Geology of the National Parks

Video Lecture


Please watch the 50-minute Unit 2 lecture featuring Dr. Richard Alley.

Unit 2 Lecture
Click for a transcript of Unit 2 Lecture Video.

Hello, and welcome to another day of Geoscience 10. This one is maybe a little bit like an old-style fraternity initiation. We make you run through something really hard to get started.

And so we're going to go cranking through a lot of material in a fairly big hurry here. You will find this material is supplied to you in the textbook, in much slower and greater detail. You also will find there's a lot of v-trips, virtual field trip slide shows that are online. So you can see the pictures that I'll show you here, in much greater detail and at your leisure.

There's actually only a few big ideas that we're going to go through here. But they may be different for you. You may have seen them in some earlier class. But if you haven't, I hope you'll find this interesting.

We have started into the big picture of how do you make mountains. After we make mountains, then later we will tear down mountains. And then we will see about how people live on earth, how animals live on earth, what the history of the planet is. So that's sort of the broad outline of the course, building mountains, tearing mountains down, living on the planet.

And we're going to start with making mountains. We'll do some earthquakes, and we will look a little bit at what we call plate tectonics, which is the vast underlying processes that make the mountains. We're going to do this by starting at one of the more interesting places that you could go for a national park, which is to go to Death Valley. OK?

So we'll look at Death Valley. We will look at how it is spreading apart. How that spreading is being driven by the heat within the planet. And then we will briefly pop by Yellowstone, just long enough to see that it's shaking, and what we can learn about that, about the interior of the earth from earthquakes and so on.

You will be given, for each of the parks we visit, one or more v-trips, virtual field trips. We would love to take you and zip out to the parks and have a good time. We haven't quite figured out how to do that. And so we've done the next best thing. We've gone ourselves. Some of our friends have gone. We've collected pictures and put them together, so that you can see virtual field trips. And so you will find these online.

This is Death Valley. Death Valley is tremendous. You're looking at the lowest spot on the continent, right here. And in the background you're looking at a spot that is more than two miles higher. It is just phenomenal.

These pictures were actually taken by Peter Fawcett, shown here. Peter worked with me a number of years ago and is now a professor in New Mexico. And he was out there in the dead of winter. You can see the snow way up high on the peaks above Peter's head. You can see a little bit of water behind Peter.

It does snow, it does rain in Death Valley. The water does come down in the valley. And in the winter it may puddle for a little while. And then, poof, it evaporates. And it's gone very, very quickly. And when it evaporates it leaves salts behind. And so you can see the white salt behind Peter, there at the picture.

That was economically important in Death Valley. Some interesting things dissolve out of the rocks. They come down in the valley with the water. They're left when the water evaporates.

And you could go and mine these. And so early people actually did mine in here. The 20-mule teams hauled boron salts, borax out. And this was used in soaps and paints and glass making and all sorts of other things. And it contributed to economic development in this area.

And also, one of our former presidents, President Reagan, earlier in his career, before he was president, sold soap. And he sold Boratine, which had borax, which came with the 20-mule teams. And so he was something of a celebrity, in part, based on Death Valley soaps.

A point in this course, you will get the joy and privilege of taking quizzes occasionally. In each time we visit a park, we're going to give you introductory material such as this. And we do this, because we find it interesting. We think you will. We hope that some day you'll be off on a road trip, and you'll get to go see these things. And this will help in your visit.

We would never, on a quiz, ask you which president sold soap. Nor would we ever ask you, which soap did the president sell? This is strictly for your edification and enjoyment.

Once we get to the point of saying, OK, geology is tearing Death Valley apart, that could show up on a quiz. But this early stuff, what's pretty, what's neat, what's exciting that you want to go see this park, this is for you. So please do not attempt to memorize the soap that President Reagan sold before he was president. That doesn't— that's just for you.

But Death Valley is something. This picture, the scale here, if you ever been to Penn State's University Park campus— and we're very impressed around here with our mountain, Mount Nittany. If you put Mount Nittany in this picture, it would tower over that little white line on the far side of the picture, almost a third of the way up that first slope.

There's nothing in the Appalachians that you could put in this picture that would look even vaguely large. You'd drop the whole Appalachians in this picture, and they would not go halfway up. This is immense. It's just truly an awesome place to get to.

You find the right place in Death Valley, such as the one you see right here, and there's beaches. During the ice age, when it was colder, the rains that came in didn't evaporate as fast. There was a big lake in Death Valley, which has now evaporated.

You go and look various places in Death Valley, and you'll find that the floor of it is salt and it's gravels and things. The streams are tearing down the mountains. They're dumping stuff down in the valley to fill it up.

