OREGON STATE UNIVERSITY
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Returning to the San Pedro/San Pablo area after visiting the salars, we came across the first, of many, major lava domes we would see on our trip. These enormous lava domes dot the APVC and represent a style of volcanism different than we had seen previously on our trip. The large eruptions that created the ignimbrites of the region seem to have given way to the more gentle, effusive style of volcanism which created these domes. It is possible that the formation of many of these domes may represent the waning stages of the magmatic system that gives rise to the APVC.
Right off the main road, Chanka was formed on the northwest slopes of Azufre Volcano, and just north of San Pedro. Like most of the lava domes, Chanka is primarily dacitic in composition, and is extremely crystal rich (30%-40% crystals). The crystal rich nature of the lavas that formed Chanka, along with its dacitic composition, lead to incredibly viscous lavas. Due to this incredible viscosity, many of the domes take on a standard "torta" or cake shape, having steep sides and flat top.
While the exact date of Chanka is unknown, it is probably post glacial in age.
Azufre Satellite image with Chanka to the west (Cc).
Chanka is of dacitic composition and high crystal content, generally over 30% crystals. The phenocryts seen are typically Quartz, Plagioclase, biotite and hornblende. There are frequent, finer grained, mafic enclaves present that show phenocrysts of Plagioclase, hornblende and some Olivine. The host lava also has a friable texture, due to some microvesiculation of the matrix.
