Okay, so J.R.R. Tolkien wasn’t a geologist.
Sure, The Hobbit and The Lord of the Rings were sufficiently in touch with the practical realities of travel through different landscapes to lend any individual location a high degree of realism (apart from the magical elements, the presence of orcs, dragons, ringwraiths, elves, and what-not). Looking at Middle-earth as a whole through the lens of natural science, though, can be a bit of a head-scratcher.
For example, you might find yourself wondering how Mirkwood could sustain such lush and plentiful vegetation when it’s surrounded on three sides by high mountains and on the fourth by a featureless plain. If it’s going to be there, why isn’t it right up next to the river Anduin, instead of nearly 50 miles to the east? And for that matter, why does Anduin go through the Emyn Muil (to say nothing of the cliffs of the Argonath) instead of, like any sensible river, around them?
The answer to all of these questions is, of course, the same: Stop nitpicking and enjoy the story.
After all, while Tolkien wasn’t a professor of natural science, he did very well with what he had. And as a credit to his sense of realism, every now and then something pops up in the natural world that sheds light on how things might work in Tolkien’s fictional one.
Take, for example, the Kilauea volcano in Hawaii. Besides the existing lava lake in the Halema’uma’u crater, there’s a second lava lake in the Pu’u O’o crater a few miles away. The second lava lake currently exists in a “perched” form, meaning that it has built up a levee system within the crater, with the currently-active lava contained (for the most part) in the levees. They have a web-cam: http://volcanoes.usgs.gov/hvo/cams/POcam/. As you can see (for now, at least), the result of all this is a relatively flat surface, probably solid basalt once it cools, with an instant moat surrounded by a good deal of desolate terrain.
In short, it would make a great place to put a Barad-Dur…except for the increasing likelihood of the whole structure collapsing before the lava has a chance to stabilize and cool down. That, of course, is a substantial disincentive to build just there…unless you had a magic ring that among other things helped control the volcano as a whole.
Granted, the Halema’uma’u crater isn’t likely to be mistaken for Mount Doom, or even Mount Slight Misfortune. Kilauea, after all, is a relatively flat shield volcano, whereas Mount Doom was a cinder cone. The current vent at Halema’uma’u is essentially a hole in the bottom of a hole.
But one can imagine a similar system at work in Mordor. And that would explain why Barad-dur takes a tumble at the end of the book (sorry!). The ring goes into the volcano and starts an eruption. The eruption reduces the pressure in the magma chamber. The pressure from the magma chamber is suddenly no longer sufficient to support the weight of that nice, flat foundation, and down goes the tower, disembodied eye and all.
Or, we could go with the book and just say that the tower fell over because it was built with the magical power of the now-destroyed ring, and not worry about causation. In other words: stop nitpicking and enjoy the story. After all, I’m not a geologist, either.
Well, it was an interesting thought exercise. Meanwhile, stay tuned for more natural fireworks from Hawaii….