A geophysical comparison of the diatremes of simple and complex maar volcanoes, Newer Volcanics Province, south-eastern Australia

Abstract

Geophysical modelling techniques are applied to examine and compare the subsurface morphology of maar volcanoes within the Newer Volcanics Province to better understand their eruptive histories and the hazards associated with future eruptions within the province. The maar volcanoes under investigation include the Ecklin and Anakie maars, and the Red Rock and Mount Leura Volcanic Complexes, which vary in their complexity, morphology, eruptive styles and host rock type. The Ecklin and Anakie maars display relatively simple geophysical signatures. Long wavelength gravity lows with corresponding magnetic highs are observed across the craters and were reproduced' during modelling with the presence of a shallow maar–diatreme at Anakie and two coalesced diatremes containing denser central vents at Ecklin. Red Rock and Mount Leura have more complex geophysical signatures, consisting of short wavelength gravity and magnetic highs superimposed on longer wavelength gravity lows. These anomalies are reproduced during modelling with coalesced ‘bowl shaped’ diatremes containing dykes and magma ponds. The complex diatreme geometries revealed from forward and inverse modelling suggest that the eruption histories of these volcanoes are more complex than their morphology would suggest. Multiple coalesced diatreme structures indicate an eruption involving vent migration, while preserved dykes within the diatreme suggest short-lived fluctuations between phreatomagmatic and magmatic eruption styles. The geometry of the diatremes is consistent with maars hosted in a soft-substrate, which likely contributed to the migration of vents observed at Ecklin, Red Rock, and Mt Leura. The shallow diatreme observed within the Anakie maar is attributed to a short-lived eruption and low water content within the granitic host rock.