Active volcanoes in Victoria, Australia!?
Intro: Unsuspectingly lurking next to the massive metropolis of Melbourne, Australia, is a region of young and active volcanoes! This volcanic province covers an area >23,000 km in the southern state of Victoria. The volcanoes here are dormant, but still active, with the last eruption ~5000 years ago at Mt. Gambier. The landscape is flat due to extensive plains of basalt, but it is riddled with features like volcanic scoria cones, maars, shield volcanoes, etc. I recently jumped on the opportunity to head across the water from Tassie to check out some of these stunning and ideal landforms on a trip lead by Dr. David Cooke and Dr. Rebecca Carey from our geology department at the University of Tasmania. Hopefully in this blog post I can share my travels to some of these young volcanic features and show what they look like and how they formed.
Science Spiel: Products of violence (Geology, Volcanology, Geomorphology)
The young, volcanically active region of Victoria is known as an intra-plate continental volcanic province. While it still is a bit of an enigma what causes this type of regional volcanic activity, volcanism is thought to be from intra-plate varying convection currents between the underlying lithosphere and asthenosphere (Davies and Rawlinson, 2014). There are over 100 eruption centres, and their products are from mafic-dominant (i.e. basaltic) effusive and explosive eruptions. Effusive eruptions produce lava flows and domes from outpouring of low volatile (i.e. degassed) magma. Explosive eruptions produce pyroclastic deposits and are volatile driven.
Southern Victoria has some uncommon example of effusive eruption products. Heading southwest along the Great Ocean Road we stopped at Airey’s Inlet on the coast to check some of these unusual features out. The basalt at Airey’s Inlet has interesting contact with an overlying limestone layer… When lava intrudes into wet, unconsolidated sediment it gets quenched rapidly, producing fluidal edges in a texture known as “peperite” (McPhie et al., 1993). This process is debatably what happened ~25 Ma at Airey’s Inlet (Cas et al., 1994).
Further northwest inland Victoria we encounter more effusion (lava) features, but instead of your characters basalt flows, what outcrops near Byaduk are lava blisters. What are lava blister? They are known as tumuli and, while they look like someone just piled a bunch of rocks together, they actually form from upwelling of lava through solidified crust, via pressure perturbations from under flowing lava.
The other eruption style mentioned above is explosive, and these usually generate buoyant plumes that result in pyroclastic fall, flow and surge deposits. They are three main types of explosive eruptions with vary due to the amount of water interaction:
1) magmatic (no external water, magmatic volatiles)
2) phreatomagmatic (magma and external water)
3) phreatic (water without direct magma contact)
Both Mt. Noorat and Mt. Eccles are volcanic complexes (i.e. tuff rings and scoria cones) with examples of combined magmatic and phreatomagmatic fall (± surge) deposits. Ash-rich phreatomagmatic layers are interbedded with scoria-rich magmatic layers that drape and mantle the topography (a signature of fall-deposits). While it is not as simple as this, or true in all areas, the scoria-rich layers may represented a drier magmatic explosive stage, whereas the intermittent ash and lapilli layers may represent a ‘wetter’ phreatomagmatic explosive stages. Within these fall deposits are scoriacious volcanic blocks and bombs (e.g. Mt. Noorat). Within some of these bombs are olivine-rich mantle xenoliths, so the eruptions may have been so violent that they entrained bits of the mantle!
Lastly, my favorite volcanic deposit (possibly because of their common association with magmatic-hydrothermal ore deposits) are maars-diatreme complexes! Maar-diatreme complexes form from extremely violent phreatomagmatic and phreatic explosions. This results in a maar (i.e. a circular volcanic crater surrounded by low aspect ratio, outward-dipping rims of phreatomagmatic base-surge and fallout deposits) and an underlying diatreme (i.e. a downward-tapering cone-shaped breccia body filled by volcaniclastic debris and collapsed wall rocks).
The characteristic circular maar craters are beautifully exposure from the air in southern Victoria, and examples include Lake Purumbete, Lake Bullen Merri, and Tower Hill.
Both Lake Purumbete and Tower Hill are ideal location to check out what maar rims look like in outcrop. High-energy ash- and lapilli-sized surge deposits with cross-beds are present at both Tower Hill and Lake Purumbete, within the gently-dipping maar rim layers.
Tower Hill had a late volcanic phase of nested scoria cones grow in the centre, and thus the maar rim deposits here have intermixed ‘dry’ basaltic scoria and ‘wet’ ash deposits. Also… Tower Hill is a great spot to see Australian wildlife, like koalas and emus!
Final Thoughts: Although it’s a lot more complicated than what I describe in this blog post, there is still heaps to learn in southern Victoria on young volcanic deposits. Not only is it geologically interesting, southern Victoria is a lovely place to visit. A common way of traveling is along The Great Ocean Road, a famous stretch of road that takes you past some of the places we stayed, like Port Fairy, and to other stunning landmarks like the Twelve Apostles…
…though don’t have your hopes up if you want to see all dozen apostles, as though yes, they are very spectacular sea stacks of eroded layers of and hard limestone, due to extensive coastal exposure and erosion there isn’t exactly twelve anymore…
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