How are geysers made?
Geysers occur in areas with high heat flow, primarily due to the presence of a magma chamber at depth (few kilometers). The magma acts like a radiator, heating the water and causing it to rise. Water is replenished from the sides (it is pulled in as the hot water rises). The pressure at the depth where this heating occurs allows water to attain higher temperatures without boiling, but as the water rises, the pressure decreases. Eventually the water reaches a pressure where the water boils; it is the expansion from boiling that drives the water upward, shooting it out of fractures in a geyser. Lots of other interesting chemical changes occur at the same time, but that is another story.
