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.

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The volcanic centres in the active Newer Volcanic Province (NVP) in southern Victoria, Australia (top; from Boyce, 2013). Map of places discussed in blog post (bottom)
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Maar rim volcanic deposits at Tower Hill in Victoria, Australia. Notice myself for scale…

Violent volcanic products

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.

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Explosively-produced interbedded lapilli-tuff volcaniclastic layers (beige and tan rocks in the distance) with an enclosed effusively-produced A’a’ lava flow (4m thick dark blocky-rock on the right edge)

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).

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Possible peperite at Airey’s Inlet. Rounded basalt clasts are within overlying limestone – Victoria, Australi
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Peperite? Fluidal-shaped basalt clasts within limestone at Airey’s Inlet – Victoria, Australia
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Schematic diagram of how lava tumuli “blisters” form (from K.G. Grimes 8-2003)
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Lava tumuli “blister” near Byaduk – Victoria, Australia

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)

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“Dry magmatic” scoria-rich layer overlying “wet phreatomagmatic” ash fall layer at Mt. Noorat in Victoria, Australia. Volcanic bombs are present within the upper scoria layer. Hammer at the left corner for scale.
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Red, oxidized scoria cone deposit of Mt. Eccles that draps the topography; a signature of fall-deposits – Victoria, Australia

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!

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Volcanic bomb with a olivine-rich xenoliths from the mantle (aka. dunite) at Mt. Noorat – Victoria, Australia

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).

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Model of a maar-diatreme volcano (simplified from Lorenz, 1986)
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Outward-dipping maar crater rim surge deposits at Lake Purrumbete – Victoria, Australia
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Ariel view of circular maar craters in Victoria, Australia

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.

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Lake Bullen Merri; a maar crater – Victoria, Australia
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Tower Hill lake; a maar crater with several central scoria cones – Victoria, Australia
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Cross-bedded maar crater surge deposits at Lake Purrumbete with local reverse faulting (soft sediment deformation?) – Victoria, Australia. Scale ~ 1 cm = 20 cm

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!

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Inter-layered ash, mixed-ash and scoria (e.g. layer I’m touching) deposits on the maar crater rim at Tower Hill – Victoria, Australia
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Volcanic bomb with an apparent impact ejecta trajectory away from centre of Tower Hill maar volcano – Victoria, Australia
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An Australian Koala at Tower Hill – Victoria, Australia

Final thoughts

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

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The Twelve Apostles rock formation (inter-layered hard and soft limestone) along the Great Ocean Road in Victoria, Australia.

…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.

Although it’s a lot more complicated than what I describe in this blog post, there is still heaps to learn and see in southern Victoria on young volcanic deposits.

-Stephanie

Check out my attempt at a video to go with this blog post
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Red oxidized scoria layer with thin ash-rich layer of a surge deposit (thanks Julie!) at Mt. Rouse – Victoria, Australia
Victoria, Australia, Melbourne, Volcano, volcanoes, geology, volcanic deposits, travel, blog, adventure, exploring, earth, exploring the earth, rocks, nature, geomorphology, maars, tuff, scoria, lapilli, ash, volcaniclastic, effusive, explosive, basalt, mafic, intra-plate volcanism, hazards, monogenetic, great ocean road, map, lava, diatreme, breccia, hydrothermall, magmatic, ore deposits, gold, copper, magmatic-hydrothermal, Tower Hill, maar rim, crater
Inter-layered ash and mixed-ash and scoria layer on the maar crater rim at Tower Hill – Victoria, Australia
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Sunrise at the beach in Port Fairy – Victoria, Australia

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