The power of Taupo
Lake Taupo lies in the caldera of an active supervolcano, the site of the world’s most violent eruption of the last 70,000 years. Just 10 km beneath it sits another lake of molten rock 50 km wide and 160 km long. With a growing need for alternative energy sources, plans for tapping this latent reservoir are hotting up.
It was a still winter morning when I drove along State Highway One winding its way north along the eastern side of the lake, past the commercial downtown of Taupo, then along the Waikato River, which drains this largest body of freshwater in Oceania. The lake was as flat and cold as sheet metal.
On a day like this, two unusual phenomena immediately grab your attention: the rocks that float on the water’s surface balls of pumice, some as large as pumpkins and the plumes of clear white steam, issuing from several places along the shore and brought into sharp relief by the morning frost. In Taupo, you always know you’re in a volcanic country.
Beyond the steamfields of Wairakei I turned right into the rolling Waikato hills, chequered with blocks of pine plantations. My destination was pinpointed by the largest feature in the landscape, a rooster tail of steam which from the distance looked like an enormous white quill. Its tip marked the construction site of the Nga Awa Purua geothermal power station, a joint venture of Mighty River.
Power and local iwi—Ngati Tahu/Ngati Whaoa and Ngati Tuwharetoa. Paul Ware, the project’s manager, and John Searle, its chief engineer, took me up to the top of a hill overlooking the construction. A gaping hole in the ground revealed steel reinforcing ribs and over them swarmed figures in dayglo overalls and hard-hats.
“The earth is hot here,” Ware said. “Two shovels deep the temperature is already 60 degrees. We had a surveyor hammering pegs into the ground and when he pulled one out, steam began to come out of the hole. Of course, we need the steam much hotter than that, and at much higher pressure, which is why our drill wells are over two and a half kilometres deep.”
John Searle has been a geothermal engineer for decades and has witnessed New Zealand go from a world leader in geothermal power generation to reliance on hydroelectricity and cheap Maui gas. Now the price of gas has doubled and geothermal is looking attractive once more.
“And a good thing, too, because there is so much untapped potential under our very feet,” said Searle. “These are small reservoirs of high-temperature steamfields which are self-discharging, meaning you don’t have to pump them. This makes them ideal for geothermal power generation.”
Searle and Ware showed me a map of the Taupo Volcanic Zone, stretching wedge-shape from Tongariro to the east coast and towards White Island, over which the geothermal hot spots plotted a dense pattern. “Within the volcanic zone the magma is relatively close to the surface,” Searle explained, “but we certainly don’t want to drill into it. They did that in Iceland, by mistake, and had red-hot magma jetting out of the well heads.”
Instead, they tap the veins of hot water that has been heated by the magma, boring into the Earth’s internal plumbing and siphoning some of its steam to the surface. But drilling deep holes is expensive. “The person who marks the X-spot on the map is effectively signing away a cheque for 5-10 million dollars,” said Searle. “Luckily, here we’ve only had three duds out of nine drill wells, which is really good going.”
At the depth of 2-3 km, the mix of water and steam is at such high pressure it can reach temperatures of 330˚C. Piped to the surface, the steam and water are separated. The steam powers the turbine that generates electricity and the water, from the separation and later from the cooling of the steam, is re-injected back into the steam-field deep underground.
As Ware explained: “It has to be a closed system, otherwise you can end up draining the steamfield so it no longer yields workable pressure, which is why there are no more geysers at Wairakei or Craters of the Moon [the nearby geothermal walk within the Park]. You can only take so much from it without upsetting its pressure balance, and our biggest challenge is finding the maximum sustainable yield which the steamfield can throw off almost in perpetuity.
“In a way, nature imposes its terms on us here,” he added. “There is no use building a $430m power station and have it run out of steam.”
Conceptually at least, the idea of geothermal power generation is elegant in its simplicity though it is not without serious challenges, and maintaining sustainable yield levels so as not to “deflate” the heat bubble is only one of them. The geothermal fluids are also laden with silica which, by careful dosage of sulphuric acid, must be kept below its “polymerisation point”. One mistake and the solidifying silica could turn into a glass-like substance, irreparably clogging the pipelines.
With its planned output of 132 MW, Nga Awa Purua is expected to provide enough power for an estimated 140,000 homes, and there is scope for many more such schemes. Yet the plant, when completed, will require only a skeleton crew to run it and has a surprisingly small visual footprint. Unless you came looking, you wouldn’t even know it was there.
