New geological map of Bay of Plenty, Taupo and Te Urewera - 07/09/2011

The geologists spent hundreds of days in the field clambering over difficult terrain – travelling to hard-to-reach areas by four-wheel drive, helicopter, jet boat, and even on horseback. They have been mapping, sampling and recording information on rock types and other geological features.

Publication of the new geological map has been led by Dr Graham Leonard and Dr John Begg of GNS Science and marks the end of one of the most comprehensive projects in the organisation’s national mapping programme – called QMAP (quarter-million scale map). The map has also had major input from co-compiler, Professor Colin Wilson at Victoria University of Wellington.

The map replaces existing geological maps of the area, published in the 1960s. It combines information from hundreds of published and unpublished maps and scientific reports, including dozens of PhD thesis studies. A great deal of new research has also been added to produce a comprehensive compilation of this geologically diverse and tectonically active area.

The large full-colour 1:250,000-scale map is tucked into a sleeve in the back cover of a 102-page companion book. The book includes stunning colour photographs of volcanic and mountain landscapes by Dougal Townsend and Lloyd Homer, illustrations of the great variety of volcanic and sedimentary rocks, and numerous diagrams summarising aspects of the geology.

The QMAP series of geological maps covers the whole of New Zealand. The project got underway in 1994 and the series of 21 maps is scheduled for completion later this year, with QMAP Rotorua being the 20th to be published.

The new map, which shows Taupo Volcanic Zone and Te Urewera geology in more detail than ever before, has been generated from a computer database using Geographic Information Systems (GIS) technology. This means it can be updated regularly as new information comes to hand.

The map accompanying Geology of the Rotorua Area covers about 20,000 square kilometres of land. It includes the Waikato River and the Rotorua lakes, Lake Taupo and Lake Waikaremoana. The map also extends offshore to include the volcanically active White Island and many faults on the seafloor.

The project provides the first complete map and eruption history of all of New Zealand’s huge caldera volcanoes from magma vents through lava and ash eruption deposits to the large lahar and sediment fans including the Hauraki and Rangitaiki plains and Tauranga basin, washed out of rivers following eruptions.

This project has helped to refine when and where calderas have erupted over the past two million years and might again in the future. Caldera volcanoes result when magma chambers covering tens to hundreds of square kilometres form and erupt in massive explosive eruptions, culminating in the collapse of the land over the vast magma chamber.

The collapse depression often fills with a lake, and this is the source of Lakes Taupo and Rotorua. The map compilers also point out that there are often tens of thousands of years between caldera eruptions in New Zealand.

The map sheet covers the central part of the Taupo Volcanic Zone, which is the most productive volcanic system known on Earth. It includes supervolcanoes that have produced some of the planet’s largest eruptions in the last two million years.

Sandstones, mudstones and limestones are also mapped, forming Te Urewera and some gentler ranges in the Waikato. These sedimentary rocks are part of the geological backbone of New Zealand dating from the late Jurassic period. This includes Tahora Formation, one of New Zealand’s best-known sources of marine reptile and dinosaur bones.

Authors of the new geological map of the Bay of Plenty are Professor Colin Wilson of Victoria University (left), Dr John Begg and Dr Graham Leonard, both of GNS Science. Photo by Margaret Low, graphic by Philip Carthew.

The book and map explore fracturing and folding in these ancient sediments and how this tells the story of a New Zealand that has formed both mountain ranges and ocean bottom over the more than 150 million years.

Hundreds of geothermal hotsprings, geysers and mudpools, a major attraction for international tourists and a taonga of local Iwi, are shown on the map. A number of the key geothermal drill holes are also shown as well as the locations of the seven renewable energy power plants that generate about 13 percent of New Zealand’s electricity. This figure is set to increase as more geothermal developments come on stream.

The map area covers the iconic volcanoes of Taupo, Rotorua, Tarawera, Edgecumbe and White Island with full details of their eruptive past. Mt Tarawera in 1886 produced our most deadly eruption to date and resulted in the burial of the world-renowned Pink and White terraces.

Some of the densest active faulting in New Zealand is found in the area covered by the map. Most of the faults formed during development of the Taupo Rift that stretches from Lake Taupo to the Bay of Plenty coast. The map shows a selection of the more significant faults in the rift and Te Urewera.

Geologists made several advances in geological knowledge during this mapping project. They discovered and dated previously unknown rock formations and tested theories about the timing and connections between eruptions and faulting.

The text describes a wide variety of natural hazards in the map area. These range from volcanic and earthquake hazards to landslides, coastal erosion and tsunami hazard.

The map includes the large landslide that dammed a major river creating Lake Waikaremoana only 2200 years ago. An even larger landslide mapped nearby has shifted an entire mountain top.

The map area includes significant geological resources that are being extracted or have potential to be developed. These include construction aggregate, pumice, zeolite, diatomite, pozzolan (used in cement-making), Hinuera building stone, and gold. The area is noted internationally for its geothermal and hydroelectric energy generation, and for its geothermal tourism.

The seven-year government-funded project involved many GNS Science geologists and field assistants. The GNS Science team acknowledges input from numerous staff and students across all of New Zealand’s universities.

The map compilers would particularly like to thank the tangata whenua o te Urewera, Tuhoe, who allowed access to Te Urewera and worked with them in the field.

Digital data from the map are already being used extensively in a number of applied and scientific projects. End users expected to benefit include DoC, regional councils, geothermal energy companies, engineers and earth scientists. People with an interest in geology and in the Taupo Volcanic Zone, and Te Urewera in general, will also find the map and text useful and informative.

The map and text are available from GNS Science (sales(at)gns.cri.nz) for $35 plus postage.