Professor Stephen Sparks said Whakaari was a "persistently active volcano" which had experienced around 250 eruptions between 1976 and 2000. Photo: GNS
A prominent volcanologist giving evidence at the Whakaari White Island trial on Monday said the tour groups to Whakaari were a "societal risk" given the probability of groups being affected by an eruption.
It is the third week of the trial into the health and safety failings in the lead-up to the eruption in 2019 which killed 22 people.
The three owners of the island, their company Whakaari Management Ltd, and two other tour operators are facing charges under the Health and Safety at Work Act for failure to take steps to prevent serious injury or death.
Emeritus Professor Stephen Sparks from the University of Bristol is one of the leading experts in his field, with his research covering the monitoring of eruptions and hazard and risk assessments.
He said Whakaari was a strata volcano, but compared to the typical volcano of this type such as Mt Taranaki or Mt Ruapehu, which have minor hydrothermal activity, it showed much more unrest.
Sparks said Whakaari was a "persistently active volcano" with an estimated 30 percent probability of eruption, which translated to roughly once every three years - compared to once every century or two for Mt Taranaki and once every decade for Mt Ruapehu, he said.
The fact that groups with dozens of people were visiting the volcanic island along with the number of hours spent there, led to an increased risk of visitors being affected by an eruption, he said.
Sparks said he had done a calculation of the probability based on the estimated 1350 hours tour groups were on the island in a given year.
"I came up with something like a 5 percent chance that a tour during the year might be affected, so that's about a one in 20 chance, so that's a very high societal risk from those sorts of numbers.
"So in other words there is a much greater chance that there's going to be a mass casualty event, than there is an individual tourist," he said.
Sparks said phreatic eruptions on Whakaari, like the one on 9 December 2020, saw currents of high temperature gas, steam and rock - known as pyroclastic density currents - being ejected hundreds of metres into the air, which would have been lethal to most people caught in the flow or struck by rocks.
"Pyroclastic density currents are the most significant hazard on many active volcanoes, accounting for almost 40 percent of volcanic fatalities worldwide," he said.
Sparks said there were many constraints on the ability to monitor Whakaari. Unlike Ruapehu and Taranaki which had a wide distribution of monitoring devices, this was not possible on Whakaari's rugged terrain. The monitoring on Whakaari was limited to the main crater and was only able to record shallow processes, he said.
He said it was challenging to make reliable forecasts on Whakaari, particularly with phreatic eruptions which could happen without warning, and occur at a depth of several kilometres which devices would struggle to capture.
The trial continues this week, with more experts expected to give evidence in court.