GEYSERS AND HOT SPRINGS 107 Geothermal waters can be analysed to determine their origin Isotopic ratios can differentiate between magmatic water (‘juvenile’ water, or water that has been brought to Earth’s surface by upward movement of rocks) and meteoric water (groundwater); geothermal waters are almost completely meteoric Meteoric water is water from rain and melting snow that has percolated downward In many volcanic areas, hot magma still lies close to the surface, long after lava and ash eruptions have ceased, and the rocks above such residual magma chambers remain hot As groundwater percolates down, heated rocks increase the groundwater to temperatures of more than 200 C, without boiling, because of the pressure In the simplest case, this water returns to the surface to form hot water springs and pools, hissing and bubbling up, or it is tapped in wells and boreholes Such developments at Rotorua, New Zealand, are well known tourist and health spa attractions Solfataras, Fumaroles, and Mofettes Vent openings in the ground allow escape of heated waters and vapours Escaping steam may mix with sulphurous gases and will deposit sulphur when its emerges at the surface Where sulphur dioxide and hydrogen sulphide are emitted, the hissing escape channels are ringed at the surface with yellow sulphur deposits Vent openings where such emissions occur are termed ‘solfataras’ (after La Solfatara volcano, Pozzuoli, Italy) and ‘fumaroles’ Vents that are sources of toxic gases are termed ‘mofettes’ In addition to carbon dioxide and sulphurous gases, inert gases such as nitrogen and argon can also be emitted These gases may be almost undetectable and are in such cases dangerous The principal gas emitted from most vents, carbon dioxide, can be lethal in high concentrations It was the cause of a disaster at Lake Nyos, Cameroon, in 1986 Lake Nyos, the mouth of an ancient volcano, released a lethal cloud of CO2 that asphyxiated more than 1700 people in nearby villages Geysers Where hot water cannot circulate freely below the surface, it will gush out intermittently as pressure builds up The water may appear muddy if it is mixed with material derived from the buried rock mass in which it is contained In the extreme case, the phenomenon of the geyser (named after Geysir, a locality in Iceland) occurs, and water and steam spurt from the surface intermittently in a high-reaching fountain ‘Old Faithful’ at Yellowstone, Wyoming, USA (Figure 2) reaches a height of 20 m, erupts every 60 minutes, and the eruption lasts for minutes It erupts so regularly that timetables have been created Figure ‘Old Faithful’ geyser, Yellowstone National Park, Wyoming, USA Reproduced with permission from Green J and Short NM (1971) Volcanic Landforms and Surface Features: A Photographic Atlas and Glossary New York, Heidelberg, Berlin: Springer Verlag