Geologists may have finally solved a long-standing enigma concerning the Colorado River’s largest tributary, which seemingly defied gravity by flowing uphill when it initially formed.
The Green River originates in Wyoming and empties into the Colorado River within Canyonlands National Park in Utah. Roughly 8 million years ago, instead of meandering around the 4,000-meter-high Uinta Mountains in northeastern Utah and northwestern Colorado, the Green River carved a path directly through them. However, a recent study posits that such a feat was impossible without a mechanism that lowered the mountains’ elevation.
“It’s a bizarre path,” noted Adam Smith, the lead author of the study from the University of Glasgow in the UK. “We know from dating evidence and other sources that the mountain range is 50 million years old, while the river has only occupied this course for the last 8 million years, perhaps even as recently as 2 million years ago.”
The Green River traverses Lodore Canyon, where it has incised a gorge with 700-meter-high walls. Previously, two competing theories attempted to explain this specific river course, but neither was particularly convincing, according to Smith.
One hypothesis suggested that the Yampa River, situated south of the Uintas, eroded the geological structure northward, creating a channel for the Green River. This would have required immense energy, which the Yampa River, being less substantial, likely couldn’t muster. “If this were plausible, one would anticipate seeing gigantic canyons cutting through all the mountain ranges, but that’s not the case,” Smith stated.
Another theory proposed that accumulated sediment temporarily elevated the Green River’s level, allowing it to spill over the Uinta Mountains and carve a path through them. Yet, existing data also fails to support this idea. “The sediment accumulations here aren’t as high as the Lodore Canyon is deep,” Smith commented.
Instead, the researchers conducting the new investigation suggest that the Uinta Mountains subsided significantly, enabling the Green River to breach them. The researchers hypothesize that a phenomenon termed “lithospheric dripping” pulled the mountains downward, followed by a rebound effect that caused the landscape to uplift again, establishing the topography observed today.
Lithospheric drips are dense regions that can form directly beneath mountains, at the interface where the Earth’s crust meets the mantle—the layer between the crust and the planet’s outer core. The weight of the mountains increases pressure at the base of the crust, leading to the formation of minerals like garnet, which are denser than mantle rock. Eventually, these minerals coalesce into a blob that slides away from the base of the crust, pulling the mountains down and reducing their height above the surface.
These lithospheric drips induce a rebound effect when they finally detach and sink into the mantle. The concept of these drips is relatively new, but evidence of their existence has been found in several locations, including the Andes. “They can occur wherever a mountain range forms and can happen at any time,” said Smith.
A characteristic sign of lithospheric dripping is a ‘bullseye’—a distinct pattern of surface uplift. Smith and his colleagues modeled the geological processes within the Uintas based on the river’s anomalous profiles and identified such a pattern.
The researchers also examined seismic tomography images—three-dimensional maps of the Earth’s subsurface created using seismic waves from a previous study. They detected a blob approximately 200 kilometers deep within the mantle beneath the Uintas, closely resembling an aged lithospheric drip, which Smith considers compelling evidence for this mechanism.
The researchers then used the depth and size of the observed drip to calculate when it separated from the base of the Uinta Mountains. They found that it likely detached from the mountain base between 2 and 5 million years ago, aligning with the model’s predictions for the timing of mountain uplift and corresponding with estimates for when the Green River first breached the range.
According to Smith, the dripping mass lowered the mountains so substantially that they presented the “path of least resistance.” He added that once the Green River began flowing over the Uintas, it continued to incise into the mountains, creating features like Lodore Canyon.
Other experts not involved in the study suggested this explanation might ultimately resolve the long-standing puzzle.
Mitchell McMillan, a research geologist at the Georgia Institute of Technology, stated that lithospheric drip tectonics is a plausible explanation for the Green River’s peculiar path.
“The most exciting aspect of this research is that it utilizes surface data to understand processes occurring in the mantle and how they can influence mountain ranges,” McMillan explained. “Regardless of whether the drip hypothesis ultimately proves correct, this study is a valuable demonstration of that approach.”
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