More importantly, geothermal does not rely on external factors rain, wind or amount of sunlight and as such is extremely reliable, with 96–97 per cent utilisation of the resource. An average hydro station, like those on the Waikato, Ware told me, usually turn over at about half of their maximum capacity. Long-term, a series of geothermal plants like Nga Awa Purua could outperform the Clyde dam, the third largest hydro station in the country.
To really understand the geology of Taupo and the firepower beneath it, you have to find a good vantage point, such as the clifftops above Kinloch or Acacia Bay along the northern shore or, better still, the 1088 m summit of Mt Tauhara above the town. Look over the lake and imagine all of it burning, belching fire and molten lava into the stratosphere, darkening skies around the world with its ash and smoke, throwing up rocks the size of houses as if they were sparks from a campfire.
According to Hamish Campbell, a senior geologist at the Institute of Geological and Nuclear Sciences (GNS), the country as we know it today is a fraction of the geological region called Zealandia a vast sunken continent that covers 3,500,000 sq km, almost half the size of Australia, stretching long and narrow from the sub-Antarctic islands to New Caledonia. The edge of two tectonic plates the Pacific and the Indo-Australian transects the foundations of the continent and its grinding and friction have given us the alps in the south and, in the north, a volcanic zone that is 350 km long and 50 km wide. Taupo sits at its epicentre.
The lake itself, with a 193 km shoreline and a surface area of 616 sq km, is actually the crater of a volcano which has been active for around 1.6 million years. Over the past 26,000 years it has erupted 28 times. However, when many volcanoes erupt, their lava, cooling as it flows out, forms a mountain around the vent, sometimes in the shape of gracefully symmetrical cones like those of Mt Taranaki or Mt Fuji. But each time Taupo erupted, it did so with such violence that it literally blew off its top. The mighty Oruanui eruption occurred around 27,000 years ago and was largely responsible for shaping the current lake. It ejected some 530 cubic km of magma and 1170 cubic km of debris in total, covering the Chatham Islands, 1000 km away, with a layer of ash 18 cm deep.
In the same way the Richter scale of magnitude is used to quantify the size of earthquakes, volcanologists developed their own comparative rating known as the Volcanic Explosivity Index (VEI). The VEI takes into account how much volcanic material is thrown out, how high it goes and how long the eruption lasts. The scale goes from zero to eight, each degree signifying an eruption 10 times more powerful than the previous.
The 2001 eruption of White Island is rated two on the VEI scale. The 79 AD eruption of Vesuvius (which obliterated Pompeii) and the 1886 blast at Tarawera (which destroyed the village of Te Wairoa, killing more than 100 people) reach grade five. Two large international events, Krakatau in 1883 and Mount Pinatubo in 1991, are rated at six, an eruption equivalent to detonating 200 megatons of TNT—about 13,000 times the nuclear yield of the bomb that devastated Hiroshima. The Oruanui eruption of Taupo is graded eight, a hundred-fold increase in the magnitude of Pinatubo and Krakatau and one of the most violent eruptions ever to occur.
The most recent eruption in Taupo’s explosive history, around 1800 years ago, unleashed a pyroclastic flow of ash and lava that swept through 20,000 sq km of land around Taupo at speeds of 600–900 km/h. It also blocked the outlet of the lake, causing its level to rise by some 35 m so that when the natural dam finally broke, the Waikato River flowed at 200 times its current rate, carving a new course for itself across the North Island and discharging into the Tasman rather than the Pacific.
Then all went quiet, though the geologists warn that it will erupt again. They monitor its activity with an array of seismographs and keep a close eye on the lake surface—a giant spirit level indicating any changes in the stability of its foundations. Meanwhile, scientists continue to try to better understand the fiery beast within, to provide warnings of its tempestuous moods and to harness its energy.
GNS geophysicist Hugh Bibby has been probing the Taupo Volcanic Zone with low-frequency electromagnetic waves to measure electrical conductivity of rocks within the Earth. His findings should help to precisely identify the hydrothermal aquifers within our geothermal fields, which could translate into more geothermal power prospects, and also determine the dynamic state of the magma whether it is in stable equilibrium, or if it’s gathering for another rumble.
Bibby has discovered that, contrary to what had been believed, the magma beneath the Taupo zone is not limited to unconnected pockets under volcanoes and geothermal areas, but forms a continuous film a subterranean lake of molten rock 50 km wide and 160 km long, several times larger than Lake Taupo itself and only 10 km under the Earth’s surface.
This might, to some, seem a horrifying discovery but it’s good news for geothermal power generation, and the inhabitants of Taupo seem to be taking it all in good cheer as well. When I visited last year, the region’s promoters had just put on a festival with fire and eruptions as the main themes. There was hot music, spicy food, explosive art and even a host of fire-breathing performers.
