Environment

How an ancient underwater eruption caused the creation of Yorke Peninsula’s historic Volcano Trail

It all started about 2 billion years ago, with an ancient underwater volcanic eruption. Images via Yorke Peninsula Council.

If you’ve ever taken a stroll along Port Victoria beach on the Yorke Peninsula, you’ve probably been amazed – or at least a little intrigued – by the vast array of different coloured rocks spanning across the coastline. Maybe you’ve wondered what sort of natural event could have caused such visually striking geological creations.

Well, it all started about 2 billion years ago with an ancient underwater volcanic eruption. Considered one of history’s great submarine eruptions, the underwater phenomenon caused vast outpourings of red and black lava, explosive eruptions, and boiling mud.

The Volcano Trail at Port Victoria offers a visual historical retelling of these events, with all the scientific info necessary to wrap your head around the unique geological formations around the coastline.

Along the Volcano Trail, there are six geosites marked by signs and numbered boulders, with each site telling the story of unique geological occurrence in that area.

Geosite 1

Geosite 1, located just south of the Port Victoria Maritime Museum, explains how the red and black rocks found under the jetty were created by red and black lava that once flowed in large underground chambers.

The red rocks, known as rhyolite, was once molten lava that flowed at 800°C, and the black rocks, known as basalt, flowed at 1200°C. Although formed at different depths beneath the earth’s surface, the two lavas flowed together to create the different rock types that now line the shore around the jetty.

Geosite 2

At Geosite 2, black basalt rocks have formed small cup-shaped “nests” that have been dubbed the “Dragon’s Eggs” due to their oval shape and original lava form.

These fascinating little rock formations, created from lava that cooled beneath the earth’s surface, are incredibly fragile (just like real eggs!), so try not to touch them while walking the trail.

Geosite 3

Further down the trail, Geosite’s 3a and 3b offer closeup looks at seabed sediments and seabed slivers. These slivers may only look like large rock formations, but they actually represent former sea floors: when you look at a seabed sliver, you’re looking at millions of years of geological change.

Best observed during a low tide, seabed slivers are fine rock layers formed from sediments washed into an ancient sea during storms or floods. Sedimentary layers have been preserved within the rocks at this site, which has been split into Sites 3a (sedimentary beds) and 3b (Seabed Slivers) for better viewing.

Geosite 4

When you arrive at Geosite 4, you’ll be met with the interesting looking Harlequin Gneiss rock formations. Formed from ancient sedimentary beds on the seafloor, these rocks feature green minerals and two distinct layers: dark brown, finer-grained layers similar to those at Geosite 3a, and coarser, green-coloured layers that may have been altered by hot fluids during the creation of mountains.

The Harlequin Gneiss leads to the famous Harlequin Stone, visible between Geosites 5 and 6, where vivid red and green rock displays the transformation from seafloor sediment to metamorphic rock through volcanic activity.

As one of the most unique rocks found in the state, the Harlequin Gneiss was chosen to represent South Australia in Canberra’s National Rock Garden.

Geosite 5 features a rare rock formation called Blasted Peperite Breccia, formed when red-hot lava interacts with cold, wet sediments in underwater volcanic environments. The rock is called peperite, a term originally used to describe rocks from the Limagne region of France covered in small black fragments that looked like cracked pepper.

However, such fragments are scarcely visible on the peperite at Port Victoria beach, which feature dark-coloured angular shards and fragments of lava rock.

This is a highly uncommon rock formation that only forms in underwater volcanic environment. As it has features unique to submarine volcanoes, this geosite is critical to the understanding of the geological phenomena along the coastline.

Geosite 6

Finally, the visually striking pink wall at Geosite 6 marks where a significant crack in the earth was created by magma which cooled and crystallised to create the wall of pink granite.

The wall is made of two hard minerals, clear white quartz and pink tabular feldspar, which have become locked together so intensely that they resist erosion. Fascinating stuff!

The Volcano Trail at Port Victoria doesn’t only offer a scenic stroll along the beach and views of Wardang Island and the Spencer Gulf. By walking this trail, you can discover the incredible history of the underwater volcanic eruption that happened around 2 billion years ago, causing the fascinating spread of rock formations and geosites along the beach.

If you’re looking to learn more, the Port Victoria Maritime Museum is located near the Port Victoria Jetty, and for a gold coin donation the volunteers there can guide you through the history of the volcanic activity, nearby shipwrecks, and Port Victoria’s Aboriginal history.

And for hikers and adventurers among us, the Volcano Trail is part of Walk the Yorke, a 500km shared walking and cycling trail stretching right around the Yorke Peninsula coastline from Port Wakefield to Moonta Bay.

WHAT: The Volcano Trail at Port Victoria

WHERE: Port Victoria SA 5573

For further reading, click here.

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