Why was Tonga’s volcanic eruption so powerful?

Jordan It happened almost out of the blue on a quiet Saturday, although in a certain sense it was right on time.

News Clip The tsunami left a trail of destruction across Tonga’s archipelago. Many parts are completely covered in ash. With communications down, neighbouring countries are frantically trying to make contact. The underwater volcanic eruption that triggered the tsunami as seen from space, lasted less than ten minutes but caused waves of more than a meter high to crash into Tonga and Fiji.

Jordan The volcanic eruption that rocked the Pacific nation of Tonga more than a week ago was many, many times more powerful than an atomic bomb. Three people are dead, dozens are injured. Countless buildings and property have been destroyed. But this was a disaster that could have been so much worse. And in its aftermath, we have a chance to learn why was this eruption the most powerful volcanic explosion on Earth in more than a quarter century? What can we learn about our climate from the fallout that still lingers in the atmosphere? And what can this explosion tell us about the powerful geologic forces moving below the surface of our world?

I’m Jordan Heath-Rawlings, this is The Big Story. Shane Cronin is a volcanologist at the University of Auckland. Hey, Shane.

Shane Cronin Hello. And how are you? I’m doing very well yourself? Yes. I’ve been extremely busy over the last few days, as you can imagine.

Jordan I can imagine. And we’re about to make you busier. Why don’t you start by telling us about this volcano that erupted? What kind of volcano is it? Where is it? How big is it? All that stuff.

Shane Yeah, sure. So it’s a volcano that not many people know about, actually, and not many people really had a great deal of interest in it until it sort of sprung to life a few years ago and created a new island. But it sits on the Tongan Arc, which is a part of the Pacific Ring of Fire. We call it an island arc because it’s a series of volcanoes along a chain. And this chain extends from the northern part of New Zealand all the way to Samoa. And so Tonga is about halfway along that Ark and Hunga Ha’apai, Hunga Tonga, were two little Islands that are the tip of a large volcano that sits about 65 km away from Nuku’alofa, which is the capital city of Tonga.

Jordan When we are talking about the eruption that made headlines last week, were we expecting it? Do we have a kind of clock for this kind of thing? I sometimes hear, and we talk about earthquakes here a lot and tsunamis, I sometimes hear that we’re overdue for these kind of things.

Shane Yeah, it’s a good question. And there are two types of clock. There’s the so called geological clock that we use as scientists. And so we tend to say things like, the last eruption of this volcano was 900 years ago, and the penultimate eruption of this volcano was 1800 years ago, and so on that kind of a time scale, yes, it was due. But on a human time scale, on a day to day person’s time scale, having an eruption due in that kind of context or that kind of probability is difficult to translate because you can’t say, oh, well, it’s due tomorrow or it’s due next week or so on. So that’s where we rely on more close to volcano geophysical modelling and monitoring. So particularly, we rely on many volcanoes around the world on seismometers to study the volcano’s current state of activity and give a kind of closer to actual forecast.

Jordan And by those statistics, did we know what was coming? Were we prepared?

Shane Well, this is where it becomes more tricky because Hunga Tonga, Hunga Ha’apai, these two tiny little Islands and a new one that joined them in 2015 are lying 65 km away from the nearest inhabited location. So there are no people living on these Islands. There is no power. There is no real way of making any seismic station work. And indeed, there are no seismic stations, even in the inhabited parts of Tonga that are currently working or have been installed. And so we’re flying blind on this volcano, with the exception of satellite data and also direct observations from local Tongan geologists from boat.

Jordan So take me back then to the day of the large eruption. Now, I don’t know much about volcanoes. I understand this was an underwater eruption. How is that different from what we traditionally think of as a volcano eruption, aside from the obvious that it’s underwater?

Shane Yeah, it’s an important point, actually, that it’s underwater. So I’ll take you back to the day to start with. And this eruption occurred in the late afternoon and during daylight hours. So that was part of the reason why we’ve captured such good information about it from satellite information. But late on that afternoon, the Tongan Geological Survey had a boat out actually observing the volcano, and they had observed that the activity was becoming vigorous. They had returned to Nuku’alofa, thank goodness. And they had provided a warning to the local people that activity was ramping up and that they should expect tsunami wave activity. Soon after that, within 2 hours, there was this very large eruption.

