The Science of Everything Podcast - Episode 111: Plate Tectonics

Episode Date: September 24, 2020

An introduction to the theory of plate tectonics, beginning with an overview of the internal structure of Earth, the differences between oceanic and continental crust, subduction and other forms of plate boundaries, the formation of volcanic arcs, the origin of the Earth's magnetic field, and the mechanisms underlying tectonic plate movement. The episode concludes with a summary of the varies lines of evidence in favour of continental drift, including seafloor spread, biogeography, and polar wandering. Recommended prelistening is Episode 74: Minerals and Rocks.   If you enjoyed the podcast please consider supporting the show by making a paypal donation or becoming a patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

listening to the Science of Everything podcast, episode 111, plate tectonics. And I'm your host, James Fodor. In this episode, we're going to discuss the theory of plate tectonics, which is the, I guess, underpinning theory that describes a wide range of phenomena in earth science, particularly the relationship of the continents and their movement about the Earth's mantle to volcanoes and the Earth's composition, the magnetic field. So it sort of all ties everything together in the, in geology. So it's a really foundational theory. And as foundational theories go, it's fairly new, dating back really to under the 1960s. So a lot of very interesting content we have to get through. In this episode, we're going to be talking about the Earth's internal composition, leading

into a discussion of the difference between oceanic and continental crust, discussion of continental plates and the boundaries between them. We'll look at the Earth's magnetic field and what causes it and how changes in the Earth's magnetic field over time gave rise to some of the most powerful evidence in favour of plate technonic theory, which leads us into the evidence in favour of plate technotics and for content of drift, and also a discussion of the mechanisms for plate movement. Recommended pre-listing for this episode is episode 74, Minerals and Rocks, which gives a bit of background about different types of rocks and minerals, which will be relevant for this episode. So, without further ado, let's make a start and begin by talking about the internal

composition of the Earth. It is not a very profound statement to say, that the Earth is quite large. And what is somewhat more interesting is to realize that the crust, the part of the Earth that, you know, we walk on and drive on and carry out agriculture on and so forth, is only a tiny thin, well, crust, are basically like a skin on the top of a very large, deep planet. So for a bit of scale here, the Earth's crust, so the outermost layer, extends somewhere between 7 to 60 kilometers down. It very much depends. on whether you're above the ocean or above land, and it varies with different parts of the Earth's surface, but, you know, let's say 7 to 60 kilometers, whereas the radius of the Earth, the Earth's

not a perfect sphere, so again, it varies a little bit, but it's something like 6,400 kilometers. So we're talking about 1% or even less of the thickness of the Earth is comprised of the crust. So nearly all of the volume of the Earth is stuff underneath the crust. and also we don't have a very good understanding of what's beneath the crust because we've never actually been able to drill down below the crust into deeper layers. So most of our understanding of what lies beneath the crust comes from indirect observations, including the behaviors of earthquakes, magnetic properties, and observing materials that have been released from the mantle and reached the surface. So there's a lot of putting pieces of the puzzle together to try to work out what's happening, given that we don't have any direct observation. of the inner layers of the earth. But nevertheless, over the decades,

we have built up a fairly good understanding of the general principles. So, let's start from the very middle, which is called the inner core. The inner core of the earth is believed to be composed of an iron-nickel alloy with some other elements mixed in. So it's basically a solid metal sphere.

The temperature at the surface of the inner core is estimated to be about 5,700 Kelvin, which is about the same as the temperature at the surface of the sun. So that is, I think, quite interesting that the center of the earth is about as hot as the surface of the sun. But the reason for that heat is quite different. The sun is heated by nuclear fusion, whereas the Earth is heated by a combination of residual

energy from the initial gravitational collapse and formation of the earth, plus heat released from radioactive decay of elements in the core. And as a general rule, as you move from the core further out to the surface, the temperature decreases. outside the inner core is what's called the outer core. The outer core, unlike the inner core, is fluid. And it consists of basically the same material, so iron and nickel. But the composition is thought to be a fluid rather than a solid.

And the outer core extends from roughly 5,100 to about 2,900 kilometers below the earth surface. So kind of a similar thickness to the inner core. it's thought that motion of charges around the inner core is what gives rise to the earth's magnetic field. So this region of the core is particularly important, but we'll come back to that a bit later in this episode. Above the outer core is what's called the mantle. Now, the mantle consists of a few different regions, and it gets a little bit confusing because there's one categorization that depends on the chemical composition of the materials that make up layer. And then there's another series of classifications that depends on the mechanical behavior of those materials.

