0n october 2, 1990, White Island flexed its volcanic muscles and provided fishermen in the Bay of Plenty, and later television viewers throughout the country, with impressive scenes of ash and rock being hurled into the atmosphere. The eruption was only a few minutes long, but it punched out a new crater (named TV1 after the channel which scooped the story) and formed an eruption column two kilometres high.
Awesome as such volcanic displays can be, they also serve to remind us of a basic geological fact: the Earth is a giant pressure-cooker whose contents are trying to get out. Usually the only thing that escapes is heat, which seeps outwards through the rocks of the Earth’s crust all the time over the whole of the Earth’s surface. The rate of heat flow, however, is astonishingly low. In fact, the Earth’s surface receives 8000 times as much heat from the sun as comes from beneath our feet, and if we could “switch off” the heat inside the Earth,the temperature would drop by only 0.01°C. Imagine an aluminium billy containing a litre of water and sitting flat on the ground. If all the heat coming through the bottom of the billy could be used without loss of energy, it would boil the water—but it would take 50 years to do it!
The problem lies in the fact that rock transmits heat only very slowly. Over those large areas of the Earth’s crust which are stable, the heat from the interior varies little. But the situation is dramatically different in the volcanic regions of the Earth. Here eruptions of ash or lava release enormous amounts of heat—up to a billion times that of the average Earth heat flow—for short periods of a day or so. Put our container of water down on one of Hawaii’s lava gushers and in seconds the water would flash to steam and the billy become a puddle of molten metal.
The movement of heat from a hot place to a cold place in a flowing fluid is known as convection, and when the fluid is molten rock the results can be spectacular. But a much more common liquid that draws heat to the surface by convection is rain water.
Most of the rain that falls on the Earth ends up in rivers, lakes and seas or returns to the atmosphere, evaporated by the sun’s warmth or transpired through the leaves of plants. But some usually manages to percolate downwards into the porous subsoils, finding its way into the joints, cracks and pores of the bedrock to become groundwater. If a bed of rock is highly porous or jointed it becomes soaked in water, and is then known as an aquifer. Natural springs form where this groundwater flows back to the surface, perhaps where a valley has cut down into an aquifer, or where a fault line has created a channel. Aquifers may be many kilometres deep, extending down to where the rocks are hot, and when cold rainwater reaches these depths it is transformed into hot geothermal fluid.
Just like a plume of heated air rising from sun-baked tar seal, heated water rises upwards through the rocks to emerge as hot springs where the fissures break the surface. The downflow of cold water and upflow of hot water forms a convective hydrothermal system. In its simplest form, such a system can occur in a non-volcanic area, where a large fault allows water to flow through a deeply shattered but narrow zone of rock. Many small hot springs of this origin are found in the valleys of the Southern Alps, where the Alpine Fault reaches deep into hot rock that has been rapidly uplifted by mountain-building processes.
The same principles of convective flow apply in the belt of volcanically active country that stretches across the central North Island. Here, however, the conditions of copious rainfall, thick, permeable and heavily faulted strata, and intense volcanic heat at shallow depths have combined to generate some of the most spectacular hydrothermal systems in the world.
The “Taupo Volcanic Zone” is the name geologists give to the whole geothermally active region that extends from Tongariro National Park in the southwest to White Island in the northeast. Northwards from White Island, this belt of volcanism continues almost without break through the archipelagos of the western Pacific, through the frozen Kuril and Aleutian Island chains, and down the Pacific seaboard of the Americas. Perhaps there should be a notice planted in the carrot fields of Ohakune to tell the world that “here beginneth the Pacific Ring of Fire.”
The Taupo Volcanic Zone is located where the crust beneath the North Island is being stretched on a plate tectonic rack. Some 100km east of New Zealand the westward-moving Pacific Plate collides with the leading edge of the Indo-Australian Plate at the Hikurangi Trench. The basaltic rocks of the Pacific Ocean floor sink beneath the lighter continental crust of New Zealand and are forced downwards beneath the underlying wedge of hot, dense mantle rocks. The downward progress of the Pacific Plate as it succumbs to closer tectonic relations with its western neighbour is marked by a zone of Earthquakes which plunges to over 2501(m deep beneath western New Zealand.
