"This geological record is an environmental time capsule, preserving a continuous 14-million-year history of climate, tectonics, and sea levels." — Associate Professor Stephen Gallagher
New Geological History Revealed for the Twelve Apostles
A study published in the Australian Journal of Earth Sciences provides a new geological history of the Twelve Apostles, the iconic limestone sea stacks off the coast of Victoria, Australia. Led by Associate Professor Stephen Gallagher from the University of Melbourne, the research establishes a more precise age for the limestone layers and details the sequence of tectonic and erosional forces that created the formations.
Formation Timeline and Revised Age
The formation of the Twelve Apostles occurred in two primary stages over millions of years:
1. Deposition of Sediments
The rock layers were deposited in ancient seas during the Miocene epoch.
- The oldest layer, the Gellibrand marl, was deposited in deep marine conditions between 16 and 14.1 million years ago.
- This was followed by the formation of grey limestone until approximately 11.6 million years ago.
- Later, yellow limestone formed as the sea level shallowed.
2. Tectonic Uplift
Starting around 12 million years ago, a shift in the movement of the Australian Plate caused compression, folding, and faulting of the rock layers. This tectonic activity tilted the layers by 1 to 5 degrees and lifted the landmass above sea level. The study identifies this uplift as the primary cause of the stacks' formation, rather than being solely the result of coastal erosion.
3. Erosion and Stack Formation
The current sea stacks and cliffs are geologically young. After the Last Glacial Maximum (approximately 20,000 years ago), melting ice sheets caused sea levels to rise by about 125 meters. Coastal erosion over the past few thousand years then shaped the limestone cliffs into the distinct pillars known today.
Revised Age Range: The study provides a new estimate of between 8.6 and 14 million years old for the limestone layers. This refined date, derived from microscopic fossils (foraminifera), narrows the previous broader estimate of 7 to 15 million years.
Scientific Significance
The study represents the first detailed geological mapping of the Twelve Apostles since 1944. It describes the limestone layers as an "environmental time capsule" that preserves a continuous 14-million-year record of climate, tectonic activity, and sea levels.
Key climate data points identified in the research include:
- A warm period around 13.8 million years ago.
- CO2 levels at the base formation estimated at 480 parts per million (ppm), compared to current levels of approximately 430 ppm.
- An estimated temperature decrease of about 4°C since the Miocene period.
Researchers aim to use this geological record to link ancient processes to modern coastal erosion rates and inform future sea-level projections.
Current Status of the Stacks
The Twelve Apostles are a popular tourist attraction. While originally twelve, only seven stacks remain standing. Two stacks collapsed in 2005 and 2009.
The study notes that further erosion may cause the remaining stacks to collapse, while new stacks may form from other parts of the cliff line over time. Five other limestone formations, referred to as the "Drowned Apostles," lie approximately 6 km offshore and are now submerged by rising sea levels.