Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1289
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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 5 minutes. It
erupts so regularly that timetables have been created
Figure 2 ‘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.
