Geology of the National Parks

Video Lecture


There are TWO Unit 3 lectures both featuring Dr. Sridhar Anandakrishnan.

Start with the first lecture. It is 40:50 minutes long.
Unit 3 lecture, part 1
Click Here for a transcript of the Unit 3 lecture, part 1.
Geosc 10 Unit 03 Lecture - Part 1

Welcome to G-Sci 10, Geology of the National Parks. My name is Sridhar Anandakrishnan, and I'll be your guide through some of the beautiful parts of the planet. Last time, we talked a little bit about Death Valley, about the spreading, about how Death Valley is going apart. This time, we're going to go slightly further north up along that same coast, go up to Crater Lake National Park, an absolutely gorgeous place, head on up the coast a little bit further, up to Mount St. Helens, go over and take a look at the Olympic National Park, and really understand why these volcanoes, Crater Lake, Mount St. Helens, why those two volcanoes are where they are.

And we're going to actually spread our picture out a little bit beyond that and look at why there are volcanoes in other parts of the world. All right? So, we'll start our talk about making mountains and making volcanoes, why they are where they are, and what they can tell us about the insides of the Earth.

Here is a screenshot that I took on my computer of a program called Google Earth. And I encourage all of you to go out there and download it. It's free. It runs on Macs. It runs on PCs. And it's a wonderful program for just looking at this planet. You can wander around to different parts. You can zoom in, zoom out. And I really encourage you to download it and to use it. It will really give you a good sense of how big this planet is that we live on.

I've circled in those sort of blue squiggly lines. They're not the best pictures in the world. But you can see that there are some places on the West Coast of the US. That's California on the bottom, Oregon in the middle, Washington state on top. And the circles represent the areas that we're going to go to, Crater Lake National Park on the bottom, Mount St. Helens in the middle. And then right up on the top, right up jammed up against the Pacific Coast is the Olympic National Park. So, as I said, go get yourself Google Earth, and you'll be able to zoom in to these spots yourself.

The other thing that you can do is you can go to your website, go onto Angel, and you'll be able to download what we call V-Trips. They're virtual field trips to these places and to other places. And they're just PowerPoint presentations or web page presentations that just have beautiful pictures of these places.

Let me take you to a couple of these places right now. We'll go take a look at Crater Lake. We'll take a look at Mount St. Helens. And then we'll come back and start the lecture proper.

I like to do this to motivate you, to say, man, these are beautiful places. I want to go see those, and I want to understand them. I want to know why there is a volcano there. So, let's hop out of this program real quick and go over and take a look at Crater Lake National Park. As I said, this is on Angel. You can see all this for yourself. I'm going to whip through it quickly. But you can find it on there.

So, this is just a shot looking down from the rim of Crater Lake through all these lovely trees, down onto that blue, blue, blue water. Crater Lake is this volcano way up high on the top of this mountain, up at 6,000 feet. And that water is as clear and as beautiful as anything.

It's just all snow melt that comes down into the crater. There are very few streams that bring in sediments into it, and no industry, no pollution, no agriculture. And so, that water is as blue and as beautiful as they get on this planet.

Here's a picture on the sides. Even though this was late in the summer, you get lots and lots of snowfall. As you can see, our friend there is wearing his T-shirt as he slides down the side of Crater Lake in the snow. Even though it's in the middle of summer, there's still lots of snow.

So, you pull back. Here's a picture of that crater. The bottom of it there is water.

You can see the far side of the crater. We're standing on the near rim. It's an almost circular feature with this little island called Wizard Island rising up in the middle of it. And then there's the far rim that you can see. And if I could pan, if this camera could pan around, you would see that rim continuing around all the way until it came back to where you are.

Absolutely gorgeous place way up high in the mountains, as I said, 6,000 feet. It's a spectacular place to go and see the world. Here's just another picture looking out over that blue, blue, beautiful water.

Here is a very garishly colored picture of the depth of the water. The purples are where the water is very, very deep. And the grey-white parts are the rim.

So, you can see that near-circular crater with Wizard Lake off on the left side there, that little dome that's starting to build. And then over in the top, there's another little dome starting to build. So, this has something to do with volcanoes. So, let's find out more.

And this is just a picture from Crater Lake looking along the Cascade ranges. And you can see in the distance one, two, three, four volcanoes, each of those perfect little cones marching up the coastline. Why? Why are there volcanoes there? Why aren't there any volcanoes here in central Pennsylvania? Why do they have four of them over there in Oregon in Washington?

That seems awfully unfair. They get all the volcanoes, and we don't get any. Let's find out why they've got them.

Here is a map of the California, Oregon, Washington coast. Each of those little triangles is a volcano. Lassen, Shasta, Medicine Lake, Crater Lake, that's the one that we're going to be talking about. And on the right, you have a little graphic depicting when they most recently have erupted.

As you probably might know, Mount St. Helens erupted in 1980. And that's marked on the right over there. Mount St. Helens has been very, very active, erupting many, many times over the course of the last 4,000 years.

And it's actually getting even more active even as we speak. The dome is starting to build today. And it might be due for an eruption in the next few years to tens of years.

Here's just a beautiful peek of Lassen Peak. It's not Crater Lake, but it's another volcanic peak. Whenever you see one of these nice triangular mountains, you should say to yourself, volcano. It's just a beautiful spot. And here's another view of Lassen Peak, looking at it from a slightly different place.

All right, here is a picture of a little car, a little 1980 Chevette going up the road to Lassen Peak. You can see all the snow there, even though it was the middle of August. I encourage you to go on to the website and just take this V-Trip. And it will encourage you to go there.

This, we went a little bit further afield. Those little white dots are sheep. When you think sheep, you should think New Zealand. And this is indeed a shot of Mount Ruapehu in New Zealand, another volcano on the other side of the Pacific Ocean.

All right, enough wandering around the world. Let's learn why those volcanoes are where they are. So, plate tectonics two, making mountains and volcanoes.

Here is a map of the world. And what we're looking at here is North America, and the US in the middle. The Pacific Ocean is on the left. And each of those little red dots is a source of volcanic activity.

