Check out my page on this crater for more info. Craters on Io are covered by lava almost as soon as they form. The mountains are isolated, no large mountain-chains like along plate boundaries on Earth. Also, many of the calderas have a depth in excess of 2 km. Chaac Patera, a volcano in the antijovian hemisphere noted for its green floor, is 2.8 km deep.
Some volcanoes are calderas, which are pits formed by a collapsing lava chamber.
Others are formed when due to lava taking advantage of a basin formed by an extensional fault or split-apart fault. One volcano, Hi'iaka Patera, was formed by a strike-slip fault where the two sides of the fault slide past each other.