How Did the Devils Marbles Form in Australia?

How Did the Devils Marbles Form?

The Devils Marbles — known as Karlu Karlu to Indigenous Australians — are giant rounded granite boulders formed over millions of years through a geological process called spheroidal weathering, where acidic groundwater dissolved the sharp edges of buried granite, rounding the rock from the inside out before erosion exposed it at the surface.

What Are the Devils Marbles?

Located about 100 kilometres south of Tennant Creek in Australia’s Northern Territory, the Devils Marbles Conservation Reserve contains some of the most visually striking rock formations on Earth. The boulders range in size up to six metres across — larger than a double-decker bus — and many appear to balance impossibly on flat bedrock or on top of one another. The site covers roughly 1,802 hectares of classic red outback landscape.

The Science Behind the Boulders

The formation process began hundreds of millions of years ago when granite solidified deep underground. Over time, rainwater mixed with carbon dioxide in the soil to form weak carbonic acid, which seeped into the natural joint lines and fractures running through the buried granite. This acidic groundwater attacked the corners and edges of the rectangular granite blocks first — the points most exposed to chemical attack — gradually rounding each block into a sphere-like shape through spheroidal weathering.

As the surrounding softer rock and sediment eroded away over millions of years, these pre-rounded boulders were slowly exposed at the surface, giving the landscape its otherworldly appearance. The rounding happened underground long before the boulders were ever visible.

Still Cracking Today

The Devils Marbles are not a finished landscape — they are actively changing. The Australian outback experiences extreme temperature swings, with rock surfaces heating intensely in the midday sun and cooling rapidly after dark. This repeated thermal expansion and contraction causes the outer layers of granite to crack and peel away in sheets, a process known as exfoliation or onion-skin weathering. Visitors can sometimes hear the boulders cracking in the heat. This living geology means the landscape looks slightly different with every passing century.

Sacred Land: Karlu Karlu

Long before European settlers named them the Devils Marbles, this landscape held — and continues to hold — profound spiritual significance. Four Aboriginal peoples share custodianship of Karlu Karlu: the Warumungu, Kaytetye, Alyawarre, and Warlpiri. To these communities, the boulders are sacred objects connected to the Dreaming — the foundational spiritual framework of Aboriginal cosmology. According to Warumungu tradition, the boulders are the eggs of the Rainbow Serpent, a powerful creator being central to many Aboriginal belief systems.

In 2008, the Northern Territory government handed the land back to its Traditional Owners, who now co-manage it with Parks Australia. Visitors are asked to respect certain areas of the site that hold particular ceremonial significance.

Why Karlu Karlu Matters

The Devils Marbles sit at the intersection of cutting-edge geology and one of the world’s oldest living cultures. Scientists study the site to better understand long-term rock weathering processes, while Indigenous rangers work to preserve both the physical landscape and the cultural knowledge attached to it. Few places on Earth offer this combination — a geological wonder still actively forming, layered with tens of thousands of years of human meaning. For anyone trying to understand deep time, whether measured in rock cycles or in generations of oral tradition, Karlu Karlu is one of the most important places on the planet.