It seems that Io’s little volcano (Tvashtar) actually isn’t — a volcano, that is.
At about 300km radius on a moon of 1800km radius, Tvashtar ought long ago to have simply split the moon in half — if it was indeed a volcano. This also opens the question of how Io managed to dredge up the energy to toss so much hot rock around.
There are many structural parallels between Tvashtar’s plume & a solar flare, which are not shared with definitely-volcanic objects, so now all we have to do is get astronomers to admit that there is (another) plasma feature kicking around the major planets. Hmmm. Not so trivial. (-: