There’s only one place you tend to get mountains: where two tectonic plates meet.

Essentially, two plates smash together and push rock up to form a line of mountains along the boundary. And sure, there’s more to it, but that’s basically the story.

Which makes it really irritating that there’s a mountain range in the wrong place. North America’s Rocky Mountains are located smack in the middle of the continent, hundreds of kilometers away from the nearest plate boundary at the edge of the Pacific Ocean. And it turns out the question of why the Rocky Mountains formed where they did is actually a pretty hot debate between geologists.

So let's take a look at what we know, and what’s still a mystery, about why one of the world’s most iconic mountain ranges is where it is. [intro ♪] The Rocky Mountains we see today started ascending around 125 million years ago during the Sevier Orogeny. An orogeny is just what geologists call a mountain building event. The Sevier Orogeny stretched all the way from present-day Alaska to Mexico.

It folded and fractured layers of sedimentary rocks, bunching them up on themselves to start building mountains. But it wasn’t the only game in town. The subject of the most debate is the Laramide Orogeny, which was largely responsible for the range in the United States.

Instead of folding layers of sediment, these mountains were built by breaking and piling up hard rock like granite. This mainly occurred a little later, between 80 and 55 million years ago. Back then the seafloor of the eastern Pacific was made up of two oceanic plates, and both were subducting beneath North America.

Sliding under the continental plate and into the mantle. On the other side of the continent, the Atlantic Ocean was in the process of opening, so that oceanic plate was pushing North America towards the west. Despite the Rockies being up to 1500 kilometers from the Pacific plate boundary, geologists do agree that this subduction zone is responsible for building the mountains.

What they argue over is how it could have happened so darn far from the coast. But they’ve come up with four hypotheses. The first is retroarc thrusting.

The idea that the North American plate was pushing so hard to the west that instead of the oceanic plate slipping smoothly beneath the continent, it got stuck.* And all that stress caused the continent to crumple like a car wreck. But this begs the question of what did the pushing. One idea is that it could have been the seafloor spreading from the Atlantic Ocean on the other side of the continent.

But when we look at rates of spreading recorded in the Atlantic seafloor, we don’t see much evidence for it being in such a hurry. And regardless of where the push came from, for this idea to work, the section of North America west of the Rockies would need to be strong enough to withstand the collision and pass the stress inland. Which, it’s not, at least it doesn’t seem like it.

There are faults out west – meaning cracks where the crust is weak. A bunch of these are older than the Rockies, so why wouldn’t the crumpling have happened there? So the second idea is orogenic float.

Maybe those faults we mentioned detached a section of the lower crust from the rest of the continent. Then it could float, essentially unattached. Then the force of the Pacific seafloor subducting could skip this section and get passed inland to the east.

But then, But in that case, where is this detached section of crust? It would have eventually smashed against the Rockies, and we don’t see any evidence of that happening at the right time. So the third is a transpressional collision.

Transpression is just a combination of the words transform and compression. It means a fault that is both moving side to side and together at the same time. This idea is that a long north-south transpressional fault existed where the Rockies are today.

North America was essentially split in two, the main continent to the east and a thin “ribbon” continent to the west. The magnetic signature in some of the rocks near the Canadian Rockies indicate they formed thousands of kilometers to the south. So that’s strong evidence for this idea.

The transform part of the fault would have moved the ribbon continent northwards, including those rocks. And the compression part would have pushed it up and over the main North American plate to build the mountains. The only catch is if a ribbon continent collided with the rest of North America, geologists expect at one point there would have been an ocean between the two.

And we just don’t see the kind of volcanic evidence that would be left over from an oceanic plate subducting before this collision. See, oceanic plates have a ton of water in them, which gets released under heat and pressure. Water lowers the melting point of the surrounding rocks and creates magma.

This means a subducting plate tends to create volcanoes. In this case they could actually end up east of the Rockies, in what we call… The Great Plains. Which we call that because of a notable lack of volcanoes and other volcano-shaped objects.

So. This is also relevant for the final idea, flat-slab subduction. While there is still a lot of debate, this is the most popular of the four.

Normally, subducting plates plunge into the mantle at a steep angle, which means they quickly get deep enough to release their water and form magma. But if an oceanic plate was younger and hotter, it could be more buoyant and not sink as easily. So the idea here is that the subducting plate slid along underneath North America for over 700 kilometers before finally getting deep enough to produce magma.

That melted rock would go on to make up the core of the Rockies, just much further inland than expected. What’s more, scraping one plate under another for that long would have transferred stress to the same area – again with the crumpling. But even though this is the most popular hypothesis, it still has a problem of its own.

There are remnants of ancient volcanoes to the west of the Rockies in both Canada and Mexico, so… it seems like that’s where the subducting plate was producing magma. Not where this hypothesis would expect. So the truth is, we don’t know the definitive answer here yet and none of these models have been able to fully explain every line of evidence.

Some studies conclude that we need a combination of these different mechanisms to explain what happened. For example, it could be that one hypothesis was responsible for the first mountains built and another took over later. Or it could be that different mechanisms were at play in different sections of the mountain range we see today.

For example, Some researchers have suggested that a “corridor” of flat slab subduction could have been responsible for the mountains in the central United States. It just doesn’t explain the mountains to the north and south. What is clear is the Earth doesn’t always do what we expect it to, and reconstructing the events that created what we see today can be pretty challenging.

But at least, it does make for some great good skiing though! If you enjoyed this episode, you are probably the kind of person who likes rocks. Well, rock enjoyers, And so to all the rock enjoyers out there, I have a thought for you to ponder.

What Imagine if an incredibly cool rock showed up at your door? And what if that happened again the next month, and the one after that? Well I don’t want to say too much so I’ll leave you with that, lovers of shiny things… and say to I’ll tell you, you should look out for our live premiere on October 2nd.

Until then, thanks for watching. [ ♪ OUTRO ]