Underwater explosion causes Tonga volcano to erupt

The eruption of the Hongga Tonga-Hanga Hawapai volcano on January 15th resulted in the largest atmospheric eruption in recorded history.Credit: NASA/GOES/NOAA/NESDIS

Researchers are beginning to piece together why an underwater volcano in Tonga was so explosive — and what happened in the aftermath. Evidence collected by two groups indicates that when the center of the volcano collapsed, it released a massive amount of magma that reacted violently with the water, causing many large eruptions and hundreds of much smaller eruptions.

The Hongi Tonga-Hanga Hawapai volcano erupted on January 15, 2022, resulting in the largest atmospheric eruption 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 sailed over 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-kilometre-wide caldera has decreased in depth from less than 200 meters below sea level to more than 850 meters.

“The volcano produced this massive new caldera,” Cronin says. It is estimated that about 6.5 cubic kilometers of rock was dumped, roughly equivalent to a ball the width of the Golden Gate Bridge in San Francisco, California. “It was a wonderful discovery,” says Taniela Kula, Nuku’alofa’s Deputy Minister of Land and Natural Resources and collaborator on the research. “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.

Researchers have mapped the mouth of the underwater volcano Tonga that erupted on January 15, 2022.

After the volcano erupted, researchers mapped the caldera, the central depression that forms when a volcano erupts.Credit: Shane Cronin/University of Auckland and Taniela Kula Tonga Geological Services

The cause of this Big Bang, Cronin says, may have been the interaction between large amounts of magma and water when the eruption began. “You have water at 20 degrees and you have magma at 1,110 degrees in direct contact,” he says. Such a large difference in temperature means that when water was forced into contact with magma by the eruption, it exploded. Each interaction pushes the water deeper into the edges of the magma, Cronin says, increasing the contact surface area and leading to more eruptions in a chain reaction.

The initial depth of the caldera was also shallow enough that the water pressure didn’t stop the eruption, but deep enough to feed the magma massive amounts of water to power the reactions, resulting in several large eruptions and hundreds of much smaller eruptions at a time. the moment. Cronin says eyewitness accounts from the day of the eruption reported “clacks and noises like artillery fire” 90 kilometers from the eruption. “I’ve never heard these sounds from erupting volcanoes before,” he says.

Ash grains recovered from Tonga after the eruption also indicate a violent interaction between magma and water. When seawater came into contact with the magma, it produced shock waves strong enough to break the grains, Joali Paredes-Mariño, a geological engineer at the University of Auckland, said in the work presented at EGU.

wipe off

A separate expedition by a team at the New Zealand National Institute of Water and Atmospheric Research (NIWA) in Auckland traveled to the volcano in April, but they did not go over the caldera. They sampled ash from the sea floor around the volcano, which showed that the eruption may have been followed by dramatic lava flows, and hot streams of ash and lava raining down the submerged sides of the caldera. Hot, flowing ash turned the ocean floor into a white desert that “wiped everything out,” says expedition leader Kevin McKay, a marine geologist at NIWA.

These underwater flows have spread for thousands of square kilometers since the volcano erupted, ripping cables on the sea floor – including those providing Tonga’s internet access, which has not yet been fully restored – and powering the tsunami waves that swept through neighboring islands, reaching To 18 meters in height. On the sea floor, nothing appears to have survived, although samples are still being analyzed to see the extent of the damage. “We don’t think bacteria live there,” MacKay says. “So we think the sediments are toxic.”

The samples collected by the NIWA team are being used to study the potential effects on ocean oxygen levels and ocean acidification, says Sarah Seabrook, a biogeochemist at NIWA.

However, not everything was destroyed. Satellite data showed a large bloom of phytoplankton in the ocean after the eruption, which feed on nutrients released by the eruption, Seabrook says. Life was thriving on the adjacent hills, which rose above the sea floor just 15 kilometers from the eruption, Mackay says. “We expected life to be destroyed globally.”

water vapor column

Other research presented by Philip Heinrich at the French Commission for Alternative Energies and Atomic Energy near Paris showed that the pressure wave from the volcanic eruption produced a tsunami as far as the French Mediterranean coast, 17,000 kilometers away, with several centimeters in sea level. recorded height. Lewis Millan at NASA’s Jet Propulsion Laboratory in Pasadena, California, also found that the eruption sent a plume of water vapor as high as 53 kilometers into the stratosphere. This plume, which now surrounds the globe, has increased the stratospheric water vapor content 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 age,” Milan says.

Some research suggests there are hints of what’s to come. Thomas Walter at the German Research Center for Geosciences in Potsdam says seismic readings indicate the possibility of a partial collapse of the caldera wall in the hours leading up to the event. “It’s a very weak hint,” he says. “But it may indicate our collapse first and then the explosion.”

Cronin agrees that there may have been some caveat. Satellite images showed part of the volcano’s prominent northern edge falling into the sea the day before the eruption. “It could have indicated early stages of a caldera collapse,” he says. This could be a crucial tool in predicting future submarine eruptions. “If we miss the big evidence that this big thing is coming, this is clearly a lesson we will take forward,” Cronin says.

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