Geology of the National Parks

Video Lecture


There are TWO Unit 4 lectures both featuring Dr. Sridhar Anandakrishnan.

Please watch the unit 4 lecture #1, Plate Tectonics III: Making Mountains & Obduction (36:10 minutes).

Unit 4 Lecture #1
Click for a transcript of the Unit 4 #1 lecture.

Welcome. This is Geology of the National Parks, GSCI10. My name is Sridhar Anandakrishnan, and I'm going to be your guide through obduction, it's a word that means collision of continents and what happens when continents collide. So, you're probably watching this somewhere in Central Pennsylvania. Maybe you're not, maybe you're off on holiday somewhere else, but State College here is certainly in the middle of central Pennsylvania. And what is remarkable about this area geologically is the so-called ridge and valley structure that stretches for hundreds and hundreds of miles to the Northeast and to the Southwest. You've seen them. You driven over them. You've had to go long ways along these ridges with Tussey Mountain on this side or Mount Nittany on this side, Bald Eagle Ridge. So, it's a very characteristic structure of this area, and it was created by obduction, by the collision of continents.

So, that's what we're going to do. Let's take a look at some pretty pictures first that will motivate what it is that we're talking about, and then we'll go to the PowerPoint presentation. This is a picture of a thunderstorm over Great Smoky Mountain National Park. Great Smoky is down in the southern US, it's just an absolutely gorgeous place. High mountains, 4,500 feet high, wooded for the most part, except for these wonderful balds that they have there, these tops of mountains that are bare for reasons that I'm not particularly clear on. But you'll be hiking through and then you'll break out into one of these balds, and you can see forever, ridge after ridge after ridge, and there's all these clouds that give it its name, the Great Smokies. And maybe there's a thunderstorm on the next ridge, or maybe it's raining over there. It really is an amazingly dramatic place. It's not that far away. It's a day's travel from here, but there's some lovely scenery in between as well. Well worth a spring break trip. You've don't have enough money to go to Cancun or something like that, time much better spent. Drive down to the Great Smokies.

Great Smokies is where the Appalachian Trail comes through, one of the most extraordinary hiking trails in the world. It goes from Springer Mountain in Georgia right up the Appalachian Mountains right up to Maine, to Mount Katahdin in Maine. And every year, there's a few people, a few hardy souls, that hike the whole thing. They start down in Georgia in the springtime and they follow the spring up through. They come up through Pennsylvania, go right up Harrisburg right up the Susquehanna Valley, and then on into New York, and then finally through the Northeast up into Maine. So, it's a beautiful place. Here's a picture of the Great Smoky Mountain Park.

There's a bridge over one of the many, many streams. You have these mountains that stick up and to the west of the Great Smokies, you've got nothing. You've got these huge planes that stretch out for miles and miles to the west, all the way through the Midwest until you get to the Rocky Mountains. And the winds build, and the weather patterns, the weather systems come down through there, and they have to climb up over the Great Smokies. And as they climb up over it, they dump all their rain down into them.

They do the same around here. We're pretty wet here in Central Pennsylvania, and one of the reasons for it is that these wind patterns and weather patterns bring a lot of water in. When you've got water, you have streams. And here's one of those streams, a close up of one of those raging torrential streams.

You can go online. This is one of our virtual trips, virtual field trips. We call them vtrips, and you can read the captions. This is just a lovely place. And you're hiking along, you'll come across one of these waterfalls. This is two views of the same one. There's some text, they're talking about why those waterfalls are where they are. So, you should have a look at this. All right?

Late autumn, things are starting to thin out a little bit, and the ridges, one after the other. You see that around here, too. One ridge after the other. You climb up on top of one, and there's another one behind it. The early European settlers were quite unsettled by this. They would get up over one ridge, and there was another one that they had to go past. Lots of lovely vegetation, mountain laurel, flame azalea. Lots and lots of animals that live in there.

This is another thing that national parks are wonderful for. They preserve flora and fauna. If we didn't have a national park in Great Smokies, who knows what would be there? Maybe it would have been logged for farmland, or a subdivision, or a highway. So, the fact that it's a national park preserves all of these wonderful critters and plants. Rhododendrons in the background. Just a lovely place. So, you should take a trip down there. So, we're going to find out why the Smokies are important to us, why the Appalachians are the way they are, why the Appalachian Trail stretches up through there.

