Plinian airfall deposits of basaltic composition.

Two widespread scoria-fall deposits, known locally as the Fontana lapilli and San Judas formation, are shown to have issued from Masaya caldera complex, Nicaragua, and to be the first documented plinian airfall deposit of basaltic composition. Calculation of eruption energetics reveals reasonable plinian values for muzzle velocity, column height, volume eruption rate and duration for both deposits. The Fontana lapilli is distinctive on the basis of its great predicted column height (50 km) and very high volume eruption rate (2 X 10 ⁵ m ³ /s). The existence of basaltic plinian eruptions implies that high magma viscosity may not be essential for plinian eruption.-A.P.