Deep regolith evolution in an active temperate margin: the roles of uplift, topography and lithology
This study demonstrates that in the active temperate forearc of South-Central Chile, deep regolith evolution is driven by a hybrid model where transient landscape stabilization preserves inherited Pliocene weathering mantles while concurrent vertical infiltration drives deep chemical depletion, revealing that slowly eroding active margins can sustain exceptionally thick, highly weathered Critical Zones despite ongoing uplift.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep beneath our feet lies a hidden world where solid rock slowly transforms into soil, a process that shapes the very ground we walk on and regulates the planet's chemistry. This zone, where living things, water, air, and rock interact, is known as the critical zone. It acts as a giant sponge and filter, storing water, supporting forests, and slowly dissolving minerals to pull carbon dioxide from the atmosphere. For decades, scientists have understood how this works near the surface, but the deep underground architecture remains a mystery, especially in places where the Earth is actively moving. In mountainous regions where the ground is constantly being pushed upward by tectonic forces, one might expect the soil to be thin and young, constantly stripped away by erosion before it can grow deep. Yet, in a specific stretch of the Chilean coast, researchers have discovered a geological paradox: thick, ancient layers of deeply altered rock sitting right beneath an active, rising landscape.
A team of scientists from Chile, France, the United Kingdom, and the United States set out to solve this puzzle in the South-Central Chilean Coastal Cordillera. They focused on a twenty-five square kilometer area where the land rises at a rate of about half a millimeter every year, a speed that usually prevents deep soil from forming. Instead of finding thin, eroded layers, they drilled and dug into the ground at 114 different spots, reaching depths of up to 44 meters without ever hitting fresh, unweathered bedrock. They collected rock samples from every two meters of depth and analyzed their chemical makeup to see how much the original stone had changed. By measuring how much of the rock's original minerals had dissolved and washed away, they found something extraordinary: the rock at the very bottom of their deepest holes was just as chemically altered as the soil near the surface. The rock had lost nearly all of its calcium and sodium, elements that usually wash out quickly, indicating that a powerful chemical transformation had penetrated deep underground, far beyond what the rising land should have allowed.
The researchers then looked at the shape of the land to understand why this deep weathering survived. They mapped out the terrain and found a distinct pattern based on slope. On steep hillsides, the soil was indeed thinner, as gravity and rain washed the top layers away. However, on the flat, gentle plateaus that sit higher up the mountain, the soil was incredibly thick, often exceeding 30 to 40 meters. These flat surfaces act like protective shields. Because the ground is so level, water does not rush down to carry soil away; instead, it soaks straight down, continuing to dissolve the rock deep below. This allows the chemical weathering process to keep moving downward, even as the entire mountain range slowly rises. The study shows that these flat, high-elevation surfaces have been stable for a long time, preserving a "weathering legacy" that started millions of years ago, while the steeper valleys around them have been actively eroding.
The team also examined the type of rock at each location, testing whether the mineral composition alone could explain the deep soil. They studied four different rock types, including granite and schist, and found that while the rock type mattered at first, it eventually became less important than the shape of the land. On the older, higher, and flatter surfaces, the soil was thick regardless of whether the rock was granite or schist. This suggests that the long-term stability of the flat ground is the most powerful factor, overriding the natural differences between rock types. The scientists propose a hybrid explanation for this phenomenon: the deep soil is likely a combination of ancient layers that formed when the land was lower and flatter, and new layers that are still forming today. The flat tops of the mountains have remained stable enough to protect these deep layers from being washed away, while rainwater continues to seep down, keeping the chemical reaction alive.
This discovery changes how we view the relationship between moving mountains and soil formation. It proves that even in an active, rising landscape, deep chemical weathering can thrive if the surface is stable enough to shield it from erosion. The findings suggest that these deep, soil-covered zones are not just passive leftovers from the past, but active systems where water continues to dissolve rock and release nutrients. By preserving these thick layers, the landscape acts as a long-term storage unit for carbon and a reservoir for groundwater. The study highlights that the history of the land is written not just in the rocks themselves, but in the thickness of the soil that covers them, revealing a quiet, slow process of transformation that persists despite the violent forces of the Earth rising beneath it.
Drowning in papers in your field?
Get daily digests of the most novel papers matching your research keywords — with technical summaries, in your language.