Hunga Tonga Eruption Likely A Small VEI 5
Information out of Tonga has been scarce since Jan 15’s violent volcanic eruption, but the puzzle is beginning to be pieced together, albeit slowly. According to volcanologist Professor Shane Cronin, Hunga Tonga-Hunga Ha’apai delivered the biggest explosion since Mount Pinatubo in the Philippines some 30 years ago (VEI 6).
University of Auckland’s Cronin said scenes on the ground immediately after the eruption would have appeared apocalyptic: “The clouds that people could see in the distance, the booming noises and then the waves coming from the first tsunami…The next step is when the ash clouds spread across Tongatapu, and that ash cloud is so dense with fine ash particles that it blocks the sun completely, so it gets really dark.”
Those ash particles were propelled to heights of 30km (98,425ft) –well into the stratosphere– where they then ballooned out, forming an umbrella with a diameter of 260km (162 miles).
“The large and explosive lateral spread of the eruption suggests that it was probably the biggest one since the 1991 eruption of Pinatubo,” added Cronin. This is a contention gathering support in some circles. However, estimations still vary widely, ranging from a mid-VEI 4 to a powerful VEI 6. The full details of the eruption likely won’t be confirmed for weeks.
Cronin and his research team camped on Hunga Tonga-Hunga Ha’apai in 2015 and noticed that the surrounding coral reef was lifting up, suggesting that magma was building underneath the volcano and could cause an eruption in the near future. The team found evidence of two previous mega-eruptions in AD 1100 and AD 200, suggesting they occur roughly once every 1000 years — making us due for a third.
Cooling Implications Explained
The Mount Pinatubo eruption of 1991 (VEI 6) pumped 15 million tons of sulfur dioxide (SO2) into the atmosphere and cooled the planet by 1C for the next year and a half. And although data on Hunga Tonga is still sketchy, it is thought the explosion released 400,000 metric tons of SO2, which would put it on par with a La Soufriere (2021) or a Calbuco (2015/16), both VEI 4s, rather than a Mt St Helens (1980) or El Chichón (1982), both VEI 5s.
The confusion, and the reason some have suggested Jan 15’s explosion was a VEI 6, is down to the sheer size of that monstrous eruption plume (with plume size being key component when indexing a volcanic eruption, along with the thickness of ejected ash and the volume of SO2). However, Saturday’s impressive 30km ash column is likely explained by Hunga Tonga being a submarine volcano: The abundance of ocean water at shallow depths greatly amplified the size of the explosion, making the eruption far larger than if the same event had occurred on land. This line of thinking, especially when combined with Hunga only releasing 400,000 tons of SO2 (a gas released by all volcanic eruptions–the measurement of which closely aligns with the amount of eruptive magma, whether that be through the flow of lava or the ejection of fine ash) adds support to Jan’s eruption being a mid/large-VEI 4 or a small VEI 5 at best, rather than anything bigger.
For reference, the eruptions of Mt St Helens (1980) and El Chichón (1982) released 1 million and 7 million of SO2, respectively, even though they had lower plume heights, of 24km and 29km, respectively. Both were documented as VEI 5s.
Still, it is likely that Tonga’s volcanic eruption will impact global temperatures, and a colder upcoming winter across the southern hemisphere is likely. This is the view of Climate scientist Jim Salinger, who said it could take a few months to kick in, but could ultimately lead to a cooling of as much as 0.5C across the likes of Australia and New Zealand, lasting until spring.
Salinger said it would be nothing like after the Pinatubo eruption of 1991, when there were some “very cold and horrible winters”. But in New Zealand, “it would mean that it might be a bit cooler than normal,” he added.
In conclusion, Hunga Tonga’s Jan 15 eruption was not the ‘big one’, it wasn’t that monster VEI 6/7 that cools the planet by 2C and instantly propels us –in combo with an ever-waning sun– into Earth’s next period of global cooling; however…
Is A Larger Eruption Imminent?
Chances are, Hunga Tonga-Hunga Ha’apai isn’t quite done yet. The volcano has continued to erupt at a low level since the weekend, and further explosive events are likely over the coming days or weeks. The question is, what does it have left?
Ordinarily, subsequent volcanic eruptions don’t tend to be as powerful as the initial explosion. However, with the collapse of the main caldera –with 95 percent of the original island now destroyed/underwater– and the infiltration of sea water into the magma chamber, Hunga Tonga is throwing volcanologists something of a curve ball.
“The concern at the moment is how little information we have and that’s scary,” said Janine Krippner, a New Zealand-based volcanologist with the Smithsonian Global Volcanism Program. “When the vent is below water, nothing can tell us what will happen next.”
Superheated magma and a huge volume of volcanic gases rose quickly and met the cool seawater, intensifying the explosion, said Raymond Cas, a professor of volcanology at Australia’s Monash University. The fear is, something similar could be setting up again — further eruptions will be intensified by the vast amount of sea water filling the caldera/chamber, building pressure.
It’s a wait and see, though. “Once the volcano is de-gassed, it will settle down,” said American meteorologist, Chris Vagasky. But we don’t know if we’re there yet. Referring again to evidence of Hunga Tonga’s historical mega-eruptions, which appear to occur once every 1000 years, previously in AD 1100 and AD 200, these ancient sequences have always contained multiple high-level explosions over a closely-spaced period; thus, similarly sized –or even larger– eruptions are expected over the coming days/weeks/months/years — your guess is as good as mine… all eyes on Tonga.
Hunga Tonga’s eruption was a true spectacle, but rarer still was the resulting tsunami.
As explained by NIWA hydrodynamics scientist and tsunami expert Dr Emily Lane, most tsunami are triggered by underwater earthquakes, only about 5 percent of historical tsunamis have been caused by volcanic eruptions,
“We haven’t seen a volcanic tsunami of this magnitude in over 100 years,” said Lane.
“This is pretty much shattering the mould. One of the tricky things about tsunamis is that they happen very infrequently; think about the Japanese tsunami and its predecessor was over 1000 years beforehand. We haven’t seen very many volcanic tsunamis. We’ve certainly never seen a volcanic tsunami that has affected the entire Pacific in this way.”
Hunga Tonga’s eruption triggered a landslide as much of the volcanoes edifice side into the ocean. The landslide had an estimated volume of 0.5km3 — one of the largest in recorded history, with that figure likely to be revised higher.
Lane said the only similar volcanic tsunami event was Indonesia’s Krakatau eruption in 1883, which killed tens of thousands of people and obliterated the island.
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