Pateras are very large, but low, volcanoes with large calderas. The calderas form when magma chambers underneath the volcano are rapidly emptied by an eruption. The empty chamber cannot support the rock above, and the roof collapses. Pateras are different from the tall shield volcanoes such as Olympus Mons on Mars or the volcanoes which formed the Hawaiian islands on Earth.
The Mbali, Mihr, Maasaw, Loki, Ra and Babbar Patera are all volcanoes which have recently erupted. Though they are similar to one another, each of them has its own unique features. The Ra Patera has many long flows emanating from it. These lava "fingers" stretch for as much as 200 km across Io's surface.
The Loki Patera is larger than the island of Hawaii. It is possible that the large black region is a lake of liquid sulfur which is about 250 km across. Another remarkable thing about the Loki Patera is the striking change in its appearance in the time between the Voyager 1 and 2 flybys.
The eruptions of these pateras are less violent and large than volcanoes like Pele, but they last much longer. The base of the plumes of these eruptions generally measures 300 to 400 km. Volcanism is associated with heating of Io by tidal pumping. Europa and Ganymede, two large satellites nearby, perturb Io in its orbit
and Jupiter pulls Io back again. The pumping action causes tidal
bulging up to 100 meters (330 feet) on Io's surface, compared
with typical tidal bulges on Earth of one meter (three feet).
Voyager 1 identified nine currently active (erupting) volcanoes on Io, probably driven by tidal heating. Many more are suspected. Voyager 2 observed eight of the nine; the largest shut down by the time Voyager 2 arrived at Jupiter. Plumes from the volcanoes reach more than 300 kilometers (190 miles) above the surface. The material was being ejected at velocities up to1.05 kilometers a second (2,300 miles an hour). By comparison, ejection velocities at Mount Etna, one of Earth's most explosive volcanoes, hit 50 meters a second (112 miles an hour).
