Io is a small body and cooled more quickly than the Earth, so it doesn't have the internal heat source that powers terrestrial volcanoes.

Instead, it is heated by tidal forces. Io orbits very close to Jupiter (keeping one face toward the planet in tidal lock). Jupiter's immense gravity pulls on Io, distorting its shape. Because Io's orbit is affected by Jupiter's other moons, Io's distance from Jupiter varies during each revolution and the distortions in its shape change.

Io's surface can bulge in and out dramatically—as much as 100 meters every day! This tugging causes internal friction, which produces heat in Io's interior. And that heat powers Io's volcanoes—making Io the hottest spot besides the Sun in the Solar System.