The Campo de la Piedra Pómez Ignimbrite covers an area of about north of Cerro Blanco and has a volume of about . It was emplaced in two units a short time from each other. They both contain pumice and fragments of country rock, similar to the Cortaderas Synthem. The most reliable radiometrically obtained dates for this ignimbrite indicate an age of 73,000 years; older estimates of their age are 560,000 ± 110,000 and 440,000 ± 10,000 years before present. The eruption reached level 6 on the volcanic explosivity index and is also known as the first cycle ignimbrite. The source vent for this eruption has not been found, there is no agreement whether the Robledo Caldera is the source. The volcano-tectonic depression northeast of Cerro Blanco has been proposed as a source. As with the Cortaderas Synthem, this ignimbrite was produced by a boiling-over vent and the pyroclastic flows lacked the intensity to override local topography. It is possible that the eruption proceeded in two phases, with a magmatic reinvigoration of the system between the two. After the ignimbrite cooled and solidified, cracks formed in the rocks and were later eroded by wind. The Robledo and Pie de San Buenaventura calderas were formed during the early activity.