Most geysers are hot springs that episodically erupt fountains of scalding water and steam.
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. The world's premier geyser fields are in Iceland, New Zealand, and Yellowstone National Park, Wyoming. Some geysers erupt from cold or warm springs and are powered by gas pressure instead of the actual boiling of water.