A land of fire
The Payunia Volcanic Field, also known as Payenia, has one of the densest concentrations of volcanoes on the planet. Sprawling across more than 5,200 square kilometers, the landscape holds at least 800 individual volcanic cones. This amounts to an average density of 10.6 volcanoes for every 100 square kilometers. The entire field sits on a high plateau, with an average elevation of around 2,000 meters (6,600 feet).
The landscape is dominated by two major volcanoes: Payún Liso and the massive shield volcano Payún Matrú. The latter features a vast caldera, a collapsed crater roughly 9 by 7 kilometers in size, that formed after a massive eruption between 168,000 and 82,000 years ago. Surrounding these giants are hundreds of smaller Strombolian cones, fissures, and vents that have covered the region in black basaltic lava and pyroclastic rocks like lapilli and volcanic bombs. Vast, dark plains of volcanic gravel, known as the Pampas Negras, create an almost lunar environment. One of the field's most notable features is the Pampas Onduladas lava flow, which stretches for approximately 181 kilometers (112 miles), making it the longest known lava flow of the Quaternary period.
Reading the basalt
The Payunia field shows back-arc volcanism. It sits about 530 kilometers (330 miles) east of the oceanic trench where the Nazca Plate subducts, or dives, beneath the South American Plate. As the Nazca plate descends, it releases water into the hot mantle above it. This water lowers the melting point of the mantle rock, generating magma that eventually rises to the surface, far behind the main Andean volcanic arc. The chemical composition of the erupted basalts, rich in olivine, shows the depth and conditions of this magma generation.
Volcanic activity in the region began in the late Tertiary period and has continued intermittently into geologically recent times. The main shield of Payún Matrú itself formed over a period of about 600,000 years. While the last major caldera-forming event was tens of thousands of years ago, smaller eruptions have occurred much more recently. Geological evidence points to eruptions around 7,000 years ago (5050 BC), and some of the youngest lava flows appear to be only a few hundred years old, with one dated to approximately 445 years before the present.