Above this subduction zone, on the eastern side, the sedimentary rocks of the East Coast Basin and the forested ranges from the Kaimanawas to the East Cape are being compressed, crumpled, sheared and thrust upwards by the titanic forces of the collision. West of the ranges the forces operate in another way, and for more than a million years plunges to over 2501(m deep beneath western New Zealand.
Above this subduction zone, on the eastern side, the sedimentary rocks of the East Coast Basin and the forested ranges from the Kaimanawas to the East Cape are being compressed, crumpled, sheared and thrust upwards by the titanic forces of the collision. West of the ranges the forces operate in another way, and for more than a million years now the strata of the crust have been stretched, thinned, cracked by faults and dropped downwards. This is exactly what happened at Edgecumbe in 1987 when some 200 square kilometres of the Rangitaiki Plains subsided by as much as two metres, and the surface of the Earth was pushed sideways. In effect, New Zealand suddenly became one metre wider at that point. These effects were graphically demonstrated where rail‑way lines plunged over the fault scarp east of Edgecumbe town. The steel rails were stretched and torn apart at the welds, leaving slivers of jagged metal on the track bed.
The physical forces of subduction which result in the crumpling and cracking, the heaving up and the dropping down of segments of the Earth’s crust also cause large-scale melting to occur deep in the subduction zone. The process is complex and poorly understood, but it is clear that wherever slabs of crust are dragged or thrust down into the underlying mantle, volcanoes become active on the surface above. Beneath the Taupo Volcanic Zone, where the crust is being pulled apart, the dense hot rocks of the Earth’s mantle have been able to rise from their usual depth of 25km or more to the base of the crust, less than 15km from the surface. As a consequence, huge volumes of the lower crust have melted to form rhyolite magma. Plumes of this liquid rock have forced their way upwards, collecting in magma chambers only a few kilometres below the surface. Every thousand years or so, some of this magma reaches the surface, welling out to build a dome of lava, or, just as often, strewing ash and pumice over the landscape in a devastating blast. The accumulated products of this volcanism fill parts of the Taupo Volcanic Zone to depths of greater than two kilometres.
It comes as no surprise that the amount of heat conducted out through the rocks of the Taupo Volcanic Zone is unusually high: rates up to 10 times the average of those in geologically sleepier parts of the world have been recorded. And when the Taupo eruption of AD 186 punched pumice more than 50km up into the stratosphere, the thermal discharge through the vent may have been 100 billion times as much. No wonder that event has been described as the most powerful eruption the world has ever known. Yet despite the high general heat flow of the area and occasional prodigious release of energy by volcanoes, scientists believe that most of the deep heat is carried to the surface by water through the region’s many geothermal systems.
Areas of intense hydrothermal discharge such as Karapiti and the geyser field at Whakarewarewa pour out heat 600 times as fast as even the hottest non-geothermal part of the Taupo Volcanic Zone. It would take all the geothermal fields in concert about 300 years to transfer as much heat to the atmosphere as the Taupo eruption did in its short but spectacular life, but time is on their side. No one really knows how long a hydrothermal system can keep going, but a big one like Wairakei may well have been boiling away for a hundred thousand years.
To understand the nature and variety of geothermal fields, we must first take a quick look at the physics and chemistry of water deep in the Earth. If we were boiling our billy of water for a brew, over a driftwood fire on the beach, and took the temperature as the tea bags dropped in, it would read 100°C. At the same time a tramper at Glacier Shelter. 2500m above sea level on the slopes of Mt Ruapehu, would find his kettle boiling at only 91.5°C. and would be well advised to stick to instant coffee. This lowering of the boiling temperature of water is due to the decreased atmospheric pressure at higher altitudes.
