The effects of the eruption in the Tonga archipelago were felt thousands of miles away. Six pressing questions about submarine volcanoes.
A mushroom-shaped cloud of ash that reached 20 to 30 kilometers high. Tsunamis that not only hit the neighboring island state of Tonga, but also led to evacuations in Japan and two drownings in Peru. A bang that could be heard as far as Alaska and pressure waves that could be measured with Dutch home-garden-and-kitchen barometers. Yes, the Hunga Tonga-Hunga Ha’apai volcano, 40 miles north of the Tongan capital Nuku’alofa, made a big splash with its January 15 eruption.
But anyone who imagines a classic volcano – such a huge mountain from which lava flows – has the wrong image in mind. Only the top of the 1,800-meter-high colossus was above the water – and even that has only been the case since 2009. Before that, the entire volcano was underwater. What kind of processes are hidden behind these kinds of hidden giants? How special was this eruption? And can we prepare better for these kinds of disasters in the future?
1. How common are these submarine volcanoes?
Very normal. ‘Most of the volcanoes on this planet are under water,’ says geophysicist Rob Govers of Utrecht University. Estimates come to about a million pieces, of which 75 thousand are higher than 1 kilometer – measured from the seabed. The vast majority of those volcanoes are not active.
2. How dangerous are the ones that are active?
The lion’s share of submarine volcanoes are quite harmless, says Govers. They are located in places where two tectonic plates – the pieces into which the Earth’s crust is divided – move apart at a rate of a few centimeters per year. ‘That produces a friendly kind of volcanism,’ explains Govers. ‘The magma is relatively clean and quite liquid, which means that gases in it can easily escape. As a result, they do not accumulate in bubbles that could cause an explosion at a given moment.’
In addition, says Wouter Schellart, professor of geodynamics and tectonics at the Free University of Amsterdam, many volcanoes of this type are several kilometers under water. ‘That thick layer of water absorbs a lot of the energy that is released during a possible explosion.’
3. How could the volcano near Tonga provide such a spectacle?
Hunga Tonga-Hunga Ha’apai is part of the so-called Ring of Fire, a horseshoe-shaped zone around the Pacific Ocean where relatively many volcanic eruptions and earthquakes take place. Here the tectonic plates do not move away from each other, but towards each other – one plate sliding under the other.
‘If such a plate moves into the depth, it takes quite a bit of water with it’, says Schellart. That water is initially locked in cracks, fissures and minerals in the rock. At some point it is released, after which it influences the chemical composition of the material that eventually comes back to the surface. The result: the magma in these types of volcanoes is much more viscous, so that gas bubbles can escape more easily. So they accumulate until the material above them can no longer withstand the pressure. “And then you get an explosion.”
What doesn’t help is that such explosions can occur relatively close to the water’s surface – as was the case with the Tongan volcano. “If the same eruption had happened several kilometers underwater, you wouldn’t have had such a big eruption,” says Schellart.
Moreover, the shallow explosion near Tonga resulted in a large amount of seawater turning into steam, says Elske de Zeeuw-van Dalfsen, volcanologist at KNMI and TU Delft. “That steam was an additional driving force behind the eruption, throwing the ash even higher into the air than it otherwise would have.”
4. How big was this eruption?
Just as we classify earthquakes on the basis of the Richter Scale, we use the Volcanic Explosivity Index (VEI) for volcanoes, says De Zeeuw-van Dalfsen. “That scale runs from 0 to about 8, with each step representing a ten times more powerful explosion.” The power of such an explosion depends on two factors: how high the ash is thrown into the air and how much ash is involved. ‘According to the most recent reports, the height here was 25 kilometers, the amount of ash is not yet completely clear,’ says De Zeeuw-van Dalfsen. “But it looks like we’re going to get to a VEI between 3 and 5.”
Due to this uncertainty, it is not yet possible to say how special the Tongan eruption was. An eruption with a VEI of 3 occurs somewhere on Earth on average once every three months, while an eruption with a VEI of 5 only occurs once every ten to twenty years. If you go one step higher, to a VEI of 6, you will come to eruptions in the category once in a lifetime (once every fifty to one hundred years). This includes, for example, the eruption of the Philippine volcano Pinatubo, which claimed about 850 lives in 1991 — thus ten to a thousand times more powerful than the eruption at Tonga.
5. What are the consequences of eruptions like the one at Tonga?
First of all, there were the tsunamis, which of course can cause considerable damage and death. ‘But these volcanoes are especially notorious for their enormous ash clouds,’ says Govers. ‘They have a major influence on everything that concerns us humans. These types of ash particles are actually very small glass particles, which can perforate your stomach or lungs if you ingest them. So you should not inhale them. And if volcanic ash rains down on your field, your whole harvest will go to the Philistines.’
Moreover, volcanic ash is quite heavy – and not all houses are designed for that. ‘In the eruption of the Pinatubo in 1991, many people did not die from the eruption itself, but because their roof collapsed due to the tens of centimeters to meters of ash that had landed on it,’ says De Zeeuw-van Dalfsen.
The question now is how much ash has ended up on Tonga. It might not be that bad, says De Zeeuw-van Dalfsen. It is known that two hundred people are cleaning the runway of the airport on the main island. ‘If there had been a meter of ash, they wouldn’t have talked about ‘wiping clean’; then they should have created.’
That same fact also seems to indicate that the amount of sulfur dioxide – the gas that is mainly released during a volcanic eruption – could be less than that. “If there had been a lot of sulfur dioxide, two hundred people would never have been able to sweep outside, because they won’t have that many gas masks available.”
Tonga would have been lucky with that, because sulfur dioxide can indeed cause major problems. ‘It is really not healthy to inhale that,’ says De Zeeuw-van Dalfsen. “If the wind is wrong, people with asthma can even suffer from an eruption that happened 1,000 kilometers away.”
6. Can eruptions like this be predicted?
If you know when a volcano is about to erupt, you can warn people for miles around. For example, they tell you: seek higher ground, in case a tsunami hits. Or: go inside, so as not to inhale the ash and sulfur fumes. (Or, if you live under a corrugated iron roof that could collapse under a layer of ash, don’t go inside.) But do we know when a volcano is about to start misbehaving?
‘We can read volcanoes better and better’, says Govers. But, he continues: then we must all be able to fit measuring instruments on it – and that is not possible with submarine volcanoes. ‘They are usually a few hundred meters deep. That makes everything you would do with normal measuring equipment practically impossible. As a result, such volcanoes are among the last to be ‘dissolved’.’
Now Hunga Tonga-Hunga Ha’apai protruded with its top above the water, which does offer possibilities. “You could put a seismometer on it,” says Schellart. ‘But a tilt meter is also very useful, for example. It measures the angle of inclination of a volcano; if it increases, the volcano is inflating itself and may soon erupt. And you really have to be able to place such a tilt meter on the side of the mountain.’
Another option is a system that does not signal the eruption itself, but the tsunamis caused by it. Such warning systems are also in use in the Pacific – but the question is how much benefit Tonga would have benefited from it. ‘The affected islands were so close to the volcano that such a tsunami can make the crossing in five to ten minutes,’ estimates Schellart. A lot of time to get away, so in this case you don’t buy with the equipment that warns you for such a wave.
