There is evidence of a large sector collapse on San Pedro, which removed the northwestern flank of the volcano and formed a large debris avalanche deposit. This avalanche deposit covers a surface area of and its front is high in the Pampa de la Avestruz. The debris flow formed by the sector collapse does not have many of the typical hummock-like hills of volcanic sector collapse deposits and instead features grooves and radial ridges. Such flank collapse occurred notably on Mount St. Helens during its eruption in 1980. A high scarp west of the eastern summit was left by the collapse of San Pedro; otherwise much of the evidence was buried by later volcanic activity. As with the mudflows, the steep slopes of San Pedro probably facilitated the onset of the sector collapse, which descended over an elevation difference of about .