The converse effect applies as the pressure increases beneath the Earth’s surface, and a miner 2500m below sea level would boil his water at 109°C. If we now imagine our deep mine abandoned, with the shaft full to the brim with water, not air, the effect is greatly enhanced: the pressure at a mere 4m below the surface would be high enough to raise the boiling point to 109°C, at 100m it would be 183°C, and at 1000m the temperature of boiling water would be over 300°C.
Because of this property of water, most of the fluid in an intense geothermal field is so hot that if it were suddenly brought to the Earth’s surface it would boil explosively. This is how steam is generated for a geothermal power station. A drillhole taps deep, pressurised, superheated water which turns into a water and steam mixture as it rises in the bore and enters the pipework at the wellhead. The steam is separated and fed into the turbines while the remaining hot water goes to waste.
The same principle drives a natural geyser eruption. If the pressure in an underground chamber of water with a restricted vent builds up high enough, it will suddenly start to discharge, causing a pressure drop and explosive boiling, which will drive the eruption until the chamber is exhausted.
In places where rocks are dry, the heat from a cauldron of molten rock (magma) a few kilometres deep in the Earth will slowly and evenly flow up to the surface. If groundwater is able to seep down towards this magma, it eventually reaches a zone where the rocks are so hot that even great pressure cannot stop the water boiling. and it all turns to steam. This steam mixes with gases from the magma chamber, and the superheated vapour heads up towards the surface. On the way it cools and condenses back to boiling water as it rises and mixes with groundwater.
In the shallower parts of a geothermal system there is not enough heat in the rocks to keep the water boiling, and the rocks will in fact cool down the rising fluid. But over decades and centuries the water itself gives back heat to the rocks, and slowly the whole mass of rock and water reaches an equilibrium in which cooling and heating are balanced. In some places however, where channels to the surface are free and open and the heat supply is great enough, a continuously boiling system can be maintained all the way down.
This is what happens in the Taupo Volcanic Zone. Cold water slips down through the cracks in the rocks like a burglar through an open door. It infiltrates the roof-rocks of the deep volcanic hEarths to steal their heat,and rapidly escapes upwards, carrying its calorific booty to the surface zones. Here, like a thief with a fistful of hot dollars, it dissipates megawatts of energy in heating up cold rock, or squanders joules in spectacular style as spouting hot springs or roaring fumaroles.
Scientists have speculated that this thermal plunder may have had a drastic cooling effect on the cauldrons of magma in their attempts to climb through the crust to the surface. So much so, that some may have ground to a prematurely geriatric halt as cooling caused the liquid rock to crystallise and solidify into arthritic immobility. Perhaps, then, hydro-thermal systems are responsible for slowing down the rate at which magma reaches the surface to cause volcanic eruptions. Maybe the people in the towns, the black swans on the lakes, and the pine trees in the forests of Taupo and Okataina have Pohutu, Champagne Pool, and the Karapiti blowholes to thank for reducing the everpresent risk of volcanic devastation that is part of life in the Taupo Volcanic Zone.
Water’s round trip from rainfall to geyser takes hundreds, if not thousands, of years. In the process it becomes a veritable cocktail of dissolved chemicals gathered from the breakdown of superheated rock.
Sodium chloride, or common salt, is the most abundant dissolved mineral, and all hot spring waters in the Taupo Volcanic Zone are, in fact, weak brines, although they bear little comparison with sea water. Pohutu geyser, for instance, is nearly 40 times more dilute than the ocean.
Deep in the Earth, where volcanic gases and exhalations from the magma chambers may have an important influence, the fluid is probably acid. However, reaction with the rocks neutralises the acid, and the deep waters that flow to the surface are mildly alkaline. Massage your hands with such waters and they develop a gentle slippery feeling as skin fats turn to soap.
