As I recall, volcanic activity on Io deposits about a millimeter of material on the surface every year. This is matter brought up from the interior and spread over the surface.
The volcanic activity is caused by tidal stress. Io's orbit around Jupiter is not perfectly circular; its distance varies as it orbits Jupiter every 42+ hours. Since tide varies with the cube of the distance, and Io is the closest of the 4 large moons, it's much more strongly affected than the other 3 large moons. The tidal stress causes it to flex, which generates heat.
One millimeter per year doesn't seem like much, but it works out to a meter every thousand years, a kilometer every million years, and 1000 kilometers in a billion years.
Since the radius is just over 1800 kilometers, that's more than enough to recycle the entire body of the moon over the 4+ billion year lifetime of the Solar System.
This assumes that material is brought all the way from the core up to the surface. I don't know (and I don't know that anyone knows) just how deep the redistribution of matter goes.
Io is massive enough that its gravity forces it to retain a spherical shape. It's not perfectly spherical (it has mountains taller than Everest), but over time it's very close. It doesn't significantly affect its orbit or destroy the moon because the eruptions aren't powerful enough to eject much matter to escape velocity; it all falls back on the surface.