And you should be able to see that those dots circle the Pacific Ocean. Sure, there's a few volcanoes in the Atlantic Ocean. Iceland in particular has quite a few. There's a few volcanoes that occur in Africa and Arabia. But most of them are around the circum Pacific, Australia, New Zealand, heading up the coast towards Japan, over to Alaska, and then down the US and Canadian coast, and then down through Central and South America.

Why? Why is it that these volcanoes are where they are? So, here's a graphic showing where all of these volcanoes are. And because all these volcanoes stretch right around the Pacific, we call it the Ring of Fire. It's a nice, evocative name.

There's another thing that happens that goes along with these volcanoes. There was a curious thing that people noticed is that if you were to take the water out of the Pacific, in the same place where you have all these volcanoes, you'd see these really deep ocean trenches. The ocean would go down 10,000, 20,000, 30,000, in some places even more, 40,000 feet. It's an extraordinary thing. It goes way, way down, as deep down into the ocean as mountains are high.

Why? Why are there trenches that go along with volcanoes, but no trenches anywhere else? That's the second thing people notice.

The third was that you get these big earthquakes in the same place. Volcanoes, deep trenches, big earthquakes, how are they all related? How do they all come together? And then the next thing is we're going to take a look at one of the great songs of all time.


By the great Johnny Cash.


So, I hope you enjoyed that. And you should be grateful that I did not try to sing that for you.

So, the overview that we're going to-- what we're going to try and go over in this class is ocean materials made of spreading ridges. We saw that last time, Death Valley, mid-ocean ridges. So, somehow ocean materials made at these mid-ocean spreading ridges, material comes up, freezes on, spreads off to the side, if you remember that from last time.

It collides with continental crust. So, all this material is being made in the oceans. It runs into the continent, and something's got to happen.

And what happens is one of two things. Either you get subduction, where that continental crust goes over, the oceanic crust goes under the continental crust. This material gets subducted.

Or you get accretion, where the continental crust and the oceanic crust run into each other, and they actually glom together to create new land. All right? So, those are the two things that can happen. Some of the side benefits, if you will, of that collision of either the subduction or the accretion is the creation of volcanoes and mountains and these deep oceanic trenches.

The third thing that we'll talk about this time is hot spots. And I'll get to that in a minute. Let's talk about subduction and accretion first.

To review, this you should remember from last time, the mantle is a deep part of the Earth, way down deep. You have those hot, relatively soft rocks that make up the asthenosphere. The upper part of the mantle and this crust are this rigid, more brittle mass of rocks called a lithosphere.

And the lithosphere, this upper part, is broken into a number of smaller plates. And these plates float over on top of the asthenosphere, and they will occasionally run into each other. Where they run into each other, that's where these mountains are made.

The convection cells, they bring up this mantle material. It freezes at these mid-ocean ridges. And then the ridges push material off to the side. So, the ocean is continually moving its crustal material off to the side.

That stuff has got to go somewhere. We don't live on an infinite plane. We live on this nice, round planet. So, if you push stuff off to the side, eventually it's got to hit something.

And this is my depiction of what the Earth looks like. If I took the Earth and took a great big hatchet and sliced through it, what I would find is I have the outer core, and then I have-- or the outer lithosphere. And then right in the middle of it, we would have the inner core, which was solid, the outer core that was liquid. And then right up at the surface, this is where we live. There's trees and houses and so on and so forth.

Right underneath that, we have the crust, which is part of the lithosphere. Underneath that, we have the mantle, which is this hot, soft rocks. And then right in the middle, you have the core, which is either solid right in the middle or liquid outside of it. So, this is a little bit a review of if I could take this hatchet and slice through the Earth and look at it from this side, this is what we think we would see.

We don't really know. Nobody's ever drilled into the Earth much deeper than a few miles, three or four miles. So, all of this is inferred from other evidence. But we think this is a pretty good picture of it. It's probably grossly simplified, but this is what we think the world looks like.

I'm going to redraw that picture here. Here we have the Earth again, sliced through. The upper part of it is called the lithosphere. Underneath that, you have the asthenosphere.

And that lithosphere is broken up into a number of little plates. I've just labeled them plate one, plate two, plate three, and so on. So, we've taken a cross-section through it. In your head, you have to imagine that this thing is a globe or a sphere, and those plates are all riding on top of the surface of that globe or sphere.

From the interior of the Earth, because of the heat inside of the Earth and the heating of the rocks that makes them low-density, that material rises up through the asthenosphere until it gets to the surface of the Earth, where it cools off and then sinks back down in these convection cells. All right? So, this should all be review if you remember it from last time.

But what goes on as a consequence of these mantle convection cells is that these plates, plate three there, for example, will slide off in one direction. And plate two will slide off in the other direction. And when plate two slides off, what happens to it? It runs into plate one. All right? You can see that plate one is in its way.

Plate two runs into plate one. Something's gotta happen there. And at that collision spot, that's where either subduction or accretion takes place.

So let's talk a little bit about this oceanic crust. We have these mantle convection cells, materials coming up from deep inside the Earth. It's deposited underneath the oceans. And it cools off, gets more dense, gets shoved off to the side. And this mantle material is basically basaltic.

Basalt is a mineral. It's heavy in silica, iron and magnesium. It's dark in color. It's relatively dense compared to continental crustal material.

Density is a key notion that I've talked about a little bit last time. I'm going to talk about it this time. You'll keep running into it over and over again during this class.

When something is dense, for its size, it's relatively heavy. So, if I have a bottle of water here, I can fill it up with water. And it will have a certain weight to it. And if I divide the weight by the volume of this bottle, I have its density.

I could pour this water out, fill it with olive oil, extra virgin olive oil, please, always. And it would weigh different. It wouldn't weigh the same as the water in here because oil has a different density than water does. You know this stuff, but it's worth emphasizing it anyway.

