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.