Astronomy Fun Fact #60
Io is one of Jupiter's largest moons and it's the most volcanically active world in the entire Solar System!
Io is pretty close to Jupiter, so the moon's interior is constantly being stretched and contracted by Jupiter's gravity. That kind of interaction is called a tidal force.
All of those tidal forces generate energy, which produces heat, and that heat has to be released somehow! So nearly all of Io's surface is volcanically active.