GNS Science unveils new explanation for volcanic forms in the Tongariro National Park with new geological map

Monday, 20 August 2018

The map's companion book explains how these three mountains were formed. Pictured is Mt Tongariro, with Mt Ngauruhoe at the top and Mt Ruapehu at the back.

Ice and lava might seem unlikely bedfellows, but new research from GNS Science suggests the two might have a close, temperamental relationship.

The Crown research institute's newly published geological map of Tongariro National Park brings new insights to the geological formation and volcanic history of this World Heritage Area.

GNS Science, the Department of Conservation, and Victoria University of Wellington collaborated to produce the map and illustrated 109-page companion book. The folded map is tucked into the book's sleeve.

Aimed at the technically-inclined, public reader, the map and book describe the structure of the park's volcanoes [Tongariro, Ruapehu, and Ngauruhoe] in detail. The volcanoes' 1000-year history is also explained.

A hiker looking across to Mount Ngauruhoe, with its distinctive cone shape.

GNS Science geologist Dougal Townsend, co-author of the map, said the publication presented a new understanding of how the volcanoes were built.

The main advance on existing understanding is on the extent to which ice-caps and glaciers have controlled the ways in which lava flows of the volcanoes have built the mountains during the last 250,000 years, he said.

Mt Ruapehu's crater lake from the mountain's summit.

Complex volcanic flows and eruption activity were unravelled with the help of precise radiometric dates and geochemical analyses.

GNS Science geologist Graham Leonard, a co-author, said a major surprise to scientists was the extent to which ice has affected lava flow paths and eruption styles during periods of glaciation.

'The recognition of distinctive rock textures indicate lava has erupted and flowed under ice, around ice and over ice in different ways,' he said.

The abundance of glacial till [unsorted glacial sediment] at mid and lower level elevations around the mountains has been another surprise for the scientists.

During interglacial periods, such as the present, when the ice has mostly retreated to highest elevations, the volcanoes were able to erupt and build more conventionally - as seen in the young, symmetrical Ngauruhoe cone, he said.

Townsend said the research suggested the retreat of surrounding ice may have caused the higher parts of the volcanoes to collapse, causing massive landslides.

'This revision has enabled a new picture to be gained of the evolution of the magma systems below Ruapehu and Tongariro,' Townsend said.

Staff and students from Victoria University of Wellington, the University of Otago, and Massey and Canterbury universities supported Townsend and Leonard with the project.

Support for the project was provided by the Department of Conservation, local iwi and the US Geological Survey rock-dating laboratory at Menlo Park, near San Francisco.

* The map and accompanying 110-page information book is available from GNS Science for $50. Visit: https://shop.gns.cri.nz/gnsgm4