The ~1245 yr BP Asososca maar eruption: the youngest event along the Nejapa-Miraflores volcanic fault, western Managua, Nicaragua

The Asososca Tephra comprises the asymmetrically distributed, well-stratified phreatomagmatic products of the 1245 + 125/− 120 yr BP Asososca maar eruption West of Managua, Nicaragua. It is the youngest eruptive unit related to the monogenetic volcanism concentrated along the N–S trending Nejapa–Miraflores active fault and intersecting E–W to NE–SW and NW–SE minor faults. The close relationship between monogenetic volcanism and the structural-local tectonic framework is evidenced by the lineament domains and vent alignment patterns identified by remotely sensed images and DEMs of the research area. The Asososca maar lake bathymetry suggests that the crater was formed by coalescing vents migrating during the eruptive event. The Asososca Tephra dominantly consists of accidental lithics disrupted from the underlying stratigraphic units observed inside and around the maar crater. Dry base-surge bedsets are dominant throughout the eruptive unit, showing facies variation from proximal, cross-stratified beds to mid-distal plane-parallel and wavy-parallel beds. Grain-size analyses indicate that base surges transported dominantly − 1, 0, and 1ϕ sized particles at the bedload in form of traction carpets, while fine ash material in continuous suspension was minimal. Juvenile fragments were only identified in the finer fractions, varying from vesiculated scoriae to dense sideromelane fragments. SEM analyses suggest that the Asososca eruption resulted from a highly efficient fuel–coolant interaction between a tholeiitic basaltic melt and an aquifer hosted in a shallow level (between 50 and 200 m) of olivine-bearing scoria ash and lapilli beds. The explosion triggered the country-rock comminution and the production of moss-like, fused-shaped, and blocky ash-shards with stepped surfaces, quenching cracks, pitting, alteration skins, and adhered particles, all indicative of phreatomagmatic fragmentation. The very recent age of the Asososca maar eruption confirms that the densely inhabited Managua area is volcanically active and that hazard studies should focus on the potential for phreatomagmatic eruptions.