And those aren't the same. So as an example, water and ice, like liquid water and solid water, have the same chemical composition, but their mechanical properties are very different in this case because they're in different states of matter. But there's other reasons that mechanical properties can differ, including temperature and pressure, which are the most relevant in terms of the internal composition. So it does get a little bit confusing because these classifications exist. alongside each other, like they don't neatly map onto each other. They're kind of just two different things. So let's focus first on the chemical composition of the mantle. In terms of the chemical

composition, there's a lower mantle and an upper mantle. The lower mantle lies directly above the outer core, from about, say, 2,900 up to about 900 kilometers below the earth's surface. The lower mantle consists of rock, of varying different compositions. And that's a bit vague because we don't understand its composition very well. The composition is thought to be quite complicated involving a variety of different elements, including iron, magnesium, aluminium, calcium, nickel, chromium, and so forth. But because it's so far beneath the earth surface, we don't really have a very good understanding of its composition. Above the lower mantle is, not surprisingly, the upper mantle. Its composition is better known than the lower mantle because we do have some samples of material

from the upper mantle that's managed to make its way to the surface. The upper mantle extends from something like 900 kilometers below the earth's surface up to below the crust. So that's somewhere between 10 to 35-ish kilometers below the surface, depending on whether you're under the ocean or under the continents. Like the lower mantle, the upper mantle is essentially made of rock, but it's rock that's under very high pressure and at a higher temperature than, obviously, it would be on the surface of the earth. because remember the temperature decreases as you move away from the core. The composition of the upper mantle is mostly mafic minerals, such as olivine and pyroxene. So if you recall from the episode on rocks and minerals, there's a distinction made between mafic and felsic rocks, and the difference there primarily relates to how enriched they are in the compound silica.

in particular mafic and ultra mafic rocks have a much lower silica content compared to felsic rocks which have a high silica content uh mafic rocks also tend to be darker in color and have a higher have a higher content of metals like iron magnesium and calcium they also melt at high temperatures so it's perhaps not surprising that the outer mantle is comprised more of mafic rocks given that it's closer to the metal rich core and also the fact that it is relatively higher temperatures, and so it's less like it to melt at those temperatures. So, again, it's important to remember that the mantle is comprised of rock. So it is solid rock. It's not melted, unless it's, you know, part of a volcano, but for the most part, it's solid.

The precise mechanical behavior, or I guess the physical properties of the material varies by different regions of the mantle. And that relates to what I said before, the difference between the chemical composition layers, where you've got upper and lower mantle, and the mechanical behavior layers. That's divided into essentially three. The mesosphere, the asthenosphere, and the lithosphere. Now here is where it gets a little bit confusing because, as I said, the layers don't map very neatly onto each other. So the lowest region of the mantle is called the mesosphere.

And the mesosphere is basically just defined as everything from the outer core up to the asthenosphere. And it's mostly the lower mantle, but also the lower part of the upper mantle. and it's not very well understood. As I mentioned before, the composition of the rocks in that region are quite variable and not well known. Now, the asthenosphere is very important, and we'll be talking about this quite a bit. The asthenosphere is the region where rocks are relatively plastic and are easily deformed, so they're kind of ductile, and therefore they bend and retain their shape. Now, it's important to understand this is not due to a difference in composition,

the mesosphere and the asthenosphere have a similar composition to each other, and are not defined in terms of their composition, but in terms of their physical property. So because of the reduced density and their reduced temperature and pressure of the asthenosphere compared to the mesosphere below, its physical properties are different. And this is very important, as we'll see, in relation to how the crust behaves, which is above the asthenosphere. The highest level, in terms of the mechanical properties, above the astinosphere, is the lithosphere. The lithosphere is distinguished from the asthenosphere by not its composition again, but by its physical properties.

So the lithosphere is rigid and brittle. So the rocks don't bend and deform. I mean, they can a little bit, but they're much more brittle and they move as kind of rigid chunks, as opposed to the underlying asthenosphere, which, again, we've said the rock is plastic and can deform more easily and is ductile and can bend. Now the confusing bit is that the lithosphere is not the same thing as the crust. And I'm going to emphasize this a number of times because it is confusing. The crust is defined as the outermost layer or region of the earth, which accounts for less than 1% of the Earth's volume,

so it's really quite small, as I mentioned before. And it's chemically distinct from the underlying mantle, because as I mentioned, the underlying mantle, at least the upper part of the mantle, is comprised of mafic and ultramathic rocks, mostly pyroxene and olivine minerals, compared to the crust, which has varying mineral composition depending on whether you're talking about oceanic or continental crust, and we'll get to that distinction in a moment. But the important point is that the crust is chemically distinct from the upper mantle, which in turn is chemically

distinct from the lower mantle below it, although we don't know the lower mantles composition very well. But the lithosphere includes both the crust and the earthership. And the earth is a uppermost part of the mantle. The uppermost part of the mantle obviously is chemically the same as the rest of the upper mantle, but it's different in its mechanical properties. And in particular, the uppermost part of the mantle is solid. So it's not plastic. It's not bendable and deformable like the as thinnospheries. And the boundary between the crust and the mantle is called the moho discontinuity. And it's defined in terms of a change in the in the velocity of seismic waves as they pass through that region of the earth.

