What are geysers

A.� Geysers are very impressive displays of hydrothermal energy. Rainwater or snowmelt seeps through cracks in the ground and meets up with hot underlying rocks.

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However, despite reaching boiling point, the water doesn't boil because of the overlying rock. Instead it becomes superheated and pressurized.

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Eventually the pressure becomes so great that event the weight of the overlying rock can't contain it and it explodes from the ground, erupting steam into the air: a geyser.

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Q.� Where am I most likely to see a geyser

A.� They occur where magma lies just below the earth's surface, particularly in volcanic regions such as Iceland or New Zealand, and places that have been volcanically active in the past, including Yellowstone in Wyoming, USA.

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Q.� Is it a one off event

A.� No, once one has formed it kick starts a cycle and the eruption of steam repeats itself. The blast of steam clears out the fissures, more water seeps into the hot areas along the fractures, heats up and starts building pressure all over again.

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Q.� How frequently do they occur

A.� It varies from geyser to geyser, some every few minutes, some every few years. The aptly named Old faithful in Yellowstone Park in the USA erupts every hour or so. The relationship between the duration of Old Faithful's eruption and interval between them illustrates the pressure-building pattern: a short eruption is followed by a short interval until the next eruption. However long eruptions of five minutes or so will mean a wait of around an hour and a half until the next one.

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Q.� So the regularly erupting geysers are the ones to look out for

A.� Well, not necessarily, some of the more unreliable performers are worth the wait. Steamboat, also in Yellowstone Park, may not erupt for years at a time but when it does it produces some of the highest blasts, up to 400 feet high.

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Q.� Are geysers permanent features

A.� No, in geological terms they're quite transient, lasting only a few thousand yeas or until a major geological event, like an earthquake, disrupts the fissures and cracks that create them.

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Q.� How many geysers are there

A.� There are around 700 worldwide, and nearly half of them are in Yellowstone Park.

Where does the name come from

The name 'geyser' comes from the Icelandic word 'geysir', which translates as 'gush'.

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by Lisa Cardy