Click for a transcript of Unit 4 #2 lecture.
Hi, welcome to GESCI 10, geology of the national parks. This is our section on building mountains, tectonics and building mountains. And we're gonna go to the Rocky Mountain National Park in the middle of the continental US, take a look at it. My name is Sridhar Arandakrishnan, and I'll be your guide through this part of it.
The Rocky Mountain National Parks are an absolutely stunning place. You have to go there. They are very near Denver. You can fly right into Denver, drive up to Boulder where the University of Colorado is. Some of my colleagues work there. And right there, the front range is of the Rocky Mountains and then this magnificent panorama of mountains stretching for thousands of miles, north 1,000 miles, north and south, and hundreds of miles to the east from there, and right to the west from there, and right up onto the Colorado plateau, all right. So it's just a beautiful place, a very dramatic setting. You come from the plains of Kansas and Colorado, and then you come along, and then here's this unbelievable mountain chain. It must have been quite something for those who first saw it.
On the map here, you're seeing a map of the western US. California is right off to the west, and then you have Nevada and Utah that make up the so-called Colorado plateau, Utah and western Colorado. And then right where that Colorado text is is the edge of the Rocky Mountains. That's where Rocky Mountain National Park is, off to the east of that.
It's all plains, very flat, lots of corn, soybeans, wheat. It is classic midwestern plains country. It's nothing like what people imagine Colorado to be. They think of, when they think of Colorado, they think of majestic mountains. Eastern Colorado is very flat. It's western Colorado that has those 12,000 foot mountains, 13,000, 14,000 foot mountains, Pike's Peak, all these amazing places, all right, so all the skiing. When you think of Colorado, most people think of skiing. Well, eastern Colorado doesn't have it. It's all in the west.
We can zoom in a little bit. And you can see the transition from Denver there in the middle. And everything to the east of Denver is all plains country. It's all flat. And you can see as you go to the west from there you get those dendritic valley patterns, those white snow capped peaks, all of the complications that always go along with mountains, you get all of these little mountain valleys with little rivers and glaciers in them.
Rocky Mountain National Park itself is right here in the front ranges. And you can see the transition there even more dramatically up into the Rocky Mountains, all of the snow covered peaks, right. So this is where we're gonna take a trip to. Let's go look at some real pictures rather than looking at satellite maps. It's a place that's easy to get to and well worth the visit.
Lots of fauna there, lots of critters, birds and mammals of many different kinds, you still have wolves up in there, still have bears up in there, lots of elk and bighorn sheep, very common, all over the place. Obviously, it's a national park so hunting is forbidden. And so it's still in very much pristine condition. It is a very popular place, you drive up in there, going to the mountain, going to the Sun Road is one of the roads that goes through there. It's one of the highest roads in the lower 48. It's up at 12,000 feet. And sometimes it's just this solid mass of RVs, and campers, and pickup trucks, and SUVs stretching as far as the eye can see.
Just pull off the road, just park, get out your car, and walk in any direction. And you walk for 10 minutes and I guarantee 90% of the people will disappear, walk for another 10 minutes and 99% of the people disappear. So few people actually get out of their cars and walk, and I don't want you to be one of them. I want you to get out and walk and walk for just a little ways, just for half an hour, 45 minutes, to experience the solitude and the majesty of Rocky Mountain National Park.
All right, here it is, here's one of these Rocky Mountains. And in the foreground you have a glacially carved lake. When the glaciers came down out of the mountains 20,000 years ago they would have covered this whole area. They would have ripped up this area and pulled up this lake, pulled up this big hole. And then when the glaciers retreated water filled in that area. Now we have these lakes. There's a whole series of lakes up, wherever you have glaciers you usually have lakes that they leave behind. Glaciers are really good at digging deep holes. And this is one of them. Here's another picture of [? Usal ?] Lake and looking up at this glacial valley behind it, really a magnificent place.
This is up on top of Flat Top Mountain. You have this classic alpine landscape, this short scrubby bushes, very hardy little shrubs that manage to eke out a living in the permafrost, in the high winds and the cold. The weather can change really dramatically. You start out in the morning, and you have blue sky, and it's 70 degrees, and you get up there then you need a jacket because the storm clouds have come up, as they have in this case.
This is looking down at a little tarn. Tarn is a Scottish word, I think, I'm not sure, for small mountain lake, usually glacial carved lake. These tarns can be just a few hundred feet across. And they have lovely cold water surrounded by all of that glacial moraine material around them, a few trees but not many.
