Drilling at Integrated Ocean Drilling Program Site U1381 on the Cocos Ridge offshore Costa Rica recovered 67 primary Miocene (ca. 8 Ma to ca. 16.5 Ma) marine fallout ash layers. Geochemical, volcanological, and geological criteria link these ashes to Plinian eruptions that carried ash to at least 50–450 km from the Galápagos hotspot. These ash layers are the first documentation of highly explosive Miocene Galápagos hotspot volcanism. This volcanism is bimodal with two-thirds of the tephra layers generated by basaltic magmas (glass compositions <57 wt% SiO2) and one-third by rhyolitic magmas. The temporal distribution of the tephra layers, inferred from sediment accumulation rates calibrated by 40Ar/39Ar and biostratigraphic ages, reveals a distinct increase in eruption frequency and hence increased volcanic activity of the Galápagos hotspot after 14 Ma which we interpret in the context of dynamic interaction between the Galápagos plume and spreading ridge.

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