Ocean floor is mostly basaltic, formed at the ridges. Because we know the Earth isn't getting bigger, as this material comes up, it isn't as if there's this material that's coming from somewhere else. It's all being recycled. It comes up, and then it goes back down. The Earth stays the same size.

What is subduction? That's a word that you ought to write down, maybe look it up in a dictionary. To subduct means to go under. "Sub," under, "duct," I'm not quite sure where that "duct" comes from. I'm not an etymologist. But subduction means for oceanic crust, as it gets denser, as it cools down, moves away from the ridge, gets denser, it starts to sink back down into the mantle.

Think of a lava lamp. If you go onto the Angel website, there's a little movie there, it's a little bit goofy, but it's fun, in which I have a lava lamp. And I show you how that material in the lava lamp rises up as it heats up, and then it goes to the top, and then it sinks back down again. The reason it's sinking down again it's getting cool, meaning it's getting dense.

All right? So, that's what's going on there. And this is to remind you, this is the goofy movie that you will find on the website of the lava lamp.

As this cooling oceanic crust moves to the side, it'll run into a continent. It might run into a continent. It sometimes does, sometimes doesn't. But in the case of the western US, it does run into the continent. It runs into California, Oregon, and Washington.

So, this cooling Pacific oceanic crust is heading eastwards. And there, bang, in its way is California, Oregon, and Washington. They're sitting there in the way, and this oceanic crust runs into it.

If it is cold enough, if it is dense enough, if it is so dense that it wants to sink down, then it does sink down and goes underneath the oceanic crust, which is low density. And it bobs up high. The continental crust is low-density and bobs up high, and so, the oceanic crust, which is higher density, will go underneath it.

If it is still relatively warm, if it hasn't had time to cool down, only recently it was formed, and it runs into the continent, it's still buoyant. It still rides high up there, doesn't want to sink down. Then when the oceanic crust and the continental crust meet, then the oceanic crust doesn't sink down. It just stays up high, and you get what's called accretion. And we'll talk about that.

As this ocean crust sinks down, and I'll draw a picture of it for you in a second. But let me get the words out, and then we'll go back and look at this picture. As this ocean crust sinks down, it carries seawater and sediment with it as it goes underneath the continental crust.

As it sinks down, it starts to heat up again. Remember, the Earth is hot inside. What's the reason for it? You know it. We talked about it last time.

The reason it's hot is because of radioactive decay inside the planet. Inside the Earth, there's all of these radioactive atoms that are slowly splitting apart, giving up a little bit of heat. And so, the inside of this planet is warm.

So, as this oceanic crust sinks down, carrying its water, carrying all these sediments as it sinks down, and starts to heat up, this added water, these added sediments help that crust to melt. If all we had was that oceanic crust, you could heat it up. And you'd have to heat it way, way up before it would melt.

But because of all these impurities that have been added, the water and the sediment, that stuff melts really quickly. All right? Most things melt better in the presence of impurities. And this is an example of one of those.

So, let's take a look at a picture of that whole thing. Here's my cross-section through the ocean. I've drawn a mid-ocean ridge.

We have material coming up from underneath. It's being deposited at this mid-ocean ridge. That's that little peak over there.

And as it cools, it gets shoved off to the right. So we have material coming up, and material gets deposited, cools, and gets shoved off to the right. There's more material going off to the left, but lets only worry about the stuff that's going off to the right.

Here's my picture of the ocean. And there's a little boat. And there's a little fishy. So now you know that's the ocean. It's got a fishy in it. And the sea floor there is the black line.

And that material is physically moving off to the right. If I could go there and put a mark in it, I would come back a year later, and that mark would have moved a little tiny bit to the right, maybe half an inch, maybe a tenth of an inch. But it would have moved a little bit.

Now over here on the right, in the green, I've drawn the continent. So this is the coast of California. This is the coast of Oregon.

What's going to happen when that black line runs into that green line? The continent is low-density. The ocean is basalt. It's high-density.

When the two run into each other in this case, because the mid-ocean ridge is way far away, the basalt has had a lot of time to cool down. It's high-density. It wants to sink back down, and it will. It will subduct down underneath the continental crust.

And in the process of subducting-- I'm sorry, I had to draw a new picture here because my last one got so ugly. In the process of subducting, it will carry down material with it. That junction there that I've circled in yellow has water, and it has ocean sediments with it.

So two things happen at that junction. The first is that the continental crust and the oceanic crust get deformed to form these trenches. So without this motion of the oceanic crust going underneath the continental crust, we wouldn't have that little dip that you see, where the black and the green meet. That part would have just gone flat across.

But because of the motion, you can imagine in your head. This, my left hand over here that's wiggling, is the ocean crust. My right hand over here is the continental crust. And as I move my hand down, down, down, it drags and makes this little triangular indentation that is a mid-ocean ridge.

So, that's why we have trenches where we have subduction zones. That's what that is. This is a deep trench in the ocean floor. If that trench is near a continent, near water, near freshwater, rivers, sediment, that trench will fill up quickly with sediments because the continent will just dump all of this material into it. But if that trench is way out in the middle of the ocean, it won't fill up with sediments, and it'll be really, really deep.

Let's draw another picture of this because my pictures tend to get very messy. So, I just erase them and redraw them. Once again, we have this subducting ocean crust going under a continent. At that junction there, you'll have a trench.

But in addition to that, you'll have sediments and water that are carried down with that subducting trench. As that material goes down, it helps that basalt to melt. As it heats up, remember, as it's going down, down, down, it is getting hotter and hotter.

Eventually, it gets so hot that it melts. And when things melt, they get low in density. When things are low in density, they rise.

And that's what those little red circles are. It's molten material that's rising up through the continental crust because now you've melted this combination of basaltic rock, water, and sediment. You've heated it up. It's melted.

It's got nowhere to go because it's low-density, nowhere to go but straight up. And that's what it does. When it gets to the top, it builds a mountain. So, when this material gets the top, it builds a volcano.

That volcano there is Mount St. Helens. That volcano there is Crater Lake. That volcano there is Mount Ruapehu in New Zealand. It's Mount Fuji in Japan. It's Mount Pinatubo in the Philippines.

