How do mountains grow?

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Climate controlled erosion is often considered the driving force in managing mountain height; however, a recent study concluded that tectonics may have a greater impact than erosion. Armin Dielforder, Ralf Hetzel, and Onno Oncken, the authors of the study, explained that the maximum potential height of a mountain is limited by what can be upheld by the tectonic and isostatic forces underneath. 

In fact, tectonics are essential in the formation of mountains. Interactions between tectonic plates at convergent boundaries, in which one plate slips under another, can lead to mountain ranges arising. Once established, the height of a mountain range is dynamic and is maintained by three forces: tectonic force, isostasy, and erosion. The tectonic force drives the creation, and then supports the mountain from underneath against gravity. On the other hand, the erosion strips away material from the top. Isostasy, also from underneath, is the ‘floating’ of the mountain range on the Earth’s mantle, shifting up and down as material is removed or added on top. Isostasy is a balancing force and acts in response to the two others. It is commonly accepted that all three influence the height, but not necessarily to the same extent or rate. In general, there are two hypotheses: one which favours the impact of climate driven erosion, and the other which credits the tectonic forces underneath. 

The basis of the climate driven erosion hypothesis is that the reformation of the topology at top of the mountain by the local climate tends to be balanced by isostasy. A dynamic equilibrium is maintained by those two forces, with tectonic forces from the convergent boundary not playing a major role. A 2009 study by Aarhus University, Denmark, analysed the observed heights of mountain ranges in various different climates, and concluded that the local climate restricts the maximum height, thus outweighing the forces underneath. A major mechanism derived from this is ‘glacial buzzsaw’, the idea that glaciers restrain the full potential height of mountain ranges by eroding the top of them at the snowline, which is maintained by the local climate. Thus, in cooler climates mountains are affected more than those in warmer climates, where there are less glaciers.

If erosion has a significant impact, the removal of material from the top of mountains would make their height less than what the tectonics underneath are capable of supporting. Dielforder, Hetzel and Onckens’ study investigated the average heights of mountain ranges across various climates, mostly in convergent subduction zones. As the tectonic force at the site of the range increased, the quantified average height of the range increased when looking at the data from mountains at convergent boundaries. This strong relationship between the heights and tectonic forces favours the hypothesis that tectonic forces outweigh the erosion caused by local climate, although it is important to note there is an element of uncertainty in the data due to assumptions made by the researchers, for example that vertical and horizontal forces underneath were equal, as discussed by Kelin Wang. Also, as the maximum mean elevation and the estimated tectonically supported elevation (the maximum height that the tectonic forces underneath could support) are similar, there is no suggestion that another factor, such as erosion, plays a major role in regulating mountain height. Erosion can affect the height of mountains, however these findings suggest that it is to a lesser extent than the forces underneath.

As Kelin Wang discusses, further research and broader studies that span even more climate and tectonic zones are needed to illuminate the discrepancies between previous studies observing the impact of climate driven erosion, and the new studies outlining the importance of tectonic forces. Discovering which force has the greatest impact is important not only to understand the maintenance of the mountain ranges, but also perhaps to extrapolate their future, especially with rising concern that shrinking glaciers exposing more and more underlying rock may lead to the shortening of mountains such as the Alps. 

Written by Arianna Schneier and edited by Ailie McWhinnie.

Arianna’s thoughts… The balance between the forces underneath and the forces above in maintaining the height of mountain ranges is still disputed. Though climate driven erosion is given more attention, it is interesting to see how modeling and calculating the tectonic forces below can give information about mountain heights. A comparison of the two different lenses and seeing how these two approaches help explain the height of the same mountain ranges would be fascinating to see. Also, would placing a greater impact on the influence of tectonic force remove some of the concern over climate related erosion, especially with the changing climate? 

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