We're going to switch out to the PowerPoint, we're going to talk about Tectonics Part Three: When Continents Collide. You get obduction — there's how the word is spelled — and we're going to plunge right into it with "Go dog go." If you can see, there's blue dog and yellow dog, and blue dog has run into yellow dog. Yellow dog is chastising blue dog for being such a poor driver. But the important thing is that the hoods of their two cars have scrunched up like an accordion. Neither one gave way. Neither one submarined under the other one. They just smashed into each other, and they accordioned up and crumpled and rumpled up.

And that's what we're seeing over here. Those rumples in the hood of the red car, and those rumbles in the hood of the yellow car, think of those as the Appalachian Mountains. That's how the Appalachians were created. Two continents collided, not go dogs, but two continents collided. And when they collided, neither one gave way. . If you remember from last time, the subduction one, when oceanic crusts and continental crusts collide, one of them gives way. One of them sinks under. And so, you don't have these types of mountains that are produced when two continents collide. When they collide, neither one wants to sink down, and so, they just crumple up, get shortened. And as they get shortened, they rise up in these peaks that we see around us here in Central Pennsylvania and down in the Great Smokies.

Here's a picture of the Appalachians. And this is a perfect shot, ridge after ridge after ridge after ridge, long skinny things. We are sitting on the hood of the blue car looking up along the hood of that car towards one of the drivers. I'm going to go back here real briefly to the go dog picture. Imagine you're sitting at that collision point, looking up along the hood. That's all it is. That's all that these Appalachians are, writ large.

Here's a photograph in West Virginia. There's a road cut where Route 68, I think, goes west through there. Now there's a wonderful visitor center where you can pull off the highway, and there's a wonderful explanation of it. You have these extraordinary curved layers. What used to be nice and flat, during the collision, they got smashed together and got turned up and turned down, and you can see that over here because the road has cut right through there, and so, they've sliced through that part of it.

So, we're going to do a quick review of what we learned last time, or the last few times. And I know we've been throwing a lot of material at you in the last few weeks, but there is a lot of material to be thrown at you. It's all available online. It's on Angel. The text is free, you just have to download it and read it, and it connects to all of this. Tectonics is driven by heat. I've said that over and over again, I'll say it again. If I ever ask you what drives tectonics, you just yell all together, heat. There are a number of plates, the upper lithosphere is broken up into eight or nine plates, and they move about on these convection cells. The convection cells are due to heat. Oceanic plates are made up of basalt, and the continental plates have more silica in them. Silica ridge, they're a lower density, they're more buoyant.

When cold ocean plates collide with buoyant continental plates, the cold ocean plates sink down. They're of high density. They just sink down, you get a subduction. When continental plates collide, that's what we're going to talk about this time. You get the Appalachian Mountain range, that stretches all the way from Newfoundland down to Alabama. In Alabama, it sinks down underneath the coastal sedimentary plane, but the Appalachians actually come up again in Oklahoma. So, they're just an enormous mountain chain.

Continents are rarely destroyed. The story gets really complicated. If you take something and you just keep piling stuff on top of it year after year, you shred it, you munch it, you fold it up, you put in a corner, you take it out, understanding its history becomes really, really difficult. The ocean is easy. It's created, it's destroyed. After 150 million years, 200 million years, all of the ocean is gone because it's been subducted and disappeared. But the continents are old. They're four billion years old. And in that time, they've had lots and lots of things done to them. And so, it becomes really difficult to know what happened in the past.

The analogy that I give is one of my colleagues— not me, of course. I have a very clean office. That's a joke. You should come see my office sometime. But one of my colleagues has a really messy office. He's an older gentleman. He's been working in the department for almost 40 years. I don't think he's thrown a thing away in that time. And you go in there, and it's piles of books and manuscripts, piles and piles of folders and papers, and rocks that he's collected, and instruments that he's collected. And so, they get jumbled up sometimes, because he pulls something out from underneath and he puts it on top. And so, understanding the relationship of these papers to each other is really difficult.

And that's the way the continents are. They've been all jumbled up, and stuff has been piled on top, and stuff has been pulled out from underneath. We're going to give it a try, though. We're going to give it a try and try and tease out the history of the Appalachian Mountains.

Here's a satellite image of the east coast of the US and the eastern Canadian Shield. And you have this very characteristic wiggly line, you can see it there, that stretches right from the southwest right up through central Pennsylvania— I hope you can recognize the Delaware River there and the Chesapeake Bay. That's where Philadelphia is. And then on up into New England, and then right off the map— that's Maine— and then right off the top would be Newfoundland and Canada. You can see that this mountain chain stretches a long, long ways, and it's got these long, linear features to it.

