Geysers are hot springs that episodically erupt fountains of scalding water and steam. Such eruptions occur as a consequence of groundwater being heated to its boiling temperature in a confined space (for example, a fracture or conduit). A slight decrease in pressure or an increase in temperature will cause some of the water to boil. The resulting steam forces overlying water up through the conduit and onto the ground. This loss of water further reduces pressure within the conduit system, and most of the remaining water suddenly converts to steam and erupts at the surface.«Geysir» is an Icelandic word meaning «to gush or rush forth».

Eruption of Lady Knox geyser, Rotorua, New Zealand. Foto: J. Alean

A lot about geysers of Yellowstone National Park