A chain of craters and "paterae" on Io

This mosaic illustrates the range of "patera" morphology on Io.
But what is a "patera"? It is an irregular depression, or a complex one with scalloped edges, but which does not have the characteristics of an impact crater. The "paterae" on Io often correspond to active volcanic centers and are in some ways similar to calderas. What's a "caldera"? "Calderas" are large, usually roughly circular depressions that form by collapse over shallow magma chambers that have been partially emptied by volcanic eruptions. However, the "paterae" on Io are different from "calderas" seen elsewhere in the Solar System. They have many straight edges and sharp angles, suggesting that they are related to fractures in Io's crust. In many cases the lava can be seen to erupt from these straight edges. Planetary geologists aren't sure whether the paterae form over magma chambers or if they result from fractures and movements in the crust, and the lava subsequently follows the fractures.