climate is dry owing to the rainshadow effect of the Andes which block moisture bearing winds from reaching Payún Matrú, and strong winds and the evaporation associated with them reinforce the dryness. In the westerly part of the volcanic field most precipitation falls during winter under the influence of the Andes, while the eastern part has most precipitation occurring during summer. The higher parts of Payún Matrú may have risen above the snowline during ice ages, and periglacial landforms have been observed. Palynology data from south of the region indicate that the climate has been stable since the Late Pleistocene. The vegetation in the volcanic field is mostly characterized by sparse bushes as well as herbaceous vegetation but few trees, and is classified as xerophytic. Soils are shallow and are mainly rocky to loess-like. Representative plant genera are Opuntia cactus and Poa and Stipa grasses. Payún Matrú is a refuge for a number of animals such as the armadillos, black-chested buzzard-eagle, condors, Darwin's rhea, guanaco, mara, Pampas fox or South American gray fox, puma and Southern viscacha. Some lizards may have evolved on the volcanoes. Eruptions The geological history of the Payún Matrú volcanic field is poorly dated but the field has been active since Pliocene at least. The older volcanism appears to be located in the eastern part of the field where ages of 0.95 ± 0.5 to 0.6 ± 0.1 million years ago have been obtained by potassium-argon dating. Lava flows have been subdivided into the older Puente Group and the younger Tromen Group formations, which are of Pleistocene to Pleistocene-Holocene age, respectively; a Chapua Formation of Plio-Pleistocene age has been defined as well. The eastern volcanism is also known as the Pre-caldera basaltic unit; a western counterpart to it is probably buried beneath younger eruption products. The first volcanic activity occurred west and east of Payún Matrú and involved the emission of olivine basalt lava flows. The long Pampas Onduladas lava flow was erupted 373,000 ± 10,000 years ago and buried parts of the 400,000 ± 100,000 years old Los Carrizales lava field; both have hawaiitic composition. The Payun volcano formed around 265,000 ± 5,000 years ago within a timespan of about 2,000–20,000 years. Its inferred eruption rate of is similar to typical volcanic arc eruption rates such as at Mount St. Helens. The main Payún Matrú massif formed in about 600,000 years, with the oldest trachytic