The Twelve Apostles stand as a collection of colossal rock formations that emerge dramatically from the Southern Ocean. Located off the shore of Port Campbell National Park in Victoria, Australia, they are the most recognized landmark along the scenic Great Ocean Road. These pillars, which can reach heights of up to 50 meters, draw millions of visitors each year to witness the raw power of nature captured in stone.
The Primary Rock Type
The towering stacks and surrounding cliffs are composed primarily of a sedimentary rock known as limestone, specifically classified as Port Campbell Limestone. Limestone is formed from accumulated materials and is relatively soft and porous compared to harder igneous or metamorphic rocks. The main constituent of this material is calcium carbonate. This composition makes the rock susceptible to the mechanical and chemical forces of coastal erosion. The distinctive yellow and cream coloring is a testament to its marine origins.
The Ancient Origin of the Material
The material that forms the Twelve Apostles originated from the compressed remains of countless tiny marine organisms. This limestone began to form approximately 15 to 5 million years ago, during the Mid-Late Miocene epoch, when the Victorian coastline was submerged beneath the ocean.
As these ancient organisms, including mollusks and bryozoans, died, their calcium carbonate-rich shells and skeletal fragments settled onto the seabed. Over millions of years, these layers of bioclastic muddy carbonate sands accumulated into thick deposits. The immense weight of the overlying sediment and water pressure compressed and cemented these fragments together, transforming the loose marine deposits into the Port Campbell Limestone that now makes up the cliffs and the sea stacks.
The Erosion Process That Created the Stacks
The formation of the isolated stacks is a result of ongoing coastal erosion driven by the harsh weather conditions of the Southern Ocean. This process began when the sea level rose following the last ice age, bringing waves against the limestone cliffs. The ocean’s energy exploits natural weaknesses, such as joints and fractures, in the cliff face.
The primary mechanisms are hydraulic action and abrasion. Hydraulic action involves the force of water and compressed air being driven into rock crevices, causing them to widen and deepen. Abrasion occurs as waves hurl sand, pebbles, and rock fragments against the cliff base, sanding away the soft limestone.
This persistent attack first gouges out sea caves at the base of the cliff. As erosion continues, the caves on either side of a headland eventually meet, forming a natural archway. When the arch eventually collapses, the remnant of the resistant rock remains standing as an isolated pillar, known as a sea stack. This cycle is continuous, with the current stacks slowly eroding at an estimated rate of about two centimeters per year, while new caves and arches form in the existing headlands.