And yet the valley is there. The valley is there, even though they're trying to fill it up. There's a little bit of sand, a little bit of sand dunes. Not too much. Usually the desert is not sandy. But there are such beautiful things as this.

There are really amazing things. You can look at this picture. You see those big rocks out on the salt flat. And those rocks have moved. And people have come and surveyed them, and they do move.

And we're still not absolutely positive how. The best picture is when it rains, you get a little water here. And then in a really cold day in the winter, it might freeze. And then these rocks are locked in the ice.

And the wind blows, and it pushes along on the ice, and it actually sort of skates these things along. But no one's sure. Racetrack Playa, the rocks move somehow. Neat place, neat place to visit.

The valley is actually growing. It's widening and it's deepening. And if you look at this picture, you can see the great mountain range towering more than two miles above the valley.

And you'll see right along the bottom, it's just a straight line. Well, that straight line is actually an earthquake fault. And here's another view of it. The near side has dropped. The far side has been raised.

These things happen. They are ongoing. It is part of our world today. And that's really what's very interesting and why we're visiting Death Valley at this point, is why are things moving? What's going on on the planet?

When this motion occurs, occasionally volcanoes come leaking up along the cracks. And so you can see volcanic craters if you go and visit Death Valley. You can see some places. This is an old, little volcanic feature, which has been torn apart by motions.

This is a slightly different motion. Most of Death Valleys' motion is sort of an up and down thing. This one's a little bit of side to side as well. And the diagram will show you what's going on. Things are moving away there.

So there's some pictures of Death Valley, if you get the chance to go. If you want to impress your friends, go in midsummer. If you want to have fun, don't. It's really stinking hot. But it's a wonderful park, big park sitting on the California, Nevada border.

Now, what's going on? Death Valley is getting bigger. And it's doing so from heat down within the planet. And so that's what we have to look at a little bit now. And then we'll go see what that does at Yellowstone.

So let's do the Death Valley. OK, Eastern California, Nevada border, the lowest, the hottest, the driest spot in the country. The water does come down. It does evaporate. It does leave salts behind. This drove various economic activity. It's largely turned off now.

And now, let's see what we do. This is a cross-section of Death Valley. If you could go in and you could take a giant saw and hack your way down and then look at it sort of as a cliff, you have spaces up above in this picture. And the earth is down below, and it's shown to you as blue. And we're going to look at it. The little line across the top is just for your reference in there.

And so what's going on, Death Valley measured? If you go in and you put in GPS receivers across Death Valley and on across Nevada and over to Utah, and then you survey what's happening, what you will find is it's very slowly, about as rapidly as your fingernails grow, the west is getting wider. One side and the other side of the valley are very slowly pulling apart.

As they do so, the valley drops down. And it drops down on what we call pull-apart faults. Because you pull it apart, and then the fault allows the drop. The fault is a break where there's motion. OK.

So the block in between is going to drop down as we pull apart. And it is measured. This is not storytelling anymore. The early geologists look at this, and they said, this has to be happening. We understand that, but we can't measure it.

It's pretty hard to measure a state spreading at the rate your fingernails grow. But we can do that now. This is directly measurable by GPS. It does work.

And so this is the picture. You can actually put out your GPS and measure that the mountains on the east of the valley and the mountains on the west of the valley are moving apart. It's actively going on. This is real. It's happening today. And as they do, they allow earthquakes. And the earthquakes drop the valley down.

Erosion takes stuff off the top. It fills it in the bottom. But then the bottom drops, and the sides are raised as the whole thing is pulled apart. And so Death Valley is getting bigger. The west is getting bigger. And the fault, the break in the rock along which there's motion, there's one on each side of the valley. And basically every valley in Nevada and Utah has this same structure going on.

If you were to find a rock layer, an old friend that you happen to know— the pretty purple rock, the pretty red rock, whatever it is— you can see the pretty red rocks scribbled in on either side. If you wanted to find that in the valley, and you drilled in your oil well— you're not allowed to drill there now, but if you were, you'd drill down— you would find that same rock layer. It's down there. And you would find it down there.

Now, this is one of the things that geologists were quite confident that this motion was going on, before they had a GPS satellite to help them measure it, because of things such as this. You say, OK, what happened to the layer in the middle? There it is. It's way down below. The valley, the west is getting wider. The bottom is dropping down.

Now, here's a map version. If you want to get in your satellite and look from above, down onto these things, Death Valley you'll see labeled way up at the top of the map there, above Las Vegas. That's sort of along the California, Nevada border. And Death Valley and through Las Vegas and on down passed Scottsdale and headed to the south, all of that region is getting wider.