So, when you get subduction, you get these deep trenches being formed. You get this melt down there that rises up somewhere back from the coast, not right at the coast. But some distance back from the coast, that melt gets hot enough that it can rise up. And when it rises up, it forms a volcano. So, that's why these things go together, subduction, trenches, volcanoes, lines of volcanoes.

You can imagine in 3D that a coastline, that green line, is a cross-section through the west coast of California. But that's just one spot on the west coast of California. If I were to go a little bit down into the screen there, I'd be up in Oregon or up in Washington, and there would be another volcano and another one and another one. And that's why there's this line of volcanoes all up the coast of California, Oregon, and Washington.

And over time, these volcanoes get bigger and bigger because there's more and more material that's continually coming up and being deposited. Now they don't keep getting bigger forever because there's erosion that will continue to make them smaller. And this process continues to make them bigger. And that fight between building mountains and tearing down mountains is what geology is all about. You'll learn all about erosion later on.

So, here we have volcanic mountains at subduction zones. So, I hope this gives you a picture of it. And I know these drawings are very crude. But they're very, very helpful for me. If you want more information, you gotta read the online textbook.

This is Mount St. Helens. This is Lassen Peak. This is Crater Lake. This is Fuji. This is Mount Ruapehu. This is all of these subduction zone volcanoes around the Ring of Fire.

OK. I'm going to draw that 3-D picture for you. I'm not a good artist. But there's some folks at the US Geological Survey that are good artists. And they have shown this process where one plate is subducting under another plate. And now you can see it in 3D.

It isn't just a single spot the way I drew it on my rather crude picture. But it heads all the way up along that suture, that contact. Where those two plates run together, the one goes under, and you just get this series of volcanoes all up and down that contact. And you get these big old trenches along there.

This type of volcanic rock, the rock that comes up at a subduction zone and forms those volcanoes, is called andesitic. it's an andesite. It's named after the Andes Mountains. There's a very noticeable mixture of basalt and these sediments that forms this andesitic rock.

Now remember at the beginning, I said there were three things that go along with subduction zones. One was volcanoes. The second was these deep trenches. The third was earthquakes.

And the reason for the earthquakes is occasionally that that downgoing slab, that downgoing oceanic material, as it's sliding down underneath the continental crust, will get stuck and get stuck. And the downgoing slab is pushing, pushing. And then suddenly, it breaks loose. And you get these huge earthquakes because of the enormous forces.

And unfortunately, you get very tragic consequences. Because one of the things that happens when these earthquakes occur is it they displace a great deal of water in the oceans. And when those water waves hit land, if there's people that are standing there, they can and unfortunately are killed by the hundreds, the thousands, and in the case of that dreadful, dreadful tsunami in December of 2004 in Indonesia, killed by the hundreds of thousands. So, these are very, very dangerous things indeed.

Trenches, we already talked about trenches. When this slab is subducting, it deforms the overriding plate, and then we get these long, linear trenches. If these trenches are near land, they get filled up with sediment because there's lots and lots of junk, if you will, that's being washed off of the coast of Oregon, the coast of Washington, the coast of California.

All those rivers are carrying lots and lots of sediments. And they'll fill up those trenches quickly. But if they're out in the middle of the ocean, then they won't be filled up quite so quickly.

Those trenches in the middle of the ocean are some of the deepest places on the planet. The Marianas Trench is the deepest spot on the planet. It's as deep as Mount Everest is high. So, Mount Everest you know is 28,000 feet high. And the Marianas Trench is actually a little bit deeper than that below the sea floor.

The trenches near continents, because of all this mud that gets washed out, tends to fill up more quickly. Marianas Trench is here, just off of the Philippines. And as I said, it's one of the deepest places on the planet.

There's a little bit of a side note here. I think I remember I mentioned last time this little symbol up in the corner there. That's my universal symbol that the choo-choo train is going off the tracks.

So, whenever you see that, you know that you can not pay attention for a minute. You can stretch. But the first, the best ever science fiction, is Jules Verne's 20,000 Leagues Under the Sea. That was a novel by Jules Verne. 20,000 leagues, that's about 60,000 miles. Jules Verne was referring to the length of the voyage that the submarine, the Nautilus made with Captain Nemo at the helm. It's a great, great read. I strongly urge you to rush off and find it.

OK. So, here is once again our picture of the West Coast of the US. You have in the blue is the Pacific Ocean and the oceanic crust associated with it. It's moving off to the east. And then in the pink, you have the continental crust. And it's staying relatively stationary. And the two will collide. Where you have that collision, you get subduction. Or further up, you get accretion. This is important to remember. Continents are the lowest density, quote, "light." I put that in quotes because it's not the weight that matters, it's the weight of a particular given volume of the material. All right?

So, I can get a pound of feathers, or I can get a pound of gold. But a pound of feathers would fill this room, and a pound of gold would fit in the palm of my hand. So, it's not the total weight of something that matters. It's the weight per unit volume. Sea floor is the denser of the two. The basalt is a more dense material. Mantle is denser still. So, I gotta always remember, it's density that we care about. The core is the densest of all. It's mostly iron.

Accretion occurs when the sea floor runs into the continent, but it's still relatively low in density. It hasn't gotten so dense that it wants to sink down. It's quote, "light." It's low-density. And so, when it runs into the continent, it doesn't sink down. It doesn't give way. It actually smears onto, it's scraped off. Some of this sediment smears onto the continent. Next time, or a little ways down the road, we'll talk about something called abduction, when two continents collide, and man, neither of them really want to give way. And you get huge mountains built that way.

Here, the oceanic crust, it has some slightly higher density than the continental crust. And so, it sort of wants to go down. But it's still buoyant, and so, it kind of scrapes along underneath, just under the bottom of the continental crust. And then all of this sediment gets scraped off and spackled onto the side of the continent. So, it makes this messy, messy story that's really hard to interpret. But that's what geology is.

