An eruption of Geysir, Iceland in the first week of August 1985. Geysir (sometimes referred to as the Great Geysir) was the first hot spring known to Europeans to throw jets of water and steam into the air and its name became a collective term as other hot springs with similar characteristics were discovered around the world, i.e. geysers.
Geysir is hundreds of years old and alternates between phases of activity and rest. Active periods are brought about by earthquakes that open up cracks and fissures that feed the geyser. As time passes, the cracks gradually become clogged up again by the precipitation of dissolved minerals in the water.
Two earthquake events in 2000 woke Geysir up from a period of slumber, but it has now been dormant for several years (as of 2011). During periods of low activity, eruptions can be stimulated by lowering surface tension with the introduction of a surfactant such as soap, which was the case in the eruption shown above. Such stimulation has ceased due to environmental reasons.
Geysir is located in the Haukadalur valley and is accompanied by a multitude of other hot springs and geysers, including Strokkur. Strokkur is a very active geyser that erupts every 5-10 minutes or so and sends fountains of water up to 15-20 m. It does not rival the Great Geysir, but is commonly mistaken for its larger sibling that is currently in a phase of rest.