Scientists lastly uncover how the Twelve Apostles have been fashioned

By Stephen Gallagher, The University of Melbourne
•
27 April 2026

Every 12 months, hundreds of thousands of holiday makers stand on the clifftop lookouts alongside Victoria’s Great Ocean Road and gaze out on the Twelve Apostles. These towering limestone stacks, rising as much as 70 metres above the Southern Ocean, are a few of Australia’s most recognisable landmarks.

Yet regardless of their fame, no-one has ever actually understood how they got here to be. Until now.

In new research printed within the Australian Journal of Earth Sciences, my colleagues and I lastly reply that query – and the story entails historic seas, shifting tectonic plates, and a metamorphosis that started hundreds of thousands of years in the past.

A window into deep time

The limestone of the Apostles incorporates an unbelievable archive of hundreds of thousands of years of historical past, and specifically local weather historical past. But it has obtained comparatively little consideration from scientists.

Each layer was laid down in shallow seas through the Miocene epoch. This interval in Earth’s historical past, marked by a transition from heat to chill local weather, lasted roughly from 23 million years in the past to five million years in the past. Each change from one layer to a different represents a change within the native circumstances equivalent to temperature, chemistry, or motion of the water.

My colleagues and I fastidiously mapped the cliffs and sea stacks utilizing high-resolution digital imagery alongside conventional fieldwork and sampling, and analysed fossils of microscopic sea creatures referred to as foraminifera trapped within the rock. I calculated that one of many stacks incorporates round 760 trillion of those fossils.

As a outcome, we have been in a position to learn the layers of rock like tree rings.

This work has given us essentially the most exact dates but for the Apostles’ limestone. Our fossil evaluation reveals the oldest layers of limestone are about 14 million years previous, and the youngest about 8.6 million.

Beneath the limestone, seen at seashore degree east of the Apostles, is an older layer of soppy, darkish materials referred to as the Gellibrand Marl. This was deposited on the backside of deeper, hotter seas round 14 million to fifteen million years in the past.

On prime of the marl, forming the majority of the cliffs and stacks themselves, is the Port Campbell Limestone. This was deposited in shallower, cooler circumstances over the next a number of million years.

From 14.1 million to 13.8 million years in the past, our fossil document captures a second when the worldwide local weather was hotter than right this moment. The layers from that point characterize a pure document of what larger temperatures and sea ranges appear like, preserved in extraordinary element on the Victorian coast.

Tectonics, tilting, and thrust faults

So how did limestone fashioned underwater find yourself standing tens of metres above the ocean? The reply lies in plate tectonics.

As Australia drifted northward after splitting from Antarctica, altering stresses in Earth’s crust compressed the area in a roughly northwest–southeast course.

Starting round 8.6 million years in the past, this compression buckled and lifted the limestone out of the ocean. It didn’t push the layers up completely straight.

If you look carefully on the cliffs right this moment, you possibly can see the horizontal layers are tilted by just a few levels. Small faults are additionally seen within the cliff faces – the scars of historic earthquakes attributable to that very same tectonic squeezing.

The cliffs are model new

Our most stunning discovering: whereas the rock itself is hundreds of thousands of years previous, the dramatic coastal surroundings we see right this moment is model new in geological phrases.

The precise sea stacks and cliffs solely took their current kind up to now few thousand years, after sea ranges rose about 125 metres following the final ice age, roughly 20,000–23,000 years in the past.

As the ocean flooded again in, waves started attacking the uncovered limestone, which had additionally been weakened by tectonic forces. The rock fractured and eroded, forming headlands, then arches, which finally collapsed to go away remoted stacks standing within the surf.

This course of continues to be taking place right this moment. There have been solely seven or eight sea stacks (attributable to disagreements about what to depend) when the Twelve Apostles got their identify – with a little bit of poetic licence – within the early twentieth century.

One collapsed in 2005 and another crumbled in 2009, leaving a typically agreed variety of seven right this moment. The relentless toll of the waves means additional collapses are inevitable, so we should pursue additional analysis whereas we are able to.

A vital local weather document

The most enjoyable a part of this analysis is not only what we’ve discovered already, however what stays to be learn in these cliffs. We are actually working to reconstruct the positive element of how local weather, sea ranges and ocean circumstances modified throughout these hundreds of thousands of years of historical past.

At a second when the world faces pressing questions on our local weather, the Twelve Apostles supply us a rare document of the place it has been and the place we could be heading.

About the writer: Stephen Gallagher is an Associate Professor within the School of Geography, Earth and Atmospheric Sciences at The University of Melbourne

This article is republished from The Conversation underneath a Creative Commons license. Read the original article.