Earth is the only planet known to have continents, although exactly how these distinct landmasses came to exist remains something of a mystery. Among the most exciting theories is the idea that a barrage of giant impacts during Earth’s infancy led stable continental portions called cratons to form, and researchers have now provided the strongest evidence yet for this hypothesis.
“By examining tiny crystals of the mineral zircon in rocks from the Pilbara Craton in Western Australia, which represents Earth’s best-preserved remnant of ancient crust, we found evidence of these giant meteorite impacts,” explained study author Dr Tim Johnson in a statement.
“Studying the composition of oxygen isotopes in these zircon crystals revealed a ‘top-down’ process starting with the melting of rocks near the surface and progressing deeper, consistent with the geological effect of giant meteorite impacts.”
Previously, researchers had tentatively mooted the notion that Earth’s cratons may have been forged during the Late Heavy Bombardment, when the inner solar system was supposedly battered by a disproportionately large number of asteroids. Based on the age and distribution of craters on the moon, scientists believe that the impact rate declined significantly between 3.9 and 3.5 billion years ago.
“That the ages of the oldest continental crust in most cratons also span the time period 3.9–3.5 [billion years ago] begs the question of whether this is coincidence or if there is a causal relationship,” write the study authors in the journal Nature.
To investigate, the researchers looked for changes in the density of a particular oxygen isotope called oxygen-18 within the Pilbara Craton. Previous studies on large impact craters have revealed that such events cause significant melting of the shallow mantle, resulting in a decrease in oxygen-18 relative to lighter isotopes.
Their analysis revealed that the Pilbara Craton formed in three stages, the first of which occurred between 3.6 and 3.4 billion years ago. Zircon crystals found within this layer were isotopically light, indicating that they may have “crystallized following an initial giant impact that ultimately led to formation of the Pilbara Craton.”
According to the authors, the collision that triggered this event would have involved a massive space rock measuring “several tens to hundreds of kilometres in diameter.” Significantly, the researchers note that a similar isotope pattern can be seen in the zircon record of some of Earth’s other cratons, including the Yilgarn Craton of Western Australia.
“Our research provides the first solid evidence that the processes that ultimately formed the continents began with giant meteorite impacts, similar to those responsible for the extinction of the dinosaurs, but which occurred billions of years earlier,” says Johnson.
“Data related to other areas of ancient continental crust on Earth appears to show patterns similar to those recognised in Western Australia. We would like to test our findings on these ancient rocks to see if, as we suspect, our model is more widely applicable.”