Later evidence has indicated that some lava flows were erupted from the summit during the Ch'aska Urqu stage. Also, a structure interpreted as a former lava lake formed close to the summit during this time. The lava lake-like structure itself is undated; one of the lava flows was dated 410,000 ± 80,000 years ago and the southern summit cinder cone is 292,000 ± 25,000 years old. This series is known as El Azufre. The El Azufre series was emplaced within a sector collapse, a collapse which generated pyroclastic deposits in the Poroto section of the southwestern flank.