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Recently at the American Museum of Natural History in New York, I saw a display set that claimed erosion makes mountains rise higher, counterintuitively. The explanation was that the material removed from a mountain due to erosion makes it much lighter. Since the mountain sits on a landmass that is floating atop magma, a lighter landmass rises higher. (Similarly, boats rise higher in water when weight is removed from them.)

I have been looking for resources online to confirm this, but none mention it. They tend to claim that pressure due to shifting tectonic plates is the cause of mountain growth, and if they mention erosion at all, they only discuss it as a possible shrinking factor.

What do actual geologists have to say about this? Does it depend on the mountain range?

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  • $\begingroup$ The answers you got so far are pretty good. I would just make a small correction: the landmass does not float on magma. There is no magma ocean underneath. It is all solid rock. However, it is viscous and can "flow" under very long timescales. Think something like glaciers, or hot (yet solid) chocolate. It is solid and if you'd hold it in your hand, you would not be able to tell the difference from any other rocks (in terms of how solid it is). $\endgroup$ – Gimelist Nov 16 at 11:27
  • $\begingroup$ Fair point, I've heard things along those lines. So the key question is, do "fluid" laws such as Archimedes's principle (which is responsible for determining how high a boat rests at equilibrium) hold for earth's crust and magma on geologic time scales? $\endgroup$ – WillG Nov 16 at 18:09
  • $\begingroup$ yes. It is the concept of isostasy. $\endgroup$ – Gimelist Nov 17 at 9:47
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This actually does happen, a feedback loop between the Himalayas and the Indian monsoon.

The Indian Plate is actively shoving itself under the Eurasian Plate, creating the Himalayas.

At the same time, monsoon storms come up against the southern side of the Himalayas. The high mountains enhance the storms' rainfall, and the mountains erode much more on the southern side than the northern side.

Material is transported from the Eurasian Plate down onto the Indian Plate, and the weight removed from the mountains causes isostatic rebound of the Eurasian Plate, allowing the Indian plate to slide underneath, enhancing the uplift of the Himalayas., keeping the monsoon rainfall on their south side, etc.

There's a more detailed explanation and some nice images here.

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  • $\begingroup$ So then, is it fair to say that the two effects work in combination for the Himalayas? I.e., isostatic rebound on its own would act to raise the Eurasian plate, but it also makes it easier for the Indian plate to slide underneath, and both these effects serve to raise the Himalayas? $\endgroup$ – WillG Nov 16 at 18:15
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It depends where the mountain is. Usually it is plate tectonics or volcanism that makes mountains higher, but in some circumstances it could be erosion. During the Ice Age,there were a couple of kilometres of ice pressing down on northern Britain. When the ice melted, it did so quite rapidly in geological terms. The ice not only disappeared, but torrents of meltwater scoured the valleys and added to the erosion. Relieved of so much weight, northern Britain, including Scottish mountains, rose up, while a see-saw effect caused southern Britain to sink. I don't think the Scottish mountains rose by any great amount, but I couldn't quote you a figure offhand. Erosion will gradually revert to its traditional role and the Scottish mountains will be ground down again and washed into the sea. The rising was a brief and temporary phenomenon on a geological timescale.

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