Origins

The andesite inclusions contained in the lava are characteristic for magma mixing processes. The eruption forming the flow may have been caused by the injection of andesite in a pre-existent homogenous dacitic magma chamber. The injection did presumably modify the crystallization processes in the magma chamber and the volatiles in the magma to the point of forcing an eruption.^[1]

The magmas that gave rise to Cerro Chao may either be the remnants of a previous magma body that gave rise to the neighbouring calderas of the Altiplano-Puna volcanic complex, or may be the sign of a new injection of magmas in the crust. The significance of these theories is controversial.^[1]

Structure

Cerro Chao is a 14 kilometres (8.7 mi)-long coulee. It has a volume of 26 cubic kilometres (6.2 cu mi) and its flow front is 400 metres (1,300 ft) high. Based on volumetric considerations, the eruption lasted about 100-150 years with an average lava flux rate of 25 cubic metres per second (880 cu ft/s). The volume of Chao is exceptional for a lava dome structure, although the lava flux rate generating it is low in comparison to a basaltic eruption like Laki in Iceland. This low flux rate is insufficient to cause caldera formation. Cerro Chao is the largest Quaternary silicic lava flow in the world.^[1] The vent location is related to an inferred fault zone emanating from one of the neighbouring volcanoes.^[5]

The flow is underpinned by a pyroclastic apron that extends 3 by 4 kilometres (1.9 mi × 2.5 mi) from the flow front. Most of it is buried beneath the flow and only on the eastern side does some material emerge; its volume is estimated at 1 cubic kilometre (0.24 cu mi). This deposit is formed from several layers of pumices separated by erosion surfaces; at least one layer may be derived from Paniri volcano.^[1]

An overlapping pair of pyroclastic cones sits on top of the Chao flow and form its eruption vent. The cone has a dense rock equivalent volume of 0.5 cubic kilometres (0.12 cu mi) of lapilli and blocks. The northern side of the cone rises 100 metres (330 ft) from terrain while the southern side is partially breached.^[1] The highest point of the cone lies at 5,169 metres (16,959 ft) altitude. Its morphology suggests that it formed from a lava dome when it collapsed over the vent.^[2]

The eruption of Cerro Chao occurred in several phases. In a first phase, Plinian–Vulcanian activity generated pyroclastic flow deposits to the south of the system. Most of the pyroclastics were formed during this phase, although some minor deposits formed from the collapse of the forming flow. A thin lapilli layer has been linked to San Pedro volcano. Explosive activity continued during the extrusion of the Chao flow, growing the pumice cone.^[1]

The flow proper is subdivided in three subunits, the first two named Chao I and Chao II. Originally subdivided because of their morphology, they most likely represent various pulses of the same eruption. They have a combined volume exceeding 22 cubic kilometres (5.3 cu mi) and are formed from a long southbound flow with some lateral spillage. The flow itself is 14 kilometres (8.7 mi) long and its flow front is 400 metres (1,300 ft) high. Its structure is massive and lobate, with lobe diameters expanding downflow from 0.5 to 1.8 kilometres (0.31 to 1.12 mi). The flows are covered by ogives (up to 30 metres (98 ft) high and with spacing of 50 by 100 metres (160 ft × 330 ft)) and some structures interpreted as fossil fumaroles.^[1] The ridges are drawn out on the western flow margin.^[2] The folds in the surface layers may have been caused by the surface stiffening more quickly than the underlying flow due to cooling.^[5] The flow surface is blocky, with blocks occasionally displaying flow banding. The lowest Chao I flow covers an area of 52 square kilometres (20 sq mi).^[2]

The Chao III flow has a smaller volume of 2 cubic kilometres (0.48 cu mi) than Chao I and II. It has less ogives than Chao I and II and forms a single lobe, 150 metres (490 ft) high. The flow overlies the pumice cone and parts of the Chao II on its eastern side. A lava dome formed above its vent and underwent several collapses, generating collapse scars. The flow is covered by weathering-derived aeolian debris from the other flows.^[1] This flow has a surface area of 13 square kilometres (5.0 sq mi).^[2]

Petrology

The Chao flow is of dacitic composition, with some non-vesicular small andesitic inclusions that are more numerous in the Chao III and upper Chao II stages, up to 5% of the volume of some Chao III lavas and vesiculated there. The lava has a porphyric texture owing to its high crystal content of 45% and displays extensive flow banding. Chao III lavas have lower concentrations of crystals. Phenocrysts in the lava contain biotite, hornblende, plagioclase and quartz. Some hornblende crystals have diameters of up to 2 centimetres (0.79 in). Apatite and zircon are accessory minerals. Based on geochemical considerations, the magmas equilibrated at depths of 7–8 kilometres (4.3–5.0 mi) and temperatures of 840 °C (1,540 °F).^[1]

Geologic history

Potassium–argon dating and argon–argon dating performed on rocks from the Chao I stage have indicated an average age of 423,000 ± 100,000 years. However, anomalous chemical compositions of the dated rocks suggest that they may overestimate the true age of the volcanics. Such alteration may be the result of the inclusion of xenocrysts or K leaching. A glacial moraine system lies on Cerro del León at 4,500 metres (14,800 ft) altitude. One of these moraines abuts Cerro Chao, indicating that the dome must be older than the moraine and thus older than the last glaciation 11,000 years ago.^[1] An active magmatic body may still exist under Cerro Chao and Paniri.^[3]