A small opening on the Earth’s surface which periodically spouts a fountain of boiling water into the air. The largest fountain height recorded was 500 m, from a now extinct geyser in New Zealand. Water beneath the mouth of a geyser is heated by conduction from surrounding hot rocks, water at the base of the column boiling before that higher in the column. Expanding vapour bubbles rise in the column of water, expelling water at the top and lowering the pressure at the base. This allows the onset of further boiling, the system being self-sustaining, until the entire column of water is blown out of the system as a water spout. The water involved carries a large load of dissolved minerals which precipitate around the mouth of the geyser as siliceous sinter. The name is from Geysir, about 45 km from the active volcano Hekla, Iceland, and was first used as a technical term in 1847 by the German chemist R. W. von Bunsen, who spelled it ‘geysir’. This spelling is still sometimes used.