Rivers underground

Featured Image: The River Styx emerging from Mammoth Cave by Daniel Schwen. From Wikipedia under a CC-BY-SA license.

Paper: Modeling cave cross‐section evolution including sediment transport and paragenesis
Authors: M.P. Cooper and M.D. Covington

It’s not easy to watch caves form. It happens slowly and out of view, so we know relatively little about cave passage erosion compared to our knowledge of how rivers at Earth’s surface work. New research suggests that the same physical erosion processes that cut river channels at the surface might also be at work underground, adding new depth to our understanding of cave genesis.

Caves form as water trickles through rocks, typically rich in soluble calcium carbonate, below ground, slowly eroding the rock and ultimately yielding open passages. Conventional wisdom holds that formation of cave passages by erosion is a reaction-rate-limited chemical process. This means that passages grow at a steady rate—depending only on the chemical makeup of the water—no matter how quickly water runs through the passage. If that’s true, cave passage formation by chemical processes is fundamentally different from the carving of river channels into non-soluble rock at Earth’s surface, where physical erosion depends on the stresses imposed by water flowing over the river bed. This means that as erosive stresses increase, for example due to more water or a steeper river channel, the river bed erodes more rapidly.

Because erosion happens slowly—and cave erosion happens both slowly and underground—distinguishing whether the formation of cave passages is limited by water chemistry or erosive power is challenging. To get around the difficulty of observing erosion processes in action, Max Cooper and Matt Covington, geomorphologists at the University of Arkansas, built a computer model to study processes of cave passage formation. Their model includes both possible erosion processes: chemical dissolution and mechanical erosion of rock. It simulates the flow of water and sediment through cave passages, showing how passages might change their shape over long periods of time.

The authors tested whether their model creates realistic cave passage shapes when the erosion rate depends on the stress of flowing water—the stress-limited case—and find that cave passages created under this assumption match observed shapes. The reaction-limited model, in which the erosion rate is independent of water flow, creates cave passages that consistently widen through time, in contrast to field observations suggesting that passages tend to reach constant widths even as they continue to erode vertically.

The new study suggests that the rate at which rock dissolves may not be the key factor determining how caves change their shape through time. But there remain two very different scenarios between which the new model can’t distinguish. It could be that cave erosion depends on water flow because rapid replacement of water saturated with the products of dissolution allows new, unsaturated water to come in and continue the process. In this case, cave erosion would still be a chemical process, but would be limited by the transport of reaction products rather than the reaction rate itself. The alternative is that caves are stress-limited because erosion is a physical rock-damaging process driven by flowing water. This case is most comparable with the majority of rivers eroding rock at Earth’s surface, but it isn’t known to what extent it dominates below ground.

Whichever mechanism for cave passage erosion predominates, it’s certain that there’s a lot more going on during cave formation than the canonical slow drip-drip-drip of water. Understanding how water sculpts flow pathways underground will allow scientists to better protect our unseen water resources from depletion and contamination long into the future.

Rivers underground by Charlie Shobe is licensed under CC BY-SA 4.0

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