rocks dated to 700,000 years ago. It is comprised by the lavic and ignimbritic Pre-caldera Trachyte unit and consists of trachyandesitic to trachytic rocks, with trachyte being the most important component. The massif may have formed a tall edifice like the Payun volcano before caldera collapse. The formation of the caldera coincides with the eruption of the Portezuelo Ignimbrite/Portezuelo Formation and took place between 168,000 ± 4,000 and 82,000 ± 2,000 years ago. This ignimbrite formation where it is not buried by younger eruption products spreads radially around the caldera and reaches a maximum exposed thickness of ; it covers an area of about on the northern and southern sides of Payún Matrú, and its volume is estimated to be about . The event was probably precipitated by the entry of mafic magma in the magma chamber and its incomplete mixing with pre-existent magma chamber melts, or by tectonic processes; the resulting Plinian eruption generated an eruption column, which collapsed, producing the ignimbrites. Different layers of magma in the magma chamber were erupted during the course of the eruption and eventually the summit of the volcano collapsed as well, forming the caldera; activity continued and emplaced lava domes and lava flows in the caldera area. These post-caldera volcanic formations are subdivided into three separate lithofacies. Basaltic and trachyandesitic activity continued after the formation of the caldera. Morphology indicates that the El Rengo and Los Volcanes volcanic cones appear to be of Holocene age, while the Guadaloso vents formed during the Plio-Pleistocene. One age from the eastern side is 148,000 ± 9,000 years ago, it comes from northeast of the Payún Matrú caldera. Uneroded volcanic cones and dark basaltic lavas indicate that activity continued into the Holocene. Oral tradition by a local indigenous tribe indicate that volcanic activity occurred within the last several centuries, although no eruptions have been observed since the European settlement. Future volcanic eruptions would be unlikely to constitute a hazard given the low population density of the area, although roads might be interrupted and lava dams might form in rivers. It is considered Argentina's 24th most dangerous volcano out of 38. Various dating methods have yielded various ages for late Pleistocene-Holocene volcanic eruptions: 44,000 ± 2,000 years ago, surface exposure dating. 43,000–41,000 ± 3,000 years ago, surface exposure dating, El Puente Formation. Basaltic lava flows of this formation reach ages of about 320,000 ± 5,000 years, implying