A new model published in 2025 by geologist Tim Johnson and his team at Curtin University suggests that asteroid impacts were the primary drivers of early continent formation on Earth. The research indicates that internal tectonic processes were not the main cause.

Earth formed 4.5 billion years ago, and continental rocks began to appear around four billion years ago. "The continents started appearing around about four billion years ago—that's the oldest continental rock we know about," Johnson said. The oldest known continental-type rocks crystallized approximately 4.03 billion years ago. Some zircon crystals found are as old as 4.4 billion years, and rare basaltic rocks date back about 4.2 billion years. The Hadean eon covers Earth's first 500 million years.

Johnson's team modeled the heat generated by the kinetic energy of asteroid impacts on early Earth. "It really is as simple as converting the size and the velocity of the impactor into energy," he said. Their model indicates that impact heating surpassed radiogenic and core heat for most of the Hadean eon by roughly an order of magnitude. The Moon, which lacks plate tectonics, has a crust containing impact craters, and these crater counts are calibrated using dated lunar samples.

Geodynamic simulations from Johnson's team suggested that Earth's crust in the Hadean eon was less than 5 kilometers thick. These models showed widespread partial melting began 2 to 3 kilometers below the surface, with melt fractions exceeding 30 percent by volume at around 5 kilometers depth. The simulations also produced a wholesale recycling of crust back into the mantle, with material descending to depths of at least 600 kilometers. The impact flux decreased exponentially over time. Between 3.9 and 3.5 billion years ago, internal heat sources became the dominant influence on the crust.

Modeling also suggests that crustal thickness reached approximately 30 kilometers by the early Archean era. "As soon as you can create thick crust and you can create a mantle lithosphere underneath, you can start building continents," he said.

Why It Matters

This new model challenges existing hypotheses about early Earth's continental formation. Previous ideas suggested that plate tectonics operated in the Hadean eon, leading to continental crust developing above subduction zones, or that Earth was too hot for rigid plates, with crust forming above mantle plumes. Johnson's model offers an alternative explanation for the heat budget of early Earth, pointing to asteroid impacts as a crucial factor. The findings propose a different understanding of the Earth's geological processes during its earliest period, which is characterized by scarce data.