“Denser” more important than “harder” for mountains?

A new paper by Braun et al in Nature Geoscience has an interesting take on making elevated areas: they suggest that you could convert a flat region into a bumpy one by eroding a uniform amount of material off the top.  The high areas would be the areas that had denser rock–say a gabbro instead of a shale. It is a cute idea: can you make mountains (well, kind of bigger hills) by uniformly eroding a flat region?

At first blush this seems counterintuitive: why would the dense stuff end up high?  Well, the key is not what is there now but what was removed.  Topography at the earth’s surface is generally in what we term isostatic equilibrium.  Basically this means everything is floating on the mantle, so when you have high mountains, there should be something light underneath (kind of like the keel of an iceberg).  So when you erode off, say, a kilometer of the crust, the material below rises up as the thinned iceberg (as it were) now has less weight keeping it down. Very nice examples of this process operating today are the rising shorelines (falling local sea level) in places formerly buried by thick icesheets.

The trick in this paper is that if you remove more weight, you get more rebound.  So an area with dense rocks, if you remove the same thickness as surrounding areas, you will get more rebound, meaning the stuff under the dense rocks (which is often yet more dense rocks) will rise up higher, all else being equal. The trick is, the “all else being equal,” because that is rarely the case. For instance, the real examples in the Braun et al. paper might have issues, as the News and Views piece by Becky Flowers indicates.

Now when you come to the western U.S., it is quite noticeable that the high peaks are nearly always crystalline rocks and not the softer, less dense sedimentary rocks that often are seen on mountain flanks. Is this the reason why?

Probably not.  First, the conditions for this effect to be really noticeable are that there has to be a pretty weak lithosphere, which in general means pretty hot.  Such places are usually tectonically active and so this effect is easily lost in the noise.  So, for instance, the Rockies, with a more rigid lithosphere, aren’t apt to illustrate this effect, a point  also evident in the much lower total erosion over the crystalline core of the ranges vs. the sedimentary flanks (such as the basins in Wyoming and in the Denver Basin). And the ranges of the Rockies are bounded by large thrust faults that created the bulk of the difference between the high crystalline mountains and the low sedimentary basins.

You’d really like to compare dense crystalline rocks with lighter crystalline rocks so as to remove differences in erodability.  There is such a place: the Sierra Nevada.  In one sense, you might see exactly what is predicted (though not in the same way as the paper presents it).  The dense rocks in the Sierran foothills were at much greater depths when crystallized than the lighter rocks near the crest; this level of exposure was fairly stable for about 30 million years (from the middle Eocene to the late Miocene), suggesting that an equilibrium had been reached.  Perhaps it isn’t obvious why this is the same effect, but imagine that you started at some point in the Cretaceous with some topography and started to erode it.  Each time you erode off a slab, the rock below rises up some–more if the rock removed was denser–but it never rises up as high as the topography used to be.  So after slicing off a few slabs of the light rocks in the eastern Sierra, you are done–the surface only came back to the level in equilibrium with the base level for the streams.  But the rocks in the western Sierra kept rising up to be sliced off, so the rocks that were the last ones to rise up were from a much deeper level–precisely what we observe.

Unfortunately even the Sierran case isn’t that clear.  The rocks in the western Sierra are older and were being unroofed before the rocks in the east were even created in some instances. And of course today everything is exactly backwards from the suggestions in the Braun et al. paper–the high part of the Sierra is where the less dense rocks are.  But this is because the range rose from something other than isostatic rebound (the amount of erosion in the Sierra is really quite small).

There could be an unexpected home for this concept, one that the authors nearly hinted at but didn’t seem to explore.  One of the enduring issues in the Basin and Range remains the shape of core complexes, mountain ranges of dense crystalline rock that were in the lower plate of (currently) low-angle faults.  Although these are often portrayed as simple flexural responses to the removal of the upper plate, the differences between extensional systems and the different shapes of these cores makes these simple explanations suspect. And the differences in isostatic rebound are even larger when the material filling in from below (asthenosphere) has crustal densities instead of mantle densities–you can even make ranges higher through erosion than their surroundings if you end up with a density inversion.

Anyways, the Braun et al. paper is a clever diversion that is probably highlighting a rather special aspect of isostasy.  Whether a clear case of this effect dominating others remains to be seen….

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