We'll talk more about that when I do an episode on earthquake, so I'm not going to get into that here, but that's how it's defined. And what that represents is a change in the composition of the rocks. So, because this is a little bit confusing, let's just recap. Remember that we're talking about a sort of a jewelistic categorization here, with chemical composition on the one hand, mechanical behavior on the other hand. And they're basically the same in terms of the core. You've got the inner core for about a thousand kilometers, which is solid nickel and iron. Then you've got the outer core for another thousand and a bit kilometers. That's also nickel and iron, but liquid.

That gives rise to magnetic field of the earth. Then you've got the lower mantle for something like 2,000 kilometers, so that's the biggest part of the earth. This is made of rock, but we don't know the composition of the rock very well, although it seems to be, have fairly enriched in terms of metal content. Then above the lower mantle, we've got the upper mantle. and that consists of maithic and ultramafic rocks. The upper mantle gives way at the moho discontinuity to the crust, which is chemically distinct from the mantle because of the composition of the rocks, which vary from oceanic to continental crust.

Overlying that, you've got the fact that the upper mantle varies in composition. The very lowest part of the upper mantle is solid rock. Then you've got the astinosphere, which is near the top, but not quite at the top, and that is plastic and deformable, and we'll see it plays a very important role in movement of continents. And then the very uppermost part of the upper mantle, again, chemically the same as all of the other parts of the upper mantle, is solid. And then above that very uppermost part of the upper mantle, you've got the crust itself. And those two uppermost layers, the crust and the uppermost solid part of the upper mantle, form the lithosphere because they're both solid, compared to the asthenosphere below, which is plastic and deformable. Hopefully that's reasonably clear. I'll put up a diagram on the Facebook so that you can have a look at how these two sort of classification schemes relate to each other.

But now that we've got a basic understanding of that under our belts, we can now talk about the distinction and differences between oceanic and continental crust. Because most of what we're going to be focusing on here is plate tectonics, which relates to the behavior of the tectonic plates as they move about, basically over the upper part of the mantle, over the asthenosphere. And so we need to understand the nature of these plates and how continental and oceanic crust differ from each other. The major player in the theory of plate tectonics are, of course, the tectonic plates. So let's start with the basic question, what is a tectonic plate? A tectonic plate is a region or portion of the lithosphere, which is able to move over the underlying asthenosphere independently of other tectonic plates, so other regions of the lithosphere. And they move very slowly, so some of them don't really move at all, others up to a fraction of a centimeter annually, so maybe a tenth of a centimeter or a hundredth of a centimeter, something

like that, which obviously is very slow, but over geologic times it can add up to quite a lot. And so over very long periods of time, the continents on Earth move around and completely rearrange over a period of hundreds of millions of years. In this episode, I'm not going to discuss in detail how that's shaped the evolution of life and the history of the Earth, that will be in a different show. But you should be aware that in the past, all the continents have been clumped together, forming what are called supercontinents, the most recent of which was called Pangaea, which you may have heard of.

Today we have a relatively separated and spaced out arrangement of the continents. But the reason they're able to move is because of the tectonic plates and how they are able to move independently of each other. So it's kind of like Earth's lithosphere has been broken up into puzzle pieces, which are able to move around each other. although the puzzle pieces aren't permanent, they can collide and break apart over a very long periods of time. Now, currently, the Earth has something like seven or eight major plates and a bunch of minor ones. It does depend exactly how you define them, and sometimes the boundary between the plates is not very well defined.

But the key concept to understand about plates is that they're regions of the lithosphere that are broken apart from each other and can move independently, or at least somewhat independently. also each tectonic plate or at least most of them consists of two different types of crust connected to each other there's the continental crust and the oceanic crust that I mentioned that before and they're distinct both by their chemical composition and also where you find them on the earth's surface so the continental crust is comprised of igneous sedimentary and metamorphic rocks and it forms the geological continents and also areas of shallow sea bed close to

their shore. So continents in the geological sense don't line up exactly with the geographical continents because there's a continental shelf that extends a little bit beyond the shoreline of the continent. But I'm not going to worry too much about that for our purposes here. We'll talk about that more when we get to talk about the ocean. But an important property of the continental crust is that it's a lot thicker than the oceanic crust. So the thickness of the continental crust varies from about 25 to 70 kilometers. That's why when you give figures for how thick of the crust is, it varies by a lot, because the continental crust is quite variable in thickness. But it's uniformly a lot thicker than the oceanic crust, which is something like 7 to 10 kilometers thick.

