Traces of the Earth’s first rocks found in WA

By Natsumi Penberthy 8 March 2010
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Rocks found in the central part of the Australian tectonic plate help to fill a 500-million-year gap in our knowledge of the Earth’s early history.

TINY TRACES OF THE oldest rocks on Earth were recently found in northern WA, helping to fill in a gap in knowledge of the first 500 million years of the Earth’s existence.

“Our feet are resting on the Earth which we don’t know well yet,” says Dr Svetlana Tessalina of the Department of Earth and Planetary Sciences at Macquarie University, and leader of the international team of researchers that found the crust remnants. Their findings, whispers of contact with the primordial crust, were published in the journal Nature Geoscience last week, and will help to enlighten researchers as to what the world’s first rocks, and the Earth itself, might have been like.

New geochemical testing

Theoretically, the first crust was formed just like a thin skin on the surface of a sea of molten rock, and covered the entire Earth a little more than 4.5 billion years ago. However, the oldest rocks known to exist on Earth are only 4.03 billion years old, leaving a huge 500-million-year gap in physical evidence.

Now, the clues unearthed by Svetlana and her team suggest that the first layer was mainly formed from dark basalt-like rocks, made from cooled magma in the Earth’s mantle. “But it was probably very unstable and we haven’t found any relic of this crust now, because it was almost completely destroyed by geological processes,” says Svetlana.

Because of the lack of solid evidence, the traces of this primordial crust were identified through new geochemical tests that allow scientists to detect tiny quantities that remain in Australia’s rocks today. To do this, Svetlana and her team took samples of volcanic, sedimentary and hydrothermal rocks from outcrops and drill cores in WA’s Pilbara region, which has changed very little over long periods of time and contains some of Earth’s oldest rocks and fossils.

The team then used measurements of radioactive decay in the rocks to date some elements within them. These were found to be roughly 4.3 billion years old – a billion years older than the physical age of the rock formations themselves, and these elements likely came from the cooled magma of the primordial planet.

Hot debate over early Earth’s appearance

Geologists have been debating for years over the appearance of the young Earth — dubbed the Hadean Earth, after Hades, the underworld of Ancient Greek mythology. One theory is that it may have superficially resembled the Moon. But ancient zircons, minerals discovered south of the Pilbara, revealed evidence of a more diverse planet with continents, oceans, and a fully operative rock cycle within the first 700 million years of Earth’s existence.

Professor Stephen J. Mojzsis, a geologist at the University of Colorado, in Boulder, U.S. supports this picture: “These discoveries have provided further credibility to the views that have emerged from studies, that…[after the crust formed and cooled the] Hadean Earth was an eminently habitable place.”

The evidence pool is still slim, but Svetlana guesses that more traces of rock remain preserved: “I think we have to continue to search for potential relics in…the Pilbara to confirm our results and to know a little bit more about the early Earth’s evolution,” she says.

Stephen believes the next obvious target is the Barberton Greenstone belt in South Africa, once linked to Australia. He says: “It makes sense that if the Earth was so action-packed way back when, that we might expect to find more old rocks and minerals.”

Illustration of the Hadean Earth by Fahad Sulehria (Website)
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