formed which then enter into shallow magmatic systems. Further, the relative thickness of the crust and narrowness of the mantle wedge mean that garnet is stable within the wedge, causing the magmas to be influenced by garnet-linked petrogenic processes. Shallower crustal components such as the locally extensive Lauca-Perez ignimbrite may have been assimilated by Parinacota as well. These crustal components contributed about 12% of the primitive magmas as erupted by the Ajata cones, while the mantle wedge contributed 83%. Fluids from the slab and sediments subducted in the Peru-Chile Trench added the remaining 3 and 2%. Climate Average temperatures at Parinacota are about , with the isotherm hovering between elevation. On neighbouring Sajama, on the summit temperatures range . The atmosphere becomes thinner and drier at higher altitudes, allowing both increased solar radiation to reach the surface during daytime and more thermal radiation from the ground to escape to the top of the atmosphere during night. This pattern determines a large diurnal temperature amplitude in the region, with variations on the scale of . Average precipitation at Parinacota is about . Between about 12 and 26° degrees southern latitude, most of the moisture that arrives was absorbed by winds over the Amazon and transported to the Andes. Thus, humidity increases from west to east, with the Pacific coastline being particularly dry. Parinacota lies within the puna seca climate region, where precipitation occurs over 7 or 8 months of wet season and results in a total amount of , most of it falling during the summer months when the Altiplano warms up under the sun, generating a monsoon-like wind current. The summer precipitation is also known as the "Bolivian winter" or "Altiplanic winter". This is an unusual precipitation pattern for Chile; most of the country has a mediterranean climate where most precipitation occurs during the winter months. The arid climate is a consequence of the activity of the South Pacific High just off the coast, the rain shadow effect of the Andes and the cold Humboldt Current in the Pacific Ocean. The dry climate became apparent in the region 10–15 million years ago. The generally arid climate of the region means that volcanoes can remain topographically recognizable for a long time, being subject to only minimal erosion. Likewise, the groundwater pools in the region tend to be fairly old, going back to 13,000–12,000 years ago. The climate was not always so