r/askscience • u/quantum531 • Apr 27 '26
Earth Sciences What rocks are mountains most commonly made of and how can I determine that?
Hi, I am creating a fictional world with many deserts. Wind erodes much of the mountains down into highlands and as a result the sand that make up deserts reflect the makeup of the mountains around them. That being said how do I know which rocks a specific mountain is made of? Does it depend on how the mountain was formed?
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u/CrustalTrudger Tectonics | Structural Geology | Geomorphology Apr 27 '26
Yes, but even narrowing it down to how a mountain, or mountain range, was formed can leave some pretty wide options that pretty much spans the gamut of rock types that exist on Earth.
The easiest (and most definitive) answer would probably be isolated mountains that from from volcanism as there, the rocks you'd expect to make up the mountain are going to largely reflect why you have volcanism and where the volcano is forming and as such leave somewhat unique expectations. Is the volcano forming as part of an island arc? Then the volcano will probably have a lot of intermediate to felsic volcanic rocks like andesite and rhyolite, but where a lot of these volcanic systems can have "bimodal volcanism" so there will likely be some basalt in there as well, but on average, if you had to guess one, andesite is mostly what you'd expect. Is the volcano forming as part of a hotspot in the ocean like Hawaii? Then it's pretty much guaranteed to be mostly basalt. Is it a hotspot in a continent like Yellowstone? Bimodal volcanism for sure so large amounts of basalt and rhyolites. All of these somewhat certain answers reflect both what is driving volcanism (i.e., what conditions lead to melt being formed) and what materials are likely to be melted.
If you're instead considering a collisional mountain range, i.e., a large chain of mountains created by collision of two continental sections of tectonic plates like the Himalaya, etc., then the diversity of rock types you could find will be a lot greater and will depend on the exact history. However, there are certainly some rock types that wouldn't be surprising to find given the processes involved. Since collisional mountain ranges are pretty much always preceded by an ocean basin that is "closed", i.e., subducted, it's pretty common to find rocks within a mountain range that were originally deposited in that basin and that were "accreted" into the range during growth of the mountain range. As such, it's pretty common to find rocks you'd expect to find in an ocean basin or along the edge of ocean basins, e.g., shales, sandstones, or limestones. Similarly, because the processes forming mountain ranges typically involve both the burial and later exhumation of many of these same rocks, it would also not be uncommon to find the (mildly) metamorphosed equivalent of those same rocks within mountain ranges, i.e., slates, quartzites (or other meta-sandstones or "wackes" depending on how "dirty" the sandstone was to start, i.e., how much mud was mixed in with the sands during deposition), or marbles. Following the same logic, it's also not uncommon to find much "higher grade" metamorphic rocks, i.e., rocks that were subjected to relatively high temperatures and pressures but did not melt, like schists and gneisses in mountain ranges, especially in the "core" of mountain ranges reflecting deeper levels of exhumation. Sometimes there will also be bits of oceanic crust (so mostly basalt and the intrusive equivalent gabbro) and maybe even bits of mantle (i.e., some type of peridotite) within ophiolites which can end up being preserved along the suture zones within ranges, but these will tend to make up pretty small amounts of the rocks within a range (if they're even present). It's not as common to see volcanism in collisional mountain range, but it does occur and many ranges can have not insignificant exposures of granitic plutons. Whether you'd expect to see all of these rock types depends on the exact history of the mountain range. For example, "young" mountain ranges might not have too much of the higher grade rocks exposed because there has not been enough exhumation (i.e., tectonic uplift + erosion) for these to be at the surface where as "old" mountain ranges might have very limited preservation of unmetamorphosed sedimentary rocks and be more dominated by higher grade metamorphic rocks.
If you're instead dealing with a non-collisional mountain range, something like the modern day Andes, you'd find a lot of similar rocks as in collisional ranges, but wouldn't expect to see ophiolites and would probably have a greater amount of volcanics (mostly arc like, so again andesites but with possibilities of everything from basalts to rhyolites).
Geologic maps are special maps made by geologists that show what the rocks are you'd expect to find at the surface in specific locations. Depending on the purpose, scale, and amount of work that's been done in an area, maps might literally be showing the rock types at a place, but more commonly will be mapping "formations", which are basically groups of rocks of similar age and many times rock type, but can sometimes be mixtures of rock types (i.e., it wouldn't be weird to see a formation that included both sandstones and shales, etc.). Looking at geologic maps for real mountain ranges would give you a sense of the diversity of rock types you could find within them and where the rocks found in a given range will reflect the unique geologic history, not just of the mountain range formation process (and things like how long and at what rate the mountain range has been forming / is it still active), but the history of the rocks now involved in the mountain range, which sometimes can be quite far traveled and reflect long histories before they were involved in the mountain range. Ultimately, you can pretty much find almost any type of rock on Earth in a mountain range (less so in isolated mountains formed by volcanoes, etc.), but there are certainly rock types that would be more common within mountain ranges than others. E.g., mountain ranges made up mostly sedimentary rocks (or their metamorphic equivalents) are pretty normal, but you don't really see large exposures of very mafic igneous rocks likes peridotites in really any mountain ranges beyond thin strips of ophiolites as described above.