Drive along Victoria’s Great Ocean Road and you’ll no doubt find a group of visitors gathering on clifftop lookouts to snap a photo of the iconic Twelve Apostles.
These towering limestone pillars, dating back many thousands of years, rise up to 70 metres from the wild Southern Ocean. Each year, more than two million tourists visit these giant landmarks, and yet up until this year, no one really understood how they were formed.
Now, a study published in the Australian Journal of Earth Sciences reveals that these structures began forming millions of years ago.
Looking into the layers of limestone
The key to understanding the Twelve Apostles is the layers of limestone that make each pillar. Each layer was created underwater during the Miocene epoch, a period in Earth’s history marked by a transition from warm to cool climates that took place between 23 million and 5 million years ago. Each new layer of rock indicates a change in weather conditions, such as temperature, chemistry or water movement. This means experts can read the layers like the rings of a tree.
The team behind the new study used high-resolution digital imagery and traditional fieldwork to uncover the secrets of the iconic Aussie landmark. This also meant analysing fossils of microscopic sea creatures, called foraminifera, trapped in the rock.
According to Stephen Gallagher, Associate Professor at The University of Melbourne, one pillar contained around 760 trillion tiny fossils.
By analysing these rock layers, Gallagher and his colleagues were able to determine that the oldest layers of limestone were around 14 million years old, while the youngest were closer to 8.6 million.
Underneath the limestone is a layer of older, softer material called the Gellibrand Marl, which was deposited when the seas were warmer roughly 15 million years ago.
The role of ancient Earthquakes
So, how did stone that formed underwater end up 70 metres above sea level? According to the study, after the Australian continent split from the Antarctic continent, it began drifting north. This change caused the Earth’s crust to compress the region, which buckled and lifted the limestone out of the sea.
If you look closely at the pillars today, you can see that the layers are not perfectly straight. Small faults are also visible along the cliff face, evidence of ancient earthquakes caused by the movement of the tectonic plates.
Dating the cliffs
Perhaps the most interesting find – according to Professor Stephen Gallagher – is the age of the cliffs and the Twelve Apostles themselves. While the limestone making up these formations is millions of years old, the dramatic coastal scenery only took shape in the last few thousand years. In fact, experts believe that the pillars appeared roughly 23,000 years ago after sea levels rose 125 metres following the last ice age.
As the sea flooded in, waves fractured and eroded the rock already weakened by tectonic forces. The stone eventually collapsed to leave only the iconic sea stacks now on display. This phenomenon is similar to the process being recorded today.
When the Twelve Apostles were first recorded in 1798 by English sailor George Bass (who actually called them the ‘Sow and Piglets’ until the Victorian Government rebranded them in the 1920s for marketing purposes), there were only nine pillars.
One collapsed in 2005, and another in 2009, leaving seven rocky structures still standing. The eroding nature of waves means that further collapses are inevitable, and will eventually reclaim the limestone back into the sea where it was formed.
Scientists believe that there is still much to learn from these structures while they still stand. By analysing the stone, they may be able to reconstruct finer details about how climate, sea levels and ocean conditions have changed across millions of years.
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