The Hidden Volcano in the Mornington Peninsula

The Hidden Volcano in the Mornington Peninsula

Imagine standing on the ridge of Arthurs Seat on Victoria’s Mornington Peninsula. Beneath your feet lies the whisper of an ancient story: a volcano that erupted — hard, fast, and loud — in the Late Devonian period, and which time has since erased all but a few brittle clues. What you see now — the resilient rhyodacite outcrops, the granite roots, the angled slopes and road-cuts — are the remains of a violent chapter in Earth’s deep past. Let’s wander together through that chapter.

The Stage is Set

Around 370 to 360 million years ago, the part of the Australian continent now called the Mornington Peninsula lay in a very different place. It was part of the Lachlan Fold Belt — a region undergoing active tectonic change: oceanic crust being forced beneath continental crust, and sediment and volcanic rocks being deformed, faulted and folded. In that magma-charged environment, heat and chemical forces were at work. The rocks that would become the Dromana Granite and the Arthurs Seat Rhyodacite were born.

*Image depicts a simplified version of Victoria 370ma with an active subduction zone shown.

*Image depicts the Dromana Granite (Red) and the Arthur's Seat Rhyodacite (Yellow).

If we step back, the pattern of events looks like this: the crust was under stress, the oceanic plate (or fragments of it) was being subducted or deformed, and that generated melting. Magmas rose. Some stalled and formed large, slow-cooling pockets of rock underground — granites. Others made their way nearer the surface and erupted as volcanic rocks. In effect, deep and shallow parts of the same system were working.

Which brings us to the granite. The Dromana Granite is a felsic (high silica) intrusive body, and at the same time the Arthurs Seat suite are silica-rich volcanic rocks (rhyolite to rhyodacite). The spatial and temporal association is strong enough that many geologists view them as two sides of the same coin: magma chamber and erupted flows. 

What The Eruption Probably Looked Like

So, with that being said, what did the eruption look like?

It’s not clear whether the volcano that erupted the rhyodacite at Arthurs Seat was a stratovolcano in the classic sense. There’s no obvious cone or crater preserved today. In fact, there’s good reason to think it wasn’t a tall, long-lived mountain at all.

Instead, the evidence points to something more subtle — a low-profile volcanic dome complex, or perhaps a collapsed caldera system. These types of silicic volcanoes don’t build towering peaks; they form pancake-like lava domes, thick, stubby flows, and ignimbrite sheets from explosive pyroclastic eruptions. Imagine a cluster of steep-sided domes surrounded by valleys filled with welded tuff and volcanic ash — more like a cluster of domes and collapse scars than a single elegant cone.

The eruption itself would have been spectacular. Because rhyodacitic magma is sticky and gas-rich, it tends to erupt explosively. Picture a massive vertical plume of ash and pumice punching several kilometres into the Devonian sky, followed by pyroclastic density currents — turbulent, superheated clouds of ash and gas racing across the landscape at hundreds of kilometres per hour. After the violence, the vent might have quieted into slower effusion: thick domes of molten rhyodacite oozing upward, glowing at night, cracking and collapsing as they cooled.

Whether the Arthurs Seat volcano collapsed after eruption is uncertain. In many silicic systems — where viscous, gas-rich magmas erupt explosively — the roof of the magma chamber can subside once a large volume of magma is evacuated. That process forms what geologists call a caldera, essentially a giant crater left behind when the volcanic edifice caves in on itself. But in the case of Arthurs Seat, there’s no clear evidence of a preserved caldera rim or collapse structure.

Instead, what we see are isolated patches of rhyodacite and ignimbrite sitting alongside the Dromana Granite, truncated and jumbled by later faulting. That suggests the original volcanic landscape has been severely eroded and tectonically disrupted. So while a collapse may have happened — as it often does in explosive felsic systems — we can’t confirm it from the surviving geology.

What’s left are the welded ignimbrites, the flow-banded rhyodacite, and the granite roots below — the skeletal remains of a once-explosive volcanic complex, now disguised as the gentle green slopes of Arthurs Seat.

