The last activity occurred 710 ± 60 years before present and formed the Chufquén scoria cone on the northern flank.
[4] The communes of Curarrehue, Cunco, Panguipulli, Pucón and Villarrica are in the area,[5] Melipeuco lies 20 km (12 mi) northwest.
[7] Sollipulli is part of the Southern Volcanic Zone of the Andes,[8] one of the four belts of volcanoes which are found in the mountain range.
[4] A number of particular landforms on Sollipulli formed under the influence of glacial ice, such as the caldera structure.
[8] Melting of the glacier risks generating lahars and putting water supplies in the region into jeopardy.
[25]Subduction has been ongoing on the western side of South America since 185 million years ago and has resulted in the formation of the Andes and volcanic activity within the range.
[9] Sollipulli volcano developed on a 600-to-1,600-metre-high (2,000–5,200 ft) basement which consists of the Jurassic–Cretaceous Nacientes del Biobío formation and the Pliocene-Pleistocene Nevados de Sollipulli volcanics, with subordinate exposures of the Miocene Curamallín and the Cretaceous-Tertiary Vizcacha-Cumilao complex volcano-sedimentary formations.
[14] Minerals contained in the rocks include apatite, clinopyroxene, ilmenite, olivine, orthopyroxene, plagioclase and titanomagnetite.
[10] It is one of the largest volcanic eruption to hit northern Patagonia,[38] but did not severely affect vegetation in the region.
[39] Tephra from this eruption has been found in neighbouring lakes[40] and in sediment cores from the Argentina Pampa;[41] layers of tephra identified in a bog of South Georgia 3,000 km (1,900 mi) away[42] as well as in an ice core at Siple Dome in Antarctica and dated to about 980 BCE may be a product of the Alpehué eruption.
[44] Pyroclastic flows from the eruption melted the caldera ice sheet, forming lahars[a] that propagated northwest away from Sollipulli.
They are brown to grey in colour and unwelded with the exception of part of the ignimbrite that is emplaced within the Alpehué crater.
[46] The eruption reached a level of 5 on the volcanic explosivity index[b][1] and led to changes in regional plant communities.
[48][49] Isla 2017 proposed that tephra from the eruption blocked the former outlet of Aluminé Lake into the Biobio River via the Rio Litrán, thus redirecting its flow into the Atlantic Ocean.
It lies within a glacial valley presently traversed by[13] the Allipén River,[23] which has cut a vertical canyon[53] into the Nevados de Sollipulli volcano, close to the intersection between two faults.
[23] Geothermal features include "spouters", geysers, hot springs and mud pools, as well as geyserite and sinter deposits[53] that form small terrace structures.
[52] The substantial ice body in the caldera means that there is a significant risk of mudflows or glacier bursts in the case of renewed activity.
[12] Conversely, a retreat of the caldera-filling glacier might facilitate the onset of explosive eruptions at Sollipulli.
[56] The Chilean geological service Sernageomin monitors the volcano and publishes a hazard index for it.
[3] Melipeuco has devised a Volcanic Emergency Plan to deal with future eruptions of Llaima or Sollipulli.