Abstract
The biotitic pyroclastic flow of Rio Liberia represents the best horizon to subdivide the large number of pyroclastic flows of the Guanacaste region. There are outcrops on the pacific slope of the Cordillera of Guanacaste, from Bijuaga, near the Tenorio volcano, as far as to Hacienda El Amo,near La Cruz, and it the Upala region on the Caribbean side. The samples and the measurements have been taken in thirty outcrops, located in an area of 2,000 km2. The original extension of the biotitic pyroclastic flow has been reconstructed to be 3,500 to 4,000 km2, and the volume of the erupted magma has been calculated to be 25 km3. We have determined the contents of pumice, lithic fragments, crystals, and ash. The chemical analyses point to an homogenous rhyolitic composition of subalkaline character. The crystal content is very high, like in the "crystal-rich pyroclastic flows." Quartz is the most abundant mineral, followed by plagioclase (An 20 to 35), biotite, hornblende, and iron oxides. Based on the amphiobole composition the crystallization has been produced by an increase of fluid pressure, linked to the liberation of thermic energy caused by the crystallization of an huge quantity of minerals.