Earth, Originally, Had A Solid Shell

New research suggests that plate tectonics, the driving force of earthquakes and volcanoes, began at a later period in Earth’s history, and not at the very formation of the planet. The findings are published in Nature.

Earth’s outer layer now consists of connecting plates, whose movement can cause earthquakes. The new study suggests that the formation of these plates is not as old as the planet itself. Photo credits: USGS.

Plate tectonics represent a key feature of our modern Earth: it constitutes a theory that describes the movements of huge tectonic plates making up the planet’s outermost shell known as the lithosphere (which includes the Earth’s crust and upper mantle). These plates are thought to be the cause of earthquakes, volcanoes, and other natural phenomena, through their sliding and other movements. The scientific community believes that these motions have been happening for hundreds of millions of years; but are they as old as the Earth? The new study suggests that plate tectonics did not begin right away at the formation of the planet (another theory known as uniformitarianism), but only later in Earth’s history. Prior to this, the planet started off with a solid shell as outer layer.

So, the Earth did not always consist of different layers: it did not originally bear separate enormous plates. Rather, at the formation of the Earth, the planet began cooling, and its first outer layer was, at that time, a single solid distorted shell, which later folded and cracked, leading to plate tectonics.

The Earth’s outer shell back in the day (4.5 billion years ago—the Earth’s estimated age) was a “stagnant lid”, says one of the authors, Michael Brown from the University of Maryland, explaining that their findings support this theory instead of the uniformitarianism one.

Brown and his team reached this conclusion after they examined rocks obtained from an ancient granitic crust in Western Australia, in a region called the East Pilbara Terrane—these rocks are considered to feature among the oldest, dating back to around 3.5 to 2.5 billion years ago. Specific types of granites were taken for the study, those whose chemical composition was linked with volcanic arcs, which are an indicator of plate tectonic activity. Basalt rocks of Coucal formation were also analysed because of previous findings showing a similarity in chemical composition in the granites of East Pilbara Terrane and the Coucal basalts, suggesting that the former might have come from the latter. Brown and his colleagues used these rocks to perform several modelling experiments to determine what might have happened when the Earth was still very young, in a scenario where it did not consist of tectonic plates.

Their findings were found to support their theory: the Coucal basalts appear to be the source of the Pilbara granites. The shift from one to another might have happened under conditions of temperature and pressure consistent with the scenario where the Earth was covered with a “stagnant lid”, that is, a single shell as outer layer. Thereafter, a number of phases came to be, resulting in the production of our first continents.

“Our results suggest the Pilbara granites were produced by melting of the Coucal basalts or similar materials in a high thermal gradient environment,” explains Brown. “Additionally, the composition of the Coucal basalts indicates that they, too, came from an earlier generation of source rocks. We conclude that a multi-stage process produced Earth’s first continents in a ‘stagnant lid’ scenario before plate tectonics began.”

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