Geology of New Zealand Field Trip Guidebook - ResearchGate

This is probably the best-known and most developed geothermal field in New Zealand. It
lies at the edge of the Rotorua Caldera (like Lake Taupo, Lake Rotorua lies in a collapsed
volcanic caldera). Among the features of Wai-o-tapu are the collapsed craters, steaming
ground, acid-sulfate and alkali-chloride pools with shocking colors, fumaroles, geysers,
hydrothermal eruption craters, and boiling mud lakes.
Acid-sulfate pools are opaque yellow-green, the color produced by suspended sulfur.
These pools generally have a pH < 3, but they are relatively cool. Water comes from surface
runoff that has interacted with altered rock and SO 2 gas emerging from the magma below. In
contrast, alkali-chloride pools are filled with water that has come from deep hydrothermal
aquifers. This water may be 22°C or warmer and are saturated in silica. They precipitate a
lovely white sinter.
The Champagne Pool is the highlight of the park. You will recall that Champagne
Pool was created about 900 years ago by a steam eruption that connected a very deep
hydrothermal aquifer with the surface . The Champagne Pool takes its name from the
bubbles of carbon dioxide that constantly effervesce to the surface; however, that’s where the
relationship to a beverage ends. The water is about 75°C and acidic. Champagne Pool is
famous among mineralogists and geochemists because it is the only place on Earth where we
can see gold and silver being precipitated from a hydrothermal fluid. For example, the bright
orange sulfur precipitates surrounding the edge of the pool contain arsenic (2%), antimony
(2%), gold (80 ppm), silver (175 ppm), thallium (320 ppm), and mercury (170 ppm) (Lynne
2003). The truly interesting observation is that precipitation of these metals seems to be
mediated by microbes (Jones et al. 2001).
We will start our day with a visit to Lady Knox Geyser. It erupts promptly at 10:15
a.m. every day with a little help from a box of soap powder. The story goes that prisoners at
the nearby penal colony were using the hot spring to wash clothes. When they dropped their
soapy duds into the spring for a rinse, they induced the first eruption, which scattered them
and their clothes into the bush.
The science behind the soap is pretty simple. The geyser has two water chambers, a
hot lower one and a cool upper one. The upper chamber is cooled by air because it has a
larger opening to the surface. The lower one is heated by magma below. Overnight, the
geyser’s two chambers develop an unstable equilibrium (cool on warm). When soap is
thrown into the upper water chamber, the lowered surface tension of the water destabilizes
this equilibrium, allowing the hot water to rise explosively, erupting up to 20 m into the air.
When the eruption is over, the chambers refill and the cycle repeats the next day for another
group of curious tourists.
As you tour though the park, have a look at the wide range of hydrothermal features and
consider the following when making notes in your notebook. Numbers refer to numbered
signs in the park.
• Primrose Terraces, between 9 and 12. What kind of material makes up this feature? What
evidence is there that it is still forming?
• Cliffs between 11 and 12. Describe these materials. How were they deposited?
• Thunder Crater, location 3. Describe this feature. How is this feature related to the
surrounding craters?
• Frying Pan Flat between location 15 and 16. Look at the nodular materials exposed along
here. What are they and how were they formed?
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