The distinctive, sulfurous smell of geothermal activity greets the many thousands of visitors who visit the hot springs, geysers and mud pools of the North Island’s Rotorua–Taupō region every year. These accessible natural features are unrivalled, apart from those at Yellowstone National Park in the USA. Although less publicised, there are numerous warm springs scattered throughout the North and South islands.
Geothermal activity occurs where hot water rises to the earth’s surface. The bubbling water or steam we see are just the tip of a column or tongue of hot water that may extend for hundreds or thousands of metres below the ground. This is known as a geothermal system.
Because geothermal waters contain lots of minerals, they conduct electricity better than other underground water. Scientists can map geothermal fields by making a resistivity survey – measuring how well electricity is conducted through the ground. Over a region, variations in resistivity help to identify where geothermal fields lie.
Geothermal water starts life as rainwater, which seeps down though cracks in the rock towards a heat source deep within the earth. Hot water is less dense than cold water, so it rises and emerges at the earth’s surface, sometimes as steam or mixed with steam. The hot water reacts with the rock it comes into contact with, and becomes enriched with dissolved minerals.
Scientists divide New Zealand’s geothermal systems into three main groups:
The major geothermal fields in the Taupō region give rise to the spectacular geysers, boiling springs, mud pools and fumaroles (steam or gas vents) throughout the region. These features are closely associated with active volcanoes. In volcanic zones such as Taupō the ground is heated by magma (molten rock) close to the surface.
Because of this intense heat source, water temperatures in the deeper parts of a geothermal system may be greater than 300ºC. The waters are under great pressure, so can become superheated well above their normal boiling temperature (100ºC at sea level).
Outside the Taupō region, the only other high-temperature geothermal system is at Ngāwhā, in Northland, which has been volcanically active in the last few thousand years.
Numerous other hot springs are associated with remnants of volcanic activity, particularly in Northland, Coromandel, the Hauraki Plains and the Bay of Plenty. Heat sources for these fields are diffuse rather than intense, producing fluid temperatures of less than 100ºC.
These smaller systems are confined to the North Island. Banks Peninsula and Otago Peninsula in the South Island are ancient volcanoes, but there is no longer any volcanic heat flow underneath them.
Warm springs (less than 70ºC) are found in non-volcanic areas of New Zealand. Faults – deep fractures in the rock – provide channels for warm water to rise rapidly from depths where it has been heated. Striking examples are the hot springs aligned along the Hope Fault, in North Canterbury, and the Alpine Fault, in the Southern Alps. At Hanmer, on the Alpine Fault, a range of thermal pools attract thousands every year.
When you see a very clear, boiling spring, you are looking at water from deep in the geothermal reservoir, freshly arrived at the earth’s surface after its journey up through the heated rock below. This kind of water is known as alkaline chloride. It is weakly alkaline (and slightly soapy-feeling) and has a high mineral content from contact with the subterranean rocks. Boiling springs, geysers and sinter deposits (from geysers and hot springs) are typical features of alkaline chloride geothermal systems.
Boiling alkaline chloride springs occur where geothermal fluids can rise quickly to the surface – often along a line of weakness such as a fault – without being modified at the surface by contact with rocks or soil, or the atmosphere. The cooking pools at Whakarewarewa are an example. They are far too hot for bathing, and need to be cooled by trickling over cool ground, or dilution with cold water.
Geysers are a rare and special class of boiling spring. They occur when underground water cannot discharge freely, but is forced through a narrow neck or opening from a larger reservoir below. The water pressure builds up, and the superheated mixture of water and steam intermittently erupts.
All rainwater contains minute traces of a radioactive isotope of hydrogen, called tritium. This is produced when cosmic rays in the upper atmosphere bombard the water molecules. Tritium decays to hydrogen at a constant rate, so it is possible to work out how long water has been in the ground since it fell as rain. At Rotorua, rainwater takes at least 100 years to emerge as hot spring water.
Sinter deposits are a characteristic feature of boiling springs. They are composed of almost pure silica, which is abundant in most rocks – either as pure silica minerals such as quartz, or combined with other elements as silicate minerals. Hot water passing through fractured rocks readily dissolves silica and other chemicals. When geothermal water pours out of a spring or geyser, it cools quickly, and starts to deposit some of the dissolved silica onto the nearest surface.