This is a road that runs, a very popular road, that runs up. There's Beaver Ponds there in the lower part of it, and then behind it a moraine, that little ridge that you can see with a little bit of a brown material on it and then some trees on top. That's a moraine. That's what was left behind by a glacier. A glacier came down, pushed up this material and left that. And we'll talk about moraines next time, or not next time but down the road.
Here's an alluvial fan. This is material that's brought down by rivers and sometimes landslides, just comes down these mountain sides, lots and lots of flora, lots and lots of fauna. So I said, all of these pictures are available online on Angel. Go and take a look at these virtual field trips, orchids, and the classic Rocky Mountain rock, this sort of grayish colored, dark gray and light gray colored rocks, metamorphic rocks cut through with igneous rocks. We'll talk about what a metamorphic rock is, what an igneous rock is over the course of this class. All right, so, let's hop over to the presentation and we'll begin our talk.
The Rocky Mountain National Parks, Rocky Mountains are a mountain range. They're quite high, 12,000, 13,000 feet high. But they're in the middle of the continent. They're 1,500 miles from the ocean. Why are they there? We talked about why the Appalachians are here. They're the collision of North America with Africa and Asia 300 million years ago. They created these high mountains. All right, that's straightforward.
We talked about why Mount St. Helen's is there, or why Mount Baker is there, or why Lassen Peak, or all of those. Those were subduction zone volcanoes. The Cascade range is a volcanic arc created when oceanic crust subducted under continental crust. The question is what's on with the Rocky Mountains? Why are they where they are?
Normally, and the short answer is, normally oceanic crust will simply subduct under continental crust. If it's dense enough and cold enough it'll just go straight down and all is well. You will get the classic subduction zone situation that we now have in the Pacific Northwest. You'll get trenches and volcanoes. Occasionally, if that subducting oceanic crust is hot enough, if it was only created recently in the last half a million years or million years, it hasn't had a chance to cool off, then it's still buoyant.
Remember hot rocks are buoyant and low density. It's only when they get cold that they want to sink. So if that oceanic crust was still hot, buoyant, low density, it would go under the continent, but it wouldn't subduct straight down. And as it got pushed off to the side it would continue to scrape underneath. And I'll show you a picture of it here in a minute, but that's, in words, that's what was going on.
I grew up in New York City, and this next cartoon perfectly encapsulates my ignorance of this country as I was growing up. This is a very famous New Yorker cover called New Yorkers View of America. And you have Ninth Avenue, 10th Avenue, all of the buildings in great detail. You have the Hudson River. They might know a little bit about New Jersey. But then the rest of the continent is just this vast blank space. And it might be, you might know, oh, there's a few mountains somewhere in the middle, and there's something off on the other coast, and then there's the Pacific Ocean. So this is a famous picture of what New Yorkers think of the rest of the country.
And to be honest I have to plead guilty to that. And I was in college, I went to school in New York City, I had a friend who had an internship, a summer internship, in Denver. And I had another friend who had a summer internship in San Francisco. And the two of them are flying out, and on the last evening as they were ready to go, I said, oh, boy, you guys are lucky. The two of you will be able to visit each other on the weekends, and I'm gonna be left here all by myself in New York City. Little did I realize that San Francisco and Denver are 1,500 miles apart. And so my ignorance was stunning back then. Hopefully I know a little bit more now but there it is.
This is our picture of how the Rocky Mountains were made. We think this is what went on with them. This is an animation that's available for you online. But the short answer is that an oceanic ridge, one of these mid-ocean ridges where material is coming up from deep inside the Earth, used to be far offshore but was subducted under North America.
But because it was so close to the edge of the continent, it's still warm. And because it was still warm, it didn't sink down. And because it didn't sink down, as it scraped along underneath the western US, it shoved up the Rockies way, way far inland, all right. So even though subduction is supposed to sink down and only produce mountains at the coast as we have in the Cascades, in this case, because that subducting slab was still warm, it went along for a long ways underneath the continent shoving up the Rocky Mountains far in the interior until eventually it got cool enough, and it did sink down far deep inside. This is the leading idea for why the Rocky Mountains are where they are and how they were formed.
We don't really know. It's a little bit embarrassing to say this, but geologists have a pretty iffy understanding of the Rocky Mountains. But this is the leading idea. This is how science works. We come up with a hypothesis, somebody did, they said we think this is what's going on, all the evidence seems to support it. But people are still working hard to try and figure out whether that hypothesis meets all the data, or if somebody can come find some new data that shall know that hypothesis is wrong, in which case we'll throw this slide out, and we'll put in a new slide.
