The neighbouring La Pacana caldera between 4.5 and 4.1 million years ago erupted some of these ignimbrites including the Atana ignimbrite. Occasionally Purico is considered part of the La Pacana system. Composition The Purico complex has erupted various different magmas, ranging from the dacitic Purico ignimbrite over rhyolitic pumices contained in the ignimbrite to the andesitic-dacitic post-ignimbrite volcanics. Dacite is the dominant component and forms a crystal-rich potassium-rich suite. Varying amounts of phenocrysts occur in the Purico complex rocks; the minerals they are formed of include augite, biotite, clinopyroxene, hornblende, hypersthene, iron oxides, oligoclase, orthopyroxene, plagioclase, quartz and titanium oxides. Additionally, mafic xenoliths are found in the Purico ignimbrite; such xenoliths are a common finding in volcanic arc rocks. They are even more common in Chascon rocks, where they might reflect the occurrence of mafic magma in the feeder system prior to the formation of Chascon. Some physical properties of the Purico magmas have been inferred from the chemistry and petrology of the erupted rocks. The dacites had temperatures of about while the andesites and rhyolites reached higher temperatures, up to . Water contents ranged from 3.2 to 4.8% by weight, while carbon dioxide concentrations were low throughout. Climate and vegetation The climate at Purico is cold (mean temperatures ), the air thin because of the high elevation and ultraviolet radiation is high. There is little precipitation in the area (about on the upper parts of the shield, decreasing to less than close to the Salar de Atacama), which mainly happens during the summer months as a consequence of the South American monsoon. Snow also falls during winter but winter snow mainly evaporates while summer snow melts. This dry climate is due to the combined effects of the subtropical ridge, the Humboldt Current in the Pacific Ocean and the rain shadow exercised by the Andes, but it was in the past interrupted by wet periods. The dry climate and high elevation mean that vegetation is scarce in the region, with cacti such as Echinopsis atacamensis and grasses occurring at lower elevations. The little vegetation that is present displays an altitudinal zonation with a lower "Prepuna" with shrubs and succulents, a middle "Puna" with grasses and shrubs and a "high Andean steppe" with bunch grass. A report in 1993 stated that red-brown cacti and brown grass grew around the foot of Purico. Conversely, the soils on the Purico complex contain a