Raising the Rockies
It is a tad embarrassing to say that we don’t fully understand the geological history of the Rocky Mountains yet, including the history of Rocky Mountain National Park. The long history of mountain building, erosion, glaciation, etc., is well-known—we can tell the story. But most mountain ranges hug coasts or are formed as coasts disappear in an obduction collision when the ocean closes and obduction occurs, whereas the Front Range of the Rockies is as far as almost 1,000 miles (1600 km) from the coast, yet the Rockies are not the direct result of obduction.
The U.S. West is a complicated region (see the Optional Enrichment section for a little more on this). The continent has been approaching and overriding the East Pacific Rise spreading ridge, which is much like the mid-Atlantic Ridge but is no longer in the middle of an ocean. The San Andreas Fault formed as the East Pacific Rise reached the trench. Before these met, subduction had been occurring beneath the western USA from push-together motion, but with a little slide-past motion thrown in. After the meeting, the subduction stopped, so the push-together stopped, but the slide-past remained to make the San Andreas Fault. To the north of the San Andreas Fault, subduction is still active, forming the Cascades including Mt. Rainier and Mt. St. Helens.
Long ago, the west-coast subduction zone started in the usual way, with the old, cold ocean floor going down into the deep mantle. But as the continent approached the spreading center, the down-going ocean floor became progressively younger, and thus warmer and more buoyant because not as much time had passed for it to cool after its volcanic origin. This warmer ocean floor didn’t “want” to go down, but it was still attached to the older, colder floor ahead of it that was going down. So, the ocean floor that went under the continent stayed high, rather than sinking, and rubbed along the bottom of the crustal rocks rather than plunging steeply into the mantle. The friction between this buoyant subducted ocean floor and the crust above, in turn, caused thrust-faulting and crustal thickening far inland (see the figure below). Because the western part of the country has been built up of many old rock bodies and sediment piles bulldozed from the Pacific, there are scars of many old faults and other geological features that have been reactivated by recent events, so mountains and valleys have formed along the old weaknesses in response to the new pushes.
As to exactly how this came to produce 14,000-foot (4300-m) peaks in the Rockies, geologists can tell the story, but it isn’t clear that any geologist could have predicted this story without seeing the rocks first. Science moves from explaining (easier) to predicting (harder), so we still have some work to do. (And we've oversimplified a bit here; see the Optional Enrichment for more.)
The video below provides additional information in support of the diagram above.
Video: The Rockies (1:57)
We've been looking at how spreading ridges under the ocean make seafloor, which moves away from them and eventually gets old and cold and sinks down. And it will be sinking down beneath a continent, often, and it will scrape things off to make the Olympic. And then it will have a big volcanic range such as the Cascades, Mount Saint Helen's, and so on.
And they will be sitting there next to the ocean, which we can draw in. And coming up from below, there will be melt to make seafloor out here. And coming up from below there will be materials that erupt at the volcanoes like that.
Now, that works all fine, but what happens when there is a little bit of a swinging down of the slab? We know that the slab is moving away from the seafloor spreading ridge, as I show here. But the slab really does have a little of this swinging down, as well, in some cases. And the continent moves to catch up with it.
And so pretty soon, the continent is going to end up getting close to the spreading ridge. And the stuff going down is then not going to be cold. It's going to be getting warmer.
And when that happens, we have to erase this, because now the materials don't want to go down anymore. You'll get something that goes more like this, with it running right underneath the edge of the continent and staying high. And where it runs under the continent and stays high, you'll get a lot of rumpling, a lot of pushing happening in this way. And so then you might expect to see something that gets pushed up like this. And we're reasonably confident that that sort of push-up is what the Rockies are.
In a few cases, as in the west, where the San Andreas Fault is, in fact, the subduction zone has been pushed all the way out and has run over the spreading ridge. And, the subduction zone and the spreading ridge annihilated each other. And then you've got the San Andreas Fault.
The Rockies, like the Smokies, were formed by push-together stresses, and the high peaks float on a thick root. Erosion of the peaks has allowed the root to bob upward, so the rocks revealed at the surface include types that formed far down in the Earth and then were brought to the surface. This includes rocks such as granite that solidified from melted rock far below, and the changed—metamorphic—rocks we will discuss below. The bobbing up of the mountains tends to drag surrounding rocks upward. If you drive toward the Rocky Mountains in Colorado from the plains to the east, you can see these dragged-up rocks adjacent to the high peaks. See the narrated diagram for more background.
