geyser - on Earth, a fountain of boiling water spraying a few tens or hundreds of feet into the air. The water is forced into the air by the pressure of its own vapor (steam), which is created by heat from surrounding hot rock. On Io, the geysers are called plumes, and the vaporizing fluid is sulfur dioxide. The plumes reach heights of tens or hundreds of miles, instead of feet, because of Io's lower gravity, insignificant air resistance, and a more efficient acceleration process believed to occur as the gas escapes from large holes in the ground.