This is the Susquehanna Valley from space. This is looking down at just the region right around here. That's the Susquehanna River, Harrisburg is right in the middle over there. And you can see these ridges, one after the other, stretching up through there, and this river slicing through them, allowing us to work out their history. This is what we're trying to figure out. Why are the Appalachian Mountains here? What's their relationship to the rest of the world?

The Appalachians are a complicated place. They aren't simple. As I said, the analogy that I give is of my colleague who has an office and it's been filled with stuff, and stuff has been removed, and it's very hard to tell the relationship between things. But we're going to give it a try. About 300 million years ago, North America and what's now Africa and Europe collided. These two huge continents— neither of which wants to sink, because they are low density. Neither of them wants to sink under the other. It's like a game of chicken. And the two, neither of them gave way. They smashed into each other. And they crumpled up to form these huge mountain chains, possibly as high as 15,000 feet. We don't really know, but we think some of those mountains might have been as much as 15,000 feet high.

It's similar to what's going on today, right now, in India and Tibet. The Indian continent is running into the Asian continent, and as the two run into each other, you have these enormous mountain chains, the Himalayan Mountain chains. The highest spot on earth, Mount Everest, is 28,000 feet high, and it's because of this collision of India and Asia.

This is a picture of that collision. The Indian land mass was way out in the Indian Ocean 70 million years ago, and it came zooming up over the last 70 million years. And today, it has smashed into the Asian plate. And in the process of those two continental massesIndia is a continental mass, Asia is a continental mass neither one wants to give way. And they just run into each other, and they crumple up, and then the rocks in between have nowhere to go but up, and so you get these huge mountain chains of the Himalayan mountains. This is a photograph of Mount Everest. Sagarmatha is what Mount Everest is called in the Nepali language. And it's an absolutely awe-inspiring sight, first climbed by New Zealander.

So, in the process of these two continents colliding, they get shortened up. It's just like the hood of the two dogs that we saw. That's an analogy, but it's a pretty good one. You take a hood that used to be longer, you accordion it together, now it's shorter. And that's what happened with the North American continent, and presumably with the European/African continent when they collided 300 million years ago.

One of two things could happen. The first is what's known as a thrust fault. This is where, when the two continents collide, you actually do get sliding of one over the other. You don't get subduction. Nothing sinks back down into the earth. But you can think of two sheets of paper that run into each other. One just climbs up and slides over the other one. This is known as a thrust fault. You can shorten up a continent by sliding one part of it over the other. And this is partly what happened in the Great Smokies area.

And one of the reasons that we know that is that there are places where there are young rocks underneath older rocks, and this is a very unusual situation. Usually, the old rocks are on bottom, and you pile younger rocks on top. It's like my colleague with his office. He gets a book, and he puts it on top of an older book. And he gets a newer book, and he puts it on top of an older book. And then he gets a newer book, and he puts it on top of that one. And so you can look at them, and oh, there's a book that he got 30 years ago, and there's a book that he got last week. And that's this normal sequence of things. Occasionally though, he'll go and he'll pull out one of the older books and read it and put it on top of this pile, and so the whole thing gets jumbled up and you don't know the sequence. That's what's happened down in the Great Smokies area, where you have older material that's ridden up on top of younger material.

Further north, around here, the rocks are more wrinkled. They look like a kicked up rug, or a sheet of paper that has been collapsed. We've got a young kitten that we just adopted from the shelter, and boy is he ever active. And he loves our rug. We don't have one of those little rug runners, the little rubber things that keep the rug from sliding around. And so he'll run as fast as he can, and he'll attack the edge of the rug, and he'll slide into it and he'll shove that rug back. And as he does it, he'll form these wrinkles, one after the other. And he doesn't seem to realize that that's a valuable rug, but his enjoyment of it simply comes from rumbling it up.

This is a similar sort of thing that happened over here. These two continents collided. Instead of one sliding under the other, what happened is the continent got shortened, but now by being crumpled and rumpled up like a rug. The rug itself is layered. There's hard layers and soft layers. And as time goes by, you get differential erosion, that we'll talk about next.

This is a cross-section. Remember what I told you a cross-section was. If I could take something and slice it open and look at it from the edge, that would be be a cross-- look at it from the end, that would be a cross-section. This is a cross-section through State College, going from the Northwest side-- the Allegheny Plateau and the Allegheny Front on one side, going down through Bald Eagle, through Mount Nittany, through Tussey Mountain, and heading off to Southeast on the right, down towards Harrisburg and down towards Philadelphia on the right. You should recognize this. If you have ever driven from here to New York City or ever driven from here to Philadelphia, you've seen this. You have to drive up over Seven Mountains and down the other side, and then there's another valley there, and you have to drive up the next one and down the next one, and that's what's going on over here.

