Water under pressure can remain liquid at temperatures above its normal boiling point, but in a geyser, the weight of the water nearer the surface exerts so much pressure on the deeper water that the water at the bottom of the geyser reaches much higher temperatures than does the water at the top of the geyser.
As the deep water becomes hotter, and consequently lighter, it suddenly rises to the surface and shoots out of the surface in the form of steam and hot water. In turn, the explosion agitates all the water in the geyser reservoir, creating further explosions. Immediately afterward, the water again flows into the underground reservoir, heating begins, and the process repeats itself.