San Pedro formed in two stages, which are known as the Old Cone and the Young Cone. The Old Cone was formed by lava and scoria and later largely buried by glacial, mudflows and volcanic deposits of the Young Cone; it forms about 80% of the volume of the present-day San Pedro edifice. One date has been obtained on Old Cone lavas by argon-argon dating: 160,000 years ago. After activity of the Old Cone ceased, glacial and fluvial erosion dissected San Pedro until the large sector collapse occurred. The so-called white airfall with a volume of as well as pyroclastic flows on the western summit may have been erupted during the collapse but this is uncertain.