Correlation and stratigraphy of dacitic ash‐fall layers in northeastern Papua

Abstract

A thick superficial cover of weathered dacitic ash that mantles the flatter ridge crests on hills and mountains for up to 30 miles (48 km.) from Mount Lamington was derived from this strato‐volcano and thins with distance from it, being about 40 ft (12m.) thick 20 miles (32 km.) away. Its mineralogy is similar to that of the oldest lavas exposed in Mount Lamington crater and to the ash and pumice erupted in 1951. Size distributions of the ash show very good sorting. Inman's coefficient σ⊘, 0–8 to 1–2, and median diameters have a small range, 0.25 ± 0.10 mm. The sorting, mineralogy, and accordance of the layers with the present topography, indicate an ash‐fall origin. Dating of charred wood fragments indicates that the upper ash layers were deposited at average rates of from 5 to 7 1/2 in. (12.5–19 cm.) per 1,000 years for periods of 4,000 to 5,500 years at a distance of 18 miles (29 km.) south of the vent. The evidence also suggests a decrease in the overall rate of ash deposition with time. The small increase in the number of ash layers in the upper part of the ash from 15 miles (24 km.) distant to the perimeter of the volcano suggests that sections through the entire ash cover are a nearly complete representation of the ash shower activity of the volcano and as such indicate a discontinuous pattern of activity with eruptive sequences alternating with periods of quiescence. The age of Mount Lamington is estimated at about 90,000 ± 10,000 years, and throughout its life explosive activity producing ash showers has involved disruption of a green hornblende‐bearing dacite magma.