The collapse event resembled the one that occurred on Mount St. Helens during the latter 's eruption in 1988 , although the Parinacota collapse was three times larger . A separate , minor sector collapse occurred on a lava dome on the southwestern foot of the volcano at an unknown time . Such sector collapses are a common phenomenon on volcanoes . The avalanche eventually came to rest in a large " L " with the long side extending along the axis of the collapse and the short side closer to the edifice pointing north , formed an exceptionally well preserved debris avalanche deposit . This deposit has a " hummocky " appearance typical for sector collapse deposits ; individual hummocks can reach sizes of and heights of , with the size decreasing away from the volcano . The formation of these hummocks was probably influenced by the pre-existing structure of the edifice ; much of the original stratigraphy of the pre-collapse edifice was preserved within the final collapse deposit . A few large Toreva blocks lie in the avalanche deposit just at the foot of Parinacota , they reach heights of and volumes of . Large blocks with sizes of up to are part of the deposit , and some of these blocks preserve details of the pre-collapse structure ; the blocks reach sizes of even at large distances from Parinacota . These large blocks dominate the avalanche deposit ; fine material is not present in the Parinacota collapse deposit , an unusual feature among debris avalanches . The avalanche deposit displays a noticeable split into two units ; the upper one is andesitic and originated from the actual cone , the lower one is derived from the lava domes beneath the present-day edifice
latter 's eruption in 1988
1980 eruption of Mount St. Helens
Parinacota (volcano)