Few geological phenomena inspire as much awe as a volcanic eruption. Eruptions are, quite frankly, extremely exciting to watch and experience. Could there then be any more exciting place to a volcanologist than the jovian moon Io (Plate 1), which has more active volcanoes per square kilometer than anywhere else in the Solar System? Io is the only body in the Solar System other than the Earth where current volcanic activity can be witnessed on such a wide scale. As a result of this high level of volcanic activity, Io has the most striking appearance of any planetary satellite.

The detection of an umbrella-shaped plume extending high above the surface of Io was the most spectacular discovery made by NASA's Voyager spacecraft during their encounters at Jupiter; in fact, it was one of the most important results from NASA's planetary exploration program. The discovery of active extraterrestrial volcanism meant that Earth was no longer the only planetary body where the surface was being reworked by volcanoes. With this exciting discovery a revolution in planetary sciences began, leaving behind the perception of planetary satellites as geologically dead worlds, where any dynamic process had been damped down into extinction over geologic time (billions of years).