Galán is a caldera with topographic dimensions of , of which about are part of the caldera proper. Such dimensions make Galán one of the biggest calderas on Earth. The floor of the caldera reaches an elevation of or about , and the whole caldera has an elliptical shape extending in the north-south direction. Only the western margin of the caldera structure appears to be a true caldera margin, however, with different landforms forming the rest of the caldera walls and the actual collapse caldera covering only a portion of the topographic caldera expression; the latter has been defined to be a volcano-tectonic depression.