In parts of South Australia, long stretches of beach are often blanketed in large patches of pink sand.
Strong swells can dump drifts of reddish grains of garnet along the shore – but the origin of these colourful crystals has until now been a mystery.
When streaks of pink first appeared in the sands at Petrel Cove, a remote beach that meets the Southern Ocean, scientists in Australia quickly worked out what the colored sand was made of, a mineral called garnet, but were surprised by its age and where it originated from.
“This journey started with questioning why there was so much garnet on the beach at Petrel Cove,” says University of Adelaide geologist Jacob Mulder.
“It is fascinating to think we were able to trace tiny grains of sand on a beach in Australia to a previously undiscovered mountain belt under the Antarctic ice.”
Earth’s crust is constantly eroding and reforming, with loosened sediments whisked away on the wind and waters getting deposited elsewhere to form new lands. If geologists are lucky, they can draw connections over huge distances and long stretches of time between deposits of similar ages with alike properties.
Garnet is a fairly common mineral, deep red in color. It crystallizes at high temperatures, usually where large mountain belts grind upwards out of colliding tectonic plates. This makes it arguably the most important mineral for deducing how and when mountains formed, as the crystals’ presence indicates the pressure and temperature history of the metamorphic rocks in which they form.
The team’s lutetium-hafnium dating showed that some of the garnet found at Petrel Cove and in nearby bedrock formations matched the timing of local mountain-forming events in South Australia.
But their results indicate it mostly formed around 590 million years ago, some 76–100 million years before the Adelaide Fold Belttook shape, and billions of years after the Gawler Craton crustal block formed.
“The garnet is too young to have come from the Gawler Craton and too old to have come from the eroding Adelaide Fold Belt,” explains Sharmaine Verhaert, a geology graduate student at the U
