# How fast does isostatic depression occur?

Imagine a mile-thick sheet of ice instantaneously appears over a mountain chain. How soon will the chain sink? Around 1 cm a year? How far will it sink?

I almost put this into Cafe Society, because this question came about because supposedly the Misty Mountains in Tolkien’s Legendarium are visibly shorter than they used to be. I wonder, if they had been magically raised in a very short time from close to sea level to around 20,000 feet, if they would visibly sink over a span of millennia. Of course, if it would not be that deep or quick on Earth, one can fanwank that the asthenosphere is weaker in Middle Earth

Now this is continental and not linked to a mountain range but (displacement of asthen.) given ice thickness of 1.5 km would be 454m.

As for the Misty Mountains, they existed as a mountain range before being raised even higher up by Melkor. Given they existed as a continental mountain range we could assume they’re located on a continental plate convergence and are continually being raised up as the plates move past each other. Now continental drift really only became accepted in the 1940s, well after Tolkien’s original conception of the range existed, so I don’t think we should take my had waving too seriously.

Finally Tolkien’s Middle Earth is ours just placed far back in time with the actions shown taking placed in roughly northwestern Europe.

Grey’s right about how far it’ll settle. The rate would likely be in the same ballpark as the isostatic rebound rate (since both are dependent on the same factors like mantle viscosity etc) which, yes, is around 1cm/yr. Possibly a bit faster us it’s sinking under load, but not likely by much. The mantle isn’t that fluid.

<nitpick> … asthenoplain … Middle Earth is flat. </nitpick>

Not after the destruction of Numenor.