What are the Devil’s Marbles?

You can understand how this odd natural feature got its name: in the middle of Australia’s Northern Territory wilderness, near the town of Wauchope, hundreds of rounded boulders are strewn conspicuously across the arid landscape. Some are piled in heaps, while others are poised atop rocky outcrops in a seemingly deliberate attempt to balance them. The Aboriginal owners of this 1,800-hectare (4,453-acre) reserve call it Karlu Karlu, which is also derived from a traditional belief that a supernatural being placed the boulders for its own unfathomable amusement. And while the reality might not involve the paranormal, the science is just as interesting…

The Devil’s Marbles started life nearly 2 billion years ago as the magma cooled in the Earth’s crust to form the igneous rock granite. On top of the granite, a thick sedimentary layer of sandstone formed that compressed the granite under its immense weight. Over millions of years, the sandstone gradually eroded, simultaneously releasing the pressure and causing the granite to expand and then crack into roughly cube-shaped blocks.

Once the sandstone was completely eroded, the angular blocks of granite were subject to erosion themselves. But granite is a significantly harder material than sandstone, so the boulders you can see now are a lot more resistant to the same chemical and mechanical weathering. A combination of water and acidic chemicals naturally present in the atmosphere rounds off the points of the blocks to leave smoother rocks, while the great difference in day and night temperatures expands and contracts the rock forms, peeling off layers to leave the ‘marbles’ we see today.