That's the wonderful thing about science is none of these slides are ever carved in stone, if you will. At any time I could just delete this slide and throw it away because somebody comes up and says, nope, I think that's wrong. So as this warm oceanic crust was subducted and slid right underneath North America long deep, deep under the continent, it shoved up the Rocky Mountains even though they're far, far inland. Whoops, sorry, going the wrong way.
The Rocky Mountains are made up of something called metamorphic rock. Metamorphic rocks are rocks that have changed from their original form. There are three main types of rocks, sedimentary rocks, which are rocks that are formed when sediments collect and over time those sediments pile up and get cemented together into a more solid mass. Sandstones, limestones, these are all sedimentary rocks, all right. Igneous rocks are another relatively straightforward type of rock. This is when you have molten material that comes up and freezes at the surface of the Earth and produces an igneous rock. These are made in volcanic zones. They are basically frozen lava and various frozen magma, very straightforward as well, all right.
Metamorphic rocks are the complicated ones. If you take any kind of rock, a sedimentary rock or an igneous rock or another metamorphic rock, and you squeeze it hard enough and you heat it long enough, it'll change its form and turn into a different kind of rock called a metamorphic rock. And where in the world can we find high pressures and high heat, deep inside the Earth, all right. That's the only place that you can get pressures and heat high enough to cook these rocks and to turn them into metamorphic rocks. So whenever you see a metamorphic rock, as we do in the Rocky Mountains and in a few places in the Appalachians, we know that those rocks, at one time, were deep down inside the planet.
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Whenever we see metamorphic rocks up at the surface then we know that those rocks, at one time, were deep inside the Earth, that some kind of rock, either sedimentary rock or an igneous rock, was carried deep down into the planet, down to miles and miles, maybe tens of miles down into the Earth. And as those depths the heat is high enough and the pressures are high enough that the rocks can be cooked and squeezed until they are a different form called metamorphic rocks. And then they come back up to the surface.
Here's some pictures of these metamorphic rocks. Unlike igneous rocks, which are more even looking, are more regular in their form, these, as you can see, have all of these grains that are growing in them. They've been folded and twisted around because of the huge pressures that squeeze them together and the high temperatures. They can be right overturned and squeezed out like toothpaste, as you've seen, as you can see in the lower one over here.
This, by contrast, are two pictures of igneous rock. And the site we've talked about, and the site quite a bit, those are the rocks that come up in the Andes mountains, and also in Mount St. Helen's, and all of these other subduction zone volcanoes. Pele's hair is a type of igneous rock that you find in Hawaii, where you have the hot spot type of volcanism, very different looking than and very different chemically than metamorphic rocks.
So why are these metamorphic rocks at the surface? As I said the only way to form these rocks is deep inside the Earth. You got to take them, sink them down miles into the Earth where the heat is high, where the pressures are high, you cook them and squeeze them and they turn into metamorphic rocks. So what are they doing up at the surface? In fact, what are they doing up at 12,000 feet up in the Rocky Mountains? How'd they get way up there? And that has to do with isostasy, deeper rocks rise up because of isostasy. So, we're gonna go to the drawing board here and take a look at isostasy again.
Remember what isostasy was, this is a cross section of a mountain. So this is trees here, and this is the very top of the mountain. If you were standing at the top of the mountain, and you were to start to drill a hole down through it, you would go part of the way down and you would be at what's the equivalent of the surface of the Earth. But you wouldn't be to the bottom of the continental crust. The continental crust continues on under there. And, in fact, it continues way, way down into these deep roots, all right. So all of these mountains that stick up high have these deep continental roots underneath them because of the requirement of isostasy.
You have to have the same mass of material above you at any point to have equal pressures. So if you imagine a line somewhere deep down in here, the amount of weight above you is the same anywhere along that red line. That's the principal of isostasy. If you're over here the amount of weight above you is the same as if you were in the middle of the continental crust over there. But because continental material is less dense, crustal material is less dense than mantle material, you need more of it. Remember that.
If you want to have the same weight of two things but one is more dense and one is last dense, then you need more of that less dense material. You need more volume of it to get the same weight, all right. And that's what these roots and this mountain above allow you to have, is you have the same weight above you but because the continental crust is less dense you have to have more of it, which means you need a mountain rising up and the roots going down. So that's the principal of isostasy. It's a little bit subtle, but I encourage you to go and read the section in the book on this because it's an important notion.
