The Masaya Triple Layer: A 2100 Year Old Basaltic Multi-episodic Plinian Eruption From The Masaya Caldera Complex (nicaragua)

摘要

The Masaya Caldera Complex has been the site of three highly explosive basaltic eruptions within the last six thousand years. A Plinian eruption ca. 2 ka ago formed the widespread deposits of the Masaya Triple Layer. We distinguish two fades within the Masaya Triple Layer from each other: La Concepcion fades to the south and Managua facies to the northwest. These two facies were previously treated as two separated deposits (La Concepcion Tephra and the Masaya Triple Layer of Perez and Freundt, 2006) because of their distinct regional distribution and internal architectures. However, chemical compositions of bulk rock, matrix and inclusion glasses and mineral phases demonstrate that they are the product of a single basaltic magma batch. Additionally, a marker bed containing fluidal-shaped vesicular lapilli allowed us to make a plausible correlation between the two facies, also supported by consistent lateral changes in lithologic structure and composition, thickness and grain size. We distinguish 10 main subunits of the Masaya Triple Layer (I to X), with bulk volumes ranging between 0.02 and 0.22 km~3, adding up to 0.86 km~3 (0.4 km~3 DRE) for the entire deposit. Distal deposits identified in two cores drilled offshore Nicaragua, at a distance of ～170 km from the Masaya Caldera Complex, increase the total tephra volume to 3.4 km~3 or ～ 1.8 km~3 DRE of erupted basaltic magma. Isopleth data of five major fallout subunits indicate mass discharges of 10~6 to 10~8 kg/s and eruption columns of 21 to 32 km height, affected by wind speeds of < 2 m/s to ～ 20 m/s which increased during the course of the multi-episodic eruption. Magmatic Plinian events alternated with phreatoplinian eruptions and phreato-magmatic explosions generating surges that typically preceded breaks in activity. While single eruptive episodes lasted for few hours, the entire eruption probable lasted weeks to months. This is indicated by changes in atmospheric conditions and ash-layer surfaces that had become modified during the breaks in activity. The Masaya Triple Layer has allowed to reconstruct in detail how a basaltic Plinian eruption develops in terms of duration, episodicity, and variable access of external water to the conduit, with implications for volcanic hazard assessment.