Each year, millions of tourists visit the clifftop lookouts along Victoria’s Great Ocean Road to admire the Twelve Apostles.
These impressive limestone formations, which rise up to 70 meters above the Southern Ocean, are among Australia’s most iconic sights.
Until recently, the origins of these structures remained a mystery. However, new research has shed light on their formation, involving ancient oceans, tectonic shifts, and a transformation that began millions of years ago, as detailed in the Australian Journal of Earth Sciences.
A Glimpse into Geological History
The limestone of the Twelve Apostles holds a rich archive of millions of years of geological and climate history, although it has not been extensively studied by scientists.
These limestone layers were deposited in shallow seas during the Miocene epoch, a period spanning from approximately 23 million to 5 million years ago, characterized by a shift from warm to cooler climates.
Changes in each layer reflect variations in local conditions, such as water temperature, chemistry, and movement.
Through detailed mapping of the cliffs and sea stacks using high-resolution digital imagery, traditional fieldwork, and sampling, my colleagues and I studied fossils of microscopic sea creatures known as foraminifera found in the rock. One stack alone was estimated to contain approximately 760 trillion of these fossils.
This analysis allowed us to interpret the rock layers similarly to reading tree rings.
Our findings provided the most accurate dating of the Apostles’ limestone to date. The fossil study indicated that the oldest limestone layers are about 14 million years old, while the youngest are approximately 8.6 million years old.
Below the limestone, visible at beach level east of the Apostles, lies an older, soft, dark layer called the Gellibrand Marl, which was deposited in deeper, warmer seas around 14 to 15 million years ago.
Above this marl layer, forming the majority of the cliffs and stacks, is the Port Campbell Limestone, which was deposited in shallower, cooler conditions over several million years.
Our fossil evidence from 14.1 to 13.8 million years ago reveals a period when the global climate was warmer than today, capturing a detailed natural record of elevated temperatures and sea levels along the Victorian coast.
Tectonic Movements and Geological Shifts
The presence of limestone structures above sea level can be explained by tectonic activity.
As Australia shifted northward after separating from Antarctica, the region experienced compression in a northwest-southeast direction.
Starting around 8.6 million years ago, this compression lifted the limestone from underwater, but not in a perfectly straight manner.
Today, the cliffs’ horizontal layers are slightly tilted, and small faults are visible in the cliff faces, evidence of past earthquakes caused by tectonic pressure.
A Recent Geological Feature
One surprising discovery is that while the rock itself is millions of years old, the striking coastal formations we see today are relatively recent in geological terms.
The current form of the sea stacks and cliffs emerged only in the last few thousand years, following a sea-level rise of about 125 meters after the last ice age, approximately 20,000 to 23,000 years ago.
As the sea returned, waves began eroding the exposed limestone, which had already been weakened by tectonic forces. This led to the formation of headlands, arches, and eventually isolated stacks as the arches collapsed.
This erosion process is ongoing. When the Twelve Apostles were named in the early 20th century, there were only seven or eight stacks, depending on differing counts.
One stack collapsed in 2005, and another crumbled in 2009, leaving the generally accepted number at seven today. The constant wave action ensures further collapses are inevitable, highlighting the need for continued research.
An Important Climate Archive
The most intriguing aspect of this research is not only the findings but the potential insights yet to be uncovered from these cliffs.
We are actively working to reconstruct detailed changes in climate, sea levels, and ocean conditions over millions of years of history.
Related: Something Else Used to Drive Climate Changes, Ancient Ice Cores Reveal
As the world grapples with pressing climate questions, the Twelve Apostles provide an invaluable record of past climates, offering clues about future conditions.
Stephen Gallagher, Associate Professor, School of Geography, Earth and Atmospheric Sciences, The University of Melbourne
This article is republished from The Conversation under a Creative Commons license. Read the original article.
