the most common type of volcanic plume on io are dust plumes, or prometheus-type plumes, produced when encroaching lava flows vaporize underlying sulfur dioxide frost, sending the material skyward. examples of prometheus-type plumes include prometheus, amirani, zamama, and masubi. these plumes are usually less than tall with eruption velocities around . prometheus-type plumes are dust-rich, with a dense inner core and upper canopy shock zone, giving them an umbrella-like appearance. these plumes often form bright circular deposits, with a radius ranging between and consisting primarily of sulfur dioxide frost. prometheus-type plumes are frequently seen at flow-dominated eruptions, helping make this plume type quite long-lived. four out of the six prometheus-type plumes observed by "voyager 1" in 1979 were also observed throughout the "galileo" mission and by "new horizons" in 2007. although the dust plume can be clearly seen in sunlit visible-light images of io acquired by passing spacecraft, many prometheus-type plumes have an outer halo of fainter, more gas-rich material reaching heights approaching that of the larger, pele-type plumes.