TRAVEL GUIDE Pilbara Rocks! Rocking around ancient region

Marble Bar.
Picture: Stephen Scourfield The West Australian
Photo of Stephen Scourfield

In the Pilbara, Stephen Scourfield finds one of Earth's most ancient blocks of continental crust...

If an hour represents the history of the Earth since its formation 4.6 billion years ago, human civilisation fills just the last minute. And the geology of Western Australia spans 4.4 billion years.

The Pilbara is one of Earth’s oldest blocks of continental crust.

Research published in Nature Geoscience details the Pilbara region as one of Earth’s oldest blocks of continental crust. It bears witness to the history of the planet.

To go into more detail, the Pilbara region began to form more than 3.6 billion years ago and research seems to show that its rocks were not formed through the more usual plate-tectonics process.

In that process, the outer layer of the Earth’s crust consists of stiff “tectonic plates” that drift across the surface, bumping and grinding at their boundaries.

But the East Pilbara rocks exhibit signs of resulting from a series of “gravitational overturn” events long before plate-tectonic processes began about 3.2 billion years ago.

Let me explain gravitational overturn. When the Earth was young it was too hot for the rigid plates of plate tectonics to form (it was more like a melted chocolate bar).

Thick piles of basalt lavas erupted and formed a dense crust, which the underlying, boiling core could barely support.

The base of the cooling crust was heated again by the hot mantle beneath and started to melt again, making quite-buoyant granitic magmas.

This all led to “unstable stratification” — low density granites overlaid by high density basalts, both layers bending and flowing in the high heat. Granitic blobs wanted to rise, basalts wanted to sink. Geological scientists call the blobs “plumes” and the reorganisation process going on “gravitational overturn”.

And this is very much the geological story of the Pilbara — all granite domes and remnants of the basalt crust preserved as the greenstone belts.

Research work has revealed that some domes originally melted 42km beneath the surface, before crystallising as granite at 20km deep. 

Uranium lead dating has revealed that they crystallised between 3.6 billion to 3.5 billion years ago.

The final gravitational overturn was more than three3 billion years ago, leaving the Pilbara crustal block finally robust and buoyant enough to survive plate tectonics to this day.

I have travelled the Pilbara with planet evolution scientists from NASA and the Caltech — the California Institute of Technology, a private doctorate-granting research university in California.

The Pilbara inspires internationally renowned scientists looking for answers to one of the planet’s great questions: just how was the platform for the eventual evolution of life created?

Fact File

ICONS OF THE PAST

The Pilbara features strongly in the Geological Survey of Western Australia’s list of the geological icons of WA…

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