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A geyser (Strokkur Geysir) erupting in IcelandRainwater may trickle down through cracks in rocks to deeper layers—on average, 2000 metres (6500 feet)—where it is heated by magma that has seeped into the Earth's crust from the mantle below. The water comes blasting back out, usually at predictable time intervals, as a fountain of steam, spray and hot water known as a geyser. A geyser can shoot up to 30 metres (100 feet) into the air. Wherever geysers are found—most commonly in the Yellowstone National Park (Wyoming, USA), Kamchatka Peninsula (Russian Far East), Chile, Iceland and New Zealand—there are also hot springs, pools of boiling mud and fumaroles, holes through which steam and other gases burst out.

Old Faithful geyser erupts{more}
Cross-section through a volcanic caldera, showing magma pooling just below the Earth's surface and fissures in the rocks....Read More >>Cross-section through a volcanic caldera, showing magma pooling just below the Earth's surface and fissures in the rocks. Geysers, fumaroles and mud pots form in these conditions.

Superheated water

Geysers are caused by water gradually seeping down through the ground until it meets rock heated by magma. This heating is sometimes called geothermal energy: heat energy created by the Earth itself. The heated water then rises back towards the surface through narrow tubes found in porous and fractured rocks.
Under great pressure from the weight of cooler water above, the water becomes superheated—that is, it remains liquid at temperatures well above the normal boiling point (100°C or 212°F at sea level). When the pressure is finally released (or the water becomes hot enough), the superheated water explodes into steam. The resulting froth of rapidly expanding steam and hot water then shoots out of the narrow geyser vent. The eruption continues until all the water is forced out of the ground, or until the temperature inside the geyser drops below boiling point once more.

Geysers are rare features. They occur only where there is a coincidence of unusual conditions: surface water, a source of intense heat and an underground system of fissures and chambers.

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