You'll also see looking farther down, past San Diego and Tijuana down there, that we get down to water. We get down to very pretty beaches, plus fun places that you could go diving. And what you find is that this pull-apart action of Death Valley is running all the way down to the ocean, down to the Gulf of California, down past Baja there. This whole area is being pulled apart. Baja, that piece out on the left, and Mexico as a whole are pulling apart. The same spreading that tends to open Death Valley is opening this whole area.

And so we've drawn in here, you can see a circle up by Death Valley and then a green line that runs down to the Gulf of California. And the Gulf of California actually is getting wider. It's unzipping. It's open.

Given enough time, it possibly will open all the way up to Death Valley. It is conceivable that you could, over time, actually have the ocean extend on up into the central part of the West there. We're not sure if it will get there or not, but that's what's going on. Is that whole thing is measured to be unzipping, with Baja moving out towards the Pacific, Mexico moving over towards the Atlantic, the space in between, the ocean basin getting larger.

Now, this is part of a much bigger phenomenon. As you might possibly imagine, if it's going— we started in a cool place, but you start moving things around on the continent. And if I move here, it's going to bump my neighbor and bump the other neighbor, you sort of get the idea that this must extend much farther along. And so what you find is this sort of spreading behavior. This pulling apart behavior happens not only at Death Valley, not only where Baja is moving away from the mainland of Mexico, across the Gulf of California, but it actually happens through the world's oceans.

The places that they pull apart from, it turns out, tend to be raised a little bit. We know that there's these big mountains next to the valley at Death Valley. And so there's high stuff associated with that.

If you go down under the Gulf of California, the center where it's pulling apart is a little bit higher than the rest of the sea floor. It's sort of arched up a little bit. And so we talk about spreading from ridges. And so that's spreading ridges. And so we find a lot of places in the oceans have these places that it's being pulled apart from, and they're a little bit high. OK?

If you played baseball, or if you play tennis— tennis would work too— but you'll find that the ball has a seam on it. And that seam sort of wraps around it. And we often say that you can follow these spreading ridges of the earth, and they wrap around the planet like the seam of a baseball. And they're raised a little bit like the seam of a baseball.

And so here is a map. And the black lines— you'll see one black line disappearing up there, pointing towards Death Valley. And then you'll see the black lines wrapped around the rest of the planet, primarily running through the oceans. If you look very carefully, way over on the far right of the picture, you can see where one black line sort of points down into Africa, towards the great African rifts.

And so there are other places that these black lines, these spreading ridges are trying to tear a continent apart, the same as is happening up in Death Valley. But for the most part, they're out in the oceans, wrapping around, and again, sort of like the seam of the baseball. If you're not a Mets fan, well, just imagine that that's whatever team you like to cheer for.

You will see that these black lines look a little bit like the edges of jigsaw puzzle pieces. And so you can sort of see between the black lines what you might think of as being jigsaw puzzle pieces. These are plates. And you'll see they're labeled in there, the Antarctic plate, the Pacific plate, the Cocos plate, and so on. These plates are going to be very important in our story of how the world works here, as we try to put this together.

You also see the Ring of Fire all the way around the Pacific. At the edge of the plates there's volcanoes. That's sort of fiery. And so that gives us a Ring of Fire. That is something we will revisit a little bit later. OK?

Where we started again was in Death Valley, where you can see one of these spreading ridges disappearing and trying to unzip the continent. And then you can follow that out and then on around. And these are some of the plate boundaries.

We will see, in a little bit, that while plates, these big chunks, the jigsaw puzzle pieces often have a spreading ridge at the edge. Sometimes they will have something else at the edge. And so some of the plate boundaries are shown by the red lines in here, but there are other sorts of plate boundaries that we will revisit.

And there's your baseball. And again, I can't say I grew up a Mets fan. I'm from Ohio, and so it's Cincinnati or Cleveland. But that's life.

All right, now, if you go to Death Valley, you find there's volcanoes. If you go to the center of the Gulf of California, you'll find it's hot along that spreading ridge. And in fact, there's a volcano there. There's melted rock, leaks up along that crack.

And as Baja moves away from Mexico, melted rock will sneak up along that crack and freeze. And then they move away. And more melted rock sneaks up and freezes, and it moves away. And more melted rock--

And so that should tell you one thing. It's hot down below, because there's heat coming up from there to drive the volcanoes. Certainly, if you see a volcano at Death Valley, there has to be a heat source. And so we're going to talk a little bit about heat. Because it turns out that all of this action on top is being driven by heat from below. So let's take a quick look at what is heat and how it moves around.

I'm sure, for some of you, this is really old hat. Some of you may have not encountered this before, and I hope you get something interesting out of it. The normal way we look at it, if you were able to look inside of me very, very careful, you could see all the little pieces of which I'm made are vibrating. There is heat in me. There's vibration going on in the atoms that make me up.