All volcanic rocks in the Taupo Volcanic Zone contain an abundance of silica, either in pure silica minerals such as quartz, or combined with other chemical elements such as aluminium, sodium, calcium and iron in volcanic glasses or rock-forming minerals. Hot water readily dissolves a small amount of silica from the porous volcanic ashes and pumices as it soaks through them, and eventually carries it in solution up to the surface. When geothermal water pours from a hot spring or gushes from a geyser, it cools quickly, especially if the water floods over a wide surface rather than through a channel. As it cools, the water’s ability to dissolve minerals is reduced, and the silica precipitates onto whatever surface is handy.
The hard, grey-white deposit that forms is called sinter. It is the freestone of geothermal architecture, and the master-mason is no more than tumbling, cooling water. Given time, it can create a cathedral of baroque buttresses as in the late Pink and White Terraces. With greater haste, but less attention to aesthetic detail, it trowels a layer of crude stucco onto a long-lost gumboot and turns it to stone.
Pure sinter gleams creamy white when freshly formed and still wet, but often trace elements tint the deposits. Salmon pinks and brick reds, caused by antimony traces, coloured the famous Pink Terrace, and can be seen now in the sinter mound at Pohutu’s vent. Antimony and arsenic sulphides give the edge of Waiotapit’s Champagne Pool its orange colour, and hidden on the shore of Frying Pan Lake at Waimangu, the sinter around the youthful geyser is a peculiar rich rusty brown, perhaps caused by unusual amounts of tungsten in the silica.
The pressure in a hydrothermal system drives the hot, alkaline brine towards the surface. Even if the fluid is not hot enough to boil under pressure at depths, it will usually carry enough heat to start boiling in the lower pressure regime near the surface. Boiling water, like all water on the loose, tries to escape at the lowest point it can, which is why hot springs and geysers are generally found on valley floors. The steam that separates from boiling water is less inhibited by gravity, and rises above the level of the hot water table through the cracks and pores in the rocks, ash beds and soils to escape to the air where it will. This explains why fumaroles jet from the very crest of White Island crater rim, and why the cliffs above Frying Pan Lake at Waimangu are wreathed in steam.
Hydrogen sulphide and carbon dioxide gases which were dissolved in the geothermal water become the steam’s fellow travellers. As the steam rises it cools and condenses to form hot water again, mixing with cold ground moisture in the process. Some of the hydrogen sulphide gas escapes into the air, but much of it reacts with the atmospheric oxygen that is always present in this humid environment, turning the ground water into a dilute solution of sulphuric acid, which attacks the host rocks. Sometimes hot acidic fluids form surface springs which are easily recognised by their dirty, turbid waters and clayey, ocherous surrounds—so very different from the brilliantly clear, green-blue waters of alkaline springs in their sinter basins.
In 1989 New Zealand geologists published an inventory of this country’s geothermal landscape. In it they classified five sites as having international significance—an “A” rating. They are Ketetahi, Rotorua, Waiotapu, Waimangu and White Island, and all are in the Taupo Volcanic Zone.
The Ketetahi hot springs occur on the northern side of Mt Tongariro. In this general area andesite magmas lie close beneath the complex of craters and vents, and the volcanic heat is so intense that much of the water that soaks down is turned into high-pressure steam, rather than superheated water. Scientists call such a system vapour-dominated, and Ketetahi is the only example in New Zealand.
The most spectacular geothermal outpourings are found not inside the volcanic craters, but in steeply eroded gulleys at the head waters of Mangatipua Stream, high on the outer northern slopes of North Crater. This area is a maze of roaring fumaroles, pits of boiling muddy water blackened by iron sulphide, and hot, decomposed ground fizzing with steam. On a cold morning after a night of hard frost the Ketetahi Valley presents a fascinating contrast of ice and fire. Within a handspan from a patch of steaming, scalding ground, bare fingers of ice clench the gravel.