But let's see what happens as we erode this mountain. Imagine, if you will, a metamorphic rock that has been formed inside this continental root. This black blob here represents the metamorphic rock. And we have to somehow bring that to the surface. How do we do that? The way we're gonna do it is we're gonna do it by analogy. We're gonna go and look at an iceberg. An iceberg is very much like a mountain and a mountain root. And there is an animation online that represents this, and I'll just do it very briefly here, and then we'll understand how it is that you can bring up this metamorphic rock to the surface.
Imagine, if you will, an ocean and floating in that ocean you have an iceberg. Ice should be white. It's a little hard for me to draw a white iceberg against a white paper here. So we're gonna make it black. The blue, obviously, represents the ocean. Blue is always ocean. So here we have our iceberg floating in the ocean. Because of isostasy a little bit of it, some of it, is above the ocean, about 1/10, and most of it is under the ocean, about 9/10 of it. OK?
That's because water and ice have almost the same density. So, if I need to have the same weight of material above me, I only need a little bit extra ice. I can go over here. I can draw my same line that I did before. I can go here. I can figure out the weight of water above me. I can go to the same point in the iceberg, and I gotta have the same weight of stuff above me. That's the principal of isostasy. And I only need a little bit extra ice, but I do need that extra ice. And so I need about 1/10 more ice because the density of ice is only a little bit less than the density of water. And so I need a little bit more ice to get the same weight, the same total weight, of stuff above me, OK. So that's isostasy once again.
But let's see how this helps us with bringing metamorphic rocks to the surface. Same picture, ocean, iceberg, but this time, for the sake of argument, I'm going to introduce a space alien into this. We're going to take a space alien, and imagine that a long time ago a space alien crashed into Antarctica and got trapped in this iceberg, and more snow fell on it and more snow and more snow, and eventually somewhere down deep inside this iceberg we have a space alien, big eyes, antenna, stuck inside this iceberg. How can we get the space alien out? Well, it's gonna happen naturally anyway. Why? Because as the top of the iceberg melts the iceberg has to bob up to replace it, all right.
Let's follow that through. Let's do a little thought experiment. Let's say I could magically take everything that's above water and simply slice it off and cart it away, all right. So we're gonna just take everything above water and slice it off and cart it away. And this is what we would be left with. And the space alien would be stuck down in here, same as before. Nothing's changed except that because of isostasy this is unstable.
That iceberg will want to naturally bob up. It has to. Because at any given depth the weight has to be the same above that spot. And in this situation the weight here is more than the weight here, all right. Water is more dense than ice. And so if you'll have the same amount of water and ice then the ice has less total weight. And so what happens next is that the iceberg bobs up in the water a little bit. Remember what we did is we just sliced off magically everything above water. Iceberg bobs up, and the space alien who is embedded in here also bobs up with that.
Let's do it again. Let's once again slice off everything that's above water. I'm gonna erase this ocean. And, once again, we slice off everything above water, and the iceberg would bob up again like this, and the space alien would now be at the surface. I hope you understand that that's an analogy. They don't really have space aliens in the Antarctica ice although according to The X Files we do. That was a terrible movie by the way. But nevertheless it did posit the presence of space aliens in the Antarctic ice.
What we do however have is that the Rocky Mountains have blobs of metamorphic rock deep inside them. And erosion slices off the top. So erosion comes along, and erosion removes the top of the mountain. And in the same way that the iceberg popped up, the roots of the Rocky Mountains also pop up. So after erosion removes the top you end up with the Rocky Mountains rising up again and the metamorphic rock along with them.
The mountains as a whole are smaller, but the metamorphic rock is at the surface or nearer to the surface. So in much the same way that glaciers can bring up these supposed space aliens with them, metamorphic rocks come up. The idea of the metamorphic rocks is a little bit more well supported than that of the space aliens, all right. We're gonna hop back to the PowerPoint here for a second, and then we'll wrap up this session.
The rocks cycle is something that is fundamental to geology. All of the rocks on the surface of the planet have been cycled and recycled and brought up. And erosion will take sedimentary rocks, and it will produce sediment which will turn into sedimentary rocks. And then subduction will carry that down. And then that rock will come back up again either as an igneous rock or as a metamorphic rock. And then it will get eroded again and produce sedimentary rock. And so this whole rock cycle says that we're always recycling all of our material on the surface. And we have to work out their history by coming up with these clever ideas and these hypotheses.
To summarize, there are three types of rocks, igneous rock formed by melting and solidification of magma, sedimentary rocks formed by erosion of other rocks and then cementing them together, and finally metamorphic rocks formed by the cooking of rocks, all right. So that's the end of this part of tectonics. We've learned about building mountains, about how you can build them at pull apart boundaries, at pushed together boundaries, and at slide past boundaries. Next time we'll move on to new material.