Atoms, as you know, smallest units of matter that are sort of recognizable as something. If you started to— please don't do this— if you started to break me up into pieces, using chemistry, using fire, using other tools that you have available to you, you could make smaller and smaller and smaller pieces down to some limit. Below that, if you wanted to break it up more, you're going to need an atom smasher or something like that.

And so the things that you can get to with a fire or your stomach or something like that are atoms. And they have, we talk about different types. I'm made mostly of carbon and oxygen and hydrogen and a little nitrogen and a tiny bit of iron and other things.

And each one of those carbons, you could pull it out, and you could weigh it. And say, oh that's carbon. And that's because of how many pieces are in it, the little protons and neutrons. And so there's a little blurb in your textbook. If chemistry is completely foreign to you, it might be worthwhile to go back and read through in the textbook. If this is old hat, great. Don't worry about it.

Now, we will come back to some of that terminology. It is true that a carbon inside of me, one of them may be a little heavier than its neighbor. And so if you've never run into an isotope in your life, you might have a quick look at that. This is going to show up for us in just a little bit.

But again, if you've never had chemistry or you completely forgot your chemistry— Oh my goodness, don't tell me about chemistry— there's about a half a page or three-quarters of a page in the text. It will get you completely up to speed. So you might have a look at that.

Now, suppose we started making it cold. The vibration slows down. It slows down. At absolute zero the vibration stops, or very nearly. There's a little quantum stuff, but don't worry about that. OK. So as you warm from absolute zero, the stuff in me vibrates and that's heat. And that's there. And it's very good, because if it wasn't going on I would freeze and that would be the end of us.

OK. So now, suppose we have some heat. There's someplace that the molecules in me are vibrating like crazy. Does the heat always stay in me, or does it go somewhere else? And the answer is, it goes somewhere else. There are other ways to get things going.

And so if something cold were next to me, if you put an ice cube on my head, it will melt. The atoms in it will start vibrating faster. The atoms in me will vibrate slower. And so I will transfer heat to it.

And so there are ways that heat is moved around in things. How does it work? One way that heat gets moved around is by what we call radiation. The lights are shining on me. I'm picking up just a little warmth from the lights that are shining on me.

It gets here by radiation through space. Radiation works great. If you wanted to get a suntan, you go out and lie in the sun, and you soak up the sun. And then you get skin cancer. And so that's maybe not a good idea.

But our cats are very clearly solar powered. Now the radiation brings energy all the way from the sun, all the way down to the cat. And the cat just sits there and soaks it up. And then the cat goes and runs around like crazy at 2:00 in the morning. And so radiation moves heat around very well through space. And as noted, don't soak too much of it up, or when you're old and feeble, you won't be happy about that.

How else do you move heat around? Conduction. If you turn on the stove burner, and you let it get hot, then you hold your hand out to one side, you feel a little bit of heat coming off the side. That's radiation actually. If you take your finger and you touch the burner, you will notice very quickly that you feel a whole lot of heat. You're feeling that heat by conduction.

The atoms in the stove burner are vibrating really fast. And when you put the atoms in your finger against the atoms in the stove burner, the collisions between the two make the atoms in your finger vibrate really, really, really fast. And then they jump out of the places where they're supposed to be, and they sizzle and combine with oxygen and disappear in the atmosphere, and your finger burns up. So don't do that either.

That is really all it is though, conduction moving heat around by collisions between a fast one running into a slow one. And then they sort of share their speed. That works really well over short distances. You touch your finger to that stove burner, and immediately you'll notice it.

It doesn't work very well over space. If I vibrate really fast, I can't make a molecule on the moon vibrate faster. It doesn't even work really well over miles and miles. If I vibrate really fast, if I had a neighbor that I could shake, and the neighbor would shake the neighbor, by the time you try to go a few miles away, it's really hard to get your neighbor to shake the neighbor to shake the neighbor to go few miles away. And so conduction works really well over short distances, not over long distances.

Convection is where we're headed. This is sort of the third big way of moving heat around. And essentially this is just taking something hot and moving it.

If you cook dinner, and the dinner's on the stove, and it's hot, and you want hot food on the table, you could go bring the hot food over. You could wait for radiation to bring the heat. You could wait for conduction to bring the heat. But I think you're going to go get the food and bring it over. OK, that's advection. But convection is just sort of that. It's the natural way for hot things to move to somewhere else, taking the heat with them.

You've probably seen convection work if you've ever cooked spaghetti on the stove. You make it hot at the bottom. It expands. It's lower in density, then it rises. It gets to the top and it cools, and then it's going to sink again. But it can't sink where it's rising, so it gets out of the way and goes around in a loop.

