[1][2] In the late Cretaceous, an oceanic trench or backarc system formed in connection with a subduction zone, situated where the Isthmus of Panama is now located.
On the Osa Peninsula, the Nicoya Complex preserves oceanic crust with basaltic lava, dolerite, gabbro, limestone, chert and argillite, obducted onto land before the Oligocene and then rearranged by Miocene wrench faults.
[3] In the Neogene the low angle subduction of the Cocos Plate led to volcanism in the now extinct Cordillera de Aguacate chain in the center of the country.
In the Pleistocene, calderas ejected huge quantities of silica-rich ash, filling the Valle Central basin, affecting the Tarcoles Gorge and generating the Orotina debris fan at the coast.
[4] In the past two million years, Costa Rican volcanoes have erupted andesite, rhyolite and dacite with geochemical patterns that suggest the magma may have come from the melting of metamorphosed basalt.