Roughly 1–2% of the flows erupted from flank vents of the western Galápagos shield volcanoes have anomalous compositions. We call these illegitimate magmas because of their uncertain parentage. Because some illegitimate magmas are compositionally indistinguishable from lavas of an adjacent volcano and erupt from the flank facing the adjacent volcano, such magmas apparently result from lateral intrusion of magma from the adjacent volcano. Other illegitimate magmas come from parts of the Galápagos plume that have incompletely mixed or result from unusually advanced melting of part of the mantle.

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