Wherever you have soft solids, really soft, or liquids or gases, and you heat them and they expand, you end up with convection cells running after a bit. And it works in spaghetti. It works in spaghetti sauce, even though that's a little thicker. As you can imagine, if we're talking about it here, it's going to work in the planet as well.

So convection, heated material moves to a new place, carrying the heat with it. Primarily because hot things are less dense. The vibration, as you heat it up, it shakes around more. It shoves its neighbors away.

Now you've got the same amount of stuff taking up more space. And so you get a little lower mass in a place, it's less dense. It tends to rise. In case you forget what density was, it's mass divided by volume.

So what happens if you have a low density thing down, it tends to rise. If you have a high density thing up, it tends to sink, and they get organized. And that's not a terribly tough thing.

You've probably seen it in the atmosphere. What happens and what's very interesting is that over the times of geology, slow times, even rocks will do this if they're very warm. And so you go down in the earth, and it gets hotter and hotter. It's mostly solid, except in a couple of places. But most of what's down there is solid, but it's soft. And you know if a blacksmith takes a horseshoe and heats it almost up to the melting point, it can be deformed. It can be worked.

Well, if you get a huge mass of stuff and you heat it up, almost to melting, it gets soft. If you heat it at the bottom, it can convect, even though it's essentially solid. And that's a cool thing.

But if I had a chocolate bar in my pocket, I'd eat it right now and I'd be happier. But if I had a chocolate bar in my pocket, it would be fairly soft. As you warm it almost to melting it gets soft. And I could deform it. I could make it do things.

And so the same is true in the Earth. It's warmed almost to the melting point. It's soft. And because it's soft, it can do things.

So that's a convection cell, here's a picture. OK? Now we're going to very briefly sort of divide the world up for you. What you see here is a diagram planet. There's a much nicer diagram in your text that you might want to look at.

The planet is layered, as described in the text. The center is sort of an iron ball. It's got an outer core, which is liquid, and an inner core that's solid, that's basically iron. Around that there's this vast shell that's got some iron, but it's got some silicon and oxygen and a few other things. And that is soft. It's not melted, but it's almost melted. And so that's soft.

And then on top there's sort of a layer that's too cold to flow very well. It prefers to break rather than flowing. And we call that the lithosphere. And at the very top of it, especially the crust, has a little bit more silicon and oxygen and a little less iron than the rest. And it's got a little water in it, little [? us, ?] and some other things. So it's fine.

But anyway, here's the picture, if you can see it. And so you've got stirring going on down below. You'll see the surface of the earth labelled there. You'll see that this is only part of the earth. Yes, the whole planet is spherical. And you'll see the center marked down below there. Just so you know what we're looking at and what's going on.

The spreading ridge is right in the middle, on top there, where it says ridge. You'll notice that the convection cell comes up and it spreads. And as it does that, a little bit of melt will leak up and freeze and then get pulled away and leak up and freeze.

And so you'll get the volcanoes in Death Valley. And you will get the spreading and making of new seafloor at those places that allow the motion of Baja away from the mainland of Mexico and so on. The process here, we've got this cold stuff on top, the jigsaw puzzle pieces, we call those plates. And the moving around of these on the convection cells we call plate tectonics. It's a big, fancy word. It's a fun word. And it's written out there for you, in case you can't spell from my speaking.

So again, pull apart. We started in Death Valley. It's pulling apart. A little bit of melt leaks up. And you go south from Death Valley into the Gulf of California. Baja is moving away from Mexico. There's pull-apart going on. A little bit of melt leaks up. Things are pulling apart, and they're doing so riding on the back of giant convection cells. The stuff on top is just rafting around on what's underneath, with the ridges at Death Valley being where we started.

Heated rock rises, less dense, cools at the surface of the earth. Then you sink back, and this gives you the completion of the loop. And this gives us action. This gives us motion. This gives us drama.

You pull apart here, it's going to run into something somewhere else. We're going to have rifts and collisions and earthquakes and volcanoes and fun. And so you get ultimately the heat of the earth making this go on. Rising rocks push aside the cool rocks at the surface. The cool rocks travel sideways. And there will be interaction when cool rocks run into other cool rocks, which we have to look at.

Where does the heat come from? A little bit of it may be left over from when the planet formed. There's some other things going on. Mostly it's radioactivity.

If you get a rock, and you put it in a Geiger counter, you'll get occasional action. There's always a little bit of radioactive stuff. Now, not huge amounts. You usually don't get wiped out by the radioactivity in the rocks next to you.

But it's a big planet. There's a lot of radioactivity. There's a lot of rock. That makes heat.

