Space probes have discovered circles in the surface. These circles are called coronae. They were formed when hot magma from the mantle pushed the ground up from beneath. This caused the surface to bulge. When the magma drained away, the bulge collapsed. What was left was a circular basin ringed with ridges. Some volcanoes may have been created by magma pushing its way up through the softened ground of a corona. As more and more lava poured through the hole, a giant peak formed.
