page_title: Parinacota (volcano)
section_title: Sector collapse
hierarchical_section_title: Parinacota (volcano) / Geomorphology and geology / Local / Sector collapse
caption_attribution_description: Vicuñas at Chungará Lake, Chile 12.11.2005 By * Herman Luyken
context_page_description: Parinacota, Parina Quta or Parinaquta is a dormant stratovolcano on the border of Chile and Bolivia. Together with Pomerape it forms the Nevados de Payachata volcanic chain. Part of the Central Volcanic Zone of the Andes, its summit reaches an elevation of 6,380 metres above sea level. The symmetrical cone is capped by a summit crater with widths of 1 kilometre or 500 metres. Farther down on the southern slopes lie three parasitic centres known as the Ajata cones. These cones have generated lava flows. The volcano overlies a platform formed by lava domes and andesitic lava flows.
  The volcano started growing during the Pleistocene and formed a large cone. At some point between the Pleistocene and the Holocene, the western flank of the volcano collapsed, generating a giant landslide that spread west and formed a large, hummocky landslide deposit. The avalanche crossed and dammed a previously existing drainage, impounding or enlarging Lake Chungará; numerous other lakes now forming the headwaters of the Rio Lauca sprang up within the deposit. Volcanic activity rebuilt the cone after the collapse, cancelling out the collapse scar.
context_section_description: Parinacota shows evidence of a major sector collapse (a giant landslide), whose deposit was originally interpreted to be a lava flow. The collapse removed a volume of about 5–6 cubic kilometres (1.2–1.4 cu mi) from the cone, plunged over 1,900 metres (6,200 ft) vertical distance and flowed 23 kilometres (14 mi) west, covering a surface area of 110 square kilometres (42 sq mi) or 253 square kilometres (98 sq mi) 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, 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 (25–60 metres per second (82–197 ft/s) ), 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, 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 400–500 metres (1,300–1,600 ft) and heights of 80 metres (260 ft), 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 250 metres (820 ft) and volumes of 0.05 cubic kilometres (0.012 cu mi). Large blocks with sizes of up to 100 metres (330 ft) are part of the deposit, and some of these blocks preserve details of the pre-collapse structure; the blocks reach sizes of 0.5–2 metres (1 ft 8 in–6 ft 7 in) 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.
  This collapse gave birth to Lake Chungará when the avalanche flowed across a westbound drainage between Choquelimpie and Parinacota, forming a 40 metres (130 ft) high volcanic dam that retained about 0.4 cubic kilometres (0.096 cu mi) of water. The formation of lakes during sector collapses has been observed at other volcanoes, including the 1988 Mount St. Helens collapse. Prior to the collapse, alluvial and riverine deposits occupied the area. In 2015 it was proposed that a much smaller lake occupied part of the Lake Chungará basin before the collapse.
  Within the hummock-like topography of the deposit, a number of other lakes and peat filled basins are found, formed by water percolating through the avalanche deposit. These lakes are known as the Lagunas Cotacotani lak
caption: Lake Chungará
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