The Earth beneath our feet may 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 roughly half of Earth’s continental crust. Some date back over three billion years and have remained largely unchanged ever since. 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 the 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 out of 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 Neoarchaean era, around 2.5 to 3 billion years ago.
To understand how this happened, let’s take a step way back in time. In the Neoarchaean, Earth was a very different place. The atmosphere contained little oxygen, and the continents were mostly submerged beneath a global ocean. But gradually, land began to poke above the waves – a process called continental emergence.
As more rock was exposed to air, weathering rates increased dramatically. When rocks weather, they release their constituent minerals, including radioactive elements like uranium, thorium, and potassium. These heat-producing elements, or HPEs, are crucial because their decay generates heat inside the Earth over billions of years.
The researchers propose that as the HPEs were liberated by weathering, they were washed into 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 driven 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’s 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 the mantle below 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 globe 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 kinds of rocks that would form from the melting of HPE-rich sediments.
Furthermore, metamorphic rocks – rocks transformed by heat and pressure deep in the crust – also record a history consistent with the model. Many cratons contain granulite terranes, regions of the deep crust uplifted to the surface that formed in the Neoarchaean. These granulites often have compositions that suggest 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.
Source: https://studyfinds.org/earths-unbreakable-continents/