This moon is the most geologically active in the Solar System with over 400 volcanoes.
The main cause of this is tidal friction. The moons Europa and Ganymede as well as the planet Jupiter, provide gravity that push and tug Io. This causes the crust to bend back and fourth and as the molecules bump together, heat is generated. This is what provides the volcanic power of Io.
Io has an atmosphere that consists mainly of sulphur dioxide. This atmosphere is stripped by Jupiter's magnetic field but constantly replenished by volcanic activity and the sublimation of sulphur dioxide ice.