Payún Matrú is a shield volcano in the Reserva Provincial La Payunia of the Malargüe Department, south of the Mendoza Province in Argentina. It lies in the back-arc region of the Andean Volcanic Belt, and was formed by the subduction of the Nazca Plate beneath the South American Plate. Payún Matrú, along with the Llancanelo, Nevado and Salado Basin volcanic fields, form the Payenia province. It has been proposed as a World Heritage Site since 2011. Payún Matrú developed on sediment and volcanic rocks ageing from the Mesoproterozoic to the Tertiary periods. It consists of a large shield volcano capped by a caldera, formed during a major eruption between 168,000 and 82,000 years ago, a high compound volcano (known as Payun or Payun Liso), and two groups of scoria cones and lava flows. The Pleistocene Pampas Onduladas lava flow reaches a length of 167–181 km (104–112 mi) and is the world's longest Quaternary lava flow. Volcanic activity at Payún Matrú commenced during the Plio-Pleistocene period, and generated lava fields such as Pampas Onduladas, the Payún Matrú shield volcano and the Payun volcano. After the formation of the caldera, volcanism continued both within the caldera as lava domes and flows, and outside of it with the formation of scoria cones and lava flows east and especially west of Payún Matrú. Volcanic activity continued into the Holocene until about 515 years ago; oral tradition of local inhabitants contains references to earlier eruptions.
Name In local dialect, the term Payún or Paium means "bearded", while the term Matru translates as "goat". The field is sometimes also known as Payenia.
Geography and geomorphology
Regional Payún Matrú lies in the Malargüe Department of the Mendoza Province, in Argentina. The area is inhospitable due to the lack of usable water and high elevation. Nevertheless, there are many paved roads such as National Route 40 which passes west of the field, and National Route 186 which runs around its northern and eastern parts. In the early 20th century, the area was prospected for copper mining. The volcano is within the Reserva Provincial La Payunia. Owing to the variety of volcanic landforms, the province was included in the 2010 Tentative List of UNESCO World Heritage Sites and a number of potential geosites have been identified at Payún Matrú itself. The active field is part of the backarc area of the Southern Volcanic Zone, a 1,000 km (620 mi) long volcanic arc and one of four eruptive belts in the Andes; the other three being the Northern Volcanic Zone, the Central Volcanic Zone and the Austral Volcanic Zone. Other volcanoes in the region include the Laguna del Maule, almost due west from Payún Matrú.
Local Payún Matrú is a 15 km-wide (9.3 mi) shield volcano whose foot coincides with the 1,750 m (5,740 ft) elevation contour and which extends mainly east–west; rising about 2 km (1.2 mi) above the surrounding terrain it covers about 5,200 km2 (2,000 sq mi) of land with lava and has diverse landforms. Ignimbrites cover and flatten its northern and eastern slopes, while in the west and south lava domes and coulées predominate; these have often rough surfaces and are difficult to traverse. The lower slopes are more gentle and covered by Pleistocene-Holocene lava flows. Wind erosion has created flutes, grooves and yardangs within the ignimbrites, such as in the western sector where yardangs reach heights of 8 m (26 ft) and widths of 100 m (330 ft). The total volume of this shield is about 240 km3 (58 cu mi).
A 7–8 km-long (4.3–5.0 mi) and 480 m-deep (1,570 ft) caldera lies in the summit region of the shield and covers a surface area of about 56 square kilometres (22 sq mi). It is surrounded by several peaks, which clockwise from north include the 3,650 m-high (11,980 ft) Nariz/Punta del Payún, the Punta Media, the 3,450 m-high (11,320 ft) Punta Sur and the approximately 3,700 m-high (12,100 ft) Cerro Matru or Payen. In the field however Cerro Matru appears smaller than Nariz. The caldera was once 8–9 km (5.0–5.6 mi) wide but erosion of its flanks and later activity have reduced its size and buried the rim below coulées, lava domes, lava flows and pumice cones that were emplaced after the caldera collapse. The exception are the northern and southern walls which are almost vertical; remnants of old andesitic and trachyandesitic volcanism crop out there. The caldera also contains a permanent lake known as "Laguna" that is fed by snowmelt and by occasional rainfall.
Matrú's highest active point field is the 3,796 m (12,454 ft) high, conical, eroded Payun stratovolcano. It is also known as Payun Liso, Payún and Payún Liso. This volcano rises 1.8 km (1.1 mi) from the southern side of Payún Matrú, 10 km (6.2 mi) away from the caldera. It has a summit crater open to the north and it has a volume of about 40 km3 (9.6 cu mi).
Payún Matrú volcanic field Aside from the caldera, the field contains about 300 individual volcanic vents with diverse morphologies distributed in a western Los Volcanes group that reaches to the Rio Grande River and the eastern Guadaloso and El Rengo groups. These fields are also known as West Payún Matrú or West Payén and East Payún Matrú or East Payén, respectively. Two further ruptures, known as "Chapua" and "Puente", have been identified east of Payún Matrú. All these groups include fissure vents, lapilli cones, scoria cones and strombolian cones. These edifices are up to 225 m (738 ft) 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 12,000 km2 (4,600 sq mi) 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 350 km3 (84 cu mi); the volcanic edifice was generated mostly through Strombolian and Hawaiian eruptions.
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