Controls on eruption style at Rabaul, Papua New Guinea – Insights from microlites, porosity and permeability measurements

Abstract

Rabaul in Papua-New-Guinea is an extremely active andesitic caldera complex that displays a large spectrum of eruption styles. Since 1878, four sub-plinian (VEI-4) and ten VEI 1–3 (effusive, strombolian, vulcanian) eruptions occurred from Tavurvur and Vulcan, the two main active vents. We study pumiceous tephra, ballistic bombs and a lava flow from five of these eruptions to investigate (1) magma ascent rates and (2) volatile escapement processes during the ascent.We measured total and connected porosities, permeability, and connectivity and related these results with measurements of crystallinity, Microlite Number Density (MND) and Microlite Size Distribution (MSD) of plagioclases and pyroxenes. From the application of existing percolation models, we find that explosive products yield a percolation threshold comprised between 50 and 60 vol% total porosity, while petrophysical parameters of the lava flow and some of the ballistic bombs are explained by bubble collapse driven by surface tension. Sub-plinian products show low phenocryst contents (5–15 vol%), microlites generated by nucleation-driven crystallization or glassy textures due to kinetic crystallization lags. Vulcanian, strombolian and effusive products on the other hand, show medium to high phenocryst contents (15–40 vol%) and microlites that crystallised by growth-dominated processes. MSDs in sub-plinian pumiceous tephra constitute a partial record of drastic magma acceleration. In comparison, MSDs in vulcanian and strombolian ballistic bombs and lava flow show a classic pattern of crystallization at a steady-state due to gradual magma ascent.We find that magma feeding sub-plinian eruptions ascends 2–3 orders of magnitude faster than magma feeding vulcanian/effusive eruptions (≥ 1–100 m/s vs 0.1 m/s). In the case of sub-plinian products, due to the important kinetic crystallization lag, these speeds cannot be estimated using microlite crystallization triggered by decompression as a proxy. Combining petrophysical and textural measurements we suggest that slight changes in initial conditions in the reservoir such as crystallinity or the presence of exsolved volatiles, can have a profound impact on the ascent rate and generate positive or negative feedback effects leading to powerful sub-plinian activity or intermittent vulcanian/ passive effusive activity respectively.