely followed by dramatic pyroclastic flows, hot streams of ash and lava that rained down on the submerged sides of the caldera. The hot ash turned the surrounding seafloor into a white desert that “brought it all out,” says voyage leader Kevin Mackay, a marine geologist at NIWA.
These streams spread underwater for thousands of square kilometers from the eruption, ripping through the cables of the seafloor – including those providing Tonga’s internet access, which has not yet been fully restored – and fueling tsunamis that swept across the islands. nearby, reaching 18 meters in height. On the seafloor, nothing appears to have survived, although samples are still being analyzed to determine the extent of damage. “We don’t even think bacteria are living there,” says Mackay. “This is how we think sediment is toxic.”
Samples collected by the NIWA team are being used to study potential impacts on ocean oxygen levels and ocean acidification, says Sarah Seabrook, a biogeochemist at NIWA.
Not everything, however, was decimated. Satellite data showed a large proliferation of phytoplankton in the ocean after the eruption, which fed on nutrients released by the explosion, says Seabrook. And in the nearby hills that jutted above the seafloor just 15 kilometers from the eruption, life was flourishing, says Mackay. “We expected life to be universally destroyed.”
water vapor plume
Other research presented at the EGU by Philippe Heinrich at the French Commission for Alternative Energies and Atomic Energy, near Paris, showed that the pressure wave from the eruption produced a tsunami all the way to the French Mediterranean coast, 17,000 kilometers away, with several centimeters at the level of the sea recorded increase. Luis Millán of NASA’s Jet Propulsion Laboratory in Pasadena, California, also found that the eruption sent a plume of water vapor that reached a height of 53 kilometers deep 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 will likely remain in the atmosphere for at least a year. “We haven’t seen anything like this before in the entire age of satellites,” says Millán.
Some research suggests that there were hints of what was to come. Thomas Walter of the German Geoscience Research Center in Potsdam says seismology readings point to a possible partial collapse of the caldera wall in the hours before the event. “It’s a very weak hint,” he says. “But it could indicate that we have first a meltdown and then the explosion.”
Cronin agrees that there may have been some notice. Satellite images showed part of the volcano’s protruding northern rim falling into the sea the day before the eruption. “This may have indicated the early stages of caldera collapse,” he says. This could be a crucial tool in predicting future underwater eruptions. “If we missed the big clue that this big event was coming, obviously that’s a lesson we’ll carry forward,” says Cronin.