In my speculative evolution project, I've created a biome where the snowy tops of mountains have been replaced with crystals.
The crystal should come from inside the planet, just like lava.
On an alien planet, what kind of geological phenomenon can create a crystal mountains range?
2$\begingroup$ crystals form very slowly in magma or other super hot fluids so the only way to have crystal mountains to have them made of crystals that formed over many millions of years in a magma pool way larger than any on earth, then somehow transported to the surface. there is just no real way to do this, refuge in magic is your only option. $\endgroup$– JohnAug 23, 2022 at 4:14
$\begingroup$ Ice is crystalline. The rocks which mountains are made of are crystalline. Apparently, the question assumes for the word crystal a special meaning. What does the word crystal mean in this question? (For example, the "crystal" on top of the mountains in the picture looks more like cut glass, which is indeed called "crystal" in fancy glassware shops, but is an amorphous solid, not crystalline. And cut glass is always artificial, cannot be made by natural processes and will lose its attractive edges if left to be eroded in the environment.) $\endgroup$– AlexPAug 23, 2022 at 6:22
$\begingroup$ Many minerals that form crystals also are easily worn away by water. That is how the crystal was formed: water wore away the minerals from the magma and transported the minerals to a pocket where the crystal dropped out of the water. That means that mountains of crystal will quickly erode. The feldspar in granite actually turns to clay by water erosion. $\endgroup$– David RAug 23, 2022 at 14:15
For the regulars here, yes, my answer to every question lately is a sulfate salt, no, it's not deliberate.
You can get enormous, obviously crystalline calcium sulfate monocrystals, e.g. in The Cave of the Crystals.
To get mountain sized crystals*, you could have a planet that had an ocean saturated with calcium sulfate (or a more soluble or colourful salt. But not a mixture of many types).
*Or at least, mountains made of big monocrystals.
Have the water disappear gradually (I don't propose how, I leave that to you), leaving a supersaturated ocean; like growing crystals in a jar, as the water evaporates, crystal growth occurs. Note: Yes, calcium sulfate is only sparingly soluble. You'd need a water cycle to keep dissolving it from a dry stock or insanely supersaturated waters from the deeps, or something of that sort. Potassium or magnesium sulfate are much more soluble.
A single huge mono-crystalline mountain or mountain range is highly unlikely due to energetic considerations: growing a single crystal requires a very controlled process which is unlikely to happen in a spontaneous way on a scale larger than 1 meter.
On the other hand we have examples of rocks which are actually a cluster of smaller crystals, the most egregious example being granite, which comes from slowly cooled down magmatic intrusions
Granite (/ˈɡrænət/) is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies underground. It is common in the continental crust of Earth, where it is found in igneous intrusions. These range in size from dikes only a few centimeters across to batholiths exposed over hundreds of square kilometers.
Granite is typical of a larger family of granitic rocks, or granitoids, that are composed mostly of coarse-grained quartz and feldspars in varying proportions. These rocks are classified by the relative percentages of quartz, alkali feldspar, and plagioclase (the QAPF classification), with true granite representing granitic rocks rich in quartz and alkali feldspar. Most granitic rocks also contain mica or amphibole minerals, though a few (known as leucogranites) contain almost no dark minerals. Thin section of granite
Granite is nearly always massive (lacking any internal structures), hard, and tough. These properties have made granite a widespread construction stone throughout human history.
$\begingroup$ Looking at the picture in the question, that would be a monocrystal with very un-crystal-like cleavage. $\endgroup$– AlexPAug 23, 2022 at 6:20
$\begingroup$ Several very rare crystals have been found that were multiple meters long. From en.wikipedia.org/wiki/Pegmatite : The largest beryl crystal ever found was from Malakialina on Madagascar, weighing about 380 tons, with a length of 18 m (59 ft) and a crosscut of 3.5 m (11 ft). $\endgroup$– David RAug 23, 2022 at 14:10