The origin of a large (>3km) maar volcano by coalescence of multiple shallow craters: Lake Purrumbete maar, southeastern Australia

• Published in: Journal of volcanology and geothermal research Vol. 254; pp. 5 - 22
• Main Authors: Jordan, S.C., Cas, R.A.F., Hayman, P.C.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier B.V 15.03.2013; Elsevier
• Subjects: Crater coalescence; Crystalline rocks; Earth sciences; Earth, ocean, space; Exact sciences and technology; Extremely large maar volcanoes; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Maar diatreme; Phreatomagmatism; Vent migration; Australia; Extremely large maar volcanoes; Maar diatreme; Crater coalescence; Phreatomagmatism; Vent migration; pyroclastic flows; Australasia; crater lakes; faults; country rocks; phreatomagmatism; aquifers; craters; tectonics; explosions; direction; maars; eruptions; volcanoes; diatremes; base surges; transport; vents; tectonic setting; migration; growth; style
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2012.12.019

Lake Purrumbete maar is located in the intraplate, monogenetic Newer Volcanics Province in southeastern Australia. The extremely large crater of 3000m in diameter formed on an intersection of two fault lines and comprises at least three coalesced vents. The evolution of these vents is controlled by the interaction of the tectonic setting and the properties of both hard and soft rock aquifers. Lithics in the maar deposits originate from country rock formations less than 300m deep, indicating that the large size of the crater cannot only be the result of the downwards migration of the explosion foci in a single vent. Vertical crater walls and primary inward dipping beds evidence that the original size of the crater has been largely preserved. Detailed mapping of the facies distributions, the direction of transport of base surges and pyroclastic flows, and the distribution of ballistic block fields, form the basis for the reconstruction of the complex eruption history,which is characterised by alternations of the eruption style between relatively dry and wet phreatomagmatic conditions, and migration of the vent location along tectonic structures. Three temporally separated eruption phases are recognised, each starting at the same crater located directly at the intersection of two local fault lines. Activity then moved quickly to different locations. A significant volcanic hiatus between two of the three phases shows that the magmatic system was reactivated. The enlargement of especially the main crater by both lateral and vertical growth led to the interception of the individual craters and the formation of the large circular crater. Lake Purrumbete maar is an excellent example of how complicated the evolution of large, seemingly simple, circular maar volcanoes can be, and raises the question if these systems are actually monogenetic. ► Reconstruction of the evolution of a very large circular maar structure ► Formation by coalescence of multiple shallow craters, not deep excavation ► The evolution of the maar was strongly influenced by the substrate and the tectonic setting. ► Multiple eruption phases, with at least one significant time break