Earth’s slowing spin may have shaped its primordial crust

Around 4.5 billion years ago, a planet-sized object collided with the newly formed Earth, throwing out the debris that would later form the moon. The collision left Earth spinning extremely fast, with days lasting just two to five hours. Yet existing models of how Earth’s first crust formed and evolved have largely overlooked how the planet’s later slowdown might have affected it.

Through new analysis, Simon Lock at the University of Bristol has revealed that as Earth’s spin slowed and the planet grew rounder, its changing shape was likely an as yet neglected driver of early tectonic activity. His findings are published in the Journal of Geophysical Research: Solid Earth.

Slowing and rounding out

Immediately after the impact that formed the moon, Earth’s rapid rotation flattened it into a squashed sphere, bulging outward at the equator. In the most extreme scenarios, its width at the equator may have been up to twice its height from pole to pole. Over time, however, tidal interactions between Earth and the moon pushed the moon into a wider orbit, gradually slowing Earth’s spin and allowing the planet to become rounder.

Almost none of the crust from this ancient period survives today, having been destroyed by billions of years of geological activity and erosion—making physical models crucial for understanding what happened. Yet the few fragments that do remain reveal a surprising diversity of rock types early in Earth’s history: usually explained by processes such as asteroid impacts or activity in the mantle recycling the original crust.

Accounting for the moon

To address the limitations of previous models, Lock combined calculations of Earth’s shape and internal structure with models of how the moon’s orbit changed over time. This allowed him to compare conventional moon-formation scenarios with alternatives that leave Earth spinning far faster, and then to calculate the possible geological consequences as its surface was stretched and compressed.

His results suggest that in the flattened early Earth, the outward pull of its spin partly counteracted gravity near the equator, likely allowing a thicker crust to form there than at the poles.

As the planet then grew rounder, the crust stretched near the poles and was squeezed around the equator, producing movements that could have rivaled those of modern plate boundaries for up to tens of millions of years. In some fast-spin scenarios, the squeezing briefly reached rates comparable to the collision that formed the Himalayas, extending all the way around the equator.

Understanding Earth’s distant past

If these calculated deformations are accurate, they would have had far-reaching consequences. As surface rocks altered by water and weathering were buried and remelted, they could have helped produce silica-rich rocks resembling those found on today’s continents.

Meanwhile, rising pressures inside the planet could have made parts of the lower crust dense enough to sink into the mantle, creating chemically distinct regions deep within Earth. Together, these processes could better explain how Earth’s rocks became so diverse so soon after its formation.

Lock’s work highlights rotation as a key yet overlooked ingredient in Earth’s earliest geology. He now hopes that by including it in future models, researchers could gain a much clearer picture of how our planet’s surface first took shape, along with the seeds of its first continents.

Who’s behind this story?

Sam Jarman

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Sadie Harley

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