So we think the eruption actually took about 1 hour. And during that 1 hour, which is relatively short, this extreme power was generated by this eruption. And the reason why the extreme power was generated was twofold. One was that there was lots of pent up gas trapped within the magma that was giving it a lot of expansion, explosive potential. And the other was the water above the volcano. So the tip of these volcanoes poke out of the water, these two little Islands. But the main big crater of this volcano is actually 6 km in diameter. And it lies between 150 and about 250 meters of water. And so what happens then when this hot gassy magma comes bursting out of the volcano, the first thing that happens is these fragments of magma encounter the sea water. And if it was very deep seawater, so if it was about 1 km deep, then these fragments would probably just burst out gently and then drop to the floor of the ocean because the pressure of deep water would suppress any further explosion. But because the water was only 150 to 250 meters deep, there was actually a super charging of the eruption. It was a sweet spot, if you like, of water depth.

So the hot magma comes out, it bursts into this water. The water is actually then coming in contact with this very hot magma. The magma is about 1100 degrees Celsius, and the water flashes to steam. And then so what happens is that steam expands something like 70 times its original volume, and it just turbocharges the eruption, and we have this enormous plume that actually had a so called ‘overshoot’. It went 55 km in the air, penetrated the lower atmosphere, the troposphere, and penetrated into the Mesosphere, which is remarkable for an eruption that shows extreme power.

Jordan That was going to be my next question, which is simply compared to other eruptions that we’ve seen recently or in the past like, how big is this?

Shane That’s another good question. It’s actually a tricky one to answer because when we talk about size of eruptions, we often talk about how much volume has been erupted from the volcano. And in this case, we are unsure of the volume because some of it went up into the air, and we can account for that volume using sort of physical models of eruption energy and spread of the ash plume and so on. And some of the material went sideways as hot currents of particles and air and probably then fell into the ocean and it’s lost to view. So the volume of this eruption probably wasn’t big. We’re kind of pitching somewhere between one half of a cubic kilometer and a cubic kilometer, although there are estimates that are varying a wee bit and we may never know. And so this sort of makes sense with the only 1 hour long eruption.

On the other side of the coin, another way of measuring eruptions is how powerful they are. So on the powerful scale, this one is right up there. This is the most powerful eruption that we have seen on Earth since 1991 Pinatubo eruption in the Philippines. The main part of the column of this one was 39 km high, only matched by the Filipino eruption. The Filipino one beats it hands down, though, because that one lasted for many more hours and days, in fact. And so it produced much more volume. This one is real big in terms of power, kind of medium size in terms of volume.

Jordan So this is me betraying my ignorance of everything I know about volcanoes except the fact that they blow up. But is this just one eruption? Is there still a worry that the area is unstable? Is it all over now except the ash in the air?

Shane Well, you certainly make a specialty of choosing the most difficult questions to answer, and these are the questions that everybody wants to know. Is it over? And we would like to know that, too. So I can tell you the sorts of things that we know so far, and I guess our pathway to that answer. So one of the things we know is the geological work that we’ve done in the past tells us that these larger eruption episodes from Hunga volcano, they may go on for ten or twelve episodes of activity. So if we track this eruption back to some of the fore-running smaller events, there were probably three decent sized eruption events, one at the end of December, one on the 13th of January, and obviously the very large one on the 15th of January local time. And so we could expect from the geological evidence to see more activity.

The other kind of way of answering that question is also seismic monitoring, but as I mentioned before, there aren’t any seismic stations in or around the volcano, so we’re blind on that front as well. We’re really just stuck with observing things by satellite, and so we’re looking for things like discolored water, new sort of surging of activity in terms of maybe creating some pumice that floats around on the surface of the water of these new submarine eruptions. And the third thing we’re doing is actually with sample material from this last eruption, what we’re trying to do is track whether or not there was new magma involved to generate this eruption or whether this eruption was simply a case of stale magma that had been sitting in the volcano and just sort of stewing to the point where the gas content got too high. With the stale magma, if that has been completely erupted, then we could expect this to be all over.

