Abstract

Two widespread scoria-fall deposits, locally known as the Fontana Lapilli and San Judas Formation, are demonstrated to have issued from Masaya Caldera Complex, Nicaragua, and to be the first documented plinian airfall deposit of basaltic composition. Data from maps of median grain size, average maximum pyroclast diameter, and average maximum diameter of lithic fragments provide conclusive evidence of the plinian nature of both deposits. The volume within the 1 mm isopach of the Fontana Lapilli is 12 km3 (3.4 km3 Dense Rock Equivalent), and within the San Judas Formation it is 1.2 km3 (0.35 km3 DRE). Theoretical eruption parameters calculated for the Fontana Lapilli and San Judas Formation, respectively, include muzzle velocity (170 m/s and 150 m/s), column height (50 km and 18 km), volume eruption rate (2 × 105 m3/s and 1 × 104 m3/s), and duration (2 h and 10 h). These are all similar to the calculated parameters for silicic plinian eruptions. Basaltic magmas are generally considered incapable of erupting in the manner characteristic of plinian eruptions. Recognition of them argues that factors other than high viscosity may be of fundamental importance.

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