with debris; the volume is not very well established. As the volcano grew, it put more and more load on relatively weak sedimentary material that the volcano had developed on, deforming it, until these sedimentary rocks gave way. The western slope might have been weakened by glacial action, further facilitating the onset of the collapse. The collapse was probably sequential from the lower part of the edifice to the summit, and it formed an avalanche of rocks that flowed down the volcano. This flow was probably laminar and extremely fast ( ), judging from the morphologies of the avalanche deposit, and it incorporated substantial pre-collapse sediments from the Lauca basin. As the avalanche descended the slopes of the volcano, it picked up enough speed to run up on some topographical obstacles. Such collapses have occurred on other volcanoes in the CVZ such as Llullaillaco, Ollagüe, Socompa and Tata Sabaya; the most recent event occurred between 1787 and 1802 at Tutupaca in Peru and was much smaller than the Parinacota sector collapse. The collapse event resembled the one that occurred on Mount St. Helens during the latter's eruption in 1980, 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 where its advance was limited by tomography, 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. As the avalanche came to rest, compressional ridges formed with axes perpendicular to the movement of the avalanche. 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