“Far over the Misty Mountains Cold…”
The dramatic Misty Mountains are some of the most prominent geologic features in Middle-earth, and will be the first stop on our geologic tour. Mountains are varied and can be formed through a number of ways, but the Misty Mountains (As well as the Grey Mountains and Mountains of Angmar) are the dramatic type of rising mountain that only occur via one method: the collision of continental plates.
So for a little geologic background, when two tectonic plates collide, one tends to run (be subducted) under the other. However, the continents float atop these tectonic plates, and continents do not fit neatly underneath one another, So when continents collide, they instead crumple, deform, and push upwards building mountains. The most dramatic example on Earth today would be where the Indian subcontinent collided with Asia, pushing up the Himalayan Mountains.
Of course, we must recall that a collision is not how the Misty Mountains were ‘made’. Specifically, They were raised by Melkor as a barrier against the huntsman of the Valar, Oromë. But we are running on the theory that the processes of the world are equivalent, so Melkor may in fact be responsible for the plate tectonics apparent in Middle-earth (among the many other things he’s responsible for).
This interaction of plates that leads to massive deformation is known as an “Orogeny”. It’s not clear what orogenic phase the Misty Mountains are in. They might still be rising, though the existence of places like Moria and absence of frequent earthquakes indicate this might be an inactive plate boundary, meaning the Misty Mountains are likely being eroded down.
So, with this in mind I put forth the idea that the Misty Mountains and its branch ranges form part of a three-way tectonic T-junction. Illustrated here:
Where the southern end of this plate boundary goes depends on later looks we’ll take at the mountains of the South (and the absolute geologic madness that is Mordor).
There is, however, one issue in terms of environment: Rain-shadow.
The issue with mountains is that they tend to get in the way of things like clouds and wind. If you look at the large mountain ranges of the Earth, you’ll notice that they tend to have deserts at one side. The arid American West, the Tibetan Plateau, the Atacama Desert, etc. This is because all that rain tends to be caught on one side of the mountains.
This rain-shadow does not occur in the Misty Mountains. We know from Tolkiens notes that Middle-earth resides at about the same lattitudes as modern Europe with the Shire standing in for Britain’s location. This would mean the bulk of Middle-earth is in range of the Westerlies, the East-West running global currents. This means that Rhovanion, home of Mirkwood and Lothlórien, should be deserts rather than lands of lush greenery. This is a bit out of my field as I am not a climatologist. But it might be possible that the rain-deprived lands of Rhovanion could be at least in-part nourished by the River Anduin and its tributaries, one of the major waterways of Middle-earth and potentially the propagator of a large river valley that feeds Lothlórien, while Mirkwood could be out of range of the rain-shadow.
Of course, it could just be Elf Magic.