These images of Io's South Pole show several volcanic features.  All of the somewhat circular marks on the surface are volcanoes which have erupted fairly recently.  The largest of these volcanoes near the South Pole is Haemus Mons.  It has a base which measures approximately 100 by 200 km.  Peaks such as Haemus Mons can be as much as 10 km tall.  The lighter rings surrounding the dark circles are volcanic material, probably some form of sulfur, which has been spewed from the central vent.