Russotto is right on about the tidal forces driving the vulcanism. It's like when you take a tennis ball and squeeze it, then release, then squeeze, then release and the ball heats up. That's EXACTLY what Jupiter does to Io. Voyager spotted something like 7 erupting volcanoes on its first pass of the moon.
Another amazing thing about Io is that it's slowly sending all its matter back to Jupiter with all these eruptions. You can actually SEE material erupt that never comes back down again.
There is evidence that the other moons of Jupiter have suffered the same as Io. Europa has strange patterns all along its surface that look like cracking and refreezing. Neat stuff
