Scientists are investigating what makes the type of volcanic blast seen at White Island last week so catastrophic – by making their own.
Pyroclastic density currents – fast-moving gushes of ash, pumice and rock that destroy, asphyxiate and burn everything in their path - are the most dangerous features of eruptions.
Around the world, they account for about a third of eruption-related deaths.
We know the two main types as pyroclastic flows - dense clouds of fragments and gases that roar down the slopes of volcanoes - and pyroclastic surges, which pack a higher proportion of gas to rock.
It was the latter type which swept across White Island on December 9, in an instantaneous, short-lived eruption that has so far left 15 people dead.
One volcanologist estimated the rapid surge would have been as hot as 200C, while inner parts of it could have reached 300C to 400C.
Because of their hostile nature, scientists have little direct information with which to study these currents, so they can build hazard models for risk forecasts.
"In particular, the mechanisms that cause hazard impacts remain poorly understood," Massey University volcanologist Associate Professor Gert Lube said.
"All of New Zealand volcanoes are able to produce pyroclastic density currents, which explains our particular interest in understanding and hopefully forecasting them better in the future."
New Zealand's pyroclastic risk
Auckland, which sits upon a relatively young volcanic field, is a particular risk spot for pyroclastic currents.
One 2017 study that described an hypothetical eruption from the shallows of Manukau Harbour found pyroclastic surges would be the most disastrous feature.
"The first pyroclastic surge causes surface damage ... complete destruction within 2.5km of the vent, severe damage to most structures and destruction of weaker structures from 2.5-4km, and some damage to weaker structures from 4-6km," the paper's authors, from GNS Science and Canterbury and Massey universities.
They found how much of Mangere Bridge suburb would be buried in 2m of material from just the first of the three pyroclastic surges.
While another eruption from the field is considered a case of not if, but when – the last event unfolded at Rangitoto around 600 years ago – scientists believe there would be enough warning signals to respond in time.
Another new study looked at the pyroclastic risk from Mt Taranaki, considered the most likely "sleeping" volcano to erupt next, with a 50 per cent probability in the next 50 years.
At Mt Taranaki and other volcanoes like it around the world, scientists long assumed pyroclastic flows could spread only about 10km to 15km.
But University of Auckland researchers recently pushed this limit out to 25km, with serious implications for populated areas within the danger zones.
The new estimates put many Taranaki towns at risk, although the main areas of New Plymouth were protected by the edifice of the older Pouakai volcano.
According to existing estimates, more than 85,000 people live within 30km of the mountain - and 40,000 in high-priority evacuation areas.
One of the most dramatic pyroclastic currents ever documented in New Zealand was that launched by an eruption at the Taupo supervolcano around 1800 years ago, leaving everything within 80km covered in ignimbrite.
A lab-made current
"In New Zealand, we are fortunate to have a unique facility where we can synthesise the conditions of real-world pyroclastic density currents in large-scale experiments," Lube said.
At Massey's Pyroclastic flow Eruption Large-scale Experiment (PELE), scientists could accelerate around six tonnes of heated natural volcanic ash and pumice material by up to 100km/h, generating currents reaching 35m long.
"Using sensors that can withstand the heat and forces of these currents we can observe and measure inside these dangerous flows."
In a new project, just awarded a million-dollar grant through the Marsden Fund, Lube and his colleagues will develop and test new computational hazard models.
It's hoped these could be enable scientists to forecast how these flows evolve as they sweep across different types of terrain.
"This research is guided by a discovery that we made in an attempt to validate existing hazard models," he explained.
"This led us to hypothesise that it is flow turbulence - or very rapid variation in flow conditions - that leads to large destructiveness in pyroclastic density currents in first place, and that it controls how destructiveness perpetuates over long flow run-outs.
"This means that the behaviour of these hazardous flows are not determined by their average conditions, for instance their average speed, heat or ash load.
"Rather, the flow and hazard behaviour will be largely governed by how rapid these conditions change and by how much.
"Through large-scale experiments, we can get the first glimpse of the mechanisms that cause the ferocity of pyroclastic density currents."
Lube expected the biggest challenges of the study would be developing techniques and sensors able to survive a flow – and then actually measuring something useful.
White Island (Whakaari)
Has been locked in a near-constant state of unrest since 2012. Has had three eruptive cycles since 1976. The uninhabited island is the visible tip of a 1.6km high, 17km wide submerged volcano, New Zealand's most active. Produces lava flows, minor ashfalls and its crater has collapsed several times. Unleashed an eruption yesterday that may have killed up to 13 people, which would make it the biggest disaster there since 11 miners died in the 1930s. Last eruption: December 9.
Dormant until 2012, the volcano produced a large eruption in 1869 which formed the upper Te Mari Crater. Another eruption 23 years later belched an immense quantity of steam, mud and boulders, and ejected material rose 600m-900m before rushing down the mountainside. Like White Island's, its most recent eruptions in 2012 were steam-driven ones, affecting a small isolated area. Last eruptions: August 6 and November 21, 2012.
New Zealand's largest cone volcano has mostly produced lava and ash in its frequent eruptions. The last explosive eruption lasted seven minutes and spread ash, rocks and water across the summit area, producing lahars in two valleys including one in the Whakapapa ski field. In contrast with the previous eruptions in 1996, there was no high ash plume to produce ash fallout over a wide area. The effects of an eruption on local tourism, particularly skiing, is large and ash can spread over large areas, especially toward the east in prevailing westerly winds. Last eruption: September 25, 2007.
Discharged red-hot rocks of lava and periodic activity that lasted for months when it erupted in 1973. In the next two years, there were explosive eruptions of ash, and blocks of lava were thrown as far as 3km. During the last violent eruption, gases streamed from the crater for several hours, producing a churning plume of ash up to 13km above the crater. This column then collapsed, causing ash and scoria avalanches that swept down the sides of Ngauruhoe, leaving trails of rubble. Last eruption: 1973-1975.
More than 85,000 live within 30km of Mt Taranaki — 40,000 in high priority evacuation areas. Risk of lahar flows, pyroclastic flows (up to 20km from eruption vent), lava flows (up to 10km) and ashfall. Could disrupt dairy farming and petrochemical industries, including reticulated supply to North Island. Last eruption: 1854
Has not erupted since human settlement of New Zealand, but has been one of the most active caldera volcanoes on Earth over 300,000 years. Largest known eruption expelled more than 500sq km of lava, ash, rocks and gas. Effect of even a small eruption could be devastating for the central North Island and its effects would be felt throughout the entire country. As well as direct damage, it would inflict severe losses on tourism, agriculture, forestry and North Island hydroelectric generation industries. Last eruption: 1800 years ago.
Auckland Volcanic Field
Home to nearly 50 volcanic centres. Mass evacuation for an unknown length of time would be essential. Planning scenarios include an ashfall over Greater Auckland, significant damage to infrastructure, airport closure and insured losses of up to $2 billion. Existing volcanoes unlikely to erupt again, but field is young and potentially active. Last eruption: Rangitoto, 600 to 700 years ago.
Bay of Islands and Whangarei volcanic fields
Bay of Islands volcanic field contains 30 vents, mostly comprising scoria cones and lava flows and domes. Little is known about the field but is likely to have erupted 10 times in the past 20,000 years. Its area is not heavily populated but Bay of Islands is a popular tourist destination. Whangarei volcanic field's last eruption included small eruptions of ash, scoria and lava. Last eruptions: Bay of Islands Volcanic Field: 1300-1800 years ago. Whangārei Volcanic Field: 250,000 years ago.