Ka Hoki whaka muri nga ra…and the days went backwards and the night became day, and day became the night. Trees shrivelled up and rocks ran like water. The earth shakes…the earth trembles. Terror ran through the land and cried out to the skies.
“We were saved by sheltering deep within a cave. We waited there for many days for the choking smoke to clear and the Sun to return. When we emerged to the light we found the world changed forever…“We speak of the Taupo Nui Atea, the huge waka that exploded…out of the darkest expanses of space and time.
“Taupo Nui Atea is a name of awesome power.”
The excerpts come from Barry Brailsford’s controversial book Song of Waitaha—Histories of a Nation and apparently relate the eyewitness accounts of a catastrophic event which in its magnitude could be likened to the Taupo eruption 1800 years ago. The elders of the Waitaha nation, a group who claim their ancestors were present at the time, commissioned Brailsford to write down some of their oral traditions passed down through generations.
When it first appeared in 1994, the book inspired throngs of new-agers to reinvent themselves as born-again Waitaha, and many applauded Brailsford for his bravery. However, his critics could barely contain guffaws of ridicule. Once a noted historian and archaeologist—he wrote the classics The Tattooed Land and The Greenstone Trails, work which earned him an MBE—Brailsford’s venture into mysticism resulted in him being accused of incompetence and cultural transgression, and ostracised from academia.
We might not give Brailsford’s passages any more credibility than we would similar evocative pieces of indigenous lore that recall, for example, a demigod carving out Fiordland with a pointed stick. But there are other shreds of evidence which, while far from conclusive, make for interesting reading.
Layers of volcanic pumice and ash are called tephra and in the tome of Earth’s geological history they make distinct and easily recognisable bookmarks. The science of reading these layers came to be known as tephrostratigraphy and in New Zealand it was pioneered by Alan Pullar. In the 1950s and 60s, Pullar, who was mapping out North Island soils for the soil bureau of the Department of Scientific and Industrial Research, developed tephrostratigraphy as a sort of weekend hobby.
In 1964, he was called to a dig conducted by an amateur archaeologist, Russell Price, on a farm in Hawkes Bay. The peat around Poukawa Lake was yielding vast numbers of artefacts broken and charred moa bones, stone tools, evidence of extensive human habitation—and Price wanted an expert like Pullar to look at it all and to date it, to confirm or deny what to him seemed unthinkable. Because Price had a problem: it was not so much what he was finding but where. Many of the artefacts were buried under a layer of undisturbed ash from the c. 230 AD Taupo eruption, and some were even below the ash of the earlier Waimihia eruption estimated to have occurred around 1320 BC.
Pullar investigated but could find no fault either with Price’s fieldwork or his conclusions. Now they both had a problem. The world of science likes its horizons gradually expanded, not blown asunder. In his report to the New Zealand Archaeological Association (Vol. 8, No 1), Pullar cautiously wrote: “If the lower layer is indeed Waimihia lapilli, then the discovery by Price of items related to man found below the pumice band raises implications almost too daring to be true.”
In the summer of 1969–70 a team of geologists from Victoria University was brought in to resolve the matter. Its leader, Bruce McFadgen, pronounced that Price’s work was flawed and inconclusive, that because the peat was dried up and cracked, the artefacts must have migrated through those cracks and thus under the layers of Taupo tephra. The case was closed.
Like so often before, I took my flyrod and the dog down the path from my friend’s house through the native bush that led to the lakeshore. But I did not fish.
I looked over the expanse of water and thought of what lay beneath, the scale of the inferno, its ferocity, the heat that could melt a body in a flash. And how grand, that with skill and knowledge, we can not only live in such close proximity to this fire but bend it to our needs also.
Not counting the Nga Awa Purua, which is to be commissioned in early 2010, our geothermal plants currently provide around 10 per cent of the country’s electricity use, about 577 MW. But the New Zealand Geothermal Association estimates that, even using only existing technology without drilling deeper and exploring more steamfields, the potential for geothermal exploitation in the country is around 3600 MW. That’s nearly 40 per cent of our total installed capacity for any form of generation and a lion’s share of the usage, remembering that hydro routinely runs at 50 per cent of capacity and wind farms at around 30.
Following the Clean Development Mechanism established under the Kyoto Protocol, geothermal resources are experiencing a worldwide resurgence, and with its expertise in the field New Zealand can once again be among the vanguard.
Humankind has long been fascinated with the idea of free energy, though it often looked to more esoteric concepts as its source. But maybe, just like in Taupo, this free energy is already available for those with the know-how to tap it. As the Waitaha story goes, Taupo Nui Atea, where rocks float and the earth steams, is an awesome power.