As a consequence of all of this, the rocks in the bottom of the ocean are never really that old. They're produced, and then they get subducted somewhere else far way. And because they're moving to the side, sure, they're moving slowly. They're only moving this much every year. And so, it takes them a long time to get any distance at all. But we got lots of time. We'll find out later on in this class that the Earth is really, really old. We've had lots and lots of time to do everything we want to do. And so, the ocean, it's produced over here. It's destroyed over there. And it might take 100 million years to do it. But eventually, all of the ocean floor will be destroyed and then recreated somewhere else.

Continents are very different. Continents are very light. They bob up. They stay up there. They never get recycled. Continents can be as old as four billion years. And we'll talk about how we know the age of rocks.

And here is a nice picture that, again, from the USGS, showing how as the Pacific plate goes down underneath the North American plate, some of the material gets scraped off and smeared onto the side. And the Olympic Mountains are produced during that scraping off process.

There's a V-Trip for the Olympics that you can go to and look at on the website. It's a beautiful spot, It rains a lot, as you probably know. It's near Seattle. And Seattle, as you probably know, is a rainy spot. So, it rains a lot over there. But as a consequence, you get lots of beautiful wildlife, lots of beautiful greenery, huge trees. And then you've got these mountains in the background. So, I urge you to go and run this V-Trip and have a look at it.

It's a beautiful thing. As I said before, when seafloor runs into these continents, the sediments get scraped off. And the Olympic National Park, where this V-Trip was taken, is a good example of that.

So, to review, the mantle is hot. The mantle is deep down inside the Earth. It's hot. You make these convection cells. Right up at the top, the upper part of the Earth called the crust and the upper part of the mantle together make up the lithosphere that's broken up into a few, about eight, major plates and one or two or a few smaller ones. And these plates float around on top of the asthenosphere. When plates meet, one of three things can happen. Either the plates run into each other, either the plates are moving away from each other, or the plates are just sliding past each other. And there's different geology that happens at each of those intersections.

What we've been talking about today is where plates run into each other. When they run into each other, if it's an ocean plate running into a continental plate, then you get subduction or accretion. And in other cases, you have other behavior. Heat from radioactive decay drives the whole thing. Thank you.

Credit: Dr. Sridhar Anandakrishnan

Now watch the second lecture (36:46 minutes long).

Unit 3 Lecture, Part 2
Click Here for transcript of the Unit 3 Lecture, Part 2.

Welcome to Geosc 10, geologies and national parks. My name is Sridhar Anandakrishnan, and I'll be your guide through volcanoes this time.

This is the second part of building mountains using subduction zone processes. Last time, we talked about what a subduction zone is, what happens when you have oceanic crust running into continental crust— you produce these volcanoes, and trenches, and earthquakes. This time we're going to go into a little bit more detail about those volcanoes. In addition, we're going to talk about a special thing called hot spots— Hawaii's a hot spot. We'll find out what a hot spot is and what's special about them.

All right. So, let's begin with a little bit of a tour of the Pacific Northwest. We saw some beautiful photographs last time of Crater Lake National Park and some of the extraordinary pictures of the destruction that went along with Mount St. Helens. This time, we'll go to a little bit more of a bucolic scene, sea shore, of Olympic National Park. Not quite so violent, but still telling us a lot about the processes that are going on.

What you're seeing on the screen here is a map of the West coast of the US— that's the State of Washington there in the middle. On the left side is Vancouver Island. Vancouver is in British Columbia, which is in Canada. If we zoom in a little bit into the State of Washington, we come close to Seattle where Boeing and Microsoft famously are headquartered. Starbucks is headquartered. I'm sure you all had your triple latte frappuccino this morning. Well, it all started in Seattle.

To the west of Seattle, you have the Olympic Peninsula. That's this big sort of rectangular-shaped mass of rocks, and mountains, and glaciers that is heavily forested, very wet, and just a beautiful place to go hiking or just simply driving around. It's also very, very heavily logged. That's the other thing to the Pacific Northwest is famous for is their logging industry —timber, paper, all those sorts of things are enormously lucrative for the Pacific Northwest. And so, some of these places that look very beautiful from the road, you go walking back a little ways, and then some timber company's clear-cut the place. So, it's a trade-off.

The Olympic Peninsula is an accretionary terrane. The sea floor has scraped off onto the North American coast. As it was scraped off and plastered against the North American coast, it created these mountains, and we're going to take a look at some of those rocks over here.

This is Mount Olympus right in the middle. Those white lines are the mountains and the snow on top of those mountains. And then, you have these deep valleys, river valleys, that cut through them, and you get enormous amounts of rainfall and snowfall there. And surprisingly, it's somewhat temperate. You get almost semi-subtropical types of plants there just because the amount of water. You get a rainforest type of environment going on over there. So, it's an extraordinary place.

We're going to switch now over to look at some photographs from the Olympic Peninsula. Here are some gulls and starfish on a big old beach rock. You can see the rock is kind of dark colored. The beach itself, the sand there is very dark in color. All of this is because the oceanic rocks that make up the Olympic Peninsula are basalts, if you remember that from last time. Basalts are dark in color and slightly higher in density, but the coloration comes from the fact that the Olympic Peninsula is made up of oceanic crust.

Here you can see the Pacific Ocean off in the distance, and all of these big, jagged rocks sticking up underneath. There's some huge cracks that are left behind and the tide smashing up against it and the big waves. You can see a big wave breaking up against that rock. It's just a great spot to wander the beach and do some sea kayaking or simply lie there on the beach.

If you walk a little bit inland, you'll go into the Hoh Rainforest where you get these enormous slugs six, eight inches long-- banana slugs. They're vast, really quite interesting looking things. Some people are repelled by them, some people are fascinated by them. You've got to go see them for yourself. It's very wet. It just rains and rains and rains constantly. You get these ferns, and you get these big waterfalls going through there. It's a lovely spot.

Here's another photograph of all the mosses and lichen hanging off of the trees. And because of all that rainfall, you have all these streams which come cascading off of the big black rocks that make up the Olympic Peninsula, and you have to make sure you take your raincoat and your rain gear and everything else with you because you're guaranteed to get rained upon on the Olympic Peninsula.

