Beneath the American Midwest, part of the continent is gradually losing its foundation. This slow geological phenomenon, invisible on the surface, puzzles scientists.
An international team has identified a novel mechanism: "drops" of rock detaching from Earth's crust and plunging into the mantle. This discovery sheds light on continent dynamics over vast timescales.
A little-known geological process
Cratons, these billion-year-old continental cores, form the rigid framework of continents. Their exceptional stability allows them to withstand tectonic movements over geological times. Yet beneath the American Midwest, seismic data reveals abnormal thinning at the base of the North American craton, indicating deep disturbance.
Study figure showing rock droplets beneath the craton plunging into the mantle.
Researchers hypothesize these flows are caused by remnants of the Farallon plate subducting beneath the craton. Credit: Hua et al., Nature Geoscience. The "droplet" imagery describes a precise physical phenomenon: under heat and mechanical stress, the craton's base softens and forms protrusions that, under their own weight, eventually detach. This process resembles the behavior of viscous materials like honey dripping, but occurring over hundreds of miles (kilometers) and millions of years.
Unlike ancient events like the destruction of the North China Craton, this one can be observed in real time thanks to modern techniques. Researchers measured that this internal erosion, though slow, permanently alters the continent's structure. These observations provide a unique opportunity to understand long-term continental evolution.
The key role of an ancient oceanic plate
The Farallon plate, which disappeared about 200 million years ago, continues to surprisingly influence North American geology. Though nearly completely subducted, its remnants buried 370 miles (600 km) deep disrupt mantle currents beneath the craton. This phenomenon creates shear forces that gradually weaken the lithospheric base.
The interaction between these plate remnants and the craton functions like an invisible erosion mechanism. Models show the plate acts in two ways: by deflecting flows of hot mantle material, and by releasing fluids that chemically alter rocks. These combined processes explain the formation of the "droplets" observed through seismology.
This discovery establishes a direct link between ancient tectonic events and current continental evolution. Researchers validated this hypothesis by comparing models with and without the Farallon plate - the droplets only appearing in the first case. Additional evidence comes from Kentucky's geological formations, whose origin is now explained by this phenomenon.
Article author: Cédric DEPOND