Io, Jupiter's innermost moon and
the most volcanically active body in the solar system, is interesting because its source of heat is believed to be tidal heating: Jupiter's gravity pulls Io inwards while Jupiter's other moons pull Io outwards. This leads to a weird, eccentric orbit, which in turn means changing levels and directions of gravity leading to movement
of Io's mass, e.g. molten rock under the surface. This causes friction that heats Io, whence volcanism.