UNIVERSITY PARK, PA — The earth 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 think 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 the Earth’s crust.
Cratons are the old souls of the continents and make up about 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 areas so resilient, even as the plates around them shift and collide.
It turns out that the key may not lie 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 the air – may have triggered a series of events that led to the stabilization of cratons.
To understand how this happened, let’s take a step back in time. In the Neoarchaic era, Earth was a very different place. The atmosphere contained little oxygen and the continents were largely beneath a global ocean. But gradually the land began to rise above the waves – a process called continental rise.
As more rock was exposed to the air, the weathering rate 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 generates heat in the Earth over billions of years.
The researchers propose that when the HPEs were released through weathering, they ended up in sediments that accumulated in the oceans. Over time, plate tectonic processes would have carried these sediments deep into the crust, where the concentrated HPEs could really make their presence felt.
Buried at depth and heated from within, the sediments would have started to melt. This would have led 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 argue, that gave cratons their extraordinary stability.
The upper crust, enriched in HPEs, essentially acted as a thermal blanket, keeping the lower crust and underlying mantle relatively cool and strong. This prevented the kind of large-scale deformation and recycling that affected younger parts of the continents.
Interestingly, the timing of craton stabilization around the world supports this idea. The researchers point out that in many cratons, the appearance of HPE-enriched sedimentary rocks precedes the formation of distinctive Neoarchaean granites – the kind of rocks that would form from the melting of HPE-rich sediments.
Furthermore, metamorphic rocks – rocks that have been transformed by heat and pressure deep within the crust – also record a history consistent with the model. Many cratons contain granulite terranes, areas of deep crust uplifted to the surface that formed in the Neoarchaic. These granulites often have a composition that suggests they were formed by the melting of sedimentary rocks.
The sequence of events – the rise of continents, increased weathering, the burial of HPE-rich sediments, the melting of the deep crust and finally the craton stabilization – all appear to be aligned.
What is remarkable is that this process may have been an inevitable consequence of large continents rising above the sea. The appearance of land set in motion a cascade of processes that culminated in the birth of cratons.
This also helps explain why craton stabilization peaked in the Neoarchaic. It was during this time that HPE-enriched sediments first appeared in large volumes, coinciding with a period when Earth’s production of radioactive heat was about twice as high as it is today, due to the natural decay of HPEs over the course of of the time.
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 harbor valuable mineral deposits including gold, diamonds and critical metals. Knowing how they came to be can support our search for these sources.
As we walk on solid ground, it is humbling to think that the 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, think about the epic journey its components 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.