Since Voyager, we know the volcanic nature of Io. Volcanoes eject materials at several hundred kilometers altitude. In fact, this ejection is probably closer to geysers that volcanoes. The blue color indicates the presence of sulfur, which after condensation falls as "snow" to the surface of the satellite.
The Galileo probe has shown us that the temperature of the volcano was very high (1000°C) and that the lava should be formed of silicates.