The discovery of erupting volcanoes on Io has been one of the highlights of our exploration of the solar system. They were first seen during the flyby mission of Voyager 1 in 1979, and have since been examined in more detail by the Galileo probe which went into orbit about Jupiter in 1995. The largest eruptions can also be monitored from Earth using infrared telescopes.

Io is the innermost of Jupiter's four large satellites, and is the only one to have a surface composed of rock rather than ice. Its radius is less than 100 km greater than the Moon's, so if heating by decay of radioactive elements were its only power supply, Io's volcanism ought to have long since died away. However, because Io is so close to such a massive planet and has an orbit regularly and repeatedly perturbed by other large satellites (the nearest of which have orbital periods twice and four times that of Io), it experiences immense tidal forces. Tidal deformation stokes so much heat into Io's interior that the present day rate at which this heat escapes to space per square kilometre is at least twenty times the equivalent on Earth.