And that heat is ultimately the big source of what drives the planet. There is a little left from the formation of the planet, a little bit from separation of the planet and freezing of things. But mostly it's radioactivity that does it.

As a wonderful story, if you're into history— don't get too bogged down on this— but if you're into history, the physicist Lord Kelvin says, OK, I'm a physicist. I'm smart. I know all this stuff. The earth started melting, and it's cooling off. And I measure how much heat is coming out of it. That tells me how old the earth is, 24 million years.

And the geologists said, oh, no, no, no, no. It's longer than that. It's lots longer than that. It's more than 100. It's— you got to go way back.

And he said, no, no, no, I'm a physicist. I have it right.

Well, he didn't know about radioactivity. He got it completely wrong, because his assumptions were entirely wrong. Darwin as well as the geologists are saying, come on, come on, we need a longer clock than this.

And actually it took the discovery of radioactivity. Marie Curie, who is shown here, and others find the energy source and say, no, there's heat down there. This will work. Your physics was right. Your assumptions were wrong. Something worth— and so geologists, we like this. So those who study the earth like this.

OK, so brief summary. Heat inside the air from radioactive decay, it drives convection cells. The cells move plates. The jigsaw pieces on top, where it's cold, they go rafting around on the surface of the planet. Where they pull apart, you get faults, you get spreading, you may get a little volcanoes leaking up. Death Valley is one. You get ocean spreading centers beyond that.

You're welcome to pause right now and take a deep breath. [BREATHING] If not, we'll go screaming right along. We're going to briefly visit another park.

And we will look at one of the implications of this. You might say, why should I care? Who cares what's going on here?

One of the reasons this matters is occasionally things that are moving get stuck, and then they move again. And when they move, they go fast. And if you're standing on top, your feet get knocked out from under you. And if you're a building, you fall down. And that's not good.

So we're going to go to Yellowstone. And we're going to look at— Yellowstone has everything. It has geysers, and it has bears, and it has beautiful trees and lakes, and it has earthquakes and volcanoes and river erosion.

Yellowstone has everything, the first national park. The first serious European Western exploration. People are there, and they've had earthquakes. The ground shook. That's cool. Sort of everything is here. It must be going on.

Yellowstone itself is a huge volcanic pit, a volcanic caldera left from giant eruptions. And there will be another one someday, with fairly high confidence. It's due, plus or minus 100,000 years, the next one is due. You can count 1.8, 1.2, and now 0.6, but we're due. Not to worry.

Those were big eruptions, really seriously big eruptions. But we're going to visit there and talk about earthquakes. You go to Yellowstone, and if you read into the history, back here a little bit, there was actually a beauty pageant going on at the Old Faithful Inn. I don't think they do this anymore.

And in the middle of the pageant, in the late 1950s, there was an earthquake. And there's this wonderful text you can read about it, in our textbook, about all these people watching the queen walking down the aisle. And the ground starts shaking. And they all screamed and ran out.

What had happened is just west of the park there was a big earthquake. And the picture you're looking at in front of you, the side that I'm standing on taking the picture, the side that the bottom of the tree is on, dropped about 20 feet relative to the other side. And in the middle of the night, it just went PFFT! And it does this huge motion.

Now you can imagine what happens. There's a hill there. And this is a serious hill. This is a couple-thousand foot high hill. And it drops, and it stops.

Well, if I had something balanced precariously on me, and I dropped, it falls down. What happened is the whole hillside fell down, and it killed a bunch of people. It was not very nice.

But there's these dramatic stories. When the hillside fell down, it's just slid across the river, and it pushed the air out of the way in front of it. And there's this story of this guy that had his clothes blown off of him in the middle of the night. He's sitting there in his tent and pfft— the things come. So it was truly an amazing thing.

You can see the pictures here. Those are big pine trees along the edge of that failure. So this entire hillside just fell off. It dammed the river. It bounced up the other side. This rock is towering over me. And this, the rock came down into the valley and several hundred feet up the other side, in the landslide that was caused by this earthquake.

Once it dams the river, it makes a lake. The Army Corps engineer says, boy, we got trouble. Because what happens, the water's going to fill up. It's going to go over the lake, go over the dam.

The dam is loose rocks. It will cut through the dam. There'll be a giant failure. It'll kill people. That happened 100 years ago, just outside the Grand Tetons. And so fortunately, the Army Corps engineers got in there real fast, and they fixed this sucker and things worked. You can still see the damage that was done by this earthquake.

There's great stories of this lady in this house. I think it was this house. The land sort of tilted. And the lake is filling up. And there was another lake just above it that had already been there.

And her house is getting flooded. And the waves are coming at her. And it's the middle of night, and she's running away. She made it. That's all right.