So about three to 10 times thicker than the oceanic crust. About 40% of the Earth's surface is continental crust compared to about 70%, which is oceanic crust. The reason why continental crust is a lot thicker than oceanic crust relates to the fact that it's different in composition. So continental crust is comprised of relatively felsic rocks. Remember that means a high silica content and that means they have a relatively lower density compared to the oceanic crust which has relatively more mafic rocks, which again means lower silica content and a slightly higher density. If you recall, mentioned before that the mantle, or at least the outer mantle, was comprised mostly of ultramathic rocks, which have very low silica content and are quite dense and have a very high melting point. And so you can think of it, although this is not uniformly true, but you can think of it like the very densest rocks have sunk down to the upper mantle and below all of the other rocks. then on top of that are sitting the mafic and then the relatively felsic rocks forming the oceanic and continental crust respectively.

So obviously there's mixing involved as well because there are ultramafic rocks on the earth's surface. But as a general rule, that seems to have been how the rocks are divided, which makes sense, obviously, because the denser matter is going to sink closer to the Earth's center because of gravity. Because oceanic crust is denser, it sits lower on top of the underlying mantle. And conversely, continental crust, because it's less dense, it's higher. And that's why the oceans are on top of the oceanic crust. I mean, you might think it's in the name, but it sort of goes the other way around. The reason the oceans sit above oceanic crust is because they have high densities,

which means they sit lower down on the underlying mantle, and therefore being lower lying than the continental crust, the water on the earth's surface, which obviously seeks the lowest level possible, lies atop the oceanic crust. rather than the continental crust. Just so you have an idea about the major tectonic plates, I'll just briefly give a tour of some of the major ones, remembering that most plates are comprised both of regions of continental crust and also regions of oceanic crust, although there are some that are just one and some that is the other.

So there's a North American plate which corresponds pretty closely to the North American continent plus a bunch of the western part of the North Atlantic, although a bit confusingly the North American plate also includes a little bit of the easternmost part of Russia. There's also a South American plate, which lines up pretty well with South America, and also includes kind of half of the South Atlantic Ocean. There's an African plate, which is Africa plus regions of the ocean, kind of surround Africa. There's a Eurasian plate, which is a very large plate, which encompasses Europe and most of Asia, except that little bit that I mentioned in the Far Eastern Siberia, which is actually part of the North American plate,

and also India, which gets its own place. It's one of the smaller ones. There's also a plate which is called the Arabian Plate, which covers the Arabian Peninsula, and also the Persian Gulf. There's a Caribbean Plate, which is quite small and covers part of the Caribbean. There's an Australian plate, which covers Australia, part of New Zealand, probably New Guinea, and also much of the Indian Ocean and parts of the Southern Ocean as well. And there's also a Pacific Plate, which encompasses most of the Pacific Ocean, although not quite all of it. It abuts against the Western Coast of the North American Plate, which is,

what gives rise to the earthquakes that San Francisco is famous for. There are a few other places as well. Antarctica has a plate and there's some smaller ones, but that gives you some idea. So it's interesting that they generally align, broadly speaking, with the continents, you know, North and South America, Africa, Eurasia and Australia and Antarctica, although there are some exceptions like Caribbean, Plate, and Arabian Plate and Indian Plate and so on. And boundaries between plates are typically locations of relative, both tectonic and also volcanic activity. And this gives rise to what's called the Pacific Ring of Fire, which is the sort of ring that surrounds the Pacific from Japan, the Philippines, down through Solomon Islands, New Zealand, and then also the western coast of the US.

And these regions are sites of many highly active volcanoes and earthquake-prone regions, precisely because you've got the tectonic plates grinding up against each other. So, where does continental and oceanic crust come from? Well, ultimately, all the material, of course, comes from deeper in the earth, and much of it has already risen to the surface over, what, billions of years really. But ultimately, continental crust arises from the relatively high silica, so felsic rocks and minerals that have come up from the deeper parts of the earth. The main way in which new continental crust can be produced these days is when you have subduction, which we'll talk about later, but basically that means destruction by bearing itself into the mantle. of oceanic crust, which causes the lighter material to melt first. So you remember I said that felsic minerals, higher silica content, have a lower melting temperature, which means that they melt first as they are buried into the earth's into the mantle.

The melted rock rises up in plumes of magma, which then, if it comes into contact with the continent, will then eventually cool and sort of add to the content of crust. And so in this way you can get sort of differentiation of the relatively more felsic parts of relatively more felsic minerals of the oceanic crust. There's also the possibility of accretion through collision of continents together and also of volcanic islands that form in the ocean and then can be accreted onto the continents. So through these processes, the continental crust has been built up over hundreds of millions of years.

