The petrogenesis of Lastarria rocks, like those of other volcanoes in the Central Volcanic Zone, involves the prolonged interaction with crustal rocks in magma chambers as well as the fractionation of certain minerals. Enriched lower crust and upper mantle might also contribute. Finally, the mixing of magma chamber contents with new and more mafic magma shortly before each eruption played an important role in rock genesis. In the case of Lastarria, this mixing occurs in a stratified magma chamber, with active convection occurring between lighter and colder upper contents and hotter and denser lower contents.