Geology of New Zealand Field Trip Guidebook - ResearchGate

Day 12—Tuesday 25 November. We will leave Tongariro behind and travel north into the
Taupo Volcanic Zone and on toward Rotorua.
Lake Taupo lies in the collapsed calderas of several enormous volcanoes that blew
themselves up in a series of massive eruptions, the last of which occurred in 186 CE [Wilson,
1980 #14]. Material from the Taupo eruptions are rhyolitic and spread over a large part of
the North Island. They include, pumice, ash and a non-welded ignimbrite (pyroclastic
deposits). Estimates suggest that the eruption launched more than 60 km 3 of material more
than 50 km into the sky. Some geologists estimate that the Taupo eruption of 186 CE was the
most violent eruption of its kind ever studied (Wilson et al. 1980). Ash from the eruption
spread worldwide producing strange skies as far away as China and Europe. An article by
Wilson and colleagues (1980) describing a novel way of dating this eruption appears at the
end of the guidebook.
Stop 1: Five Mile Bay Recreation Area
From this stop we can look across Lake Taupo to Western Bay, site of some of the most
recent volcanic activity in the area. Looks peaceful doesn’t it? We want to look at three
things here. First, the sand along the beach. Grab a handful and look carefully, definitely use
a hand lens if you have one. The second thing to look at here are the deposits that make up
the lake shore terraces to the east of the current shore line. Finally, notice the large boulders
scattered along the shore. Notice that they are all about the same distance from the shore.
Notebook Assignment: Describe the sand on the beach in detail, paying particular attention
to its composition. Compare this sand with other sand you’ve described in your notebook
from other places we have visited. Next, describe the terrace deposits. Finally, have a close
look at the large boulders. After you have made some good observations, suggest how the
two sets of observations are related.
Some have suggested that the shoreline boulders actually floated into place. Although they
seem too dense now, when hot and full of gas, some argue, they might have floated to the
strand line, cooled and been deposited.
Stop 2: Taupo
We’ll stop briefly to have a look at this touristy town. The guidebooks say that Taupo “rivals
Rotorua as the North Island’s capital of adrenalinised action.” It is a big tourist spot, both for
Kiwis and foreign visitors. It is also New Zealand’s lake trout capital.
Stop 3: Wairaikei Geothermal Power Plant viewing area
Wairaikei was the first geothermal power station developed in New Zealand. Exploratory
drilling began in 1950 and power generation started in 1958. The station produces about 150
MW of electricity. Power is derived from hot water extracted from wells that penetrate an
impermeable cap of sedimentary rocks into hot, fractured volcanic rocks below. The wells
produce water at about 260°C, sufficiently hot to flash to steam at surface pressures. The
steam drives turbines that produce electricity.
In a world worried about burning fossil fuels for energy, geothermal seems like a
good alternative for those places, like New Zealand, lucky enough to have high heat flow
near the surface. However, geothermal energy is not entirely benign. Removing large
amounts of water from the underground aquifer can cause the landscape to subside. This is a
significant problem if your home—or town—happens to be built over the geothermal field.
The water itself can also be problematic. Hydrothermal water may be acidic, contain lots of
45