Volcanic plumes are the most impressive manifestations of volcanism. It was fitting, therefore, that the first detection of active volcanism on Io was of a volcanic plume (Figure 1.2, Morabito et al., 1979). In the wake of discoveries made by Voyager, the Galileo mission greatly advanced understanding of Io's plumes and revealed the importance of their role in the resurfacing of Io.

Large volcanic plumes, which can reach heights of hundreds of kilometers, and their resulting surface deposits are the most visible indicators of ongoing or recent volcanic activity on Io. A variety of mechanisms form these plumes. The largest plumes are the result of explosive volcanic activity with the greatest excess pressures produced by an abundance of volatiles in the magma. Smaller plumes are formed by the interaction of recently erupted lava and surface deposits of sulphur and SO2. Even smaller plumes are formed by the relatively quiescent escape of volatiles from erupted lava and fumaroles.

Explosive activity on Io and Earth

Explosive volcanic activity is driven by the release of volcanic gases dissolved in magma at high pressure, the interaction of magma with external volatiles (e.g., with groundwater on Earth or a deposit of sulphur or sulphur dioxide on Io), or a mixture of both processes.