It's not known whether the overall amount of continental crust has been increasing over the last few hundred years or whether it's sort of fairly stable, at least in sort of in relatively recent geological time. Obviously, that's not something that happens in any humans scale. Now, unlike continental crust, which typically is very old, oceanic crust is always quite new. And when I say quite new, I mean, created within the last few hundred million years. The reason for this is because oceanic crust is constantly being sort of created

and then destroyed and recycled in a process called seafloor spreading. Now, we'll be discussing this a little bit more just momentarily. So I don't want to quite get into this at the moment, but just bear in mind that continental crust is generally quite old, billions of years old, much of it is billions of years old, and is probably on the whole not being continually created, whereas oceanic crust is typically quite young, only a few hundred million years old at most, is involved in a continual process of creation and destruction under the oceans. One final principle that I want to mention is the principle of isostasy, which refers to the gravitational equilibrium between the crust and the mass.

mantle. Remember that the earth's crust sits on top of the mantle, on top of the uppermost part of the mantle, which is also part of the lithosphere being solid, but all of that then sits on top of the asthenosphere, which is plastic and ductile, and is the underlying substrate that then the tectonic plates move about on. Isostasis refers to the fact that because lithosphere is essentially floating on top of the asthenosphere, then the thicker and less dense concedental crust will sit higher up than the thinner and more dense oceanic crust. And furthermore, it means that if there are thicker regions of the continental crust, like for example, because of mountain ranges, that will need to be balanced out by those parts of the continental crust extending further

down into the mantle, because basically if you have to support more weight at that part of the crust, then there needs to be more displaced mantle, just like whenever you sit anything on top of water that you're floating on water, the buoyancy force is equal to the mass of the displaced material. So if you want to support more floating mass, then you have to displace more material. So it's the same thing with the continental crust. Basically, if there's a mountain range, then if that region is in equilibrium, then that region of the crust must extend further into the mantle. Not all regions are always in isostatic equilibrium.

That is something that occurs over time. So if a mountain range forms, then that region will tend to sink over time further into the mantle. Or equivalently, if there's a region in which a glacier has melted, then weight has been removed in that region will gradually rise over time. And this is called glacial rebound, which is something that happens at the end of an ice age. So this is quite interesting, and so something that's maybe not obvious, that shapes the landscape over time. Having discussed the basic idea of what tectonic plates are and the difference between the oceanic and the continental crust and a bit about how they're formed,

now I want to talk about what happens at plate boundaries. And this is kind of where the interesting stuff happens when tectonic plates collide, or at least come up against each other. Now, the first type of plate boundary that I'll talk about is a divergent plate boundary. This is when the two plates are moving apart from each other. This mostly happens along boundaries between tectonic plates in the middle of the oceans. There are some divergent boundaries that occur through continental crust, but mostly it occurs in regions of oceanic crust. And the reason for this is because of the mechanism of seafloor spreading that I mentioned before. So here's how it works.

There are particular regions along the Earth's surface where there are plume, of magma, these are called hotspots that project up to the earth surface or nearly to the earth surface. And as they do so, of course, they're bringing up magma. They're bringing up molten rock, essentially, and that has to go somewhere. So that rock, sometimes it erupts in terms of volcanoes, but often it just kind of comes to the surface and then pushes to the side the plates on either side. Obviously, they're going to these hotspots sort of push up at weak spots, and this is at the boundary of two tectonic plates. So if you have two tectonic plates next to each other, meeting in the middle of the ocean where they each consist of oceanic crust,

so two bits of oceanic crusts from different plates next to each other. Under that is an upwelling of magma, or in many positions at least, there's an upwelling of magma from further down in the mantle. That upwelling magma pushes the two plates apart from each other, and then as the magma sort of reaches the surface, or at least close to the surface, it cools and solidifies and adds on, forming a new region of oceanic crust. So this is why this is called a divergent boundary.

because the plates are being pushed apart and then kind of also accreted onto at either end. And this occurs at what I call the mid-ocean ridges. If you look at maps of the surface of the ocean, what you'll find is that there are series of mountains on the ocean floor. They don't project over the surface of the ocean, but you can see them if you map the ocean floor. And these extend, probably the clearest example, is right down the middle of the Atlantic Ocean, because basically right down the middle of the Atlantic Ocean is where a number of tectonauton. plates meet each other. In the north, it's the Eurasian and the North American plate, and then further south it's the South American and the African plate. And at those regions, you have a series of

