Geology of Western Nicaragua
Masaya is one of 18 distinct volcanic centres that make up the Nicaraguan portion of the Central American Volcanic Front (CAVF). Formed by the subduction of the Cocos Plate beneath the Caribbean Plate, along the Mesoamerican trench, the CAVF runs from Tacaná volcano in Guatemala to Irazú in Costa Rica. In western Nicaragua, the CAVF bisects the Nicaraguan Depression from Cosiqüina volcano in the northwest to Maderas volcano in Lago Nicaragua. The Interior highlands to the northeast make up the majority of Nicaragua. Western Nicaragua consists of 4 principal geological provinces paralleling the Mesoamerican trench: 1) Pre-Cretaceous to Cretaceous ophiolitic suite; 2) Tertiary basins; 3) Tertiary volcanics; and the 4) Active Quaternary volcanic range.
An ophiolitic suite is found in the Nicoya Complex, which is comprised of cherts, graywackes, tholeiitic pillow lavas and basaltic agglomerates. It is intruded by gabbroic, diabasic, and dioritic rocks. The Cretaceous-Tertiary basin is made up of five formations of mainly marine origin. The Rivas and Brito formations are uplifted to the southeast and are overlain in the northwest by a slightly tilted marine near- shore sequence, the El Fraile formation. This in turn passes north into the undeformed Tamarindo formation, a sequence of shallow marine, lacustrine and terrestrial sediments interspersed with ignimbrites. Northeast of the Nicaraguan Depression, the Coyol and Mataglapa formations, run from Honduras to Costa Rica and still show evidence of some volcanic centres, distinguishable as constructional landforms.
Quaternary volcanic rocks are found mainly in the Nicaraguan Depression and form two major groups: the Marrabios and Las Sierras formations. The Marrabios Cordillera starts in the northwest with Cosiguina volcano and continues to the southeast with San Cristobal, Casita, La Pelona, Telica, Rota. El Hoyo, Monte Galan, Momotombo and Momotombito volcanoes are built upon ignimbrite deposits from the nearby Malpaisillo caldera. South-east of Lago Managua lie Chiltepe, the Nejapa alignment, Masaya, Apoyo and Mombacho which overlie the Las Sierras ignimbrites, erupted from the Las Sierras Caldera surrounding Masaya volcano. Further south in Lago Nicaragua, Zapatera, Concepcion and Maderas volcanoes mark the end of Nicaraguan section of the CAVF (Van Wyk de Vries, 1993).