The Waikato region contains about 70 per cent of New Zealand’s geothermal areas. These areas contain natural hazards such as hot springs, boiling mud pools and unstable ground.
Geothermal systems are places where the earth is hotter than surrounding areas, due to the presence of hot rock or magma near the earth’s surface. Almost all of New Zealand’s geothermal systems are hydrothermal systems, which means ground water from the surrounding area is heated by hot rocks and rises to the surface.
Scalding hot water
Springs are found throughout New Zealand where ground water comes to the surface. In geothermal areas this water is hot from its contact with hot rocks and can include geothermal springs, seeps, lakes and streams. In some cases, the water is hot enough to scald the skin, resulting in serious burns and even death.
Some hot springs are known as geysers, and occasionally erupt a mixture of steam and boiling water into the air. Many hot springs erupt very infrequently and unexpectedly. Geothermal water contains silica, which sticks to glass surfaces such as spectacles (glasses), camera lenses, watch faces, and car
windscreens causing permanent damage.
Hot lakes and springs can contain microscopic organisms (amoebas) that cause the disease amoebic meningitis. They're found in some geothermal water that has come in contact with soil, and are a danger if the water is not adequately chlorinated. You can get the disease if water containing the organism enters your nose (for example if you put your head under the water). From the nose, it quickly travels to and infects the brain. Amoebic meningitis can quickly kill a person, with symptoms including:
- severe and persistent headache
- sore throat
- nausea and vomiting
- high fever and sleepiness.
See a doctor immediately if you think you've got this disease.
Geothermal chemicals in water
Geothermal waters are thought to have healing properties, due to the minerals they contain. Minerals include arsenic and mercury, which are toxic, making geothermal water undrinkable. They can also get into the food chain, and high levels of mercury may build up in trout found in geothermal streams and lakes. Some geothermal water may be either very acidic or very alkaline, and may irritate the skin, corrode clothing or oxidise jewellery.
Like many other geothermal features, boiling mud pools are often unpredictable. A larger than normal eruption can suddenly occur, sending mud further than expected. The mud is boiling hot and will burn clothing and skin.
Steam and other gases
In some geothermal areas, although no water can be seen, steam will rise from holes in the ground called fumaroles. In some places the steam will be hot enough to burn you. The steam from fumaroles and hot springs may also drift across roads, preventing motorists from seeing oncoming traffic and other hazards.
Geothermal areas also produce toxic gases, including mercury and methane. Hydrogen sulphide is one of the major geothermal gases. It is heavier than air and will collect in confined spaces such as hollows, caves, and sometimes rooms. It may reach concentrations high enough to be fatal.
Geothermally altered ground
Rocks and soils that have been altered by geothermal activity may be at risk from subsidence and landslides. Geothermal ground may consist of a thin hard crust covering an underground cave, or tomo. The crust may give way under the weight of a person, sometimes into a scalding geothermal stream.
Geothermally altered ground can be prone to landslides as it can lose its underlying structure. One example of this is the Hipaua Cliffs beside State Highway 41 near Little Waihi at the southern end of Lake Taupo. These cliffs are unstable and prone to landslides, particularly after heavy rain.
Natural hydrothermal eruptions nearly always occur completely without warning. These eruptions can be violent enough to threaten people’s lives. Hydrothermal eruptions are caused by hot water rapidly changing to steam underground. This can occur as a result of:
- altering the amount of ground water in a geothermal area (for example, through land drainage)
- reducing the flow of heat from the area (for example, in sections of ground covered with roading material).