How Geysers Work
For geyser to occur there must be heat, water, and a plumbing system. A magma chamber provides the heat, which radiates into surrounding rock. Water from rain and snow works its way underground through fractures in the rock.
As the water reaches hot rock it begins to rise back to the surface, passing through rhyolite, which is former volcanic ash or lava rich in silica. The hot water dissolves the silica and carries it upward to line rock crevices. This forms a constriction that holds in the mounting pressure, creating a geyser's plumbing system.
As superheated water nears the surface, its pressure drops, and the water flashes into steam as a geyser. Hot springs have unconstricted plumbing systems.
Select Anatomy of a Geyser to learn more.