The summit of Llullaillaco is formed by a small cone with about four associated lava domes, which reach lengths of and have abrupt walls. Large dacitic coulees emanate from the summit of the volcano and are young in appearance. One extends north of the volcano and the other one south. Their overall length is and their flow fronts are up to thick. Morphologically, these flows are reddish-black aa lava flows and feature blocks with sizes of . The southern lava flow is long and was fairly viscous when it was erupted. It almost reaches a road southwest of the volcano. These flows feature levees and ridges. At first they were considered to be Holocene, but argon-argon dating indicates they are of late Pleistocene age. Some traces of glacier activity are found in the summit area.