If it’s fresh, new magma, there’s very good likelihood that ongoing magma supply is continuing at the volcano, and that we’ll get ongoing activity. There’s still a good depth of water over the top of the volcano, so it could still have that extra water boost. But we’re anticipating if new magma comes in to replace that magma that’s been erupted, that it will have better pathways for the gas to escape. So when magma rises up and the top part of the volcano has been riven by this previous explosion, you can imagine the top part of the volcano is now going to be quite leaky. So when new magma rises up, then the gas has a chance to escape from that magma and leak out the top of the volcano like a safety valve. And that may mean that the next eruptions are more gentle.

Jordan In terms of the immediate aftermath and then the days following the large eruption, in the immediate vicinity, I know there’s nobody right at the site, but what has happened to Tonga and how are they doing right now? How are they coping with this?

Shane Yeah, it was a pretty serious event for the 105,000 people of Tonga. So tsunami waves were affecting most of the coastlines facing the volcano and particularly the low-lying areas. A couple of the Islands have had extensive damage with tsunami, and there were three fatalities. In addition, most of the Islands are covered in volcanic ash, particularly the Southern Islands. So the so called Ha’apai group of Islands and the Southern Islands around Tongatapu. So these areas have had anywhere between 2 and 6cm of ash on them. So there’s a lot of houses and crops and everything covered with this ash. It’s just everywhere. It’s really affecting food supply and water supply and just actually people getting around the place and actually breathing without having to breathe in these fine particles.

Jordan What does it mean for, I guess the geology of the region, for lack of a better way to describe it, does something like this eruption or series of eruptions shift something permanently or does it sort of gradually go back to something like normal?

Shane Yeah, that’s a good question. I would say flippantly that things would just continue on as normal based on the geological record, because we have seen across the Tongan area over the last six and a half thousand years evidence for around 25 similar sized eruptions. And this is based on radio carbon dating and past work of me and some of my students. But the other side of that question is maybe not so flippant in that we are seeing a number of quite large earthquakes in and around the volcano, and that may be the region surrounding the volcano kind of readjusting to a rapid change in the situation there. So that volcano has lost a lot of material. It may be a lot more unstable than it was before in terms of the top part of it being riven by this explosion. So over the large scale, the Tonga scale, there’s probably not going to be a lot of change.

Maybe near the volcano. There could be some tectonic change, there could be some readjustments, some earthquakes or even some instability and maybe even collapse of part of the volcano.

Jordan So far, I’ve asked you for help understanding this based on what I don’t know. I’d like to ask you, as somebody who studies this stuff in depth and full time, what was the most fascinating and interesting thing to you about this particular eruption, what was unique about it, and what could we learn?

Shane I think the most amazing thing for me was just how extremely explosive this was, not only in terms of the vertical height, that’s one thing. But for me, the most amazing thing was how widely spread the ash plume traveled, it basically spread laterally extremely rapidly. Within 30 minutes, the width of the ash plume was more than 350 km in diameter. That’s remarkable energy.

Jordan What drives that?

Shane Yeah, that’s driven by these internal gases and the magma and water interacting, creating this incredibly strong, powerful explosion. The thing that drives me from my geological perspective is that I’ve camped on this island. I mean, I was there for five nights with a group of students and post docs and we climbed all around these Islands and took samples from them and we were seeing deposits that were describing very large eruptions in the past. And I think it’s fantastic to see one of these types of events actually take place in front of our satellite eyes, but essentially in front of our eyes and sort of relate that to those geological records. It gives us a good validation.

We were lucky enough to get samples from these Islands before half of them were blasted to pieces. So we’ve got this really interesting situation where we’ve got these materials that we can examine and do some deep dive into these materials and try to understand what’s driving the cyclicity of these volcanic eruptions. And that’s been something that’s fascinated me throughout my career, no matter which volcanoes I worked on, is seeing these patterns and activity that lends some degree of predictability. And if we can understand through all kinds of geochemical studies and Geophysical studies how to learn about that repeatability of the volcanic cycle and to learn about what signals an impending large event, this would be very, very helpful for us across the world in forecasting volcanoes.

