The January 15 eruption at the underwater Hunga Tonga Hunga Ha'apai volcano - observed in this satellite image - was the largest in nearly 140 years. Photo / JMA
The January 15 eruption at the underwater Hunga Tonga Hunga Ha'apai volcano - observed in this satellite image - was the largest in nearly 140 years. Photo / JMA
This month, scientists also reported how January 15's spectacular eruption of the underwater Hunga Tonga-Hunga Ha'apai volcano pushed a towering ash plume into the mesosphere – more than 50km above Earth – adding to an ever-growing list of mind-blowing records.
Yet they've yet to answer perhaps the greatest question of all: how a volcano with a history of low-magnitude eruptions suddenly produced the planet's biggest blow in nearly 140 years.
Lying submerged in the South Pacific between two islands, about 65km north of Tonga's main island Tongatapu, the caldera volcano is known to have erupted several times over the last century – most recently in 2009 and 2014.
Well before that, there's evidence of two much bigger bangs - one in 1100 and another in 200AD – suggesting its larger eruptions occurred each millennium.
Precisely what kicked off the latest episode – and why it came suddenly after a month-long period of mild eruptive activity – remains unclear, Otago University volcanologist Dr Marco Brenna said.
"But from what we have learnt thus far, something is likely to have happened within the magma reservoir that led to its destabilisation."
In a new study, supported with a Marsden Fund grant, Brenna will lead an international team of scientists to reconstruct the sequence of events that led to it.
To help solve the puzzle, the scientists will draw on a unique collection of samples - namely crystals created within the volcano's plumbing system - that were collected following the January eruption and older events.
The ongoing eruption of Tonga's Hunga Tonga volcano appears to be the most powerful and violent eruption of the 21st century. pic.twitter.com/VK0A1kQUSq
Though tiny, these record magmatic processes and could still tell us much about the environment they were formed in, such as its depth, how much magma was mixing at that time, and how quickly that magma was rising.
While experiments run at Rome's National Institute of Geophysics and Volcanology will recreate a minuscule magma chamber, US colleagues at Columbia University will quantify the role of magmatic gases in the build-up.
In revealing these mechanisms, Brenna said we stood to gain a deeper insight into what sparked the event - and perhaps what could trigger similar big eruptions in future.
"Being able to unravel the sequence of events at Hunga will inform future hazard scenarios," he said.
"The eruption changed the dynamics at Hunga volcano quite dramatically - there is now an 800m hole in the middle of it - so the results might not be directly relevant to the near future eruptive activity there," he said.
"There are however many other similar volcanoes in Tonga but also elsewhere that could have the potential to generate similar eruptions, and improving hazard models for similar eruption sequences will be globally beneficial."
The new study comes as a recent expedition to the area, led by University of Auckland volcanologist Professor Shane Cronin, found no indication that another major blow was imminent.
The Hunga Tonga-Hunga Ha'apai eruption on January 15. Photo / Supplied
The two islands the system sat between 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.
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 told the Herald last month.
"But there seems to have been a bit of an upsurge in activity in nearby volcanoes."
That included some observed swelling at Fonuafo'ou, a submarine volcano about 20km north of Hunga-Tonga, and signs of small eruptions at Tofua, a remote volcanic island known for its fiery history.
