Periglacial phenomena are observed on Llullaillaco, commencing at altitude and reaching their maximum around on the Chilean and on the Argentine side. There, solifluction and cryoplanation surfaces are developed, including lobe-shaped ground and block ramparts. These landforms have been mapped on the northwestern side of the volcano. Patterned ground is also common. Permafrost is found at higher altitudes. Cryoplanation and solifluction landforms are also observed on Iris and Mitral. Other than on periglacially influenced terrain, the ground at Llullaillaco is formed mostly by lavic rocks and block debris, which are frequently buried by tephra.