When parts of the Earth no longer have volcanic activity on the surface, the hot magma underground can still heat water.
Hot water builds up pressure and shoots out of a hole or crack in the ground as geysers.
The water that comes out of the ground leaves an empty space underground where it just came from. The empty space fills with water, is heated by the hot magma and again builds up pressure. When there is enough pressure the geyser once again shoots up through the ground. The Learning Company, Inc.