The Devil's Marbles.

Driving up the Stuart Highway (linking Adelaide with Darwin in Australia) near Tennant Creek, one comes across a field of strange balls of granite glowing red in the setting sun. Clusters of marbles, some of them balancing, others cut in half like with a sword, are strewn about a shallow sandstone valley. They vary from half a metre to seven metres across, and orthoclase feldspar megacrysts up to 8 centimetres long are dotted in the rock. A parks service campsite with toilets and an honesty box sits in the middle of the formations.

Known as Karlu Karlu to the indigenous owners, The reserve is a registered sacred site, and one tale that can be told to outsiders relates to their origin: The Devil man Arrange walked through the site making a hair string belt, and clumps of hair fell on the ground as he passed by that turned into the boulders. He also spat on the ground, creating the central part of the reserve. Appearing out of the heat haze in a flat and empty landscape, it is easy to see why they play an important role in the dreamtime songs of several tribes.

The geological aspect of their formation is due to erosion. A molten granite pluton bubbled its way up through the sandstone 1.64 billion years ago, and slowly crystallised and stewed. The weight of the sandstone was gradually removed by erosion after an uplift event, taking the pressure off the granite, which expanded and cracked to form rectangular blocks bounded by joints. As they were uncovered, water infiltrated into the blocks, which began to weather both chemically and mechanically.

Exfoliation (also called onion or spheroidal weathering) then shaped the rocks into marbles, as they expanded and shrunk in the extreme alternations of heat and cold in the desert's daily cycle. The erosion takes place at the edges, peeling them off and leaving a rounded remnant. The temperature related expansion/contraction (called solarisation weathering) is so extreme that some of the marbles have split in half, like the one in the photo.

Loz

Image credit: Kaya Polekatt.