The Jovian moon Io, its small size notwithstanding, is one of the most geologically active objects in the solar system. It is close enough to Jupiter that it is subject to constant tidal stresses, which generate a tremendous amount of heat in the moon’s interior. While most airless moons are scarred and pocked with impact craters, the surface of Io is quite smooth, due to its constantly being paved over with volcanic ejecta. Were Io much closer to Jupiter it would simply be torn apart.
Now the exploratory probe New Horizons, passing by on its way to Pluto, has sent home a remarkable image of an enormous eruption on this tormented moon. You can see it here.
Io is a truly remarkable place, unique in many ways. If you’d like to learn more, Wikipedia offers a good gateway article about this amazing little world.