But if you're a geologist, you can go and look at the types of rocks that form the different mountains. And what's interesting here is that these layers, what used to be a sequence of nice flat layers, that have now been squeezed together and crumpled up, have eroded in slightly different ways. Where you used to have very hard rocks, were the high places, and those cracked and you went down into the softer material, and those just eroded right down to what are now the valleys. So paradoxically, it's where the very hardest rocks used to be very, very high up, those are what have broken through, and now we have very deep valleys there. And the ridges in between are made up of what used to be the somewhat softer areas.

Eventually, the collision stopped. These things are colliding, they're getting pushed together. Eventually, the collision stopped, and then a spreading began. Similar to what's happening in Death Valley today. Death Valley is a mid-continental spreading ridge, and that's something that also happened in this area. About 150-200 million years ago, you started to have spreading apart of this in similar fashion to what happens in Death Valley. And the Atlantic Ocean was formed. As you started to rip it apart, and then the waters rushed into that low spot that was created where the continent was spread apart.

The mountains stopped being pushed up. You're no longer shoving them together, so you've stopped shoving these mountains up into the air. As soon as you stop doing that, those mountains will start to be eroded downwards. Mountains are very rapidly eroded. This is something we'll talk about more down the road when we're talking about erosion. But when you have these high mountains, the winds blow on, the water rains on them, the glaciers build on them, and they slowly get scraped off. That's what erosion is. Erosion is simply scraping off pieces of this mountain and making it go away, and depositing them in the low spots.

But for some very interesting reasons, the mountains are still fairly fine. There are 2,000, 3,000, 4,000 foot high peaks. They used to be much higher. They used to be 15,000 feet. But even though they're very old, they haven't been scraped down to sea level yet. And we know about rates of erosion, and so we should have been able to scrape down those 15,000 feet to sea level by now, but we haven't These mountains are still up 2,000, 3,000, 4,000 feet. Why is that?

It's something called isostasy. Mountains have very deep roots. When we squeeze those continents together, the continent bulged upwards, but it also bulged downwards. When you squeeze them together, it isn't as if the bottom was flat and it's just the top that bulged up. When you squeezed it together-- think of silly putty. When you squeeze that together, it bulges out in both directions. And that's what happened with the Appalachian Mountains. Some of it bulged upwards, and some of it bulged downwards. And as the mountains are scraped off on top, these roots ride up, scrape off some more. The roots ride up, scrape off some more, the roots ride up.

And so to get this mountain scraped down to sea level, you don't just have to scrape away the stuff above. You've got to scrape away the stuff below. So I'm going to go to the drawing pad and illustrate this notion.

Obduction is the collision of continents. And about 300 million years ago, we had the North American continent, we had the African/European continent, and the two of them collided. They ran into each other. And over time, as they kept pushing against each other, you had to give way. The one had to give way. Neither of them wanted to give way, but something had to happen. And so you've got shortening of the continent, and one of the ways they shortened is that the continent rumpled up like a rug.

This is still Africa here, and this is still North America on the left. They've shoved into each other, and they have rumpled up. But in the process of rumbling upwards-- I'll go to a different color here-- they've also rumpled downwards. Wherever you see a high spot like this, they also have a low spot underneath them. You squeeze them together, and in the process of squeezing them, you squeeze upwards, and you squeeze downwards. So I'm going to zoom in on that and show that in a little bit more detail.

This is a view of one mountain peak. And it has a deep root that might be three times as deep as the mountain is high. So even if the mountain is three miles high, then the roots might be 10 miles deep, or sometimes much deeper even than that. So it depends on the density contrast between crust and mantle. After the collision, you've now created a mountain, let's say 15,000 feet high. But at the same time, you've created this deep crustal root that goes deep down into the mantle, that is squeezed downwards at the same time that the crust has bulged upwards.

Over time, the collision ended, and so the mountain stopped growing. And in fact, erosion took over, and the mountain started to be scraped away. But the mountain isn't gone yet. Even though we've scraped away and scraped away and scraped away, we've kept eroding that mountain away, it's still there. After all this time, it's still there. And the reason for that is as you scrape material away, more material bobs up from underneath. And that's the principle of isostasy. That's what we're going to talk about next.