Here's another beautiful shot of a nice waterfall and some lichen and moss on it.

This is a photograph pulled back, and now we're looking at the higher peaks in the interior. This is a shot looking up towards Mount Olympus right in the middle. You can see some snow patches. And if you go right up to the top, Blue Glacier is a well-studied glacier. University of Washington folks have had a long-term project. They've got a hut up there and you can hike up to the hut and you can do all sorts of studies on the glaciology of that glacier. And here you have a deer sleeping in the underbrush looking a little suspiciously at the photographer.

Here's some kids playing on the beach. You can see, again, the very black color of the sand, an indication of the basaltic nature of the peninsula. Here you have these lighter colored material, which these are sedimentary rocks. These are material that came off of land, was deposited in the ocean. And then because of these huge forces involved, the collision of the Pacific plate and the Continental plate, these forces have turned these rocks up on their side and deformed them, and tortured them like this— broke them up into— and you see a big crack running through them. It's all very indicative of all the very large forces involved in producing the peninsula.

This is a close-up of some of those rocks. These are a very special type of rock called turbidites where you have material that goes whooshing down and jumbles up all the sands and gravels and so on, and you're left with a very layered rock. It comes out like that. And just the beautiful Pacific sunset with a gull flying off in the distance. Well worth a visit. If you're in Seattle anyway to go to the Space Needle or have a cappuccino at Starbucks, take a drive out to the Olympic Peninsula.

This slide by now should start to make sense to you. It's the first time we've seen it. We hopefully have put everything together by now. This is a cross section, a very idealized cross section, through the ocean on the left, and a continent on the right. You can think of this as the coast of Washington State or the coast of Oregon State, but it really represents a more generalized situation. This could be the Philippines, this could be Japan, it could be Alaska, it could be one of very many places.

In the middle, you have an oceanic spreading ridge where hot rock comes up, meets the surface, cools off, is shoved off to the right and to the left. When that material, that oceanic crustal material, this basaltic material cools off, it gets much, much more dense. As it gets shoved off to the right, it collides with the continental crust, which is much less dense. And during the collision you get subduction of the oceanic crust underneath the continental crust.

In the process of the subduction, some of the seawater and sediments get carried down with the subducting crust, the subducting plate. And as they get carried down, they melt and they come up as a volcano. That's all going on to the right of the screen over there. And we're going to look into this in a little bit more detail.

When basalt, water, and sediments heat up, they melt. The basalts by themselves don't really want to melt, but you mix in this water, you mix in the sediments, and at relatively low temperatures that mixture starts to melt. If you didn't have the seawater, if you didn't have the sediments mixed in with it, that subducting plate would have to go much, much, much deeper before it started to melt. But because the subduction process has taken the seawater, and taken the sediment and grabbed it, and pulled it down along with it, this mixture melts more readily.

And you've seen this yourself when you take snow out on the streets of State College, or whatever town that you're in. You mix salt with it. You mix these impurities in with it. It'll melt at a much lower temperature and you can drive your car over it. So, most materials, if you make them less pure, if you mix in all these other chemicals with it, like the sediments and like the water, they'll tend to melt more readily, and that's what happens with this.

That mixture, once it's molten, now has become much less dense, and it will rise up to form a volcano. That's what's going on in the left of this image over here. This subducting plate has carried seawater and sediments down with it, and at a relatively shallow depth because of that mixture, because of those impurities that are mixed in to basalt, you get melting, and that starts to rise up to form a volcano. That's the material, that's the picture along the right-hand side of the image.

So, as this mixed basalt, seawater, sediment mass has melted and it rises up, it forms andesitic volcanoes. It was named after the Andes Mountains and is just a type of rock that you find in the Andes. It turns out it's the same type of rock that you have in the Cascades, that you have in Mount Ruapehu in New Zealand, that you have in Alaska in the volcanic belts. It's very typical of these subduction zone volcanoes, so, we all call them all andesitic volcanoes.

As this magma rises up, it wants to polymerize— polymerize is a word you need to know. All it means is turning into these long chains or bonds of material. It wants to clump together to turn into this solid material. That's all that rock does as it goes from being molten to being solid is it's polymerizing. It's turning into a solid by making these long chains, these long compounds of the chemicals. It makes these long stringy units, sort of like lumpy oatmeal.

As this magma comes up, it comes up the top, it freezes into these long ropy, stringy lumps. And more material comes up, it comes out the top of the volcano, it immediately freezes on to the side, and you get these very steep-sided volcanoes. I'm going to draw you a picture now. I'm going to switch over and take a look at this process.

This is our subducting slab. On the left, we have the ocean, as I keep saying, and on the right we have continental crust. And the subducting slab is taking along with it some sediments and water. At a relatively shallow depth, this mixture of basaltic rock, which is the slab, of sediments and of water will start to melt. It will melt, become less dense molten rock, and it will rise up. So, this molten rock is rising up to the surface.

When it gets to the surface, it will polymerize, which is simply a fancy word for freeze, turn into a rock, turn solid, turn into a polymer. As more material comes out, it will come out the top of the volcano, and spill down the sides. I'm going to zoom in on this and draw it on the next page. This is the surface of the earth. You have this molten rock coming up. It polymerizes at the surface. It just freezes on. More stuff pushes through that, comes to the surface, spills down, and freezes on. And more stuff pushes through that, spills down the sides, and freezes on. And more stuff. Eventually you build a tall, steep volcano, known as a stratovolcano. V-O-L-C-A-N-O. Stratovolcano means tall, steep volcano. So, this is the process by which all of the subduction zone volcanoes are made.

You have molten rock, which is formed deep inside the earth where that subducting slab off the bottom of the screen there. The subducting slab has melted because of the addition of the sediment and the seawater. You have this molten material. It rises up. Where it breaks through the crust, it freezes on in the air. And when it freezes on, more stuff breaks through that and spills down the side, and over and over and over and over again. And you end up with these tall, steep stratovolcanoes. And this process goes on and on and on.

