Boiling mud, hot pools and Hobbits; Geothermal systems in Taupo, New Zealand

Continuing on from my first post on The Volcanoes of Taupo, this one will focus all about the hot, dangerous and ever changing geothermal systems of the Taupo Volcanic Zone. As I was saying before, the link between volcanoes, geothermal systems, and precious metal formation is quite strong, as many gold deposits are formed via the interaction of hot hydrothermal fluids with their surrounding rocks.

Thus making our way through the active Taupo Volcanic Zone (TVZ) in North Island, New Zealand was a fantastic opportunity to see some of the processes in action that contribute to mineral deposits and drive geothermal energy (which is both clean and renewable, and accounts for >15% of New Zealand’s source of electricity). With my last post talking about the broad tectonics and overview of the TVZ, this one will focus a little closer on more of the unique and bizarre features associated with geothermal systems. And of course, once again another Lord of the Rings mention has to be made… though not quite in the TVZ, Hobbiton is a short drive away and a chance to see one of the most detailed, preserved movie sets out there.

Hot Waters. Cool Rocks. Bad Smells.

Geothermal systems are regions where a nearby magmatic heat source has caused the surrounding rocks and circulating waters to be heated to higher temperatures. Hydrothermal (hot-water) convection is driven by the difference in the hot, rising column of water with the surrounding cool water. Hot springs, geyser, mud pools, and fumaroles (gas vents) are the surface expression of these geothermal systems. While the waters circulate through the ground, it incorporates magmatic volatiles such as H₂O, HCl, CO₂, H₂S, and SO₂ that gives the waters unique physical and chemical characteristics (Bibby et al., 1995). Therefore, the three vital requirements for geothermal activity are water, heat, and permeability to the surface.

The Taupo Volcanic Zone has around 20 known geothermal systems (Bibby et el., 1995). Though similar features can be seen in most, each ones hosts unique features.

Orakei Korako, also known as “The Hidden Valley”, was our first stop and is most notable for its series of silica sinters. Sinter is a precipitate of amorphous silica (SiO₂) from cooling geothermal waters, and at Orakei Korako it imposingly cements old fault scraps, leaving the “stair step” appearance. Other elements or compounds incorporated into the sinter, as well as algal and bacterial growth like conophyton, are responsible for the departure from the usually white colour of silica sinter.

Next up was Wai-O-Tapu, one of the largest geothermal systems in New Zealand where much of the activity is centred on a 900 year old explosion craters. Wai-O-Tapu has an extensive variety of different types of bizarre geothermal features. For example, acid-sulphate waters dissolve the overlying rocks, crating collapse craters like Devil’s Home.

Wai-O-Tapu has many large alkaline springs, like the famous Champagne Pool; a neutral, Cl-filled hydrothermal explosion crater. The orange rim around Champagne pool is sinter from outflow and this has been tested to contain abnormally high amount of Au, Sb, As, Ag, Hg, among others, making this a compelling analogy to a modern gold/precious metal deposit (Giggenbach et al 1995).

Finally, the last geothermal area we visited was the Waimangu volcanic valley. Youthful and unique, Waimangu is the world only known geothermal field to have formed in historical times from an eruption on June 10^th 1886. It is beautifully maintained as a natural, eco-tourism park. A highlight is “Frying Pan Lake” (you can guess where the name came from…)

There are (mainly) two types of geothermal water that differ from surface waters due to the hot chemical interactions with rocks on their way to the surface. These are weakly alkaline, chloride water (rich in Na and Cl), and the acidic sulphate water (due to the trapped underground water going through chemical reactions and boiling, which leads to the oxidation of hydrogen sulphide to sulphuric acid). Frying Pan Lake is quite odd as it consists of an unusual mix of the two acid-sulfate-chlorite waters with a pH <2.5.

Further along the trail, but still hydraulically connected to Frying Pan Lake, is Inferno Crater. As a site of an old eruption through the flank of an ancient rhyolite dome, Inferno Crater has a distinctive horseshoe form and a brilliant blue colour that’s due to its shallow depth (30m) and fine suspended silica particles (Houghton 1982).

Final thoughts

With my quick review of the major geothermal sites, it really goes to show how diverse, bizarre and alien the Taupo Volcanic Region can be. Anyone who gets the chance to go to New Zealand should relish the opportunity to immerse themselves in this active, true geological wonder. It is well worth putting up with the rotten egg smell >_<

And wait, to finish off as I promised, I had to make a necessary stop just a hour north-west of Rotorua, and visit the meticulously  preserved Hobbiton set from the Lord of the Rings and Hobbit movies. I won’t write about all the different scene and sets I saw here, but instead just end the post to enjoy all these pictures and embrace the Middle Earth-ness like I did.

Until next time New Zealand… Cheers!

-Stephanie

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