UNIVERSITY PARK, PA – The ground beneath our feet can feel solid, stable, and seemingly eternal. But the continents we call home are unique among our planetary neighbors, and their formation has long been a mystery to scientists. Now researchers believe they may have uncovered a crucial piece of the puzzle: the role of ancient weathering in shaping Earth’s “cratons,” the most indestructible parts of our planet’s crust.
Cratons are the old souls of the continents, forming approximately half of the Earth’s continental crust. Some date back more than three billion years and have remained largely unchanged since then. They form the stable hearts around which the rest of the continents have grown. For decades, geologists have wondered what makes these regions so resilient, even as plates shift and collide around them.
It turns out that the key may lie not in the depths of the Earth, but on its surface. A new study from Penn State and published in Nature suggests that subaerial weathering—the breakdown of rocks exposed to air—may have triggered a chain of events that led to the stabilization of cratons billions of years ago, during the Neoarchean era, about 2.5 to 3 billion years ago.
To understand how this happened, let’s go back in time a bit. In the Neoarchaean, Earth was a very different place. The atmosphere contained little oxygen and the continents were mostly submerged under a global ocean. But gradually the land began to rise above the waves, a process called continental rise.
As more rock was exposed to air, the rate of weathering increased dramatically. When rocks weather, they release their constituent minerals, including radioactive elements such as uranium, thorium and potassium. These heat-producing elements, or HPEs, are critical because their decay has generated heat inside the Earth for billions of years.
The researchers hypothesized that as the HPEs were weathered, they were washed into sediments that accumulated in the oceans. Over time, plate tectonic processes would transport these sediments deep into the Earth’s crust, where concentrated HPEs could really make their presence felt.
Buried deep and heated from within, the sediments would begin to melt. This would lead to what geologists call “crustal differentiation”—the separation of the continental crust into a lighter, HPE-rich upper layer and a denser, HPE-poor lower layer. It is this layering, the researchers say, that gave the cratons their extraordinary stability.
The HPE-enriched upper crust essentially acted as a thermal blanket, keeping the lower crust and mantle below relatively cool and healthy. This prevented the kind of large-scale deformation and recycling that affected the younger parts of the continents.
Interestingly, the timing of craton stabilization worldwide supports this idea. The researchers point out that in many cratons, the appearance of HPE-enriched sedimentary rocks preceded the formation of distinctive Neoarchean granites—the types of rocks that would have formed from the melting of HPE-rich sediments.
In addition, metamorphic rocks—rocks transformed by heat and pressure deep in the crust—also record a history consistent with the pattern. Many cratons contain granulite terranes, regions of the deep crust raised to the surface that formed during the Neoarchean. These granulites often have a composition that suggests they formed from the melting of sedimentary rocks.
So the sequence of events—the emergence of continents, increased weathering, burial of HPE-rich sediments, deep crustal melting, and finally, craton stabilization—all seem to line up.
Remarkably, this process may have been an inevitable consequence of the rise of large continents above the sea. The emergence of the earth set in motion a cascade of processes that culminated in the birth of the cratons.
This also helps explain why craton stabilization peaked during the Neoarchean. During this time, HPE-enriched sediments first appeared in large volumes, coinciding with a period when Earth’s radioactive heat production was about twice what it is today due to the natural decay of HPE over the course of the weather.
The implications of this work extend beyond simply understanding the ancient past. Cratons are more than just geological oddities – they are important habitats for life and contain valuable mineral deposits, including gold, diamonds and critical metals. Knowing how they formed can inform the search for these resources.
As we walk on solid ground, it is humbling to think that the very foundations of our continents owe their existence to the slow, patient work of weathering and erosion billions of years ago. The next time you pick up a rock, consider the epic journey its components may have taken—from mountain to sea to deep crust and back again—all culminating in the world we know today.
StudyFinds Editor-in-Chief Steve Fink contributed to this report.
