The Taupō Volcanic Zone (or ‘TVZ’ for short) is a term widely used by the New Zealand and international geothermal and geoscience community, but it isn’t an area you will find on your everyday map.
In GNG, we are focussing on the central part of the TVZ (from Kawerau to Tokaanu), where we aim to delineate potential supercritical resources, and explore prospective locations to access supercritical fluids. This part of the TVZ has the highest heat flux, huge-scale rhyolitic volcanism, and is where we infer the deep-seated intrusions are that drive the geothermal systems.
But what does all this mean? Read on to learn about the TVZ, and why it is important for New Zealand’s geothermal future.
Where is the TVZ?
The Taupō Volcanic Zone is located in New Zealand’s central North Island. It runs from Mt. Ruapehu in the south-west to Whakaari (White Island) in the northeast, and covers an area of around 30 km x 300 km.
What is the TVZ?
The TVZ is a localised zone of high surface heat flow, with abundant volcanic, tectonic and geothermal activity.
And what does this really mean?
Heat is naturally generated deep within the earth, and moves slowly and continually to the surface. In areas away from volcanic or geothermal activity, the ground temperature increases by about 25-30°C for every kilometre depth.
In the TVZ this geothermal gradient is doubled, to about 50°C per kilometre. The increased heat flow reflects magma coming into the base of the crust (about 15 km deep) that is driving volcanic and geothermal activity.
The TVZ contains six active volcanoes, and many more dormant and extinct volcanoes. In the central part of the TVZ (Kawerau to Tokaanu), there are 23 high- temperature geothermal fields. Faults and fractures act as channels for heat to flow to the surface. In places, permeable formations store some of the rising fluids, forming reservoirs of heat. In addition, many hundreds of low-moderately thermal springs occur, related to young volcanism, or deep faults and tectonic features.
Why does the TVZ occur?
New Zealand is located at the confluence of two tectonic plates: the Pacific and Australian. Under the TVZ, the Pacific Plate is being subducted (i.e. carried down) below the Australian Plate.
This tectonic movement causes the crust to stretch, like a piece of chewing gum! This means the crust under the TVZ is thinner than normal, and may be as little as 16 kilometres thick (where it is typically over 30 kilometres thick in other areas). This stretching also means magma is closer to the surface (ca. 6-8 km), and can more easily generate volcanic and geothermal activity than in places where the crust is thicker.
The crust below the TVZ represents a vast source of renewable geothermal energy. Heat from TVZ geothermal fluids is used for generating electricity, and for direct heat applications ranging from timber drying, space heating and cooling, food processing, milk drying, greenhouse heating, aquaculture and bathing. There are 19 geothermal power stations, and over 200 examples of commercial uses of geothermal energy in this zone.
As Aotearoa-NewZealand moves towards a low-carbon economy, our energy sector will undergo a fundamental transformation to meet decarbonisation targets. As renewable technologies gradually replace petroleum-based systems, the TVZ will see growth in the use of existing ‘conventional’ geothermal resources. And, potentially, we will access the near-limitless base-load energy supply stored in hotter, deeper supercritical resources.