Pure silica is white, but sinter often contains traces of impurities or micro-organisms which produce beautiful coloured forms. Some of the more common colours are pink, from iron oxide, and grey to black, from iron sulfide (pyrite).
Sinter deposits can also take specific shapes:
If deep geothermal waters are prevented from reaching the surface quickly, we see a different set of features. The trapped waters may boil at depth, and a mixture of steam and volcanic gases (mainly carbon dioxide and hydrogen sulfide) will rise towards the surface. This is known as an acid sulfate geothermal system, because the hydrogen sulfide oxidises to sulfuric acid. The Craters of the Moon region, just north of Taupō, is an example of an acid sulfate system.
Geysers are a rare and spectacular phenomenon: hot springs which intermittently eject jets of boiling water and steam into the air. Their name is related to the Icelandic word geysa, meaning ‘to gush or spout’.
Geysers are rare and precious features – there are only about 1,000 active around the world. Of these, about half are in Yellowstone National Park, in Wyoming, USA. The only other major geyser fields are in Chile, Iceland, Russia’s Kamchatka peninsula, Alaska and New Zealand.
In the 19th century there were five major geyser fields in the North Island: Rotomahana, Whakarewarewa, Ōrākei Kōrako, Wairākei and Taupō Spa. The Rotomahana geyser field was destroyed by the 1886 eruption of Mt Tarawera, and most of the remaining geysers have been damaged or affected by human activity, especially withdrawing steam or hot water for heating. Whakarewarewa is now the only major remaining geyser field.
In the 19th century about 220 geysers were recorded in New Zealand. By 2004 only 58 geysers survived, some very small.
Geysers can form when hot water welling up from an underground reservoir has to pass through a narrow or constricted vent to the surface. The vent acts like the safety valve of a pressure cooker. The water in the reservoir becomes highly pressurised and superheated above its boiling point. Eventually, bubbles of steam form and rise through the constricted vent to the surface. This displaces some of the overlying water at the surface, which in turn relieves the pressure in the reservoir. The heated reservoir water flashes to steam, and the frothy mixture of expanding steam and boiling water is sprayed out of the vent. After this eruptive phase, the reservoir begins to fill up from below, and the cycle starts over again.
Although there are more than 500 hot springs at Whakarewarewa in Rotorua, the most obvious features are the seven geysers on Geyser Flat, where there is almost always some activity. The geysers are aligned north–south along a buried fault, through which hot water escapes to the surface.
Pōhutu, the largest geyser, regularly erupts to a height of 15–20 metres, and sometimes much higher. Prince of Wales Feathers, a few metres north of Pōhutu, began spurting in June 1886, after the Mt Tarawera eruption – probably triggered by earthquakes. Originally it was known as the Indicator, as it normally played shortly before Pōhutu erupted, but in 1901 it was renamed in honour of the royal visit that year. Since 1992 it has played almost continuously.
Although visitors to Whakarewarewa will almost always see geysers playing on Geyser Flat, most of the others that made the area famous are no longer active. In particular, the Waikite and Wairoa have disappeared, although you can see their vents, surrounded by sinter. Waikite was one of the highest above sea level (315 metres), built on a prominent sinter mound. The southern end of Fenton Street (the main street of Rotorua) was designed to offer travellers a view of the geyser.
Lady Knox geyser erupts every morning at 10.15, when it is deliberately activated by pouring soap into its vent. The area close by was originally part of a prison camp, and it is said that the prisoners discovered the action of the geyser when they were washing their clothes.
The spectacular Wairoa geyser spurted up to 50 metres high. Eruptions could be triggered by soap, which appears to have two effects: it reduces the surface tension of the water, and a reaction between soap and mineralised water creates nuclei for soap bubbles to form on. The geyser was sometimes artificially induced for important visitors.
Fumaroles are steam and gas vents. They are common on the flanks of active volcanoes as well as in geothermal fields, where temperatures are generally close to the boiling point of water.
