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Location-Location-Location

TVZ map and cut through

The Taupō Volcanic Zone sits over the subduction zone boundary where the Australasian and Pacific tectonic plates meet.

New Zealand’s geothermal activity occurs due to high heat flow in the Earth’s crust along the Pacific – Australasian tectonic plate boundary. Faults and fractures channel heat to the surface creating volcanoes, hot springs, geysers, heated soils, hot streams, and fumaroles.

The most well-recognised geothermal area in New Zealand is the Taupō Volcanic Zone (TVZ). This is a 100 km wide by 350 km long volcanic region of the central North Island.

Māori legend says that geothermal fields in the central North Island were created when Te Pupu and Te Hoata, goddesses of fire, emerged from the Earth’s core in search of Ngātoroirangi who had been stranded freezing on Mt Tongariro. Wherever they surfaced, they left geysers, hot springs and mud pools, leaving the path of geothermal activity that remains today.

Warm and hot waters are also discharging from springs throughout the country, including Ngawha and Northland, East Cape, the Hauraki and Auckland regions, around Tauranga, and along the Alpine Fault in the South Island such as Hanmer Springs.

Champagne pool

Champagne Pool at Waiotapu. Photo Credit: GNS Science.

Why are geothermal areas colourful?

The brightly coloured rocks and soils in geothermal areas arise in different ways, due to chemicals in the soil and the presence of pigmented microorganisms.

Hot, acidic gases and fluids interact with rock to form colourful clay minerals. For example salmon pink is cinnabar (mercury sulphide), orange is realgar (arsenic sulphide) and yellow to grey colours can be from sulphur-based minerals.

When geothermal water discharges out of a spring or geyser, it cools quickly and forms what is known as a sinter deposit. Pure silica is white, but sinters often contain traces of impurities or microorganisms which produce beautifully coloured forms. Some of the more common colours are pink from iron oxide or microbes, and grey to black, from iron sulfide (pyrite).

Waimangu spring

Hot spring at Waimangu. The yellow and green colours are associated with photosynthetic microorganisms. Photo Credit: Jean Power.

Microorganisms inhabit geothermal areas and can survive in very high temperatures. The type of microbes will depend largely on the temperature and the chemical composition of the surrounding water, and each is pigmented differently. At lower temperatures photosynthetic pigments produce blues, reds and greens, while some higher temperature microorganisms have carotenoid pigments that produce pinks, reds, yellows and browns.

What is that smell?

That rotten egg smell in geothermal areas is due to hydrogen sulphide gas being released into the atmosphere. Fortunately if you live here, or visit often, you quickly get used to the smell!