I'm going to switch back to the PowerPoint very briefly here, and then we'll come back to this picture drawing. As the mountains are eroded, they remain high because the material from below is floating up at the same time. It's like an iceberg floating in water. You probably have heard this expression, 9/10 of an iceberg is below water and only 1/10 is sticking up. And if you remember that movie The Titanic, with Kate Winslet and Leonardo DiCaprio or somebody. The two of them are on the Titanic, they're riding along, and it smashes into the great iceberg and disappears and all of that. It's because only a little bit of it is above water, and a big chunk of it is underwater. Ice is slightly less dense than water, and so it mostly sinks down into the water, but a little bit of it sticks up.

You can do this experiment for yourself. You can go home today, get yourself a glass of water, take an ice cube out of the freezer, and just drop it in. And if you look at it, you'll see most of it is underwater, and a little teeny bit will stick up above water. If you want to do it a slightly different way, you can go up to your bathtub, take a bath, put in lots of hot water, put in some bubbles, get your little rubber ducky out, and put your rubber ducky in. You'll see that the rubber ducky floats way up. A little bit of it underwater, a little bit of it above water.

That's the principle of isostasy. Different things that have different density contrast. An ice cube in water, a rubber ducky in water, a mountain in mantle, a crustal mountain in mantle. Each of these will float in the fluid. A little bit of it will be below water, and a little bit of it will be above water. And the amount that is below water and the amount that's above water depends on the density difference between them. Ice and water have almost the same density, so most of the iceberg or ice cube is below water, and a little bit of it sticks up above water, and the Titanic can run into it because you can't see it. So that's what we're going to talk about.

We're going to show you a picture of what this iceberg looks like. Before we do that, let me review a little bit about plate collisions. We saw in the first week, pull apart, that was Death Valley. Last time or two times ago, we saw subduction. Crater Lake, Mount St. Helens, all of those. We're seeing obduction this time, and we talked about slide past very briefly when we talked about the San Andreas Fault. In slide past tectonics, sometimes these two plates, when they're sliding past each other, they won't slide very smoothly. You'll have a kink in the boundary, and where these two are sliding past each other, you'll get these big mountains being built.

This goes along with the theme of all the action is at the boundaries. It's where the plates are running past each other, it's where the plates are running next to each other, it's where the plates are running into each other. That's where all the action is, that's where the mountains are being built. So we talked about obduction. We've built the Great Smoky Mountains, and we're going to come back to isostasy in a minute.

Let's briefly take a detour to slide past tectonics. If there's a kink where these two boundary plates are running past each other, once again, you'll have these great mountains being built. The San Gabriel Mountains near Los Angeles are a perfect example of that. The San Andreas Fault wants to go straight, but it's got a little kink in it and you get these big mountains being built. All of these are examples of isostasy. Where these mountains are built, you get these high mountains, and you get these deep roots underneath. Any time you've got a high mountain on a continent, you've got a deep root underneath it.

We're going to take a little detour here. If you remember, I told you when you see that little symbol up in the corner, that means we're running off the tracks and we're going to wander off and talk about something slightly different. During World War II, there was a proposal to build an aircraft carrier from an iceberg, because it would be unsinkable. The Germans could come along-- this was an Allied project, a US and British project. The Germans could come along with their submarines and fire their torpedoes at these aircraft carriers which is made out of an iceberg, and it wouldn't sink. It would just float. And this was an actual project. It would be unsinkable.

They thought big during War II, they really did. They were in a struggle for their lives, and for their way of life, and they would do whatever they needed to do to survive. And they came up with what now seems like a perfectly ridiculous idea, but it was actually taken forward quite a ways. It was called project Habakkuk, not quite sure why Habakkuk. There was a small one that was built on Lake Louise. As a glaciologist, I find it very interesting, things that people do with ice. But this was quite a visionary project that never went anywhere.

Back to the track. We're going to talk about the Rocky Mountains next. That's going to be another example of mountains built within a content. We've seen how obduction can create the Appalachian Mountains, how they will rise up high and they'll have deep roots. And as you scrape away the top of the mountain, the bottom will rise up to take its place, and so you've got to scrape away some more, and more and more has to come up. So that was obduction. You can also produce mountains at a kink in a slide past boundary.

So now, I think we've run into just about every type of mountain building mechanism. The last one that we're going to look at is one of the most unexplained, one of the most enigmatic. It's the Rocky Mountains, right in the middle of the continent. Why do we have these mountains way in the middle of the continent? That's what we're going to do next time.

Credit: Dr. Sridhar Anandakrishnan

Now watch part 2, Plate Tectonics III, Mountain Building & Obduction (34:47 minutes)

Unit 4 Lecture #2
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.

Credit: Dr. Sridhar Anandakrishnan

Want another look?

Check out the Unit 4 Overview, Obduction used in the online lecture here.