If the steam diffuses generally upwards through the soil rather than following well-defined pathways, areas of steaming ground result. The vividly coloured rocks and soil in many geothermal areas are the result of hot, acidic gases and fluids interacting with the rock. This produces clay minerals tinted by trace amounts of minerals. Some of the more unusual shades include purple (from cinnabar – mercury sulfide), orange (from realgar – arsenic sulfide), and yellow to grey (from sulfur).
The gases escaping from fumaroles can be rich in hydrogen sulfide. When the hot gases come into contact with the atmosphere they cool and oxidise, and can form yellow crystals of pure sulfur around fumarole vents. The extensive sulfur deposits around the fumaroles of White Island (Whakaari) were mined until the 1930s.
Mud pools are an icon of New Zealand scenery. They form where steam and gas rise to the surface under rainwater ponds. The acidic gases attack surface rocks, forming clay. The clay-rich soil mixes with the pond water to produce a muddy, steam-heated slurry, or mud pool.
Rainfall affects the appearance of mud pools. In dry conditions, the mud is thick and sticky, and small mud volcanoes may form. When rainfall is high, the mud is much more fluid and the pool may look more like dark boiling water.
Large holes in geothermal areas are known as craters. The more common type is the collapse crater. This is formed when the rock beneath the surface has been dissolved away by acid waters so that the ground collapses. In such areas the ground resonates, and the possibility of collapse is a good reason not to wander off defined tracks.
If excess steam pressure builds up under the ground, a violent hydrothermal eruption will blow out debris and create an eruption crater. At Waiotapu thermal area there are more than 20 eruption craters. Radiocarbon dating indicates that these were formed 700–800 years ago, and were possibly triggered by a major eruption of Mt Tarawera, about 15 kilometres to the north-east. Eruption craters are sometimes difficult to distinguish from collapse craters, but it is usually possible to find a surrounding layer of material that has erupted from the crater.
Small hydrothermal eruptions have occurred at Kuirau Park, near the centre of Rotorua, for over 100 years. As a result the area has been made a reserve: after an eruption the affected area is fenced off, and the vegetation grows back again within a few years.
Old newspapers in Rotorua and Taupō have many accounts of thermal activity breaking out in unexpected fashion. There are stories of cold taps running from hot to boiling, and steaming cracks opening up on concrete floors. This is the price of living in an active geothermal area.
Spectacular hydrothermal eruptions occur regularly when the steel casing of neglected geothermal bores corrodes or is uncovered during excavations, allowing high-pressure steam to escape. A few new buildings have been unknowingly constructed over old vents and uncapped wells, which have later been reactivated.
Māori lived in geothermal areas from the earliest days of settlement. Ngāwhā (boiling springs) were used for cooking, and waiariki (warm pools) were used for bathing, laundry and relaxation. Puia (geysers) were probably treated with caution. Food was preserved using the available heat in the ground, and the mud from some pools was found to have medicinal properties. Vividly coloured clays such as kōkōwai (red ochre) were used as paints and dyes.
Life in geothermal areas was not without its hazards. Henry S. Bates, visiting Ōrākei Kōrako in 1860, recorded in his diary that a Māori child had fallen into the Te Mimi-a-Homoaiterangi geyser.
The gases carbon dioxide and hydrogen sulfide are ever-present threats in geothermal areas. These ground-hugging gases are heavier than air, and can accumulate to dangerous concentrations.
Carbon dioxide is not particularly toxic, but concentrations around 10% can cause asphyxiation by excluding oxygen. Hydrogen sulfide is very toxic, and a concentration of 500 parts per million will kill a person within minutes. A number of deaths in Rotorua have been due to hydrogen sulfide poisoning. The most common death traps are excavations in geothermal ground, or poorly ventilated bathing pools.
In built-up areas like Rotorua, much of the ground is covered with roads, car parks, pavements and buildings. Unable to escape through these structures, the gases are channelled under the surface to emerge wherever they can. One study of Rotorua buildings found that geothermal gases were seeping through cracks in floors, walls and skirting boards. In one house on the edge of the Arikikapakapa thermal area, life-threatening levels of both hydrogen sulfide and carbon dioxide were found near a crack in the floor. Dead insects and birds were found on the property, and the building was declared uninhabitable.
