the discovery of volcanic plumes at pele and loki in 1979 provided conclusive evidence that io was geologically active. generally, plumes form when volatiles like sulfur and sulfur dioxide are ejected skyward from io's volcanoes at speeds reaching , creating umbrella-shaped clouds of gas and dust. additional materials that might be found in the volcanic plumes include sodium, potassium, and chlorine. although striking in appearance, volcanic plumes are relatively uncommon. of the 150 or so active volcanoes observed on io, plumes have only been observed at a couple of dozen of them. the limited area of io's lava flows suggests that much of the resurfacing needed to erase io's cratering record must come from plume deposits.