Video: Red Rocks (2:36)
We're going to learn some things about metamorphic rocks now. But we're going to start with my finger shoved up through a piece of paper. And when I shoved the finger up, it dragged the paper up along that curve you see indicated by the red arrow. We've been talking about mountain ranges formed by squeezing together, that gives you high peaks, floating on a thick root. And how once you get those peaks up there, erosion tends to take them off. But then the root comes bobbing back up to make the peaks almost as high as they were. You might think that if you had some layers of rock nearby that they would be dragged up along the rising mountains, and in fact they are. And you can see this in many places. One of the really good places to see it is along the front of the Rockies. So this is just outside of Denver, at a place called Red Rocks. The mountains are just off to the left of this picture, and they bobbed up like that. And you can see these dragged-up rocks that are so spectacular along there. You find this all along the front of the Rockies. This is down at Garden of the Gods near Colorado Springs. The dragged-up rocks are in the foreground and in the back is Pike's Peak. Pike's Peak here is granite that solidified way down in the earth, and it is surrounded in many places by metamorphic rocks. Now metamorphic rocks most commonly start out as mud, as a mud rock that's called shale. And if this is buried way deep in the earth, it is heated, it is squeezed, it gets chemically active fluids, it changes. By the way, that is the toe of my running shoe for scale, so you know how big it is. And when it changes, you may end up with either melted rock altogether, you get granites, or you can end up with metamorphic rocks. And here are some metamorphic rocks at the bottom of the Grand Canyon. You find things like this in the Smokies, the Rockies, and elsewhere. So it really is true - the mountains get high by being squeezed horizontally. They have deep roots. Erosion takes off the top. The bottom comes up. Sometimes you see where the next-door rocks were dragged up, and eventually, in the heart of the mountain range, you see something that once was very, very deep.
Cooking the Earth
Think about cooking. If you mix up a bunch of ingredients to make a cake batter, throw the mixture into a pan, and put it into a warm oven, the cake you obtain will not be very similar to the mixture you started with. Grill a steak or a meat substitute, and the original cow part or vegetable-based material will come out quite different. Marinate the steak or meat substitute before grilling, and more differences appear. It is common knowledge that a material that is stable in one environment will change if it is placed in a different environment. This is true of everything (and everyone!) on Earth.
The Earth has a great range of conditions. The inside of a mountain range is hotter, has higher pressure, and is less affected by acidic groundwaters than the surface. Materials that are stable at the Earth’s surface (such as the clays in a piece of shale) are not stable deep in a mountain range. The minerals change, grow, and produce new types even without melting. This process is called metamorphism. Metamorphism makes rocks that many people consider to be especially pretty (see the video on “Toothpaste Rocks” from the Grand Canyon), produces some wonderful gems, and contributes rock names that make good puns. (The Geoclub at Wisconsin liked puns and used a metamorphic rock, a volcanic rock, and a sedimentary rock in claiming that geologists are “gneiss, tuff, and a little wacke.”) You can read a little more about rocks and minerals in the Enrichment section.
Toothpaste Rocks: Grand Canyon National Park
Metamorphic rocks—those cooked and squeezed deep inside a mountain range—are often especially pretty. At the bottom of the Grand Canyon, you can see such rocks. They were formed long ago, and many miles down, and then reached the surface as erosion removed the mountains above and the deep roots of those mountains floated upward. Later, these rocks were buried again under sediments from oceans, rivers, and wind, and finally revealed to us as the Grand Canyon was carved by the Colorado River. Some people—including Dr. Alley—think that these rocks are so beautiful that they're worth the overnight hike into the canyon all by themselves!
Video: Toothpaste Rocks - Grand Canyon National Park (2:22)
[MUSIC PLAYING]
This is the most glorious place. Just look at this place. We're sitting on these old rocks, these seriously old, 1.7 billion-year-old rocks.
Everything around us that's black didn't quite melt. Everything around us that's pink actually did. That was molten magma squirting into cracks. And the stuff that didn't melt was like toothpaste. It was so soft, because it was so hot, that it just flowed and crinkled and folded, and--
It's been bent, and one can follow any of these layers along. And you see that they wiggle, and they come around, and then they come out here and back. And so these rocks were really, really hot. They were almost up at the melting point.
And they were being squeezed. There was mountain building of some sort going on that caused them to have squeezing and to be pushed from here to there. And as they go, very often, you get something like this folding. If you take a phone book and squeeze it, it'll fold. And in the same way, when you squeeze these rocks, you end up folding them.
The other part that's interesting, here, is that we can see these beautiful things so very well because this stream has come over them. And it's eroded them, and it's polished them. And the surface that we're on is very smooth.
But this is the heart of the mountain. This is what it would look like if you could get down in a mountain range somewhere, down there about 5 or 10 miles. And that's what we're standing on.
Migmatite. M-I-G-M-A-T-I-T-E. It's not quite magma. It's migma. Mixed magma. And you'll see all of these awesome morphs and wiggles and the little things through here.
This melted. This didn't. This melted. This didn't.
Oh, this layer-- that's beautiful. These things have been really, really hot.