The earthquake shook up Yellowstone. It changed the timing of the geysers. It did all sorts of things.

And so there are earthquakes. And we know they matter. They matter in natural places. They kill people. They cause troubles.

This is a different earthquake. 1964, Alaska had a big one. And you can see one of the things that happened there.

We usually build buildings to stand up against gravity. We make them so they don't fall down. In an earthquake, they shouldn't fall sideways.

And we usually don't build things to be strong against falling sideways. Well, you notice what happened? Things moved, and that bridge truss there just fell sideways— pfft— it fell off. It's an interesting mess.

Here's another one. This one is from the World Series earthquake in California, '87, I believe it was. An again, sort of we build things not to fall down. But it shook, and it sort of slid off, and then it— So this isn't good. You really wouldn't want to drive your car over the middle span of that bridge right now.

And when you shake the ground and things move, you get cracks and that's not good. And it's not good for houses. And this one, the road is a little perched on these little— you put a real strong thing, and it sits on a post. And then you put a road on top of it, and then it shakes and whoop, the post just poked through the road. Whoop, I didn't design it that way, did I?

So earthquakes are a very bad thing when they happen. And this is also the World Series earthquake. You can see the road is not going to be useful for a little while there.

So what's going on with earthquakes? The land moves. We've seen that it pulls apart. If it's pulling apart somewhere, somewhere else it must be running together. And occasionally there's sliding passes that go on.

And so this is a diagram of the West. This is a diagram of what you might see going on for the World Series earthquake, for the San Francisco earthquake, for the San Andreas fault. What you have out there, in fact, is that there's sort of the far-western piece of California is headed for Alaska. And so it's sort of sliding along. And the join between that far western part and the rest of the country is the San Andreas fault.

So let's see if we can walk through this. We're going to see what happens when a fault get stuck and then it breaks. And what happens is it gets stuck for a while, and things sort of bend. And then eventually, they let go. And when they let go, things happen. And so let's see what we can do here with this.

OK, so we have plates. They're moving. The North American plate is probably what you're sitting on. If you're in Pennsylvania, for example, you're on the North American plate.

But the very far western part of California is out on the Pacific plate. There's a lot of motions. Everything is sort of going that way, and then there's a little of this.

But if we just focus on how they're moving compared to each other, the Pacific plate is headed for Alaska and the North American plate is not. And so there's a little of this slide pass going on. The break between them, the San Andreas fault. And so you can just take that blue line as the San Andreas and the Pacific plate moving relative to the North American plate.

Now let's start. You have a fence or a road or a building or something. You build it across the San Andreas fault, and then things are moving. OK?

So what's going to happen is the different sides are trying to move, but that fault is not letting go. You're trying to go. Maybe they're moving a little bit, but they're not going real far.

And so you actually may start things bending. Far away from the fault it's moved. Right at the fault it's stuck. The rocks, they sort of get locked around each other, and they're getting bent.

Well, eventually what's going to happen is that bend that we have circled there, something's going to break. You can't bend rocks forever. And when it breaks, [POP SOUND], OK? There you go.

And so you go, rrr, stop. And that snap is the earthquake. It's been caught on pumps or rumps or jumps in the rock. And then it breaks and it lets loose.

What happens then? If something snaps, these are big chunks of rock. I mean, sort of everything from San Francisco to LA just moved north by 20 feet. Now, if that happens, it sort of elbows its neighbor a little bit. And so other sorts of things are going to be going on with that snap. And it could get very interesting. And it will release energy.

And so when it goes 20 feet north, it shoves what's just north of that, and that shoves what's just north of that. And the truth is, on a big earthquake, the whole planet will get shaken a little bit. It's not just shaking yourself. It's shaking lots of other places.

Where are earthquakes? Well, the red dots here are earthquakes. This happens to be 1978 to 1987, fairly large quakes. And what you'll find is that these quakes actually outline the plates, the big tectonic plates. The jigsaw puzzle pieces moving around on the planet are pretty well outlined by these red dots, as you can see by comparing to this one.

And so if you sort of remember what this looks like and remember what this one looks like, you will be able to see, and you can compare them on your own time. You'll be able to see that the plates are primarily outlined by the earthquakes. The action is mostly at the edge. There's a few places in the middle that get more interesting, but not too many.

We have been looking at a couple things out there, the pull apart at Death Valley, the slide pass. If that is confusing you a little bit, there's sort of a little of both going on. There's a little of this sort of thing. And so, in fact, we're not telling you anything that's wrong, but we don't want to get this too complicated. So we'll try to keep it—

So now, the rocks shake. They shake their neighbors. They shake their neighbors. This sort of pushes things off. And so you get deformations that run away. And this makes waves, and the waves knock down buildings and do other sorts of things.

