Geology of Costa Rica

Arenal volcano is situated in the Central American volcanic chain, on the boundary between the northern and central Costa Rican segments. The Cocos Plate is being subducted under the Caribbean Plate along the Mesoamerican trench northeast of Arena l. Costa Rica consists of six principal geological provinces paralleling the Mesoamerican trench: 1) the Cretaceous to Middle Tertiary ophiolitic suite; 2) Tertiary basins; 3) Tertiary volcanic ranges; 4) Active Quaternary volcanic ranges; 5) Intra-arc b asins; and 6) the Caribbean coastal plain.

(1) A Cretaceous to Middle Tertiary ophiolitic suite is contained in the Nicoya Complex, which is comprised of cherts, graywackes, tholeiitic pillow lavas and basaltic agglomerates. It is in turn intruded by gabbroic, diabasic, and dioritic rocks.

(2) The Tertiary basin is composed of sediments of mainly marine origin, intercalated with volcaniclastic deposits.

(3) The Tertiary volcanic range is made up of a block-faulted horst, the Sierra de Tilaran y Abangares, extending southeast into the Montes del Aguate. Composed of andesitic and basaltic flows, volcanic agglomerates and tuffs, this range is part of the late Miocene-early Pliocene Aguacate Volcanic Group.

(4) The active Quaternary Volcanic ranges comprise the Cordillera de Guanacaste to the northwest and Cordillera Central to the southeast. The Cordillera de Guanacaste contains five stratovolcanoes (Orosi, Rincon de la Vieja, Miravalles, Tenorio and Arenal), eruptive products of which have compositions that vary from basaltic andesite to andesite. Large-scale rhyolitic and dacitic tuffs crop out on the southwest flank of the Cordillera and overlie part of the Aguacate Volcanic Group and Nicoya Complex. The Cordillera Central consists of four stratovolcanoes (Poás, Barba, Irazu, and Turrialba), deposits of which have compositions that vary from basalt to dacite and andesite. Extensive mudflows and volcanic ash deposits crop out on the southwest side of the Cordillera.

(5) The Intra-arc basins comprise the Arenal graben to the northwest and the Valle Central to the southeast.

(6) The Valle Central basement consists of slightly folded Oligocene a nd Miocene marine sediments, overlain by tuffs, lavas and ignimbrite sheets of the Cordillera Central.

(7) Finally, the Caribbean coastal plain is a sedimentary basin of Early Tertiary age composed of river alluvium and lahar deposits from the volcanoes of t he Cordillera Central. Some small Quaternary cinder cones also are found in the coastal plains near Tortuguero.