Extending the symbolism of eggs as a metaphor for life and reproduction, recent research reveals the Earth itself may have once had an egg-like structure. According to a report from the University of Maryland, the plate tectonics that now define the Earth’s geology may have begun later in the planet’s history. Before the plates began moving and colliding to define the surface we know and love today, the Earth’s crust likely consisted of a solid but deformable shell encasing a molten liquid interior.

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The research, a joint effort between the UMD’s College of Computer, Mathematical and Natural Sciences, Curtin University and the Geological Survey of Western Australia, was recently published in the journal Nature, and represents the latest in a longstanding debate over the Earth’s geological history. One side of the debate says plate tectonics began right after the Earth started to cool (known as uniformitarianism), while the other proposes the planet went through a long phase with a solid shell enveloping it.

This latest study clearly favors the latter view. Models for how the first continental crust formed generally fall into two groups: those that invoke modern-style plate tectonics and those that do not, says Michael Brown, a professor of geology at the University of Maryland and a co-author of the study. “Our research supports the latter ‘stagnant lid’ forming the planet’s outer shell early in Earth’s history.

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Coming to this conclusion was no easy task. Brown and his team studied rocks collected from the East Pilabara Terrane – a large area of ancient crust located in Western Australia. As old as 3.5 billion years, these rocks are some of the oldest on the planet. The researchers looked at the granite and basalt rocks for signs of plate tectonic activity, such as subduction of one plate beneath the other.

As UMD explains it: “Plate tectonics substantially affects the temperature and pressure of rocks within Earth’™s interior. When a slab of rock subducts under the Earth’s surface, the rock starts off relatively cool and takes time to gain heat. By the time it reaches a higher temperature, the rock has also reached a significant depth, which corresponds to high pressure – in the same way a diver experiences higher pressure at greater water depth.”

In contrast, a stagnant lid regime would be very hot at relatively shallow depths and low pressures. Geologists refer to this as a “high thermal gradient.”

According to Brown, the results showed the Pilabara granites were produced by melting rocks in a high thermal gradient environment and the composition of local basalts shows they came from an earlier generation of source rocks supporting the ‘stagnant lid’ theory of the Earth’s early formation.

Images via Robert Whitehead, domdomegg