A new study led by Tim Johnson of Curtin University argues that a high frequency of asteroid impacts during Earth's Hadean period provided sufficient heat to melt the early crust, facilitating the formation of buoyant continental crust.

Geologists have long debated how continental crust first appeared, with the oldest known rocks dating to about 4.0 billion years ago. The scarcity of Hadean evidence has forced researchers to rely on models and indirect observations.

The research uses crater counts on the Moon, which lacks plate tectonics, to estimate the flux of impactors larger than 10 kilometers in diameter. Scaling this flux to Earth suggests thousands of such impacts per year, delivering energy that would have heated the upper mantle.

Modeling shows that kinetic energy from these impacts, when converted to heat, exceeded radiogenic and core heat by roughly a factor of ten for most of the Hadean. This impact heating kept the crust thin—less than 5 kilometers—and largely molten, with partial melt fractions exceeding 30 percent at depths of 5 kilometers.

Because the lithosphere was not rigid, subduction and plate tectonics could not operate under these conditions. As the impact flux declined exponentially between 3.9 and 3.5 billion years ago, internal heat sources became dominant, cooling the mantle and allowing the crust to thicken to about 30 kilometers.

The thicker, cooler crust eventually supported plate tectonics, a prerequisite for the emergence of continental rocks that appear in the geological record around 4.0 billion years ago. The authors note that the argument rests on physics‑based modeling in the absence of direct rock evidence, but they emphasize the need to consider model outputs rather than dismiss them due to a lack of samples.

Future discoveries, such as newly dated mafic rocks from the Nuvvuagittuq Greenstone Belt, could test the model. If confirmed, the impact‑heating hypothesis would provide a new explanation for how Earth's first continents formed.