Eyeballing a map of the world, it seems that most mountain ranges that don't occur along continental fault lines run parallel to coastlines. Is there a reason for this?
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3 Answers
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Mountain ranges are usually formed as orogeny where tectonic plates collides, known as convergent boundaries.
The continental plates have less density than the oceanic plates and the buoyancy results in that they are mostly above sea level. Continental sea floor is known as continental shelf, but usually, it doesn't reach far from the coastline. Therefor the continental-oceanic boundaries often runs sub-parallel to the coastlines we see today.
At convergent boundaries where oceanic crust and continental crust collides, the mountain ranges can therefor form right at the coastline. Vulcanism is also important in the process as volcanoes forms where oceanic crust is subducted under the continental plates.
Some of the mountain ranges that today are located far from the coast, e.g. the Urals are formed from earlier convergent boundaries between continental plates that today are incorporated in larger plates.
Convergent boundaries explains most of the mountain ranges (e.g the Andes, Himalaya, the Alps, Nihon Arupusu) but there are however some curious exceptions, e.g. the Scandinavian Mountains that are not located near any convergent boundary and might be the result of regional uplift. However, there is an interesting ongoing discussion on how these processes work.
This is a simplified answer that I hope inspire to further studies in tectonic and geology.
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1$\begingroup$ I see - so a good way to understand it is that areas where there are mountains were once the locations of active continental margins. Thus the resulting mountains are parallel to that coastline. A good example of this is the Appalachian Mountains: They were formed when what is now the East coast of N. America was the leading edge and collided with what is now Africa (the Alleghanian orogeny). Due to changes in the continent's drift, this is now a passive continental margin, but the mountains remain. Does that sound correct? $\endgroup$– nmvasqAug 8, 2015 at 13:31
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1$\begingroup$ Yes, the Alleghanian orogeny was one of the orogenies that formed the units that today forms Appalachian Mountains at the convergent boundary between Euramerica and Gondwana. However, we should remember that the orogeny itself took place 3-400 Ma, and the shape of the range we see today has little resemblance of the range formed at the time, erosion is a force just as efficient as tectonic when it comes to shaping the earth. $\endgroup$– user2821Aug 9, 2015 at 17:18
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When two plates collide, if the one going down, if it is already under the sea, produces a depression which naturally will be filled with sea water whereas the other plate is raised by the collision originating mountains One typical example is the Andes.
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1$\begingroup$ Please do not repeat what is already in other answers. $\endgroup$ Aug 11, 2015 at 7:44
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Mountain ranges are usually formed as orogeny where tectonic plates collides, known as convergent boundaries.
While this is true in certain situations, it doesn't begin to explain how the vast majority of the earth's mountains are formed. In the global map below:
we see that the edges of many tectonic plates are edged by spreading ridges and subsequent mountain building. Obviously, most mountain building is actually the result divergent tectonics, and not convergent. Curiously, there is no interesting conversation about "how these processes work".
Nevertheless, it is interesting that Mid-Ocean Ridges as well as most continental margin mountains are aligned north-south. However, there is little reason to believe that mantle convection and thus plate motion MUST operate in this fashion and that it has always done so.
There is plenty of paleomagnetic evidence for landmasses moving in a wide variety of directions in the geologic past, that resulted in the creation of supercontinents. Today's N/S orientation may simply be an artifact of how the last supercontinent was broken up, and not evidence of the orientation of a permanent deep, driving force.
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3$\begingroup$ My answer was an answer to OP's question, not a general discussion about tectonism or mantle dynamics, it simply answers why most terrestrial mountain chains are subparallel to coastlines. However, exceptions are also mentioned and I acknowledged that the answer was simplified. Please read the question and the whole answer before downvoting. Additional answers are always welcome and encouraged, but as usual, they should aim to answer OP's question or add relevant knowledge. $\endgroup$– user2821May 11, 2017 at 3:46
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1$\begingroup$ Gosh. I can only encourage you to read the question and try to formulate the most suitable answer you can. This forum has been joyfully free from trolling and aggression and I think it's better to keep it like that and encourage peoples interest in Earth Science. 'Eyeballing a map of the world' and 'continental fault lines' suggests that OP was interested in terrestrial mountain chains, however, I don't know OP but that was my interpretation. I'd have welcomed an additional answer about how MOR (naturally) runs subparallel to coastlines and accordingly upvoted it. $\endgroup$– user2821May 11, 2017 at 4:53
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2$\begingroup$ "most continental margin mountains are aligned north-south" What? What about the Himalayas? The Alpine Orogeny? Also, the Rockies/Pacific Coast, Andes, Himalayas, and Alpine Orogeny are all caused by plate collisions. That takes care of most of the 4000m mountains, leaving the East African rift and some isolated volcanoes that aren't caused by collisions. The statement you quoted from @Tbb seems pretty accurate. Lastly, since when did mid-ocean rifts cause mountains? I don't see any mountains in the mid-Atlantic. Overall -1 $\endgroup$ May 11, 2017 at 19:21
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2$\begingroup$ @kingledion I'm with KnobScratcher on this one actually. The mid ocean ridge is called like that because, well, it is a ridge. The longest mountain chains on earth are submerged. This is irrelevant for the OP because obviously the question is about subaerial ridges. $\endgroup$– GimelistMay 12, 2017 at 10:08
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1$\begingroup$ @Michael Fine, poorly worded on my part. But the OP is obviously talking about subaerial mountains, so the only (large group of) mountains of that sort caused by spreading is the East African Rift. $\endgroup$ May 12, 2017 at 12:13