Story: Hot springs, mud pools and geysers
Page 5 – People, animals and plants in geothermal areas
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.