A parasitic vent named La Poruña lies on the western foot of San Pedro, its formation was probably influenced by a normal fault that runs in that area. It is a wide and high scoria cone on top of a field of lava flows which extend as far as west of San Pedro and from the cone. The cone's western side slumped and lava flows issued from the breach. In addition, a lava dome at an elevation of lies on the southwestern flank of San Pedro and also seems to be a parasitic vent. Andesite lava flows emanate from this area as well. They are the largest found on San Pedro, with volumes of and a surface area of ; they reach a distance of c. from the vent.