Cerro Porquesa
Cerro Porquesa is an approximately high rhyodacite lava dome in the Andes. The lava dome is of Pliocene/Pleistocene age with little glacial features on the younger domes indicating young ages.
The dome was formed in at least three different eruption stages, with each stage contributing about two or three different lobes. Further, a rhyolitic ignimbrite with 69.5% SiO2 may be linked to the domes. It fills a valley in the south of the complex about thick. This ignimbrite is dated 0.73±0.16 and 0.63 +0.92/-0.63 mya by potassium-argon dating in biotite, although with low precision.
This lava dome is located 20° in a volcanic gap named Pica gap. In this gap volcanic activity younger than 2 mya isn't found and where lead isotope ratios in rocks change with the radiogenicity of the isotope ratio decreasing northward. Porquesa has intermediary isotope ratios. The lower ratio was principally imparted by the low-radiogenic Pb upper crust. Samples and the appearance of the domes in aerial photographs indicate a homogeneous composition with about 68% SiO2.
