The science behind the Tonga volcanic eruption, tsunami

The first satellite images of the Pacific nation of Tonga after Saturday's volcanic eruption emerged Tuesday. The photos show the islands coated in ash. Despite the violent explosion, the Tongan government has reported just three deaths. The status of two smaller islands in its chain is still unknown. John Yang takes a closer look at the science behind the volcano, its explosion and aftermath.

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  • Judy Woodruff:

    Today, the first satellite images emerged of the Pacific nation of Tonga following Saturday's volcanic eruption.

    The photos show the islands coated in ash. Despite the violent explosion, the Tongan government has so far reported just three deaths. The status of two smaller islands in its chain is still unknown.

    John Yang takes a closer look at the science behind the volcano, its explosion and aftermath.

  • John Yang:

    Judy, scientists say the explosion revealed some of the mysteries of underwater volcanoes. It produced a tsunami that was felt on the coasts of Japan and the United States, 5,000 miles away, and triggered an oil spill in Peru. It set off a lightning storm that lasted seven hours and had 400,000 strikes. It was heard as far away as Alaska.

    But for all the ways it made itself felt around the world, its damage was relatively confined. Tonga bore the brunt of the volcano's fury, blanketed with ash that is now contaminating its water supply.

    Michael Poland is a research geophysicist with the U.S. Geological Survey's Cascades Volcano Observatory in Vancouver, Washington.

    Mr. Poland, thanks so much for joining us. As we talk about, it was so dramatic, this event, that huge plume that was visible from space, the tsunami, the atmospheric shockwave which rippled around the globe, but you say that the data actually showed this was a relatively small eruption.

    Explain that.

  • Michael Poland, U.S. Geological Survey:

    Yes, it's a very confounding event, in that the amount of material that came out of the ground was not especially huge.

    It was perhaps the kind of thing we might see every few years from a volcano somewhere on Earth. But it produced an outsized explosion, really a massive explosion. And that has to have something to do with the interaction with ocean water, and to produce that really massive tsunami.

    So it's having really outsized impacts for the amount of material that came out of the ground.

  • John Yang:

    And what was in that material? What did come out of the ground?

  • Michael Poland:

    Well, there was a lot of ash that came out, of course, as with most volcanic eruptions, and SO2 gas, among other types of gas, water vapor, and so forth.

    And that sulfur dioxide really allows us to pinpoint where the plume is and how big it is, because we know roughly the amount of sulfur gas that's dissolved in the magma. So it's a bit like opening a soda and having that carbon dioxide come out of the soda. When the gas comes out of the volcano, you get a lot of SO2 that comes out of that magma. That allows us to track the plume and know roughly how big an eruption is.

  • John Yang:

    And you talk about the reaction with the ocean water. Does that also — or what's the explanation for it being heard so far away, heard as far away as Alaska?

  • Michael Poland:

    Yes, that's really difficult to understand at this point.

    It had to have something to do with that sort of interaction. Perhaps a large amount of gas-rich magma being suddenly exposed to cold ocean water generated just a really massive explosion, and it was heard obviously, very, very far away. That's something I think is going to be the subject of an awful lot of research in the days, years to come.

  • John Yang:

    And the tsunami, I think a lot of people's familiarity with tsunamis really comes from the 2004 Banda Aceh tsunami, which was the result of an earthquake, of a seismic event.

    Is this common, a tsunami also common with volcanic eruptions under underwater like this?

  • Michael Poland:

    Volcanic eruptions that are right underwater or right near the water can generate tsunamis. It's not something we see a lot of. Certainly, we see the earthquake-induced tsunamis much more commonly.

    There's a lot of study that went into earthquake-induced tsunami, especially after 2004. And we have come to understand those sorts of processes a lot better than we did before. But volcanic tsunamis being rare give us fewer opportunities to study them.

    So we don't understand the volcanic tsunamis quite as well. So a bit like that airwave that traveled around the Earth, this tsunami is also going to be studied quite intently to understand more about what generated it. Was it some displacement of the seafloor? Was it perhaps the impact of an ash plume on the water?

    There are many potential mechanisms for why the tsunami was so big. And understanding that is going to be key to being able to forecast these sorts of events in the future.

  • John Yang:

    As I hear you talk, it sounds like there's a lot we don't understand about underwater volcanoes, and that this is suggesting ways or new research, as you say, to examine this one.

    What sorts of things will you be looking at and your colleagues be looking at it in this event?

  • Michael Poland:

    I think the key here is that magma-water interaction.

    And so there's going to be all kinds of attention paid to how magma and water interact. What happens when you put these two different materials in contact with another really in a dynamic way. So we're going to be looking at modeling studies of those sorts of interactions, studying the ash particles to see whether that can provide some clues as to how the interaction took place, and then modeling this explosion, modeling the tsunami, to try to understand more about the nature of this interaction, because, obviously, there are submarine volcanoes in other places.

    And they're tremendously hard to study, because they are obscured. It's relatively easy to study volcanoes on land compared to these underwater volcanoes. But, clearly, they present a hazard. So we need to understand more about that hazard.

    This event, as tragic as it is for Tonga, might help us understand these kinds of interactions in more detail. And that will help us in the future.

  • John Yang:

    It's fascinating stuff.

    Research geophysicist Michael Poland, thank you very much.

  • Michael Poland:

    My pleasure.

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