of Payún Matrú. All these groups include fissure vents, lapilli cones, scoria cones and strombolian cones. These edifices are up to high and are associated with lava flows and pyroclastic units; the vents in the Los Volcanes group are spread across two separate belts. Wind-driven ash transport has formed ash tails at individual vents. Older lava flows have pahoehoe surfaces with lava tubes and pressure ridges, while Holocene flows are more commonly aa lava with blocky surfaces. Some flows have reached the Rio Grande River west of Payún Matrú, damming it; the river later cut through and formed table-like landforms and canyons. One of these is a slot canyon known as La Pasarela, where the structures of lava flows such as joints in the rocks and vesicles are clearly visible. The entire field covers an area of over and some of its flows have reached the Llancanelo Lake north of Payún Matrú and the Salado River in the east. The estimated volumes of the entire Payún Matrú volcano are as large as ; the volcanic edifice was generated mostly through Strombolian and Hawaiian eruptions. The cones are aligned along easterly or northeasterly lineaments which correlate with geological structures in the basement, and appear to reflect the tectonic stresses underground. Among these lineaments is the La Carbonilla fracture which runs in east–west direction and crops out in the eastern part of the field; in the central sector it is hidden by the caldera and in the western it is buried by lava flows. The La Carbonilla fracture is a fault that appears to have been an important influence on the development of the Payún Matrú complex in general. Fissural ridges and elongated chains of vents and cones highlight the control that lineaments exercise on the volcanic eruptions. In the summit area, pumice cones are aligned along the caldera rim. Among the cones in Payún Matrú are the Plio-Pleistocene (5.333 million years ago until 11,700 years ago) Morados Grandes east and the cones around Pihuel volcano northeast of the field, respectively; the Guadalosos, La Mina and Montón de Cerros cones in the northern part of the field; and the Holocene cones in the eastern and western part of the field. Among these the Los Morados, Morado Sur and Volcán Santa María cones in the eastern and northeastern part of the field are uneroded and are probably of recent age. These cones are