The summit of the volcano is an elliptical caldera wide and deep, formed by collapses of the summit and explosive eruptions. The caldera walls are made of lava flows bearing traces of hydrothermal alteration; the caldera floor is covered by lava flows and pyroclastic debris from explosive eruptions. It contains one or two ash cones with a triangle-shaped crater wide and deep; its walls are fractured and hydrothermally altered. Geophysical surveys have indicated the presence of an even larger buried caldera in Ubinas.