Io boasts some of the most impressive topography in the Solar System, with mountains higher than Mt. Everest and volcano-tectonic depressions deeper than the Grand Canyon on Earth. The shapes of these volcanic depressions, associated shields, and volcanic cones and the morphology of lava flows on Io's surface yield important clues to the nature of the magma and the interior processes that generate the observed geomorphology.

Paterae on Io and calderas on Earth

The most common volcanic feature on Io, a patera, is currently defined by the International Astronomical Union, guardian of planetary nomenclature and feature names, as an “irregular saucer-like crater.” This may be a misnomer because few saucers are perfectly flat, as the floors of many paterae appear to be. Io's paterae have steep walls and arcuate margins and, geomorphologically, are unique to Io. However, in some respects they do resemble some calderas formed on Earth, Mars, and Venus (e.g., Radebaugh et al., 2001). In some cases, notably at Maasaw Patera (Figure 1.7c), the craters are nested and look strikingly similar to nested calderas at the summit of Mauna Loa, Hawai'i, on Earth, and Olympus Mons on Mars (Figure 15.1a, b).

Four hundred and twenty-eight paterae have been mapped within a region covering about 70% of Io's surface (Radebaugh et al., 2001). Locations are shown in Plate 13b.