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Arthur's Seat: Victoria's Ancient Devonian Volcanic System Revealed

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Arthur’s Seat: Victoria’s Ancient Devonian Volcanic System Revealed

Clip title: The Hidden Volcano in Australia Author / channel: OzGeology URL: https://www.youtube.com/watch?v=st4B8l6q0lI

Summary

Arthur’s Seat, an iconic landmark on Victoria’s Mornington Peninsula, holds a deep and dramatic geological secret: it is the eroded remnant of an ancient, explosive volcano. Approximately 370 to 360 million years ago, during the late Devonian period, this region was part of the Lachlan Orogen, a highly active tectonic zone. The current landscape, with its resilient rhyodacite outcrops, granite roots, and distinctive slopes, serves as a sparse but vital clue to a violent chapter in Earth’s history, a story largely obscured by millions of years of erosion and subsequent geological forces.

The formation of Arthur’s Seat was intrinsically linked to the subduction of oceanic crust beneath the continental crust, a process that subjected the region to intense tectonic stress, including deformation, faulting, and folding of existing rock layers. This dynamic environment facilitated the generation of magma. Some of this molten rock cooled slowly deep beneath the surface, forming the Dromana Granite, a felsic (high-silica) intrusive body. Simultaneously, other batches of magma rose closer to the surface, erupting explosively to create the silica-rich volcanic rocks known as Arthur’s Seat Rhyodacite. Geologists therefore recognize the Dromana Granite and Arthur’s Seat Rhyodacite as complementary products of the same ancient magmatic system.

The eruption itself would have been a spectacular and violent event. Due to its sticky, gas-rich nature, rhyodacitic magma tends to erupt explosively, likely producing massive vertical plumes of ash and pumice, followed by devastating pyroclastic density currents that raced across the Devonian landscape. Unlike classic towering stratovolcanoes, the evidence suggests Arthur’s Seat formed as a low-profile volcanic dome complex or perhaps a collapsed caldera system, as no distinct cone or crater is preserved today. Over vast timescales, erosion and later tectonic faulting, notably the Selwyn Fault, severely truncated and jumbled the original volcanic structures, leaving behind only isolated patches of the erupted rhyodacite and ignimbrite.

The eruption of Arthur’s Seat was triggered by a complex interplay of regional and local tectonic forces. While the overarching mechanism was the subduction of oceanic crust, local factors such as crustal extension or collapse along deep faults, like the re-opened Selwyn Fault, provided crucial conduits for the magma to ascend. Chemical analysis of the rocks also indicates a component of partial melting from older crustal rocks, rather than a purely mantle-derived source. Ultimately, understanding this ancient volcanic system, which involved heat, fluids, and fractures, is not merely an academic exercise. These same processes are fundamental to the creation of mineral systems, influencing the formation and distribution of valuable resources like gold and other precious minerals, and offering profound insights into the planet’s enduring geological evolution.

Video Description & Links

Description

morningtonpeninsula victoria australia Hidden beneath the rolling hills of Victoria’s Mornington Peninsula lies a forgotten chapter of Australia’s volcanic past. In this video, we explore the Arthurs Seat Rhyodacite, a remnant of a massive Devonian volcanic system that erupted around 370 million years ago. Once part of a fiery landscape shaped by fault-controlled magma chambers and explosive rhyodacite eruptions, this region is now a quiet coastal escape. Yet the rocks tell a story of subduction, mountain-building and tectonic chaos that transformed the ancient sea floor into a land of erupting volcanoes and rising granite domes.

We’ll uncover how the Selwyn Fault Zone acted as a deep-crustal magma conduit, feeding molten rock toward the surface and giving rise to the now-eroded volcano of Arthurs Seat. Through field observations, geological maps and modern research, this video reconstructs how the Dromana Granite once sat beneath an explosive rhyodacite volcano — a system comparable to modern-day dome complexes and caldera eruptions. You’ll learn how this lost volcano formed in the wake of the Tabberabberan Orogeny, when the Australian continent was still welded to Gondwana and Victoria was an evolving patchwork of shallow seas, volcanic islands and rising highlands.

Join me as we trace the evidence of ancient volcanism in the Mornington Peninsula, from the flow-banded rhyodacites on Arthurs Seat Road to the granite exposures near Dromana. Together, we’ll piece together how this long-eroded volcano once erupted in the open air, shaping the foundations of modern Victoria. This is the story of The Hidden Volcano in the Mornington Peninsula — a geological mystery buried in plain sight, revealing how one of Victoria’s most iconic landscapes began in fire, ash and tectonic upheaval.

Study Used To Construct This Video: Petrology and origin of a Devonian volcanic complex on Arthurs Seat, Mornington Peninsula, Victoria, Australia: multum in parvo: https://www.tandfonline.com/doi/full/10.1080/08120099.2025.2538585#summary-abstract

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🌏 About OzGeology The core mission of OzGeology is to make geology exciting, accessible, and inspiring for everyone. Instead of presenting rocks and earth science as dry or overly academic, OzGeology brings stories of the planet to life, revealing how every mountain, mineral, and landscape tells part of Earth’s grand adventure. The goal is to help people see the world differently, to understand the dynamic forces shaping Australia and beyond, and to spark curiosity in the next generation of geologists. Through engaging storytelling, field exploration, and clear explanations, OzGeology turns the study of our planet into a journey of discovery rather than a classroom lecture.

00:00-00:29 - The Lost Volcano In The Mornington Peninsula 00:30-01:22 - The Tectonic Setting of Victoria At The Time 01:23-01:52 - The Felsic Rocks That Exist Today 01:53-04:07 - What Did The Volcanic Eruption Look Like? 04:08-04:37 - The Selwyn Fault Line 04:38-07:04 - Why Did The Volcano Erupt? The Trigger 07:05-07:41 - Why Should You Care? Mineral Riches. 07:42-08:38 - Conclusion & Patreon / YouTube Member Thank You!

Tags

geoscience, geology, earth sciences, earth science, geological, geosciences, geologist, australia, volcanology, australian, aussie, mineral discovery, geological exploration, mineralogical exploration, exploration geology, ozgeology, Mornington Peninsula volcano, Arthurs Seat geology, Arthurs Seat Rhyodacite, Dromana Granite, Devonian volcanism Victoria, Selwyn Fault Zone, hidden volcano Australia, Victoria geological history, ancient Australian volcanoes, Gondwana tectonic history

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