Geothermal areas are more than just rocks, minerals and hot water. They are also home to unique and remarkable plants, animals and micro-organisms. Geothermal ecosystems are protected because they are so rare and easily damaged. There is also strong – and growing – scientific interest in how inhabitants of geothermal areas adapt to living in some of the most extreme conditions on earth. Geothermal areas can be highly acidic and toxic, because of the presence of dissolved mineral components such as arsenic and mercury.
Different organisms have different abilities to adapt to high temperatures. The least heat-tolerant group is the animal kingdom, which generally has an upper limit for survival of about 50ºC.
At the Craters of the Moon thermal area, vegetation grows in zones controlled by temperature. Steaming ground may be as hot as 97ºC within 5 centimetres of the surface. No plants can survive this, so the ground is completely bare. The most heat-tolerant plants are mosses and lichens, which can survive ground temperatures of 70ºC. Clubmoss (Lycopodium cernuum) is actually a tropical moss species, which thrives here because the warm ground and steam keep the killer frosts at bay. Prostrate kānuka (Kunzea ericoides var. microflorum) is a low, spreading variety of the kānuka shrub that only grows in geothermal areas. It can tolerate ground temperatures of up to 55ºC.
Geothermal features are fragile and easily damaged. Hot springs and geysers are sensitive to any changes in their underground water supply, and are easily affected by natural events such as earthquakes and landslides. Over the last 100 years, however, the impact of humans has been vastly greater than that of natural change.
The traditional Māori activities of cooking, bathing, preserving food and heating caused only minimal disturbance. The human impact on geothermal features began early in the 20th century, when wells were sunk in Rotorua to help meet the tourist demand for hot water for bathing and heating. This accelerated after the Second World War. The population increased and there were no effective controls on the drilling and discharge of bores for domestic and commercial heating. By the 1970s the water levels were dropping in Rotorua, and there was an obvious deterioration in the geyser activity at Whakarewarewa.
New Zealand’s first geothermal power station was built at Wairākei, near Taupō. By the time the first stage was commissioned in 1958, the geysers at Geyser Valley and Taupō Spa had disappeared. When the Ōhaaki–Broadlands field was drilled, the Ōhaaki–Ngāwhā boiling pool declined.
Much of the Ōrākei Kōrako thermal area was flooded in 1961, when the Waikato River was dammed and Lake Ōhakuri was formed to generate hydroelectricity. Two hundred hot springs and 70 geysers were drowned. The area remaining above ground contains only a small proportion of the geothermal features.
Although geothermal power was thought to be an environmentally friendly, renewable resource, by the 1970s there was growing public concern about the irreversible damage to surface features, especially geysers. The situation was compounded by legal uncertainty about how geothermal development was regulated and the responsibilities of local and central government.
In a landmark legal case in 1982, the Court of Appeal affirmed that geothermal exploration involved natural water, and that a water right was therefore necessary. Over the next decade this led to a regulatory regime under regional councils, who have responsibility for monitoring and preserving natural geothermal features.
The government started monitoring geothermal activity in the Rotorua area in 1982. Three years later the Ministry of Energy confirmed that there had been a dramatic decrease in geyser activity, related to a 30% decrease in heat flow between 1967 and 1980. Bores within 1.5 kilometres of Pōhutu geyser were closed in 1988.
Since then the water level and pressure in the Rotorua thermal area have generally increased. The geysers and hot springs are no longer dwindling, but the major geysers such as Waikite and Wairoa have not recovered.
Concerned about the lack of a comprehensive summary of geothermal features, the Geological Society of New Zealand undertook an inventory of all New Zealand’s geothermal fields, and prioritised sites. They paid particular attention to rare or unique features.
Five geothermal fields – White Island (Whakaari), Rotorua, Waimangu, Waiotapu and Ketetahi – were considered to be of international significance, and their complete preservation (including protection from further drilling) was recommended. This has generally been followed in subsequent regional plans.
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Houghton, Bruce F., and others. Inventory of New Zealand geothermal fields and features. Miscellaneous publication 44. Lower Hutt: Geological Society of New Zealand, 1999.
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