The Lonely Mountain is apparently a free-standing mountain that isn’t part of a mountain range. It does even seem to have foothills.
Geologically speaking, how plausible is it’s existence?
Very plausible if it’s an extinct volcano. That might also explain why it has chambers inside it.
Ah.
Would an extinct volcano be likely to bear the lodes of gems and precious metals that TLM yielded?
Gold and silver deposits and crystal forms of minerals are usually produced due to volcanic processes.
Mt. Kilimanjaro is a dormant volcano and the highest free-standing mountain on Earth (not part of a mountain range).
It’s definitely true that volcanic areas are great places to hunt for precious stones and metals but the Lonely Mountain was a depository for treasure rather than a source of it. However if you’re into gold and diamonds, then a region like that is the place you might want to set up.
Yeah, those square mountains around Mordor really looked odd to me. They’re much much weirder than the Lonely Mountain.
I always thought Mordor looked weird, too, but it was more the fractal nature of the map that got me. It was like Tolkien said “okay, the big baddie lives here,” (places finger on Udun), "in a square area surrounded by mountains. Nope, not big enough. Lemme draw another box around it. It’s a lava plain. But where do they get food? Lemme draw another box around that! On more than 1 zoom level it’s a box of mountains with another smaller box in the northwest corner.
And if the author had read the Silmarillion he’d know that the Misty Mountains, lover of right angles, was also created by Melkor so should also get somewhat of a pass.
There are some pretty substantial mountains in the world that aren’t in mountain ranges, but I am pretty sure they’re all volcanic.
I don’t see why Lonely Mountain even had to be an EXTINCT volcano. Maybe it just don’t erupt during the events of the book.
Not fully exitnct.
The ‘dragon’ sleeping inside is a mythologic explanation of a smoking mountain that, within memory, destroyed the towns surrounding it.
cf the Balrog, the anthropomorphic personification of the malice of nature on high mountains. Although it lives in the root of the mountain, the surface of the mountain exposes it’s existence.
Remember Middle Earth was actually crafted. It was not the result of science.
Yeah, I don’t get the article that goes on about how ‘mountains don’t work like that’. These are mountains that were explicitly built into place by godlike beings, they aren’t natural formations that are a result of geological processes. And it’s not ‘we have some myths passed down in oral tradition and written down thousands of years later’ like a lot of Earth mythology, within the context of the books the events of the creation were written down by people who were there or talked to people who were there when it happened.
I’ve never understood the desire some geeks have to force science into explicitly fantasy worlds. It’s one thing to assume that real-world physics applies if the world doesn’t say anything about it, but when a work explicitly states that the mountain ranges were constructed by godlike beings it’s just absurd to deny it.
It’s not that unusual. Mount Monadnock in southern New Hampshire stands pretty much alone, and it isn’t an extinct volcano – it’s just harder rock than the other summits nearby were, so they eroded away. In fact “monadnock” has become a geological term for such isolated mountains. (Elsewhere in the world the term is inselburg. Obviously, there’s a bunch of them around.)
The Arkenstone was found in the Lonely Mountain. Probably some, but not all, of Smaug’s hoard was too.
Volcanos are not good places to look for precious stones, or metals. Not even diamonds, even though they are volcanic in origin - the kind of eruption that brings diamonds to the surface is not quite the nice conical volcanic mountain-making kind. Much more likely to leave a lake than a mountain. But the Arkenstone is not a diamond so who knows…
Other precious stones and most semiprecious ones are not volcanic, but many can be found around volcanic areas, in their metamorphic aureoles, such as in skarns and pegmatites.
Some metals occur in volcanoes themselves but rarely in any economically-recoverable amounts, but again, the area around the extrusion is a much better bet for mineralization than the volcano itself. And most terrestrial igneous-associated metal deposits are associated with these aureoles (see e.g porphyry copper and gold) or with intrusives - giant bodies of magma that never make it to the surface. I emphasized terrestrial there because there is one kind of extrusive “volcano” that is heavily metal-bearing - the marine hydrothermal vent deposits known as “black smokers”. But these don’t become lonely mountains when they eventually make it to being part of the continents.
Safer just to stick with “a wizard did it”
For the record, although I’m the one who posted the article, and I think it’s interesting for what it says about our world’s geography, I pretty much agree with your point of view here.
Nope. Eru did it, with help from the Valar. 
He’s correct about the Mountains of Mordor, but there are a few places where mountain ranges meet at close to right angles similar to those between the Misty Mountains and the White Mountains and Grey Mountains, such as the Alps and Apennines and Dinaric Alps or the Himalayas-Hindu Kush-Tian Shan.. So those are odd but not completely implausible.
IIRC, the usual scenario is a caldera forms after the magma chamber empties making these nice, vertical cracks that go deep into the crust. This allows surface water to seep down, get heated and mineralized, then rise back toward the surface leaving the minerals behind in concentrations rich enough to exploit.
At least, that’s how the copper, silver, and gold deposits in southern Arizona formed.