One of the biggest mysteries concerning the origin of many recent volcanic ash deposits in NW Argentina has been solved. New data and interpretation about a major eruption — spreading more than 100 km3 of ashes over about 500.000 km2 — occurred around 4200 years ago in the Cerro Blanco Volcanic Complex. This eruption was the biggest of the last five millennia in the Central Volcanic Zone of the Andes and was possibly one of the largest Holocene eruptions in the world.
This discovery offers researchers an excellent, extensive regional chronostratigraphic marker for reconstructing mid-Holocene geological history over a wide geographical area of South America. The recognition of this significant volcanic event may shed new light on interpretations of critical changes observed in the mid-Holocene environmental, paleontological and archaeological records.
An improved understanding of the widespread ash-fall deposits occurring within the Upper Quaternary deposits in NW Argentina is key because they can serve as potential stratigraphic markers and provide a temporal framework for estimation of sedimentation rates, topographic reconstructions, assessment of landslide hazards, and archaeology. Moreover, they allow for modeling of the environmental geochemical impact of volcanism. It was thought that these deposits were related to multiple eruptions from the Central Volcanic Zone of the Andes due mainly to the limited and sometimes controversial available ages of proximal volcanic products in the potential source areas of the Puna. This situation led us to conduct new research on these deposits.
The Cerro Blanco Volcanic Complex, encompassing an elevation ranging between 3500-4600 m, is part of the Cordillera de San Buenaventura volcanic field in NW Argentina (26°45′S, 67°45′W, Fig. 1). It is situated on the southern border of the Altiplano-Puna Plateau, where active volcanism occurs within the plateau and along its margins. 4200 years ago, the activity of this volcanic complex produced lava-domes, proximal pyroclastic flow deposits, and proximal and distal fall deposits of rhyolitic composition.
Location map of Cerro Blanco volcano. Figure courtesy José Luis Fernández Turiel
During the climax of the eruption, a Plinian column of ash and gases reached more than 30 km high. Once in the stratosphere, more than 100 km3 of ash were dispersed by strong winds to the east, affecting an area of more than 500,000 km2. The thickness of the resulting deposits exceeded several tens of meters near the vent to more than 30 cm near Santiago del Estero (at 400 km from the source). This thickness decrease is not linear because it is greatly affected by the relief. Thus, 3-4 m thick deposits were produced at places more than 200 km from the volcano.
Another impressive impact of this eruption was the pyroclastic flow deposits that filled as far as 35 km of valleys located around of Cerro Blanco.
The implications of these findings of the Cerro Blanco eruption reach far beyond providing an excellent chronostratigraphic marker for reconstructing mid-Holocene geologic history for an extensive area of South America. On the other hand, it is unlikely that an extensive tephra deposit would remain on the Earth’s surface for very long without reworking and redeposition. The interaction of ash with wind and water in the large affected hydrological basins mobilized huge amounts of both particulate material and chemical elements to the Chaco-Pampean Plain (Fig. 1). The impact of this eruption on the environmental, palynological, faunal and archaeological records are open questions that may help to prevent hazards associated with these large eruptions.
These findings are described in the article entitled The large eruption 4.2 ka cal BP in Cerro Blanco, Central Volcanic Zone, Andes: Insights to the Holocene eruptive deposits in the southern Puna and adjacent regions, recently published in the journal Estudios GeolĂłgicos.