Because of the yellow colour of Io's surface, it was once thought that most of Io's lava flows were emplaced as molten sulfur. However it is now recognised that the global dispersion of sulfur and its oxides from explosive eruptions means that colour is no guide to composition in this case. Furthermore, infrared observations of active lava flows and other eruptions have revealed that the temperature is far too high for molten sulfur but consistent with molten rock as on Earth.

Io appears to have retained little or no water. The role played on Earth by water as the source of most volcanic gas is on Io taken up by sulfur, and Io's eruption plumes are powered mostly by the explosive expansion of sulfur dioxide. Io has virtually no atmosphere, so ejected particles follow ballistic trajectories, imparting an umbrella-shaped profile to the plumes. Io's whole surface is stained yellow by sulfur and various oxides of sulfur that have been distributed by eruption plumes. Io's surface pattern is continually changing as a result of this process.