SIMON FRASER / SCIENCE PHOTO LIBRARY SIMON FRASER / SCIENCE PHOTO LIBRARY
The Strokkur geyser at Geysir in Iceland. Every few minutes this hot spring throws a column of steam and superheated water 20-40 metres into the air. A geyser consists of a deep central tube with ramifying branches leading from it. Juvenile water (emanating from cooling magma) accumulates in this system and, owing to the pressure-head of the water column, superheating of the liquid takes place. When the critical temperature is exceeded, the liquid passes into the gaseous (steam) phase, whereupon the whole mass boils vigorously and fountains into the air. Strokkur ("The Churn") is close to the eponymous Geysir ("The Gusher") geyser which is no longer active.
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