Quetrupillán lies on the border between the Los Rios Region and the La Araucanía Region, in the Southern Volcanic Zone. Together with Villarrica and Lanín it forms a northwest-southeast alignment of volcanoes, which may be a transcurrent fault. The Cordillera El Mocho and Quinquilil volcanoes are likewise situated on this alignment, both are deeply eroded composite volcanoes of small dimensions. Other volcanoes in the Southern Volcanic Zone have similar alignments, such as Nevados de Chillán and Puyehue-Cordon Caulle. In comparison to Villarrica, Quetrupillán has been less active and its eruptions were also smaller than Villarrica's, with no large pyroclastic flows found at Quetrupillán. Quetrupillán is a high composite stratovolcano with a wide caldera and a shrinking glacier cover. The volcano contains in total two nested calderas, the larger of which is wide, and a field of lava domes, maars and pyroclastic cones that occupy a surface of. These subsidiary vents include the scoria cone Huililco, the Volcanes de Llancahue and the Volcanoes de Reyehueico. Fissure vents of Pleistocene-Holocene age occur on the southern side of the volcano. The small volume of the main Quetrupillán edifice and the widespread vents may reflect the interaction between the volcano and the Liquiñe-Ofqui fault, which generated fault-controlled secondary vents. A number of eruption products show traces of ice-lava interactions. A geomagnetic anomaly at shallow depth south of the volcano may be a pluton associated with a resurgent dome. Huililco scoria cone has produced two lava flows and is considered to be also part of the Quetrupillán volcanic complex. Three different formations make up the basement of Quetrupillán: The Triassic Panguipulli, the possibly Cretaceous Currarehue and the Miocene Trápatrapa formations and plutonic rocks. These are plutonic and volcaniclastic rock units.
Composition
Volcanic rocks at Quetrupillán have a bimodal composition, ranging from basalt to andesite, and overall more silicic than the rocks erupted by Villarrica and Lanín. Unusually for the region, trachydacite also occurs at the volcano. These contain phenocrysts of plagioclase and pyroxene, with additional microphenocrysts of ilmenite and magnetite. Based on the composition, it has been inferred that the magma reservoir of Quetrupillán contained a mush of crystals, from which magma was repeatedly mobilized following the injection of fresh magmas that reheated the mush. Fractional crystallization of basalts generated trachytic melts.
Eruptive history
Eruptive activity at Quetrupillán commenced before the ice ages. The first phase of activity involved the formation of calderas and stratovolcanoes; later during the ice ages lava flows and ignimbrites were emplaced. Finally, the present stratovolcano with its caldera was emplaced towards the end of glaciation; parasitic vents formed even later and produced lava flows. Quetrupillán has erupted pyroclastics, which have formed flow and pumice deposits east of the volcano. Several phases of volcanic activity have been inferred from the deposits; most of them feature either pumiceous or scoriaceous pyroclastic flow deposits with varying contents of juvenile lapilli, lithics and ash fall deposits.
The Moraga sequence was formed 12,720 ± 40 - 12,690 ± 40 years before present during one rather prolonged eruption.
The Puala sequence was formed 10,240 ± 40 years before present.
The Trancura sequence was formed 8,680 ± 40 years before present and has a similar composition to the Avutardas sequence.
The Carén sequence was formed 3,800 ± 30 years before present.
The Correntoso sequence was formed 2,930 ± 30 years before present.
The Trancas Negras sequence was formed 2,060 ± 30 years before present.
The Puesco sequence was formed 1,650 ± 70 years before present, during the largest known eruption of Quetrupillán. This eruption created a high eruption column and deposited about of rock. A volcanic explosivity index of 4 has been assigned to this event.
The Carén sequence was formed 1,380 ± 30 years before present, it is the youngest explosive eruption of Quetrupillán.
In addition, three tephras in neighbouring lakes dated to 16,748 - 16,189, 15,597 - 12,582 and 12,708 - 12,567 years before present may originate from Quetrupillán but they have also been attributed to Sollipulli. All these tephras are of rhyolitic to rhyodacitic composition and the eruptions that generated them have an estimated volcanic explosivity index of 3. Reports exist of eruptions during the 19th century, one eruption was reported in 1872. Explosive activity has a recurrence interval of about 1,200 years, which given the age of the last event carries significant implications with regards to the volcanic hazard of Quetrupillán.
