Large patches of pink sand can be found on the beaches of South Australia, but the source of these colorful crystals has remained a mystery. Garnet, a rare mineral in beach sand, is typically destroyed by the ocean’s waves and currents. The presence of garnet in beach sand suggests a local source of garnet-bearing rock. However, the search for this rock led geologists to a previously undiscovered mountain range buried beneath the Antarctic ice sheet.
Garnet forms deep in the Earth’s crust under conditions similar to those in which diamonds are formed. One way garnet can reach the surface is through kimberlite pipes, volcanic structures that often contain diamonds. While there are kimberlites in South Australia, they are far from the coast and not abundant enough to be the source of the beach garnets. Another possibility is that the garnets are a result of erosion from the Adelaide Fold Belt, a mountain belt that developed between 514 million and 490 million years ago. The Gawler Craton, a large slab of ancient rock beneath South Australia, is another potential source of garnet.
To determine the age of the beach sand garnets and identify their source, researchers analyzed several hundred grains of garnet and found that the majority formed around 590 million years ago. This age ruled out both the Gawler Craton and the Adelaide Fold Belt as sources. Instead, it suggested that the garnets had traveled from a distant location without being ground down and had been stored locally before ending up on the beaches.
One possible solution is Hallet Cove Conservation Park, where sedimentary rocks formed around 280 million years ago during an icy phase of Earth’s history. Glaciers and icebergs can transport rocks over long distances without damaging their internal structure, making them a suitable means of transportation for garnets. Additionally, garnets found in glacial sediments on Kangaroo Island were also dated to around 590 million years ago, suggesting they were transported by ice flow.
During the Late Palaeozoic Ice Age around 280 million years ago, Australia and Antarctica were connected in a landmass called Gondwana. Glaciers from the Transantarctic Mountains in East Antarctica would have brought ice northwest, but these mountains did not experience garnet-forming conditions until 60 million years after the garnets in the pink sands formed.
The only known outcrop of rock with garnets of the right age is near the Skelton Glacier in Southern Victoria Land, East Antarctica. However, this small outcrop could not have produced the large volume of garnet found on Australian shores. It is believed that a vast area buried beneath a thick ice sheet contains abundant garnet that grew in an unknown mountain belt around 590 million years ago. Unfortunately, sampling this rock under the ice sheet is currently not possible. The theory is that millions of years of ice transport eroded the bedrock beneath and transported the ground-up rock, including garnets, northeastwards towards the coastlines of Antarctica and Australia.
Eventually, the transported rock was delivered to the South Australian coast around 280 million years ago and stored in sedimentary deposits like Hallet Cove. Over time, erosion released the garnets into the sea and onto the beaches of South Australia.