mountains. And those mountains are basically a product of the fact that this is a, this whole big line is a giant divergent region, where the plates are moving apart from each other. And in the middle is upwelling magma, which is forming basically new rock and accreasing onto the oceanic crust on either side. As it does so, it pushes the plates apart. What's interesting is that this happens in a very systematic way. So what has been discovered is that the material of the ocean crust the closest to the mid-ocean ridges is very new. If you move just a little bit further away from that, it's slightly older, move further away from that, slightly older again, and so on and so forth, until you get as far as you can to basically where the oceanic crust

meets the continental crust of that technonic plate, or possibly a different tectonic plate, and that will be the oldest oceanic crust that is found. So it's a very consistent relationship the further, you move away from that medeoceanic ridge, the older the crust gets. And the way that this is determined, like you might ask, well, how do we know how old the crust is? The reason is because we can detect a banding pattern in the alignment of magnetic domains in the minerals in those rocks. So we can do this by basically measuring the orientation of the magnetic field of the minerals that were crystallized when those rocks solidified. And what we find is that exist an abandoned pattern.

So sometimes basically it's pointing to the north and sometimes it points to the south, or the north pole and the south pole. And the reason for this is because of a phenomenon that the Earth's magnetic field flips polarity periodically. And this is something we'll talk about in a little bit more when we get to talking

about the causes of the Earth's magnetic field. But basically, throughout history, the polarity of the Earth's magnetic field shifts randomly from point to the north to point to the south and so forth. And this can be observed if you look at the banding pattern of the frozen alignments of the magnetic domains frozen in the rock and the oceanic crust.

And by observing the patterning of the flipped orientations and then correlating that against other rocks, particularly continental crust and other records that also preserve a record of the flipping polarity of the field, we can determine the relative age of the rocks. And what we can find is that there is a very regular banded pattern where you see that this flipping pattern matches up pretty much exactly with the distance away for the mid-Atlantic ridge. And really the only way that you can explain this is that new crust is being created at the mid-oceanic ridge, that is at the boundary between the tectonic plates and is pushing the plates apart so that the further away you get the older is that crust. And eventually that crust is destroyed when it moves to a boundary with the continental

crust. We'll talk about that in a moment. But this discovery of seafloor spreading was basically the key discovery that was necessary to, for scientists to accept plate technonics. Because you may know, I'm not going to get into the history here, but the theory of continental drift, the continents were moving around, was developed, I think, early in the 20th century, but it lay kind of dormant and widely ridiculed actually for decades because a geologist thought, well, this is a silly idea, how can continents move? There's no known mechanism for that, and there were a bunch of other problems, and that there wasn't really very strong evidence for it until seafloor spreading was discovered in the 60s. So this is one of the sort of biggest milestones in the history of geology, I would

say this discovery. So very important to understand how that works. And this, of course, is the reason why oceanic crust doesn't get very old, because it's constantly being created along the meteoceanic ridges and then destroyed through a process called subduction. So that's now the next process that will move to. So remember, we're talking about the different types of plate boundaries, and the first we started talking about was a divergent boundary where you have upwelling of material from the magma, and then that pushes the plates apart and forms new material. Mostly this happens at boundaries between oceanic crust, although not 100%. That's a divergent boundary. Another important type of boundary between technonic plates is called convergent boundary. Obviously,

that's when plates are coming together towards each other. And when this happens, you usually get a phenomenon known as subduction. Subduction occurs at convergent boundaries in which one technonic plate is forced under another and sinks into the mantle as they sort of come into each other. Now, because oceanic crust is, as we know, denser, and also sits lower on the underlying mantle in comparison to continental crust. When you have continental crust and oceanic crust converging together, the oceanic crust pretty much always subducts under the continental crust. You can also have subduction of one oceanic plate or region of crust under another, so that's also possible, but it's just that when you

get oceanic converging with continental, the oceanic will almost always subduct underneath the continental. So basically this means that the oceanic plate is forced under the other one. It buries itself down into the mantle as it's pushed further and further down and then gradually it kind of melts and is destroyed and is mixed up with the rest of the mantle. So this is how oceanic crust is destroyed. I mentioned that it's created at the Meteorceanic ridges via sea force spreading. It's destroyed primarily through subduction underneath either other oceanic crust or also under continental crust. Now I mentioned before that when subduction occurs, what tends to happen is that the relatively more volatile materials that are incorporated into the oceanic crust, such as the more felsic minerals, in addition to the water that's brought in, because it's oceanic crust, so there's some water that's brought in. The increased temperature and pressure leads to this water being squeezed out and kind of boiled out as well as some of the other volatile minerals being pushed out, forming plumes of magma, which then rise up to the surface.

So basically what's happening is the oceanic crust is being subducted down. It's being pressurized, heated up, squeezed. And in that process, water that's been imbued in the minerals, plus some of them more volatile, generally more felsic, having a lower melting temperature, minerals come up, they melt and form plumes, which then rise to the surface and form volcanoes. As a result of this process, it's very common when you have a subduction of one plane under another to form what are called island arcs. Obviously, this is when there's a subduction of ocean, so this goes in the ocean.

