Complex eruption processes and deposits of basaltic fissures: insights from the ~37 ka Budj Bim volcanic complex, Southeastern Australia

Abstract Throughout Earth’s history, eruptions from volcanic fissures have been responsible for some of the largest outpourings of basaltic magma, impacting climate and society. In continental monogenetic settings, these eruptions can occur on short timescales and with short notice after long quiescence. Their deposits are often quickly destroyed or buried as the eruption progresses, limiting understanding of eruption triggers and hazards uniquely garnered from these tephra layers and lavas. The ~37 ka Budj Bim Volcanic Complex (BBVC) in the active intracontinental monogenetic Newer Volcanics Province (NVP), southeastern Australia (~4.5 Ma to present), is a rarely preserved volcanic fissure system that provides new insight on how magma ascent dynamics impact eruptions. The BBVC comprises a 2.2 km long array of basaltic scoria cones, spatter cones, an elongate open crater with steep inward-sloping walls and a substantial lava-tube-fed flow field. Newly logged eruption sequences, petrography and geochemical analysis reveal five eruption stages, involving vent-shifting along a dyke-fed fissure, and rapid changes in eruption styles between Strombolian and micro-Plinian activity interspersed with concurrent and intermittent phreatomagmatism. The eruption culminated in the establishment and then breach of an elongate lava lake, feeding the > 30 km long culturally significant Tyrendarra lava flow (UNESCO World Heritage site). The BBVC eruption would have evolved similarly to the 2014–2015 Bárðarbunga-Holuhraun fissure eruption in Iceland, but with increased complexity due to its intraplate continental setting and feeder dyke interaction with aquifers and near-surface faults, resulting in greater level of phreatomagmatism and explosion depths potentially down to 320 m. This finding must be considered to improve future eruption scenario assessments in the NVP and builds new understanding of complex volcanic fissure eruption behaviour that can be applied to similar systems around the world.