The eruption of Cerro Chao occurred in several phases. In a first phase, Plinian-Vulcanian activity generated pyroclastic flow deposits to the south of the system. Most of the pyroclastics were formed during this phase, although some minor deposits formed from the collapse of the forming flow. A thin lapilli layer has been linked to San Pedro volcano. Explosive activity continued during the extrusion of the Chao flow, growing the pumice cone.