Late last month a group of volcanologists ascended Mt Pirongia on a field trip held in conjunction with the Geosciences 2019 Conference hosted by University of Waikato.
Led by Oliver McLeod (PhD student studying Mt Pirongia) and Dr Adrian Pittari (lecturer in Earth Sciences at University of Waikato), the field trip was a brief summation of highlights from Oliver's PhD study of Pirongia Volcano.
It included the growth history of the volcano and its collapse events that have shaped the mountain's distinct morphology.
With 23 attendees, it was the largest of the conference field trips.
Those on the field trip included some highly influential New Zealand geologists, including Nick Mortimer (GNS Science, Dunedin; who established that New Zealand is part of the continent Zealandia), Hamish Campbell (former resident geologist for Te Papa and expert on Chatham Islands), Alex Nicols (lecturer, Canterbury), Peter Otway (Antarctic explorer), volcanology PhD students, a glacial expert, Tom Davies (Pirongia Restoration Trust), and Bruce Hayward (author of numerous geological books).
First stop was Ruapane Peak, a 40-minute climb, and once at the peak the group got to see Pirongia's unique basalts, which contain large black crystals of a mineral called augite.
Following this was the reveal of the new 1:25,000 geological map of Pirongia Volcano — a map created by Oliver and which shows the mountain's 2.5 million year volcanic history.
This was followed by a short talk on Pirongia's history given by Oliver.
After lunch in Pirongia township the group carried on south to Oparau, driving down Pirongia West Rd to observe lava flows and volcanic domes on the edge of the old volcano.
The last series of stops focused on a very large collapse event which occurred 1.6 million years ago.
This collapse destroyed about quarter of the volcanic landform and triggered a 20km long debris avalanche that flowed into and blocked up the Kawhia Harbour.
Resurgent volcanism followed the collapse, forming two new volcanic vents around Pirongia summit and the cone.
Throughout the day there was keen discussion within the group. Topics included the challenges a geologist faces with interpreting ancient volcanic landscapes and what they represent in terms of the original volcanic structure and whether or not the Kawhia harbour was completely above sea level at the time of Pirongia's eruptions 2.5 million years ago.
The present shallowness of the harbour certainly means that a 'dry' valley around that time can't be ruled out.
A new book and accompanying 1:25,000 geological map,Geology of Pirongia Volcano will be released early next year through the Geoscience Society of New Zealand.
¦See www.gsnz.org.nz miscellaneous publications for details.