The Nullarbor Plain in Australia, one of the largest magnetic anomalies in the world

The Nullarbor Plain is located along the coast of the Great Australian Bight, which stretches for 1,160 kilometers to the south of the country, between Capes Pasley and Carnot.

It is an arid and desert area considered to be the largest limestone outcrop in the world, with an area of ​​about 200,000 square kilometers.

Considered uninhabitable by Europeans, the plain was used by semi-nomadic aborigines as a place of temporary transit, and today it is part of a natural park. Crossing the Nullarbor is for Australians one of the most exciting experiences that can be lived, and many display stickers with phrases on their vehicles I have traversed the Nullarbor as proof of his feat.

The Nullarbor plain, in a semicircular light tone next to the coast / photo Public Domain on Wikimedia Commons

But apart from the adventurous attraction, the area is also home to one of the most exciting mysteries of geology, the so-called Coompana anomaly.

Detected by reconnaissance aircraft during the 1970s, the subsurface magnetic signatures of the plain still intrigue researchers, prompting multiple theories to be proposed.

And it is that Coompana is one of the largest magnetic anomalies in the world, with a maximum width of 50 kilometers, caused by the remnant magnetism of the deep igneous rocks that were formed at a different time than the surrounding rocks.

The anomaly lies about 200 meters below the surface, where the magnetism of the rocks at that depth is opposite to that of the surrounding ones. One of the explanations being considered for this phenomenon is that these rocks would have formed in the northern hemisphere, then moving south with the movement of tectonic plates.

Coompana anomaly: magnetic field on the left, gravity field on the right / photo Csiro

Another of the proposed theories suggests that these rocks formed at a time when the Earth’s magnetic poles were reversed. And a third hypothesis suggests that the rocks would be the remains of a gigantic meteorite impact.

The latter would be supported by the fact that the entire Nullarbor plain is home to large meteorite deposits, many of them discovered near the Mundrabilla farm (the largest of all those discovered in Australia was found here, weighing about 12 tons). . And also because the shape of the anomaly is reminiscent of an impact crater.

In addition, the intensity of the magnetic field in the anomaly is extremely low, increasing progressively as we move away from the edges. Moreover, in recent investigations carried out with a magnetometer just 80 meters above the ground, small anomalies were also detected in shallow rocky structures, spread throughout the area surrounding the Coompana anomaly.

Location of the Nullarbor Plain on Google Maps

Unlike the main one, which is 200 meters deep, these small anomalies are just below the surface layer and their presence could be associated with the existence of heavy minerals or the coveted rare earths.

The negative severity anomaly detected in Coompana currently suggests two possibilities to researchers. Either it is a gigantic collapsed caldera, or an impact structure after which magmatic intrusions would have occurred in its outer ring.


Gondwana Geoscience / CSIRO blog / Looking into a ‘Blue Hole’ (Clive Foss et al.) / Wikipedia.