Global Mercator Mosaic

Io's volcanic plains are shown in this Voyager 1 image mosaic which covers the area roughly from latitude 60 degrees N. to latitude 60 degrees S. and longitude 100-345. North is up. Numerous volcanic calderas, lava flows, and volcanic eruption plumes are visible here. The composition of Io's volcanic plains and lava flows has not been determined, but they could consist dominantly of sulfur with surface frosts of sulfur dioxide or of silicates (such as basalt) encrusted with sulfur and sulfur dioxide condensates. The bright whitish patches probably consist of freshly deposited sulfur dioxide frost.

http://photojournal.jpl.nasa.gov/catalog/PIA00319