How were the Devils Marbles formed?

The incredible Karlu Karlu / Devils Marbles Conservation Reserve contains a series of giant granite boulders scattered around the valley south of Tennant Creek.

Karlu Karlu, Northern Territory

The Warumungu Aboriginal people believe them to be the fossilised eggs of the Rainbow Serpent, and the formations receive over 100,000 visitors every year, each one eager to gaze upon the mysterious boulders.

Some of the boulders stand a massive six metres high, which leaves many to wonder: just how did these spectacular rock formations take shape?

It went a little something like this.

The initial formation

Karlu Karlu / Devils Marbles are situated on an eroded nubbin, a small hill with a core of bedrock and smaller rocks dotting its surface. This majestic formation consists of granite stone that makes a top layer of the formation. The formation then runs through the surface below, and is surrounded by an abundance of sandstone.

What makes this formation so incredible is that it started to take place millions of years ago due to hardening magma underneath the Earth’s crust. Heavy layers of sandstone that sat atop of the granite placed huge downward pressure on the granite, which, after time, was lifted by folding of the area’s Earth caused by tectonic shifts. This also fractured the sandstone and allowed the granite to shift towards the Earth’s surface.

The pressure continued to diminish, and the granite soon expanded and caused cracks to form in the Earth’s crust. This is when the larger formations began to split into gigantic, square blocks.

Marbles forming

What happened next is why this place is such an awesome sight to see. The marbles began to form when the blocks became heavily exposed to water. The blocks began to decay once exposed to the water. The layer of loose material that surrounded the blocks began to erode due to the water and wind’s influence.

Ongoing weathering caused the blocks to become rounded or, as they’re known today, marbles. A particular chemical process caused the blocks’ surfaces to expand and then contract. This resulted in a process called exfoliation, in which thin layers of rock start to come off the block. This chemical process has more effect on edges and thus rounded the blocks.

Karlu Karlu / Devils Marbles is a wonder to behold. If you find yourself heading towards the Tennant Creek region, they are something not to be missed.

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