How big is the January eruption of the Hunga-Tonga volcano? Four months of intensive science only raised the bar. You could point to the sonic noises that interrupted the night in Alaska, 6,000 miles away. Or maybe to the tsunamis in the Caribbean, caused by a rare form of acoustic wave that jumped over the continents and stirred the seas. Space has also changed in space, NASA scientists said earlier this month, with explosion winds accelerating to 450 miles per hour as they left the farthest layers of the atmosphere. This briefly redirected the flow of electrons around the planet’s equator, a phenomenon previously observed during geomagnetic storms caused by the solar wind.
Therefore, when the researchers began searching the bottom of the ocean that immediately surrounded the volcano, they expected to find a bumpy landscape. It would surely have been reshaped by an explosion and covered with debris. Scientists believe the explosion was the result of a flammable recipe: hot gaseous magma collided with cold, salty seawater. But how did those two ingredients merge with such force? Some of the leading theories have focused on the idea of a landslide or other collapse of the slopes of a volcano that helped water fall into a magma chamber. It would also help explain the tsunami that killed three people on the nearby islands of Tongan. A huge shift in the submarine rocks also means squeezing out a huge amount of water.
A team of scientists from the New Zealand National Institute for Water and Atmospheric Research, or NIVA, recently noticed something different. Using acoustic instruments mounted on a ship to map the seabed, they discovered that the terrain had indeed changed - it was now covered with at least enough ash to fill 3 million Olympic pools. But other than that, not everything is so different. The slopes of the underwater volcano are still largely the same as before the eruption; the same characteristics continue to be conceived by the surrounding seabed. Within 15 kilometers of the volcano, some of these landmarks are still teeming with life, with starfish and corals clinging to rocky submarines. “The first thing we did was circle around the volcano, and I’ll say, ‘What the hell?'” Recalls Kevin McKay, a marine geologist from NIVA who led the expedition. “It just defied expectations.”
One area they did not dare to enter was just above the caldera, a depression left behind when the volcano erupted. Mackay’s large research ship full of scientists and crew did not dare to sail there - not because of the danger of large explosions, but because of minor burps of gas that could rise from the place of the eruption. “Those gas bubbles can crash ships, and they’ve done it before,” he says. But they suspected complete destruction. The islands that rose from the sea just before the eruption were torn apart by the explosion, indicating a crater below the surface.
The day after the NIVA team published its findings, another group of researchers from Tonga Geological Services and the University of Auckland helped fill out the map. Using a smaller boat that was less at risk of bubbles, the team came out over a caldera with a similar set of acoustic instruments. Yes, there was a hole in the line. Rap is 4 kilometers wide and 850 meters deep, and is surprisingly limited, bordered by the original slopes of the volcano. “What we have now is a very big, very deep hole in the country,” Shane Cronin, a volcanologist at the University of Auckland, told a news conference in Tonga. “It helps us understand why the explosion was so very, very big.
These two sets of observations help scientists reconstruct a huge underwater explosion unlike any they could have studied before. The picture reveals that it looks like Hunga exploded right in the air. As the caldera disintegrated in the early stages of the eruption, it probably led to a flood of seawater that met with deep regions of magma, causing a chain reaction. More seawater, more magma, more explosions.