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Start:

  • Normal planet, very similar to Earth. The main difference is that the continents are placed differently.
  • Magic could play a role in the formation of mountains but I prefer a scientific explanation.

Process:

  • The activity inside the planet makes surface plates move. At a specific place, two plates are colliding and this creates a large mountain range.
  • It is a subduction zone of several thousand kilometers long.

Result:

This question already addressed the maximum height of a mountain. But what about the size of a mountain range? Given sufficient time and tectonic activity the rock will keep compressing and will form several parallel mountain ranges. How large could it get with peeks over 4000m?

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  • $\begingroup$ Maximum size of a mountain range would ultimately depend on the size of the plates involved. Not sure if I can answer beyond that. $\endgroup$
    – Twelfth
    Oct 31, 2014 at 22:33
  • $\begingroup$ Earth has at least two veeeery looooong mountain ranges; one is the Alpide mountain belt extending west to east from the Atlantic to the Pacific across Eurasia, and the other is the American Cordillera extending north to south across the Americas from the Artic to the Antarctic. Towards the eastern end of the Alpide belt is the Tibetan plateau, over 1000 km north to south. Each of those belts extends over just a little less than half the circumference of the Earth. What was the question? $\endgroup$
    – AlexP
    Oct 25, 2019 at 8:54

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The maximum vertical height is already addressed so the question is then the maximum width and/or length of the mountains.

A subduction zone could be the maximum of half the planet for a solid plate or the whole planet's circumference for soft plates forming bernard cells (which is essentially a torus-shaped planet with a small inner diameter in the limit of size). These could be extended temporarily by instabilities (wiggly edges). Those limits are for a linear mountain range. It's perfectly plausible to have a circular one or a network though if that counts. In such a case you could have a checkerboard pattern of upthrust and subducted plates. There's no real limit if you don't limit the planet's size. A lower amount of "instabilities" increases likelihood of larger ranges while reducing the chance of the super long "wiggly-extrema".

An example of instability would be rock having a non-uniform shear strength or non-uniform plate sizes. As we drag our maximum shear strength lower and widen the range of values within a plate we get more cases where "edge wear" can occur on the mountain range. The "edge-wear" in a plate collision would be the tips of the mountain range being scraped off the plate or a wiggly mountain proper being flattened against a subduction zone.

So length is kinda variable. A solid upper limit would be the setup with the most bernard cells you could get for your given material strength. But a lot of specifics can drag the limit all the way down to just half the circumference (our basic hard plate case).

Maximum width of a mountain is already set by the material strengths and was covered in the answer to the tallest vertical height. When you achieve the maximum height you'll have the largest width. Anything wider next to the mountain will be past an inflection point in the slope that measures our true width.

Example: You have a pile of sand and make it as tall as you can by adding to the peak. It will have a specific angle of slope. It will be a convex mountain. If you add material at the base around it then it will not be structurally part of the sand pile. The part where your added sand at the base makes the mountain convex and is under its own separate structural support.

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