COLLECTION: MONOGENETIC VOLCANISM Eruption processes and deposit characteristics at the monogenetic Mt. Gambier Volcanic Complex, SE Australia: implications for alternating magmatic and phreatomagmatic activity Jozua van Otterloo & Raymond A. F. Cas & Malcolm J. Sheard Received: 10 July 2012 / Accepted: 3 June 2013 # Springer-Verlag Berlin Heidelberg 2013 Abstract The ∼5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 erup- tion points aligned along a fissure system. The complex stra- tigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation inten- sities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and tem- poral sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, vis- cosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters. Keywords Alternating eruption styles . Phreatomagmatic . Magmatic . Grain size image analysis . Mt. Gambier . Newer Volcanics Province . South Australia Introduction Recent research demonstrates that great complexities can exist in the origin, behaviour and evolution of small monogenetic volcanoes, including multiple depositional facies and related eruption styles (Houghton and Schmincke 1989; Houghton et al. 1999; Valentine et al. 2007; Clarke et al. 2009; Genareau et al. 2010), compositional variations (Németh et al. 2003; Blondes et al. 2008; Smith et al. 2008; Brenna et al. 2010; Needham et al. 2011) and changes from fissure eruptions to shifting multiple eruption points (Self et al. 1980). The urban sprawl of some cities and towns has expanded around the edges of monogenetic volcanic fields (for example, Melbourne, Australia) or, in some cases, occur entirely within them (e.g. Auckland, New Zealand). A better understanding Editorial responsibility: I.E.M. Smith, Guest Editor This paper constitutes part of a topical collection: Smith IEM, Nemeth K, and Ross P-S (eds) Monogenetic volcanism and its relevance to the evolution of volcanic fields. Electronic supplementary material The online version of this article (doi:10.1007/s00445-013-0737-y) contains supplementary material, which is available to authorized users. J. van Otterloo (*) : R. A. F. Cas Monash Volcanology Research Group (MONVOLC), School of Geosciences, Monash University, Clayton Campus, Building 28, Room G44, Wellington Road, Clayton, VIC 3800, Australia e-mail: jozua.vanotterloo@monash.edu M. J. Sheard Department for Manufacturing, Innovation, Trade, Resources and Energy (DMITRE), Geological Survey of South Australia Division, GPO Box 1264, Adelaide, SA 5001, Australia Bull Volcanol (2013) 75:737 DOI 10.1007/s00445-013-0737-y