Geophysical and volcanological insights into the subsurface morphology and eruptive histories of complex maar volcanoes within the Newer Volcanics Province, Western Victoria

Abstract

In order to better characterise the eruptive histories and the subsurface structures of maar volcanoes, forward and inverse geophysical modelling has been combined with a detailed study on the geology of the volcanic centres. The maar volcanoes under investigation include several maars within the Red Rock Volcanic Complex (RRVC), Ecklin maar the Mount Leura Volcanic Complex (MLVC) and Anakie. High resolution gravity and magnetic data was acquired across each of these maars in a series of cross-cutting traverses and the data was subsequently modelled in two and three dimensions to understand the subsurface morphology of the volcanic vent and its feeder dykes. Models of the maars subsurface structures were constrained by the regional geology, pyroclastic deposits, petrophysical properties and the interpretation of gridded geophysical data. Varied geophysical responses are observed across each of the maars surveyed, indicating the complex and variable nature of the subsurface volcanic vent, even when they are similar in surface morphology. Where corresponding gravity and magnetic lows are detected across a maar crater, it is suggested that all the available magma was erupted and the maar diatreme (subsurface collapse structure) was not intruded by any dykes. The gravity low arises because of lower density lake sediments and pyroclastic debris infilling the diatreme. The lack of any intrusive dykes or remnant vents within the diatreme suggests that plenty of groundwater was available for phreatomagmatic explosions. Maars with corresponding gravity and magnetic highs indicate a large volume of subsurface basalt is present, resulting from the ponding of magma at the surface of the vent. This results from a lack of groundwater for magma to interact with during the eruption, which facilitates magma rising upwards through the diatreme where it can be fragmented at shallow levels.