Slight changes in the rate of lava flow from Io's mantle and the heating of cooler crust below the surface create the mountain-forming faults. Maps of all the mountains and volcanoes on Io's surface were also made. Tohil Mons on Io. The top image was taken at low resolution and a low Sun angle during Galileo's third orbit, in 1996. Because the Sun is low, topographic features on the mountain can be recognized from the shadows they cast.
Labels indicate the peak of the mountain and two volcanic depressions, called paterae.
The topography is almost indistinguishable, but many more details can be discerned. Of course, volcanoes are notoriously unstable, so if volcanism falters in one region, the surrounding crust begins to heat. This causes the crust to expand, generating compressive thermal stress in the crust, which in turn forces the crust apart, forming faults and mountains. This helps explain why concentrations of mountains are seen on Io that are separated from concentrations of volcanoes. The researchers propose that similar events may have occurred on Europa, another satellite of Jupiter, and the early Earth.