and increased in the Ojos del Salado region, until Maricunga volcanism ceased 4 million years ago. This shift coincided with a gradual flattening of the subduction process since the Miocene and was accompanied by change in crustal and mantle properties that are reflected in the isotope ratios of erupted volcanic rocks. During the Quaternary, volcanism formed the edifices of Cerro Solo, El Fraile, Incahuasi, El Muerto, El Muertito, Falso Azufre, Nevado San Francisco, Nevado Tres Cruces and Ojos del Salado, which together cover over half of the area. Apart from the large volcanoes, many smaller mafic monogenetic volcanoes developed in the area, especially east of Ojos del Salado. Pleistocene volcanism was limited to the Ojos del Salado area, where recent faulting offset volcanic rocks. The large dimensions of Ojos del Salado indicate that magmatism was focused here. Composition Volcanic rocks erupted by Ojos del Salado form a calc-alkaline potassium-rich suite of dacitic rocks, with occasional andesite and rhyodacite. Earlier in the geological history of the region more mafic magmas also erupted. The rocks contain phenocrysts like augite, biotite, hornblende, hypersthene, opaque minerals, plagioclase, pyroxene and quartz. Magma mixing phenomena produced olivine and pyroxene xenocrysts and amphibole reaction rims. Climate Detailed climate data do not exist for the area. The Puna de Atacama region has an extreme climate with strong wind, high elevation, a dry climate and high insolation; the area is just south of the Arid Diagonal. Temperatures at lower elevations can exceed but mean annual temperatures only reach . Mean winds at Laguna Verde reach maximum speeds of in winter, on the mountaintops they can exceed and can impede climbing attempts. Winds blow strongest in the afternoon. The winds produce aeolian landforms such as aeolian sediments, dunes, gravel pavements, abraded rocks and megaripples at lower elevations, and redeposit snow. Annual precipitation consists mostly of hail and snow. It either amounts to less than per year or reaches per year. Compared to sites farther north falls primarily during winter although snowfall is common in summer. Precipitation probably peaks at where the cloud base lies; above that elevation it decreases to about . Snow cover in the area is sporadic and quickly sublimates, which hinders its measurement; the average snow cover is less than thick. The dry climate prevents the development of substantial glaciers in the region; only farther south at Tronquitos does more extensive glaciation begin although temporary ice and