Fig. 1: Map showing the location of Masaya caldera and proximal volcanic and tectonic features.

The current active centre of Masaya volcano is highlighted in red. Sampling locations for the FL and MTL eruptions are indicated by blue and red squares respectively. The inset map shows the location of the study area. The stratigraphic column shows the Plinian eruptions of Las Sierras-Masaya volcanic system and the 1670 and 1772 eruptions which produced lava flows22. The 50 cm isopach for the Fontana Lapilli eruption2 and the 40 cm isopach for the MTL eruption (subunit II of Pérez et al.3) are highlighted in yellow. Structural features in the Masaya area18 are highlighted in white. The maps were made using GeoMapApp85 (http://www.geomapapp.org).