However, occasionally what happens is you get this stratovolcano. For example Mount St. Helens, and you have some hot, molten material rising up through it. So, this is molten rock. Let's see if we can't spell rock properly. Molten andesitic rock, it's rising up through there. It gets to the top, and there's a cap on top of this volcano that prevents the molten rock from getting out. So this rock is rising up, and for one reason or another you have a hard, impervious cap on top of the volcano, and this molten rock is trapped underneath this cap. And in the case of Mount St. Helens, more and more material is rising up underneath there, and this cap was bulging and getting pressurized.

So, let's switch back briefly to the PowerPoint presentation and we'll discuss how this then leads to those catastrophic Mount St. Helens explosive volcanic eruptions.

The magma, as this molten rock, as it's rising doesn't freeze within the earth. It stays liquid inside the earth. It's only when it gets to the surface that the CO2 and water that's inside escapes up out of the magma, and that's when it freezes on. And that's why you get these steep volcanoes. The stuff has molten all the way up. As it gets near the surface, the water and CO2 are released from it into the atmosphere, and then what's left behind is plain old basalt, and it freezes on, and you have a nice, wonderful steep volcano.

Sometimes the rock forms a cap, and when that happens, the pressure starts to build. You've got this magma coming up inside, you've got a cap over the thing, so the steam, the CO2 can escape, you're starting to build to a disaster. That's what happens with the stratovolcanoes. It's like releasing a cork on a champagne bottle, or on a Coke bottle, if you're under 21. Pressure, the CO2 is dissolved into the Coke. And when you pop the top on that, it fizzes out. It's all liquid in there as long as it's under pressure. You've got a cap on that Coke bottle. As soon as you pop that Coke bottle, it fizzes out all over your hand if you've shaken it up a little bit.

That's a very small example, but that's exactly what goes on with the volcano. You've got a cap on it. The magma's coming up from underneath, the CO2 and the water can't escape, the pressure builds, the pressure builds, the pressure builds. Something happens to crack the cap. In the case of your Coke or Pepsi bottle, you simply pop the pop top and that's it. In the case of Mount St. Helens, a small earthquake cracked the cap, and the CO2 and the water came shooting out, the pressure dropped and everything exploded. Mount St. Helens, 20th of May 1980. Coke bottle writ large. Champagne bottle, disastrous champagne bottle. This is another picture of Mount St. Helens again. Just a massive destructive force.

The magma contains water and CO2. If there's a cap over, it can't get out, the pressure builds, something comes along. A small earthquake that cracks that cap, and now suddenly all of that pressure that's been built up starts to get released. And then it just goes on and on, releases more, which opens the cracks some more, release some more, open the cap some more, and the whole thing takes off and you get this huge explosion.

That's all it is. It's relatively simple. You can't predict perfectly when it's going to happen, but it happened over and over and over again. Mount St. Helen's has exploded a number of times the last 1,000 years. All the Cascade volcanoes have. And so you have this cycle of pressure building up and an explosion. In the case of Crater Lake, that explosion was so enormous it just blew the whole top of that mountain off and all that's left is that big, round lake.

There's a different kind of volcano, one that isn't quite so destructive. These are the hot spot volcanoes. These are the volcanoes of Hawaii. Hawaii's the best example of them. There's a few of them around the world, but Hawaii is the best example of them. It's the one that's easiest to get to, it's the one that has a national park associated with it. Sometimes a plume of magma will come up from deep inside the asthenosphere.

Now, this is not this same as a subduction zone. If you remember, subduction zone volcanoes, the stratovolcanoes, was when you had oceanic crust going down. And at a relatively shallow depth, because of the mixture of water and sediments, that basalt would melt and it would come up. The hot spot volcano's very different. In a hot spot volcano, that material is coming from deep inside, hundreds, maybe even 1,000 kilometers inside the asthenosphere. Maybe even as deep as the core-mantle boundary.

We really don't understand hot spot volcanoes very well. They're one of the really most enigmatic features of the natural system today for volcanologists and for Earth scientists. So, they're really a fascinating thing. Why are they produced so deep inside in those places are not in other places? We don't really know.

But for some reason, deep down inside the asthenosphere, this plume of magma comes up, and this magma, because it comes from so deep inside the Earth, picks up a lot of iron. And that iron also prevents polymerization. It keeps the magma molten. Unlike stratovolcanoes, iron can't escape. With the stratovolcanoes, CO2 and water could escape and magma would freeze instantly. With these hot spot volcanoes, they come up, the iron can't go anywhere. The iron's not going to just evaporate and disappear into the atmosphere.

And so, even after the magma comes up to the surface, you still have it spreading out. It doesn't freeze instantly. You've probably seen pictures of Hawaiian big lava flows that spread for miles and miles, big rivers of lava that just spread for miles and miles. They haven't frozen on. And the reason they haven't frozen is because you have so much iron mixed in with them that they stay molten even at really low temperatures.

So, these lava flows stay molten long, long— even though they're up in the cold air, they still stay molten because of all this iron that's mixed in with them. And in the end, you end up with these broad, gentle sloping mountains known as shield volcanoes. Stratovolcano's are this big, steep ones. Why? Because the magma comes up, water and CO2 escapes, and the magma instantly polymerizes, and you just get these steep volcanoes because the magma can't run very far away from the mouth of the volcano.

In Hawaii, on the other hand, the magma comes up, it's got lots of iron in it because it comes from deep inside the Earth, comes up to the surface, comes into the cold air, and it still runs way off to the side before it finally freezes. And then more comes up and it spreads way off to the side, and you get these gentle, broad, large volcanoes, like a gladiator's shield. That's why they're called shield volcanoes. If you were to look at them in cross section, they'd be broad and rounded.

These hot spots are fixed in location inside the Earth. But as we talked about before, the surface of the earth is moving around all the time. These plates on the surface are sliding to east and to west all the time. But the hot spot itself is fixed. And so as the overriding plates goes by, the positions of volcano changes because the hot spot is fixed and location, but the overriding plate is moving. And so you get these series of volcanoes, one after the other. And I'll show a picture of that here.

