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Geologic Structure
The ground floor of Las Tres Vírgenes volcano holds the oldest rock in the region which is a granitic rock of the Cretaceous era. Above this intrusive rock layer is a volcanic cover from the late Cenozoic era that includes andesite from the Santa Lucía Range, and Esperanza basalt. The Santa Rosalia Formation is beneath this volcanic cover which is consists of shallow-water marine deposits identified by fossiliferous sandstone. Above all these layers, the region consists of a variety of pyroclastic particles from different stages of its most recent volcanic activity. These rocks produced at Las Tres Vírgenes volcanic region have characteristics like those of the calc–alkaline magma series. The volcanic complex has also emitted andesitic to dacitic magmas from 300 to 22ka.