Nine eruption plumes were observed over a period of 6 1/2 days during the Voyager 1 encounter with Io. During the Voyager 2 encounter, 4 months later, eight of these eruptions were still active. The largest plume viewed by Voyager 1 became inactive sometime between the two encounters. However, a major eruption occurred at Surt sometime between the two encounters and deposited an ejecta blanket comparable in size to those associated with the largest plumes. The plumes range in height from about 60 to over 300 km with corresponding ejection velocities of about 0.5 to 1.0 km/s. Where topographic information exists (plumes 1 and 8), the plume source is located on level plains rather than topographic highs and consists of either fissures or calderas. With the exception of plume 1 (Pele), the brightness distribution monotonically decreases from the core to the top of the plume. The shape and brightness distribution together with the pattern of the surface deposit of at least plume 3 (Prometheus) can be simulated by a ballistic model in which the ejection velocity is constant (0.5 km/s) and ejection angles vary from vertical (0¿) to 55¿. The brightness distribution of plume 1 is probably better explained by a shock front near the top of the plume. Numerous surface deposits similar to those associated with active plumes probably mark the sites of recent eruptions. The distribution of active and recent eruptions appears to be concentrated in the equatorial regions and indicates the volcanic activity is more frequent and intense in the equatorial regions than the polar regions. This suggests that the depositional rate is greater and the surface age younger in the equatorial regions, which may account for the darker polar regions. The geologic setting of certain plume sources and the very large reservoirs of volatiles required for the active eruptions suggests that sulphur volcanism rather than silicate volcanism is the most likely driving mechanism for the eruption plumes.