Earth's slowing spin may have shaped its primordial crust

Around 4.5 billion years ago, a planet-sized object collided with Earth, causing it to spin extremely fast with days lasting only two to five hours. New analysis by Simon Lock at the University of Bristol reveals that as Earth's spin slowed and the planet became rounder, its changing shape influenced early tectonic activity. His findings, published in the Journal of Geophysical Research: Solid Earth, suggest that the planet's shape changes were an overlooked factor in the formation of its early crust. The study accounts for the moon's orbit changes and Earth's internal structure to calculate the geological consequences of its flattened state.

Lock's research combines calculations of Earth's shape and internal structure with models of the moon's orbit evolution. This allows him to compare conventional moon-formation scenarios with alternatives that leave Earth spinning far faster. His results show that in the flattened early Earth, the outward pull of its spin partly counteracted gravity near the equator, allowing a thicker crust to form there than at the poles. As the planet 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.

The findings imply that Earth's early crust was shaped by the planet's changing shape, leading to diverse rock types and geological processes. The few surviving fragments of the ancient crust reveal a surprising diversity of rock types, usually explained by asteroid impacts or mantle recycling. Lock's work highlights rotation as a key yet overlooked ingredient in Earth's earliest geology. He hopes future models will include rotation to better understand how Earth's surface first took shape and the formation of its first continents. The study provides insights into the planet's distant past and the processes that shaped its early crust.

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