It is common knowledge that mountain ranges were created by tectonic forces, but how their height is maintained today is a matter of debate. A widely held view is that climate-controlled erosion limits their height1,2. In a paper in Nature, Dielforder et al.3 take a different stance. They show that, at least for mountain ranges that are near convergent tectonic-plate boundaries, tectonic force has a dominant role in controlling height.
The mountain height discussed by the authors is that of a smoothed version of the actual mountain topography, in which high peaks and deep valleys are omitted. The natural processes that maintain this mountain height can be simplified into three types (Fig. 1). The first is lateral support of mountains from tectonic force, which either prevents mountains from falling apart under their own weight or pushes them farther up against gravity. The second is climate-controlled erosion, which limits mountain height by removing material from high elevations. And the third is a process known as isostasy, which keeps mountains afloat above the hot and soft mantle material in a similar way to icebergs floating in water. For the purposes of this discussion, we can ignore local-scale processes such as volcano growth due to magma activity.
Scientists all agree that the three main processes work together to maintain mountain heights in a dynamic fashion. Complications arise because the different processes might not keep pace with one another4. The scientific debate focuses on whether erosion can outpace tectonic force, or vice versa. The isostatic response to the other two processes is thought to be sufficiently prompt to keep pace, and therefore is usually not questioned in this debate.
The most representative of the models in which erosion controls mountain height is known as the glacial buzz saw. On the basis of observations of topography and glacier distribution, this model postulates that glacial erosion, in concert with isostatic uplift, keeps mountain heights at about the elevation of the climate-controlled snowline, regardless of the tectonic force at work2.
Dielforder and colleagues now propose a different model that puts tectonic force in the driving seat. The primary tectonic force near convergent plate boundaries is provided by the major geological fault at the boundaries (Fig. 1). The authors estimate this force for various plate boundaries, using estimates of the strength of the associated faults and various thermal and mechanical parameters appropriate for each region considered5. The quantification of all of these parameters requires several assumptions to be made, thereby introducing some uncertainty into the estimates.
However, Dielforder and co-workers’ most important assumption is used to forge a link between tectonic force and mountain height. The authors assume that stress in the crust directly beneath the mountains is in a neutral state — that is, horizontal compression due to the tectonic force and vertical compression due to the weight of the rock column are the same6. Because the weight of the rock column is proportional to its height, the tectonic force estimated for each plate boundary can thus be used to predict the mountain height that it can support. The authors find that the heights predicted by their model agree well with observed elevations. They therefore conclude that today’s mountain heights are maintained by the tectonic force, regardless of climate conditions and erosion rates.
If tectonic force is the dominant control, how do we explain the previously reported correlation between mountain heights and climatic conditions2,7? The answer might lie in the regions examined by Dielforder and co-workers: most are subduction zones (areas in which one tectonic plate is sliding beneath another; Fig. 1), which, with the exception of the Andes, do not host very high mountains. Perhaps the previous results can be reconciled with the current study if a broader range of tectonic environments is examined.
Alternatively, if erosion is the dominant control, the tectonic force is still needed to balance the weight of the rock column. Areas that exhibit large topographic relief, such as that from a submarine trench to a mountain top (Fig. 1), generally need both isostasy and tectonic force to balance the weight. If a mountain range is kept low by climate-controlled erosion, does this implicitly indicate that the tectonic force is small? A related question is whether a given mountain height can be associated with only one possible value of the tectonic force. The answers require an understanding not only of the force, but also of the strength of the rocks.
Dielforder et al. provide a crucial argument in the mountain-height debate, but their perspective comes with its own dilemma. Tectonic force raises mountains by crushing and piling up crustal rocks. To keep up with erosion, it has to keep the crust on the verge of compressive failure. In accordance with commonly accepted ideas about the strength of the brittle upper crust, compressive failure requires the horizontal stress to be much higher than the vertical stress8–10 — whereas Dielforder and colleagues assume that horizontal and vertical stress are of the same magnitude (a neutral state) beneath the mountains.
To solve this conundrum, the authors speculate that the crust in mountainous areas has almost no strength because it contains very weak faults, so that neutral stress is not far from failure. But if the crust is so weak, why don’t the mountains collapse, and why don’t these areas become plate boundaries? There is evidence that crustal stress is indeed almost neutral near some subduction zones10, but it is not clear whether it is commonly neutral beneath high mountains. The mountain-height debate thus leads to a crustal-strength puzzle. Dielforder and colleagues’ work suggests that much observational and theoretical research is needed to understand crustal stress and strength if we hope to resolve the issue of mountain height.
Nature 582, 189-190 (2020)