The mountain is a volcano. While the date of the last eruption is not known, the degree of erosion suggests an age of about 8.3 million years. Originally the mountain was about higher and had a volume of . Pacha Qullu is a Pliocene age volcano. Its slopes range 11-16°, formed by lava flows and pyroclastic material. Erosion has carved radial gullies into its flanks, at whose ends alluvial fans have formed. The volcano has suffered a sector collapse, resulting in the formation of a wide and breach.