There's various kinds of waves. If you're hearing me, I am speaking and the sound is going to a microphone. And what I'm doing is essentially compressing and expanding the air. It's called a push wave or a sound wave or a P-wave. It very clearly goes through gases and liquids, as well as through solids. Because you can hear me, because it's going through the air and getting to the microphone.

There's another kind of wave. If you were to grab a rope and shake it, it sort of this kind of thing that— actually, imagine for a moment going to the football game, getting in the stadium, and doing the wave. Now what's a wave do? You're part of the wave. Yay, yay, yay. Your part of the wave is up and down.

But the wave moves around the stadium. So in that case what you're doing and what the waves are doing, you're going in different directions. That kind of wave is called a shear wave or an S-wave.

And it turns out that unless your neighbor really wants to do what you want your neighbor to do, they don't go through liquids or gases. Because if you move— yay— and your neighbor doesn't cooperate, you can't really grab your neighbor and grab your neighbor up. And so it turns out they do not go through liquids or gasses, oddly enough.

So two kinds of waves are generated. The P-wave, compressional push, they will go through liquids or gases. Sound waves, they are squeezing and then expanding. The shear waves don't go through water or air. They're more like doing the wave in the stadium. You go up and down, but the wave goes around.

Now, if you go look in the textbook you find out the earth is layered. And you find out the outer core is liquid. We say so. It's right in there. Why in heaven's name would we say a silly thing like this?

One thing that can happen, as you can see in the diagram down there, you make an earthquake. If you put out a listening device that listens for this kind of waves and this kind of waves, if you put it near the earthquake, you'll hear both kinds. P-waves and S-waves come winging by.

If you put your listening device on the other side of the planet from the earthquake, you won't hear any S-waves. You'll only hear P-waves. And no matter where the earthquake is, as long as your listening device is on the other side of the planet, there's a zone that doesn't get any S-waves. Now, S-waves won't go through liquid.

And so you start looking down there. And you can see the diagram. And what will happen is that any S-wave that hits that outer core disappears. It gets soaked up. It makes heat ultimately.

And so because every earthquake has a zone on the other side of the planet that there's no S-waves, there must be a ball of liquid in the middle of the planet that keeps the S-waves from getting there. So the seismic energy leaves the earthquake. It goes zipping through the planet. And if it's a big earthquake, you will hear it on the other side of the planet.

And we say that the outer core is liquid, because none of these S-waves or shear waves get through it. And we can look at the global map of earthquakes, listen, because now there's listening devices all around the planet. And the other side of the planet never gets one, so there must be a ball of liquid in the way.

S-waves don't go through the outer core which is liquid. Now, the textbook will tell you why we think there's a solid inner core in there that's even a little more complicated. But this much of it basically make sense.

Earthquakes, we talk about earthquakes. We say, wow, that was a nine, that was an eight, that was a seven. And we get really, really excited if it's an eight rather than a seven.

Why? Well, we sort of cheat in the way we do this. Sometimes scientists get lazy, and we really like talking about nine and eight, rather than 100 million and 10,000. And so we sort of play an interesting game in how we scale these things.

When you move up one in how big an earthquake was, that really means that the ground moves 10 times more. We just do that because it's easy. And it turns out that to move the ground 10 times more takes about 30 times more energy.

So if you have a one earthquake, it's not very much. A two earthquake actually is 10 times more ground motion and 30 times more energy. A three is 30 times more energy than two. A four is 30 times more energy than the three. A five is 30 times more energy than a four.

And by the time you get up to these big ones, they're bigger than all the nuclear arsenals of the world. They're really, really, really bad things. And they are out there occasionally.

What this turns out is that most of the action, most of the damage is in those few really big ones. There aren't many big ones, but they are so huge that they really dominate what happens. And so there's thousands and thousands of earthquakes, but you don't hear about them until that one rarity when, oh, the whole world shakes.

So quick summary, earthquakes mainly at plate boundaries, not entirely. Faults get stuck, then they suddenly slip. When that happens, the energy is spread away on waves that cause a lot of [? destruction. ?] You can measure them to get information about how the earth works. When we say that it was a little bit bigger, we mean it was a lot bigger. And most of the action is from the few big events.

And so we're started on this very interesting— interesting to us, I hope to you— this very interesting journey to understand why there are mountains. And what you can see is that the heat of the earth is stirring things. It's moving things around. It's causing things to come up and down, to stick and slip.

There's action. You can surely see that there's more action coming. And we will get to that more action next time.

Credit: Dr. Richard Alley

Want another look?

Check out the Unit 2 PowerPoint Presentation used in the online lecture here.