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A hot spring that at regular intervals discharges a mixture of water and steam, often many metres high. In the geyser, the water flows through a narrow vent to a geothermal reservoir where it is heated by the nearby presence of magma. But the water pressure in the chimney above the reservoir acts like the lid of a pressure cooker, so that the water is heated above the boiling point without turning to steam. Eventually, though, some of the water starts to boil and rises upwards, forcing part of the cooler water out of the chimney and reducing the pressure in the chimney, causing the superheated water to rapidly turn to steam and burst out. Then cooler water from the surface flows back into the geyser and the whole heating process repeats again.