Alkaline chloride systems
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:
- Small cones, resembling miniature volcanoes, can form around geyser vents.
- Terraces form when the discharge from a hot spring flows over a suitably sloping surface. The fabled Pink and White Terraces, at Lake Rotomahana near Rotorua, were probably the world’s most famous example.
- Smaller-scale forms include ‘geyser eggs’, the rounded masses found close to geyser vents such as Waikorohihi geyser at Whakarewarewa.
Acid sulfate systems
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.