Jordan In terms of the atmosphere and how quickly it spread, as you mentioned, I believe the explosion generated like a sort of unprecedented atmospheric shock wave and that’s how it spread so far, is that right?

Shane Well, the atmospheric shockwave was one thing and the so called pressure wave traveled around the world and that’s been remarkable in terms of how far that was. And the other remarkable thing was people hearing the shock wave or hearing the pressure wave and an associated explosion in Alaska, which is just remarkable. But the tsunami and the tsunami generating mechanism in association with the eruption, with the shockwave, with potential change of the top of the volcano, all of this, these are totally unanswered questions that we really are struggling with at the moment.

Jordan Well, this might be another one of those unanswered questions, but I’ll just explain, in the new year we did an episode of this podcast talking about potential climate change solutions and one of them was geoengineering. And one of the popular theories about that, at least so far, obviously not tested in practice, is that we could release some stuff into the atmosphere that would block some of the Sun’s light and heat from reaching us and I saw a couple of things in the aftermath of this eruption that seemed to suggest that we couldn’t figure out how to test this in real time ethically, but we might be forced to test it in the aftermath of this eruption. Does that make sense to you? Is that even possible?

Shane Yeah, it does make sense to me in the respect of the two things that volcanoes do that may influence climate is that they eject a large amount of both solid material and gas. And so what tends to happen then above these volcanoes when they have these very high energy eruptions like this one, the gas actually combines with water in the atmosphere to create aerosols. And sulfur dioxide is very good at that. And these little aerosols, you can think of them as tiny little droplets of water with sulfur dioxide dissolved in them. And when these are high in the atmosphere above 25 km, and some of them are still sitting there in the atmosphere from this eruption above 35 km, what happens is that the little droplets reflect the sunlight back into space, so they do have a cooling effect. And similarly, tiny little particles of ash, when they are ejected up high into the atmosphere, they may stay high in the atmosphere for quite some time. And so these little particles of ash do the same. They kind of reflect the heat from the sun back into space.

So the Pinatubo eruption I mentioned before in terms of being extremely powerful, that eruption produced a very large amount of sulfur dioxide and a very large amount of fine ash, and that had a cooling effect, a net cooling effect that was seen across the world for a couple of years. Now with this particular eruption in Tonga, the cooling effect is not expected to be large because the volume of material compared to Pinatubo was much smaller. Nevertheless, this has injected these little sulfur aerosols, plus the fine ash particles very high into the atmosphere. And so there will be those particles and those aerosol droplets reflecting solar energy back into space. We just won’t be able to measure that because of the normal variability of climate.

Jordan My last question is what are we hoping to learn from this practically and how long might it take to do that?

Shane Yeah, well, right after speaking to you, actually, I’m having a meeting with a group of people who are trying to organize ships to be able to go back and measure what’s happened at the volcano under the water. So do the bathymetry there. And what we’re really wanting to learn is what actually drove the explosivity of this thing. Was it a collapse of the upper part of the volcano we call so called ‘caldera collapse’? Was it a collapse of the flank of the volcano, something like the 1980 Mount St. Helens eruption? Or was it simply that just sheer combination of hot, gassy magma and seawater? And so that’s probably going to take us a little while to get that bathymetry survey done and also the potential to try and drag some samples off the ocean floor. Probably we’re looking at a year or two of analytical chemical work to try to get to the bottom of why was this eruption so powerful and how can we sort of identify other volcanoes around the world that might have a similar degree of power if they were to fire up one day out of the blue?

Jordan Thank you so much for this, Shane. It was really insightful. I learned a lot.

Shane Thank you for your interest, Jordan. I was really interested to answer your questions.

Jordan Shane Cronin, volcanologist at the University of Auckland. That was the big story, for more head to thebigstorypodcast.ca, find us on Twitter @TheBigStoryFPN. Talk to us anytime via email, thebigstorypodcast@rci.rogers.com [click here!].

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Thanks for listening, I’m Jordan Heath-Rawlings, we’ll talk tomorrow.

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