Galileo color images showing two volcanic plumes on Io. A plume erupting over Pillan Patera was captured on edge of the moon (see main image and inset at upper right). The plume was 140 kilometers (86 miles) high. The Galileo spacecraft will pass almost directly over Pillan Patera in 1999 at a range of only 373 miles (600 kilometers). The second plume is erupting over Prometheus, seen near the center of the moon and near the boundary between day and night and the inset at lower right. In the inset image, the shadow of the plume can be seen to the right of the vent. The plume is about 45 miles (75 kilometers) high.
Two types of eruption plumes have been observed: Prometheus-type and Pele-type.
| Prometheus-type | Pele-type | |
| Plume heights | 50-120 km | up to 300 km |
| Plume character | optically thick, dark jets | optically thin (transparent) |
| Deposits | bright halos, 200-600 km diameter | dark halos, 1000-1500 km in diameter |
| Eruption Velocities | about 500 m/s | up to 1000 m/s |
| Duration | months to years | days to months |
| Location | common near equator | restricted longitudes |
| Temp. of associated 'hot spots' | about 450 K | about 600 K |
Ejection velocities for explosive eruptions are estimated to be 500 to 1,000 meters per second. Plume diameters can be as much s 1,000 km. In December 1996, Pele's plume had at a height of 460 km. Most of the plume-producing eruptions are near the equator (between 30 degrees north or south). Two of the eruption sites, called Pele and Loki, are associated with calderas. Explosive eruptions can continue for at least a few days but some wane after a few hours. Sulfur dioxide gas may be the driving force of the explosive eruptions.
High-resolution image of part of Io showing lava flows and other volcanic features on Io.
Earth-based monitoring of thermal emissions on Io have been interpreted as eruptions of surface lava flows. In 1996, two effusive eruptions produced about 3 square km of lava at eruption rates of 10,000 to 1,000,000 square meters per second. Eruption temperatures were greater than or equal to 1130C (Strawberry and others, 1997). Eruptions on Io may produce pahoehoe and aa flows, possibly as overflow from lava lakes or from fissure eruptions.