A Kiwi-led expedition has been observing signs of increased activity and eruption at Tofua, a volcanic island in Tonga about 96km north of the Hunga-Tonga volcano. Photo / Shane Cronin
A Kiwi-led expedition has been observing signs of increased activity and eruption at Tofua, a volcanic island in Tonga about 96km north of the Hunga-Tonga volcano. Photo / Shane Cronin
A volcano that produced the planet's largest explosion in nearly 140 years now appears to gone into a period of dormancy, says a Kiwi scientist in Tonga who's just finished surveying it.
But there are signs of fresh activity at nearby volcanoes - including the famous Tofua, which has a similarly fiery history.
The January eruption of Hunga Tonga–Hunga Ha'apai - the largest in more than three decades – created an initial tsunami wave that reached more than 90m high, along with a pressure shockwave that travelled around the globe multiple times.
In Tonga, at least four people were killed and more were injured.
Here in New Zealand, cannon-like booms were felt and heard in homes across the country as local climate stations picked up strong, fleeting spikes in air pressure.
The big blast – dramatically captured by satellite - also sent at least 55 million tonnes of water vapour into the stratosphere, where scientists recently warned it could cause a small and brief spike in global warming.
For now, there appeared to no indication that another major blow was imminent, said University of Auckland volcanologist Professor Shane Cronin, who's been surveying the volcano's underwater vents this week.
Sitting between the islands of Hunga Ha'apai and Hunga Tonga - and about 65km north of Tonga's capital Nuku'alofa - the volcano's seafloor base spans 20km in diameter and rises about 2000m toward the sea surface.
The eruption at the underwater Hunga Tonga Hunga Ha'apai volcano, observed in a January 15 satellite image. Photo / JMA
Earlier this year, Cronin led an expedition that mapped the entire caldera - a deep depression in the top of the volcano, where the centre collapsed inwards on itself – created by the eruption.
The survey showed the base of the depression was 860m below sea level, and the 4km-wide caldera had filled with an unknown depth of fresh volcanic material.
The two islands themselves were now just a fraction of their former size, after an estimated 6.5 cubic kilometres of material was blown out of the central volcano.
Cronin said the latest survey showed that, while small fumarole vents closer to the sea surface were still producing gas, activity at the much larger vents deeper down had effectively died down.
"So, it looks like Hunga-Tonga has gone into a period of quiescence, or dormancy, which is good news," he said.
"But," he added, "there seems to have been a bit of an upsurge in activity in nearby volcanoes."
When his team mapped Fonuafo'ou - a submarine volcano about 20km north of Hunga-Tonga, that's gradually eroded below sea level since last erupting in 1936 – they found it be sitting at a shallower level than expected.
"There seems to be a bit of swelling going on at that volcano, and there were several shallow fumarole-like submarine gas plumes - so it's one we're going to keep a close eye on for the next while."
This new bathymetry map reveals the shallow summit area of Fonuafo'ou, a submarine volcano located nearby the recently-erupted Hunga Tonga volcano. It has a series of faults and cones around it, indicating that it also has an incipient caldera of the same size as Hunga Tonga. Image / Shane Cronin
Next, the team travelled to Tofua, a volcanic island famously associated with the 1789 mutiny on the Bounty, and which Cronin described as an "emergent version" of Hunga Tonga.
"It's exactly the same dimension as Hunga Tonga, in that it's a large caldera – but the rim of the caldera sits at about 500m above sea level."
Following local reports of increased activity, the team carried out offshore gas surveys that suggested it was daily emitting about 300 tonnes of sulphur dioxide (SO2), which Cronin considered high for a quiescent volcano.
At the northern part of the caldera is a cone, named Lofia, that's produced most of Tofua's historical eruptions.
Here, the team observed large amounts of blue gas – indicating hydrogen sulphide reacting with the atmosphere to create SO2.
"When there's lots of hydrogen sulphide being released, this can mean lava is near the surface."
Sure enough, drones flown over a crater in the caldera revealed a new lava lake – something that hadn't been seen there for years.
More gas measurements at Tofua - also suggested a daily SO2 output of more than 500 tonnes, or about double the normal rate.
"That's typical of mild eruption with one of these volcanoes."
Our objectives for our latest @AutScience @EnvUoA @ScienceUoA Tonga mission with @marta_ribo were to 1. Check ongoing state of activity at Hunga (caldera map below joint NZ - Korean Polar Res Inst effort), and 2. Check state of nearby Tongan volcano pic.twitter.com/WevV9PGX3c
As he climbed to the edge of the crater, he immediately noticed signs of fresh tephra, along with scattered lava bombs.
"You can imagine cowpat-like bits of molten material all over the place, but very fresh and black – so it's likely these were erupted out within the past two weeks, and not any longer."
All the while, small eruptions also appeared to be going on within the crater lake, throwing up material about 100m and causing a glow in the night sky that's been reported by locals.
"Clearly, Tofua is in much more of agitated state than it has been over the last decade, which means we need to monitor this one more closely too," he said.
"While it's pleasing to see activity at Hunga has died down, it remains of concern that neighbouring volcanoes appear to have become more active."
Hunga-Tonga is part of a string of volcanoes forming the Tonga Arc, formed by the Pacific tectonic plate subducting under the Indo-Australian plate.
"This arc has been unusually quiet over the last couple of decades, and geological history tells us large eruptions occur here frequently – so it's perhaps not surprising we're seeing this uptick in activity."
The latest expedition was a joint effort between the University of Auckland and Auckland University of Technology, and included PhD student Annahlise Hall, marine geologist Dr Marta Ribo-Gene and Tonga's chief geologist, Ta'aniela Kula.