What we're looking at in the left picture is a fixed hot spot. The material's just always coming up in the same spot over and over again. But the plate above it is moving. And as that plate moves, a new volcano comes up in a different spot. And then the plate moves again and a new volcano comes up, and the plate moves again and a new volcano comes up. Those older volcanoes slowly get eroded and disappear as they go off to the side.

And that's why Hawaii is this long chain of islands. You have the big island where there's current eruption going on, and then you have this whole chain of islands off to the northwest where older volcanoes have been carried off to the northwest and eroded one after the other. Hawaii is the best-known hot spot, but there's a whole bunch of them around the world. Iceland is another one, and the Galapagos Islands, and so on. So, as I said, they're very poorly understood, but they're very intriguing nonetheless.

This photograph here is showing that chain of islands. Hawaii is in the bottom left, and then you have this long line of volcanoes stretching off to the Northwest known as the Hawaiian Ridge. Midway Island and the emperor seamount chain, that will continue on, off to the north over there. And this is simply showing the Pacific plate as it moved off to the northwest carried volcano after volcano after volcano off into the distance.

So, hazards. Volcanoes are hazardous. We know that. You can see that. All the pictures that I've showed you of the ash, and the heat, and the blast. That's the biggest hazard. Don't be on a volcano. If there's a danger that it's going to explode, go somewhere else. That's the first thing you can do.

The second is you can't just go 100 feet away, or 1,000 feet away, or even 10 miles away, because this blast from the volcano can kill you even if you're 10 miles away. The gases are hot, 300 degrees plus, and that blast is fast— hundreds of miles per hour, it comes shooting down the side of the mountain so it'll knock you over. The gases are heavy, and so they flow along the ground. They don't go straight up. They flow along the ground. It's known as the nuee ardente, a glowing cloud in French.

There's a famous tragic example on the island of Montserrat where one of these clouds came blasting through and killed everybody in the town, except for one guy who was locked up in the basement of the prison, and the heavy, thick walls of it saved him because it didn't get as hot in there. But everybody else in the town was killed.

The second kind of hazard are the ashes and cinder. Those can spread much further away. Those can spread tens of miles, as much as 50 or 70 miles away from the central parts. These are the pyroclastic flows. All of the smaller pieces that spread out they can clog engines, that can bury houses and roads.

Finally, landslide and avalanche. All of the heat generated from this will melt glaciers and then you'll get floods that go along with them. So, these are some of the hazards associated with volcanoes.

Another hazard that's a little bit less well known, but is still quite dramatic, is the CO2. The carbon dioxide that comes up from one of these volcanoes can kill you in high enough concentrations. Lake Nyos in Cameroon in Africa is on a hot spot, and the lake itself was filled up with CO2 until a small earthquake triggered a turnover of the lake. All the CO2 escaped out all at once, and there was a couple of villages near the lake where everybody died because they were suffocated. There was too much carbon dioxide and not enough oxygen in the air, and they just died of suffocation.

So, CO2 seeped into lake. Something disturbed the lake and the CO2 escaped, and it suffocated hundreds of people that were living downhill from the lake. It was quite a tragic event back in the '80s. Nowadays, they just pump the CO2 out to keep it from building up, but this could happen in other places as well.

The third type of hazard, so you have all the hazards associated with living right near a volcano— nuee ardente, pyroclastic flows, those kinds of things. You have the second type of hazard, the CO2, the gases build up. The final type of hazard is a tsunami. A tsunami is a huge ocean wave that occurs when an earthquake occurs under water, and then that earthquake can generate a big ocean wave. A large volcano can also generate one of these huge ocean waves.

There was an island in Indonesia, Krakatoa, that blew up. And when Krakatoa exploded, it created this enormous cloud of dust and debris that travelled around the world. But another thing that happened was when that island exploded, because it was an island, there was a huge wave that was generated that spread out from Krakatoa that killed thousands and thousands of people.

The tsunami that was generated in 2004 was an earthquake-generated tsunami. An earthquake in a subduction zone let loose and generated a tsunami that spread across. We're going to show you a movie now, an animation, of what happens when that tsunami occurs.

This is an animation of the surface of the water in the Indian Ocean in the minutes and hours after that earthquake. As you can see, the effects of it spread outwards from the earthquake location. They do it relatively slowly, it took a couple of hours for it to spread right across the Indian Ocean, but for the island of Sumatra, which is right there— I'm going to run this animation again for you. For the island of Sumatra that's right there, the waves were enormous.

Let's take another look at this. This is right after the earthquake, and as that wave spreads out, it piles up against the island of Sumatra, and you get these huge waves that spread way inland. And this wave also spreads to the west across the Indian Ocean, and there's Indian [? Sulan ?] off to the left of the screen. And if this animation were to run for long enough, you would see that that wave would eventually hit the island of Sri Lanka and the coast of India.

We're going to run it one more time. And you can see the wave as it spreads out. There it goes, it hits the island of Sumatra, and the waves build up on the coast, and that's where 250,000 people died so tragically the day after Christmas, Boxing Day, December 26, 2004. It was a truly monumental disaster.

So, we need to make sure that we prevent these kinds of hazards, these kinds of disasters from happening. What happens is as the wave gets closer and closer to shore, the sea bottom is getting closer and closer to the surface, there's less room left over for that water. So, as this wave comes along, it builds up and builds up and it strikes with great force, flooding out villages, people, roads and infrastructure are gone. And once roads and infrastructure are gone, all the people that survived the direct effect of the wave, now have to deal with disease and hunger. Quite often the water wells are polluted, and so they don't have clean drinking water and it's just a disastrous situation.

So, next time we're going to talk about collision of continents. This time, we were talking about collisions of oceans with continental crust. Next time, we're going to talk about collision of two continents and what happens when two continents collide. Stay tuned.

Credit: Dr. Sridhar Anandakrishnan

Want another look?

Check out the Unit 3 PowerPoint Presentation used in the online lecture here.