A geological countdown
The Twelve Apostles are limestone stacks undergoing constant, measurable decay. The primary agent is the wave energy of the Southern Ocean, which erodes the base of the stacks at an estimated rate of 2 centimeters per year. This process, known as hydraulic action, forces water into cracks within the soft Port Campbell Limestone, gradually widening them. The limestone itself, formed between 5 and 15 million years ago during the Mid-Late Miocene epoch, is composed of the compacted skeletal remains of marine organisms.
The formation sequence shows coastal geomorphology. The initial assault from waves and wind carves caves into the coastal cliffs. Over thousands of years, these caves deepen until they break through the headland, creating an arch. When the arch becomes too wide to support its own weight, the top collapses, leaving an isolated pillar of rock, a sea stack. The tallest of the remaining stacks reaches a height of 45 meters. This entire process is expected to continue, with new headlands on the mainland slowly being carved into the next generation of stacks as the current ones disappear.
A history of collapse
Despite the name, there have never been twelve stacks in this specific formation. The name was adopted in the 20th century for tourism, replacing earlier names like the "Sow and Piglets". At the time of the renaming, there were nine stacks.
The most dramatic recent event in this erosion timeline occurred at approximately 9:18 AM on July 3, 2005. On that morning, a 50-meter-tall stack, the second from the main viewing platform, collapsed into the sea in a matter of seconds, leaving eight stacks. Another smaller stack reportedly collapsed in 2009. These events are not isolated; London Bridge, a nearby double-arch formation, experienced a partial collapse in 1990, leaving two people stranded on the newly formed stack until they could be rescued by helicopter. In 2016, researchers discovered five more submerged limestone stacks, dubbed the "Drowned Apostles," 50 meters underwater and 6 kilometers offshore. These likely formed about 60,000 years before the current Apostles and were submerged by rapid sea-level rise at the end of the last ice age.