A comprehensive field and ⁴⁰Ar/³⁹Ar investigation of Neogene volcanism in Costa Rica (>200 new age determinations) provides new insights into how the Central American volcanic arc evolved since the early Miocene. Arc magmatism dates back to at least 24 Ma, but volcanism has been strongly episodic, with important peaks at 16-17, 4-6, 1-2, .4-.6, and <.1 Ma, and regional lulls in volcanism between .1 to .4, 2.1 to 3, and 6.5 to 10 Ma. The modern composite volcanoes (e.g. Poas, Barva, Miravalles) were built mainly during the last two peaks. Gabbro to granodiorite plutons widely exposed in the Talamanca Range, Tilaran Range, and Central Valley have emplacement ages ranging from ~17-3.5 Ma. Most plutons are 7-10 Ma, emplaced during an apparent gap in volcanism. Concordant Kspar, biotite, and hornblende ages from a number of these plutons indicate that they cooled quickly from >500° C to <200° C.

There has been a 30° counterclockwise rotation of the arc from its middle Miocene position to the present volcanic front, with a pole of rotation centered on southern Costa Rica. The early-middle Miocene arc (24-15 Ma) extends from SE Nicaragua through the San Carlos region to the Talamanca Range in southern Costa Rica. 15-10 Ma arc rocks are known only in southern Costa Rica and may be largely buried to the north. 6-3.5 Ma arc rocks extend the length of Costa Rica, more or less coincident with the modern arc. At ~3.5 Ma, volcanism shut off in southern Costa Rica, due to subduction of the Cocos Ridge. The CCW rotation of the arc in Costa Rica between 15 and 8 Ma is attributed to shortening in southern Costa Rica coeval with extension in NW Costa Rica and Nicaragua, accompanied by trench retreat in the north.

Previous workers suggested that the Costa Rican arc evolved from a more primitive Miocene marine arc dominated by tholeiitic compositions to the modern calc-alkaline arc dominated by andesite to rhyolite. We see no dramatic difference between the Miocene and modern arc, either in terms of average silica content or range of compositions. Silicic ignimbrites are widely exposed in Costa Rica, more that is generally appreciated. For example, ignimbrite eruptions in the vicinity of the Central Valley are now known at .33, .49, .57, .92, 1.59, 2.1, 4.0, and 6.1 Ma. Large ignimbrite eruptions pose a significant hazard to the densely populated Central Valley region.