Why does Io have so many volcanoes? Io has hundreds of volcanoes, many of which are active. It is one of the most volcanically active bodies in the solar system. Io is heated up by the strong gravitional pulls of Jupiter on one side and the large moons Europa, Ganymede and Callisto on the other. This gravitational tugging stretches and bends Io causing it to heat up. Io's surface is covered by large sulfur lava flows and irregularly shaped mountains. It is about the size of our moon.