what is thought to cause IO's volcanism?

Answer 1

In earth, the main responsible for our tides is the moon, a small body in the sky, but capable to deform the whole seas in the earth. Now imagine a giant like Jupiter instead of the moon, its enormous gravity pull would deform not only the seas, but the planet itself. This is what happens in Io, the deformation produces friction and consequently heat, which melts the rock core and produces the volcanism.

Answer 2

It is also due to the fact that Io is within Jupiter's radiation field which adds more warmth to the environment on Io. But like JulienCC said, gravity tides are the biggest contributor to Io's volcanism.

Answer 3

Io is the nearest Galilean moon of Jupiter, the biggest planet of the solar system.
Io's volcanism is due to a gravity effect, the difference of the attraction (tide) between two points of Io is so important that it deform and heat (by friction) Io.

Answer 4

Io is caught in a gravitational tug-of-war between Jupiter and the other large moons of Jupiter (mostly Ganymede and Europa). The moons pull Io one way, Jupiter pulls it the other, and this back-and-forth gravitational tension has caused the interior of Io to heat up. That (plus the tidal effects already described by other answers) are the main causes of Io's activity.