The volcano that just produced the planet's biggest bang in three decades could keep erupting – even with similar-sized events – for weeks or months, a Kiwi scientist says.
Saturday's eruption, which sent shockwaves and tsunamis hundreds of kilometres across the Pacific, marked the latest explosive chapter in Hunga-Tonga-Hunga-Ha'apai's long and fiery history.
The volcano itself is a submarine volcano that breached sea level amid a 2009 eruption, and lies submerged between two islands, about 65km north of Tonga's main island Tongatapu.
About five to six kilometres beneath it lies a magma reservoir that fueled previous eruptions, and which scientists suspect is also driving this bout.
University of Auckland volcanologist Professor Shane Cronin and colleagues earlier discovered evidence of two huge blows at Hunga – one in 1100 and another in 200AD – which suggested a recurrence interval of 900 to 1000 years.
"The current eruption seems to be one of these large events, which fits with the timing since the last of these," he said.
Those ancient episodes were likely comparable in scale to Saturday's spectacular event, which registered as a five on the Volcanic Explosivity Index (VEI).
As these had also played out over eruptive phases, rather than a one-off blow, there could be more action to come over the next weeks to months.
Cronin said the latest eruption was remarkable for several reasons.
One was rapid lateral expansion of its eruption cloud - dramatically captured in satellite images – which came with tsunami and atmospheric shockwaves.
That indicated large volumes of gas-charged magma had been suddenly belched from the sunken volcano, which itself had probably been transformed.
"Further eruptions from this caldera during this episode could generate new tsunami and widespread ashfall, especially if there caldera has further collapses or landslides."
Cronin also noted that the type of magma spewed from Hunga was of an intermediate composition – andesite to dacite – similar to that generated by Ruapehu.
"Based on the past compositions, the magma is not especially rich in volatiles such as sulphur or fluorine," he said.
"Hence, this volcanic ash is not especially toxic, but notably all volcanic ash can produce acid rain and acid leachates."
He expected the event to have carpeted surrounding islands with layers of ash centimetres thick.
"People of Tonga must also remain vigilant for further eruptions and especially tsunami with short notice, and should avoid low lying areas."
Niwa hydrodynamic scientist Dr Emily Lane said tsunamis created by eruptions weren't as common as those triggered by big quakes, and only accounted for about 5 per cent of historical events.
Still, there were plenty of ways volcanoes could kick off tsunamis, ranging from underwater eruptions to part of a volcano falling into the sea, as happened at Indonesia's Anak Krakatau in 2018.
It wasn't yet clear precisely what occurred at Hunga.
"When we see what is left of the island after this eruption is over, we can start to put together the pieces of what happened."
Otago University volcanologist Dr Marco Brenna said Hunga had blown several times over the last century: in 1912, 1937, 1988, 2009 and 2014.
"Little is known of the early 1900s ones, but of the last three, the erupted volumes appeared to be increasing."
While the latest episode began not unlike its predecessors in December, over the past few days, it generated much larger explosive events.
"It is too early to say at this stage how the situation might evolve."
Could Auckland's fibre reveal volcanic rumblings?
Meanwhile, scientists have launched a new study which will use Auckland's fibre network to help detect volcanic activity under the city.
Auckland is built on a volcanic field with 53 recognised volcanoes across the Auckland Volcanic Field, (AVF), posing a substantial natural hazard to New Zealand's largest city.
Previous eruptions have differed in time and location, making it challenging to predict future events across the field.
While 11 GeoNet seismometers located around Auckland provided good recordings of seismic activity, they didn't capture low-magnitude earthquakes, nor did they offer the variability that could be used to study the AVF.
In their EQC-funded project, beginning in April, Victoria University scientist Dr Calum Chamberlain and his team use fibre-optic technology known as distributed acoustic sensing (DAS) to detect smaller quakes of less than magnitude 1.
"We know there is likely to be an eruption in the AVF sometime in the future and we think that earthquakes might provide some insight into volcanic unrest in the area," Chamberlain said.
"Being able to accurately monitor seismic signals is essential to forecast the location and timing of an eruption."
DAS used "dark strands" of fibre networks, or sections not used by telecommunications, to monitor seismic activity.
Chamberlain's team will laser pulses to repeatedly measure the length of the cables, allowing them to record stretching in the cables due to passing seismic waves.
The DAS method allowed them to resolve shaking about every 10 metres along the fibre.
Chamberlain said the project would build national capability for this important emerging technology, offering new ways to detect seismicity wherever communications fibres were installed.
"The tools we develop will provide high-resolution ways of detecting and monitoring volcanic and seismic hazard in urban environments, not only in Auckland but across New Zealand."