Geology of Volcán Arenal
Arenal has a volume of only 15 km3 and is the smallest but most active of seven historically active Costa Rican volcanoes. The tectonic setting of the volcano is disputed, with some authors suggesting that Arenal overlies a tear in the subducting Cocos plate and others believing there is a smooth transition in the orientation of the Wadati-Benioff zone, thought to lie 150 km below Arenal. The small truncated and dormant volcano, Cerro Chato, lies approximately three kilometres southeast of Arenal. Arenal is most likely directly tapping a lower to mid-crustal magma chamber, possibly located at a discontinuity which lies at a depth of 22 km.
Three stages of differing magma compositions at Arenal are believed to coincide with variations in eruptive activity. Stage-1 zoned magmas likely resided in the magma chamber prior to the 1968 eruption. A new magma intruded into the chamber resulting in the ejection of the stage-1 magma in July 1968. It subsequently mixed with the more mafic parts of stage 1 to produce stage-2 magmas. Stage-3 magmas (mid-1974 to present) are the product of continued mixing and fractional crystallisation along the walls of the conduit and chamber. Each change in stage appears to correlate with a variation in the cumulative volume of extruded material.
The rocks around Arenal range in age from Pliocene-Pleistocene to Holocene and are divided into five lithologies: 1) undivided Pliocene-Pleistocene volcanics, 2) Chato lava flows, 3) Arenal lava flows, 4) undivided tephra from Arenal and Chato, and 5) sedimentary deposits. The oldest rocks are from the Venado Formation, consisting of Miocene continental shelf deposits. These are overlain by Miocene-Pliocene deposits from the Aguacate Volcanic Group, which are in turn overlain by local Holocene alluvium deposits. These alluvium deposits are typically found in the Lago Arenal area and along the margins of major rivers in the region.