Geysir (), sometimes known as The Great Geysir, is a geyser in south-western Iceland, that geological studies suggest started forming about 1150 CE. The English word geyser (a periodically spouting hot spring) derives from Geysir. The name Geysir itself is derived from the Icelandic verb geysa ("to go quickly forward"). as a mention in Gesta Danorum, his work finished about 1206. However the oldest definitive accounts of the hot springs at Haukadalur in their present form, date back to 1294, in the chronicle Oddaverjaannáll, when earthquakes in the area caused significant changes in local neighbouring landscape, creating several new hot springs. Changes in the activity of Geysir and the surrounding geysers are strongly related to earthquake activity. In records dated 1630, the geysers erupted so violently that the valley around them trembled. The name "Geysir" was first mentioned in written sources in 1647; as unusual natural phenomena were of great interest during the Age of Enlightenment, the term became popular and has been used for similar hydrothermal features worldwide since then.
thumb|upright|1873 drawing of Geysir
In 1809 and 1810 the eruptions were about every 30 hours and up to high. but it stopped all activity around 1908. Initially, Geysir eruptions were taking place on average eight times a day. By July 2003, this activity had again decreased to around three times per day. Large eruptions after this became so rare that one in 2016 was considered newsworthy.
Geology
A current geological definition of a hot water geyser is "…a hot spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapor phase" which is a technical correction of historic definitions that mention steam and a column in air.
Ownership of the area
Until 1894, the Geysir area was owned by local farmers. In that year the area was sold to James Craig (later Lord Craigavon), a whiskey distiller from Ulster and a future Prime Minister of Northern Ireland. Initially, he erected large fences around the site and an entrance fee was charged for visitors wishing to view the geysers.