Scientists piece together what led to the massive volcanic eruption in Tonga

AYESHA RASCOE, HOST:

The island nation of Tonga got its internet back today. Repairs were finally completed on an undersea cable that was severed last month during an explosive volcanic eruption. Now, as NPR's Geoff Brumfiel reports, one scientific group has new clues as to what caused it.

GEOFF BRUMFIEL, BYLINE: For a few weeks, the volcano had been rumbling. Residents could hear it in the distance sometimes, low like thunder. And then on the afternoon of January 15, things got crazy. There was a big earthquake. Gas and steam shot skyward. And minutes later...

(SOUNDBITE OF VOLCANO ERUPTING)

BRUMFIEL: That was what one social media user recorded in Fiji over 250 miles away. The blast was detected all over the world. This shockwave actually circled the Earth for days afterwards.

SHANE CRONIN: It's a remarkable explosion.

BRUMFIEL: Shane Cronin is a volcanologist at the University of Auckland in New Zealand.

CRONIN: But it's also quite a mysterious one just at the moment as we try and figure out, hey; what did create this?

BRUMFIEL: Cronin has been working with the Tonga Geological Services. He's gotten samples of volcanic ash that blanketed nearby islands.

CRONIN: And we're seeking clues in those ash samples. You know, what went on during that explosive eruption period?

BRUMFIEL: Now, he says, they have some. First, the magma, or liquid rock inside the volcano, was filled with tiny, star-shaped crystals. The crystals were small because the magma was still hot and fresh from deep within the Earth, and that means...

CRONIN: The magma that drove this explosion rose very, very quickly.

BRUMFIEL: It came from miles below in a matter of minutes. What caused this rapid rise? Well, Cronin and his team suspect it was probably that first earthquake. He thinks it was actually a landslide that sent an entire flank of the volcano's rim to the ocean floor.

CRONIN: That would also explain why we've had some damage, for example, to the undersea cables east of the volcano.

BRUMFIEL: Without the weight of the volcano's rim to hold it down, the magma shot up, and water filtered downward into the cracks and crevices in the remaining rock.

CRONIN: That actually magnifies the explosion, and it makes it even bigger.

BRUMFIEL: Because the rock can - for a short time, at least - hold the water and steam and magma together, allowing pressure to build and build until finally it blows.

CRONIN: It's also the way in which weapons explosions are magnified - by compressing the accelerant inside a tight container.

BRUMFIEL: Researchers think this blast was as powerful as some of the biggest nuclear weapons ever made. Cronin says what happened at Tonga matters. Globally, there are more volcanoes under the sea than there are on land. He hopes that this eruption can teach researchers a lot more about the dangers they might pose. Geoff Brumfiel, NPR News. Transcript provided by NPR, Copyright NPR.