Clearly, Hunga involved an unusually explosive recipe that may not be easily replicated. For about a month, the eruption had progressed as expected — moderately violent, gas and ash-laden, but manageable. Then everything went aside. That appears to be due to at least two factors, Cronin says. One was mixing sources of magma with slightly different chemical compositions down below. As these interacted, they produced gases, expanding the volume of magma within the confines of the rock. Under immense pressure, the rocks above began to crack, allowing the cold seawater to seep in. “The seawater added the extra oomph, if you will,” says Cronin. There followed a massive explosion—two actually—that blew trillions of tons of material directly through the top of the caldera, some of it apparently all the way into space.
Both explosions generated large tsunamis. But the biggest wave came later — possibly caused, Cronin thinks, by water pouring into the mile-deep hole that had suddenly been dug out of the seafloor. “That’s really something new for us,” he says — a new type of threat to consider elsewhere. Previously, scientists thought that such a volcano could only really cause a large tsunami if one side of a caldera collapsed. The bottom line, he says, is that submarine volcanoes are more diverse and, in some cases, more capable of extreme behavior than anyone thought.
But the process of composing the eruption has also highlighted the challenges of studying submarine volcanoes. A typical mapping expedition involves a large, fully manned research vessel equipped with multibeam sonar that maps the seafloor for changes and a battery of water sampling instruments that look for chemical signs of ongoing activity. But boating across a potentially active caldera is risky — not so much because the volcano could explode, but because the bubbles of gas bubbling up could sink a ship. In Tonga, researchers solved that problem with smaller ships and an autonomous vessel.
Even Tonga, which has been visited four times in the past year due to an influx of research funding for groups studying the eruption, is unlikely to receive another major manned mission in the coming years, Cronin says. The cost is just so high. It would probably take decades to examine every volcano in detail, even just those in the Tongan arc. This is unfortunate, Walker says, because these kinds of expeditions are one of the few ways scientists get close enough to see how volcanoes behave. An ideal scenario would involve more funding for those missions, as well as investment in improving new technology, such as the autonomous ships, which can be tricky to operate in the treacherous open ocean.
Without them, scientists are stuck watching from a distance. This is difficult to do when trying to observe underwater events, but not impossible. Satellite technology can discover objects known as pumice rafts — sheets of floating volcanic rock that float on the surface of the water — as well as algal blooms, which are fueled by the minerals released by volcanoes. And the USGS, as well as counterparts in Australia, are installing a network of sensors around Tonga that can better detect volcanic activity, combining seismic stations with sound sensors and webcams that look for active explosions. It will be a challenge to keep it going, Lowenstern says — a matter of keeping the systems connected to data and power sources and making sure Tonga can man the facilities. He adds that Tonga is just one of many Pacific countries that could use the aid. But it’s a start.
One of the benefits of studying Hunga volcano so closely is that researchers have now identified new volcanic features to watch out for. In the coming years, Cronin envisions a process to identify which volcanoes need more attention. On their final 2022 Hunga voyage, Cronin’s team took advantage of time on the ship to visit two other submarine volcanoes in the area, including one about 100 miles to the north with a mesa-like topography similar to Hunga before his outburst. The maps will provide a foundation for future studies that manage to get out on the water, a way for researchers to find out just how much action is happening beneath the sea and rocks. So far, Cronin reports, the ocean is calm.