And these island arcs are basically a line of volcanoes or a chain of islands that are formed by volcanoes, produced by the material that's rising up and forming an island due to the subduction of one tectonic plate under another. And some famous island chains that are formed in this way include the Aleutian Islands and the Corral Islands, all of Japan, as well as the Raikyu Islands. the Philippine Islands and the Andaman Islands of India and various islands around the Solomon Islands, Papua New Guinea, New Hebrides and Tonga. What you may see is that most of these lie along the Pacific Rim of Fire, which is the big region where you have the Pacific Plate interacting with the Australian Plate, as well as the Filipino Plate and then for the North, the North American Plate. So all of these regions have given rise to important island chains.

as a result of the volcanoes spewing out material that then can form new regions of continental crust. A related phenomenon is called a hotspot. Now this is basically the same thing, you know, a volcano spewing out material which then forms an island, except the difference is that they're not produced by subduction. Instead, they're thought to be formed by a region of the underlying mantle that's just unusually hot, hence the name hotspot. As far as I can tell, this is a bit controversialist of exactly how they're formed. But the idea is that there's a region of the Earth's mantle that continually produces unusually sort of hot upwellings of material. And these kind of burst through at particular parts of the Earth's surface.

Could be under the continental crust or it could be under oceanic crust. And as the tectonic plate moves across the mantle relative to that hotspot, which is sort of fixed relative to the mantle, who is relatively speaking, the hotspots position on the Earth's surface moves. So it's not that the hotspot moves. it's actually more that the tectonic plate moves across the hotspot. This is how you get other island arcs, such as the Hawaiian Islands or Iceland at Yellowstone, is also thought to be one of these. But the Hawaiian Islands are a very good example because they lie near the middle of the Pacific Plate.

So you can't explain them in terms of subduction. Instead, they're thought to have been formed as a result of this hotspot, which still gives rise to active volcanic activity in that region today. And it is also why you can see that there's a sort of a series of islands and there's also other islands and atolls that extend beyond the current-day main Hawaiian islands that were produced as a result of the hotspot being in relatively different locations as the tectonic plate has moved. So the Pacific Plate's moving today to the northwest,

and that's why the chain of the Hawaiian Islands is kind of directed from the northwest down to the southeast where the larger islands are today. So that can be explained by the relative motion of the Pacific Plate. So we talked about divergent boundaries and convergent boundaries. Another important type of boundary is called a transform boundary. So if a divergent boundary is when plates are moving away and a convergent boundaries when moving towards each other, if plates are moving sideways relative to each other, that's called a transform boundary.

So neither plate is being subducted under the other, nor is there a production of new material at the Mid-Oceanic Ridge or similar. Transform faults usually give rise to a lot of earthquake activity because that's where you have essentially friction between the plates and they sort of move relative to each other and can produce significant activity at the earth's surface, but they don't result in production of a new continental plate or oceanic plate material. Probably the most famous example of a transform vault, in this case a continental transform fault, is that which exists between the Pacific Plate and North American plate at the San Andreas Fault in California, which is a very large

region where they kind of just move relative to each other and obviously produce a lot of earthquakes in that region. One final example of a type of plate boundary, which I'll just briefly mention, is a passive margin. This is a transition between oceanic and continental lithosphere that's not a plate margin. So this is not exactly a plate boundary, but it's more just a transition between oceanic to continental crust that occurs in one tectonic plate. Because remember, a tectonic plate can be made up of different combinations of continental and oceanic crust. It doesn't have to be all one or the other. Because they have different compositions, there is a sort of a transitional region of crust that connects the two. So remember, oceanic crust is thinner and denser, whereas continental

crust is thicker and less dense. So it does transition into the two, and that there's sort of a little region of kind of like a trench that separates the two just between the ocean and the continent. And this region is where you typically, this is the region of the passive margin. and it tends to accumulate large quantities of sediments from the ocean and also from the continent bringing sediments down and sort of accumulating there next to the oceanic crust. So again, there's no active plate boundary there, but it is still an important feature of the transition from continental to oceanic crust.