A hundred kilometres north of Ketetahi are Waiotapu and nearby Waimangu, two fields which are connected at depths of a kilometre or so, but which have distinct surface expressions. Waiotapu, a Crown-owned scenic reserve, is noted for its large chloride spring and associated sinter terraces, mud volcanoes, rugged collapse pits, explosion craters, and the obedient Lady Knox geyser which erupts on schedule when her barrel is loaded with soap and sackcloth wadding like an old Kentucky flintlock.
The international importance of Waiotapu rests mainly on Champagne Pool and its wide sinter apron. The pool formed some 900 years ago when a series of hydrothermal explosions blew craters and fissures deep into the geothermal system, allowing chloride water to flood to the surface. Ever since, this flowing spring has been lining its vent with silica, and depositing sinter over the flats of Artists’ Palette and Primrose Terrace, as well as down the cascade of the Bridal Veil Falls. The lime-green waters hiss as millions of tiny carbon dioxide bubbles rise and pop at the pool surface.
The waters are enriched with gold, silver, mercury and thallium which precipitate along with the orange sulphides in ore-grade quantities. Geothermal wells drilled into the system in the 1950s, but now abandoned, discovered small quantities of galena and sphalerite (the ores of lead and zinc). The Waiotapu hydrothermal system is, in fact, an ore body in formation, and scientists have speculated that gold mineralisation is occurring now deep beneath Champagne Pool. This alone makes Waiotapu something special.
The present thermal activity at Waimangu, a few kilometres north of Waiotapu, dates only from the time of the 1886 eruption of Mt Tarawerathe most recent outburst from this 18,000-year-old volcano. No doubt a hydrothermal system already existed at depth in the general area, but there were no previous accounts of hot springs at the site of what is now the Waimangu Valley.
Like many a vigorous youngster with energy to spend, this field has led a turbulent life. The name Waimangu comes from the famed “black water” geyser that erupted at the northeast end of Echo Crater from 1900 to 1904. With each explosion heaving tonnes of mud and rocks from the vent in eruptions that towered to 460m, it was hardly likely that Waimangu would lead a long life, and indeed this James Dean of the thermal fields lived hard and died young. Subsequent history of Waimangu Valley has been marked by sporadic hydrothermal blowouts, and the present Frying Pan Lake fills the hole left after a series of lethal blasts in 1917. The most recent eruptions occurred in 1981.
On some days, visitors who cross the footbridge over the hot stream from Frying Pan Lake and climb the wooden steps into Inferno Crater are greeted by the sight of opal-blue warm waters lying 12 metres below outlet level. On another day they may see the lake full to the brim, sluicing scalding water out through its over flow and down through the scrub to join Waimangu Stream. Inferno Crater is hydraulically linked at some shallow depth to the lower and larger Frying Pan Lake, which also has an oscillating temperature and discharge. Together, Inferno and Frying Pan show the most spectacular rhythmic rise and fall of any known natural lakes, hot or cold, anywhere in the world.
Just 25 kilometres north-west of Waimangu is the Rotorua hydrothermal field, with its 1200 geothermal features, including 120 alkaline chloride hot springs and five geysers. The salient feature is Geyser Flat which contains the last remaining cluster of major geysers in New Zealand, including Pohutu,which ranks with the best in the world. The thermal spots of Rotorua are found in private, public and Maori tribal land scattered across the city, though the geysers are all within the Whakarewarewa Scenic Reserve.
Marking the northeastern end of the Taupo Volcanic Zone is White Island, Whakaari, a privately owned scenic reserve lying 50km offshore from Whakatane. The island is merely the exposed, sharp end of a complex of volcanic formations which comprise the submerged White Island massif—the only active volcano at this end of the zone.
Few active craters with a resident, high-temperature hydrothermal system are as easily accessible as White Island, and over the years it has become something of a volcanologists’ playground.
The instability makes White Island all the more intriguing. The configuration of vents, conduits, fumaroles and hot springs can change quickly and dramatically. Old vents disappear or become buried, while new ones, like TV1, form in a matter of minutes, and in spectacular style. Such areas really are the heartbeat of the Earth.