Click 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 1982. 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.
[MUSIC PLAYING - "RING OF FIRE" BY JOHNNY CASH]
By the great Johnny Cash.
[MUSIC PLAYING - "RING OF FIRE" BY 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.