Strokkur Geyser erupting. A geyser is a deep natural well in a geothermal rock fissure. The water lies in a large chamber at the well's base, and is heated by surrounding rocks until it is well over its normal boiling point. The normal boiling of the water is prevented by hydrostatic pressure. At a critical temperature the superheated water spontaneously boils, resulting in the eruption seen here. These eruptions repeat periodically. Strokkur Geyser is in Haukadalur valley, in south-western Iceland. It erupts every three minutes, and the eruptions can reach over 10 metres in height.