In some locations, the material apparently condenses into particles or crystals of sulfur flakes, and falls from volcanic plumes to the surface. Galileo scientists are now studying whether heating of the volatile, sulfur dioxide-rich plains by encroaching hot lava might account for the persistent plume activity observed near Prometheus. Yet the snow blanket appears to be fairly uniform across the surface of the moon, and is not concentrated at the poles like the ice caps on Earth or Mars. Researchers are still unclear on why this is. Most interesting are the appearance of volcanic calderas on Io's mountain tops.
Calderas are the pit-like scars of shallow lava eruptions.
Unlike most volcanoes which build mountains, a caldera forms when a wide area of land collapses after being undercut by a wide pool of magma. Planetary geologists aren't sure whether Io's paterae form over magma chambers or if they result from fractures and movements in the crust, and the lava subsequently follows the fractures. The calderas, which are also less circular than similar structures on Earth, seem to be linked to cracks in the moon's crust. In one area, a string of the structures follows a line across the surface, taking a right-angle bend. The more traditional volcanoes of Io are still unearthly in their heat.