Beneath the Mornington Peninsula runs one of Victoria’s great hidden giants — the Selwyn Fault, a deep fracture that cuts through the ancient crust like a scar from an old battle. During the Devonian, when the crust was under tension after millions of years of compression, that fault reopened. In doing so, it provided a ready-made conduit for rising magma. The molten rock that became the Dromana Granite collected deep along this fault zone, while more mobile batches of the same melt shot upward through its fractures to erupt at the surface as the Arthurs Seat Rhyodacite.

*Image shows the Selwyn fault, along with magmatic intrusions that utilized it as a pathway to ascend during the Devonian.

So, why did it erupt? Subduction, thinning crust, or something else?

This is a good question and one worth diving into. In many volcanic cases, particularly in present-day settings, you get an oceanic plate subducting under a continental plate; water from the sinking plate triggers melting of the overlying mantle and crust, and volcanic arcs form. In the case of the Arthurs Seat complex, the tectonic setting is part of the broader “Tasmanides” story of southeastern Australia: oceanic‐crust accretion, terrane collisions, subduction, folding, uplift.

However, the exact trigger for the Arthurs Seat event may involve more than a simple “plate goes down, magma comes up” model. Some of the geological literature suggests that Late Devonian volcanism in central Victoria is associated with cauldron structures (large collapsed volcanic centres), crustal extension (stretching) after or during orogenic events (mountain-building), and melting of older crustal rocks then erupting. For example, the record of rhyolite-rhyodacite cycles in central Victoria shows three cycles of silica‐rich volcanism associated with subsidence of the crustal block.

So, we might infer that the Arthurs Seat volcano and its granite feeder were formed in a transitional tectonic regime — one where subduction may have been happening or had recently ceased, and the crust was beginning to collapse or extend. That extension allowed magma to ascend, but the magma itself may have been derived from partial melting of older crust (rather than purely a mantle source). The fact that the Arthurs Seat rocks show mineralogical features like cordierite and sekaninaite (indicators of peraluminous, crust-derived magmas) supports a crustal melting component.

In short: yes, subduction of oceanic crust may have been the large-scale driver of heat and melting, but by the time this particular volcano erupted, the local trigger was likely a collapse or extension of the crust (a cauldron or subsidence setting) that let the magma out. The system, therefore, is a mixture of “subduction legacy” plus “crustal melting plus magma chamber plus eruption”.

What Remains Today and How We Interpret It

When you walk the ridge at Arthurs Seat, you are walking on the roof of a long-extinct volcano plus the rocks of its intrusion. The tiny rhyodacite outcrops that do exist are the hardened remains of flows, domes or ignimbrite sheets. They may show flow banding, quartz and feldspar phenocrysts (those larger crystals that grew while the magma lingered underground before eruption), and in places you might find signs of thermal or hydrothermal alteration. The granite is the deep root. Overlying sediments are largely missing, any cone (if one existed to begin with) is gone, the pyroclastic blanket is almost fully eroded. All that remains is a skeleton.

The significance and what it might mean for you as an explorer:

But why should you care? Because even though the volcano is long gone, the processes it records are the same forces that create mineral systems, shape landscapes, and control where we find ore. A felsic volcanic system like Arthurs Seat implies heat, fluids, fractures — and those are often ingredients for gold or other precious mineralisation.

On the landscape side, the ridge of Arthurs Seat and the Mornington Peninsula are not just scenic; they are the exposed skeleton of a volcanic system. By understanding the plumbing, you understand why the rock is where it is, why the outcrops look the way they do, why the faults and structure matter.

Final reflection

So there it is: The Lost Explosive Volcano of the Mornington Peninsula. Hidden beneath tourist trails and eucalypts, wearing the skin of the current landscape but born in fiery eruption. The granite roots, the rhyodacite flows, the faults and ridges—they are all parts of that story. For the layperson you can see a beautiful ridge with strange rock outcrops. For the technical you can map intrusion-extrusion relationships, study mineral chemistry, interpret tectonic regimes. And for explorers of gold, quartz and ore systems, this volcano is another chapter in the deep-time detective work many of us, myself included, enjoy so much.

Here's the video we made on this on the OzGeology YouTube Channel: