Master's study to probe secrets of devastating Taranaki lahar

The lure of the mountain has proved too much to resist for former Taranaki man Ben Dixon.

On completing his Bachelor of Science degree at Wellington's Victoria University, majoring in geology, at the end of 2011, Dixon, 22, was prepared to put study behind him.

But when the opportunity arose to study Mt Taranaki for a master's thesis, the outdoor enthusiast knew fulltime work would have to wait.

Dixon is on the ground in eastern Taranaki this summer collecting rock and earth samples for his research on the Ngatoro Formation, which runs eastward from Mt Taranaki, through Midhirst and Tariki, Kaimata and on to the Waitara River.

It is associated with one of Taranaki's largest volcanic eruptions, the Inglewood eruption, 3600 years ago.

"I was prepared to go and work, but this kind of fell into my lap. I always thought that if I was going to do a master's thesis, I wanted to do it in Taranaki and on a subject I didn't know anything about."

While Dixon's supervisors have previously mapped the Ngatoro Formation, it has not been studied in depth.

The likely effects of a lahar careering down the mountain's eastern slopes is a part of the research which, once completed at the end of the year, will be gifted to the Taranaki Regional Council to assist with its volcanic hazards planning.

While it was initially thought the lahar deposit on the mountain's eastern side was formed during the Inglewood eruption, research has put that theory into doubt. What is interesting about the Ngatoro Formation is that it appears a catastrophic lahar may have flushed down Mt Taranaki's eastern slopes years after the actual eruption, Dixon says.

"We believe there was a period of time between the eruption and when the lahar deposits occurred. We have got a few hypotheses at the moment. The main hypothesis is that there was a lava dome growing on the top of the mountain, like a mushroom growing over time."

Evidence suggests that the dome may have eventually given way, resulting in a lahar that devastated the eastern Egmont National Park boundary.

Mt Taranaki is overdue for another eruption, with the last recorded in 1755.

A buried forest dating back some 3600 years on the eastern boundary shows the forest floor, then a layer of tephra (airfall material from a volcanic eruption) around the base of the charred trees, before a layer of soil. It is that organic layer Dixon says indicates there was a period of time between the Inglewood eruption and the subsequent lahar. On top of the soil and charred trees is a 10-metre-thick deposit of rocks - some the size of cars - left behind when the lahar tore through.

"It indicates massive amounts of material coming down - basically like a flood on steroids.

"These lahars decimate everything in front of them. It tore down a massive forest," Dixon says.

"If we can look at how these lahars behaved in the past, we have a better idea of what they will do if the mountain erupts again."

While not directly part of his study, such a lahar would have considerable impacts on people, property and infrastructure, Dixon says. Farmers closest to the park boundary would predictably be the worst affected, with the amount of volcanic material diminishing the further east it spread.

While working on his thesis, Dixon has also been employed part-time as an engineering geologist for a Wellington-based consultancy firm.

Taranaki Daily News