We talked about convergent plate boundaries in the context of an oceanic plate and a continental plate and where you have the oceanic crust which subducks under the continental crust and gives rise-top whirling magma which can form. island arcs or just volcanoes if it's over the over a continental region. But it's also possible for two continental plates to have convergent boundary between them. What happens then? Well, that is a bit more complicated because usually, because they're both formed a lighter and thicker continental crust, usually you don't have full subduction of one under the other or you might have a bit of

partial subduction. But more what tends to happen is they just kind of crumple into each other and form a big mountain range because the material's pushed upwards. Some of it might be subducted, but a lot of it is just kind of pushed upwards and crinkles. And probably the best example of this is the Indian plate, which is moving northwards and pushing into forming a convergent boundary with the Eurasian plate. And this is the cause of the Himalayan mountain ranges, obviously the highest mountain ranges on earth. They're formed by the buckling and pushing upwards of continental crust as a result of a convergent boundary. And this region is out of isostatic equilibrium because it's been pushed up so high. So over time, what we would expect is those mountain ranges to be eroded and also that region

to sink a little bit lower on overlying a stenosphere. Okay, so that concludes a discussion of the different types of plate boundaries and how that relates to seafloor spreading and volcanoes, island arcs and so forth. Now I want to talk a little bit about Earth's magnetic field, because I mentioned before that this is produced by the liquid iron and zinc metals in the outer core, but I didn't really explain how. And partly that's because we don't really know how. It's only been in the last few decades that we've got some understanding of how this might work. It's not something that can be directly observed, as I mentioned,

because we don't have access to anything further down than really the uppermost part of the mantle. And so primarily our understanding of the behavior of the liquid outer core is based on computer simulations. But it's thought that what happens is you have electric currents being formed as a result of the convection currents formed in the mixture of molten iron and nickel that's in the outer core. So as the convection currents are sort of swirling around, carrying heat away from the inner core up towards the mantle, they combine with the Coriolis effect of the rotation of the earth to kind of twist around. So you don't just have, you know, simple rolling convection currents, but they form complicated twisting sort of helical patterns. and it's this complicated motion of charged particles around the Earth's core that gives rise to a magnetic field. We know that moving charged particles give rise to a magnetic field. That's a basic principle of electrodynamics.

The process that's thought to be at work here is called a geodynamo. And this is something that I don't understand very well, and I don't think is understood very well, but I thought I'd mention that. But for the long and short of it, it's complicated coiling motions of currents moving in the Earth's liquid outer core that gives rise to the magnetic field that surrounds the earth. And as I mentioned, the polarity of that field spontaneously flips periodically over geological time. It often takes tens or hundreds of thousands of years, but it can be nilus of years sometimes, or it can be shorter. It's highly variable. And it just seems to occur spontaneously without any particular reason, just because of the chaotic motion of the liquid moving around. Contrary to popular belief, the magnetic poles of the earth don't line up with

the geographic poles. So currently the north magnetic pole of the earth is located somewhere in northern Canada. And also they move around. So the location of the magnetic poles wander spontaneously. So this is different from flipping the polarity. That's when the north and south flip between each other. But every year they move around and they've been moving quite a lot in recent years, I believe. So that doesn't normally matter for using a compass very much because they still point more or less in the direction of the north, but there are places on the earth surface where if you use the compass, it would actually point to the south, or it would point east or west, depending on where you're standing. And I mentioned before that the spontaneous reversals of polarity of

the magnetic field are recorded in the formation of the banded pattern of the ferromagnetic minerals in the oceanic crust, not just in the oceanic crust, but particularly there, which then it can be measured in terms of which direction they're pointing in, giving rise to the pattern, which was used to discover seafloor spreading. One final point, just to a point of clarification, we talk about the north magnetic pole, but in fact, the magnetic pole that exists near the north pole of Earth is actually a south magnetic pole, which is confusing. But the reason for that is because when you have a compass that has a magnet in it,

the north pole of that magnet points to the north, and of course it will point towards the opposite pole. So the north pole in your compass points towards the south pole of the Earth's magnetic field, which is in the geographic north. So it is a little bit confusing, but I just thought I would put that clarification in there. Okay, so we've talked about the different regions of the Earth's internal composition. We've talked about the plates and the different types of crust,

oceanic and continental and the different plate boundaries. I thought that I would discuss some of the mechanisms that cause plates to move around, because we've said that the plates move with respect to each other and that they move around really completely over the course of hundreds of millions of years. but what's actually causing them to move. So this is still a topic of ongoing research. Again, we can't study much of the mantle directly, only the very uppermost layers we have samples of,

and in particular the astinosphere, which is the plastic deformable region of rock in the upper mantle, which is what the lithosphere sits on. We can't study that directly, and therefore our understanding of how it all works is quite imperfect. But it is thought that there's two main factors that contribute to movement of the plates. one is mantle convection and the other is what's called slab pull so i'll talk about both of those first

mantle convection so i mentioned that the inner core contains convection currents which are basically moving heat from the solid inner core out to the mantle and it's the same it's thought to be the same thing with the mantle as a whole it's thought that over very very long periods of time because most of the because all of the mantle is solid but nevertheless over very long periods of time there's a creeping motion of the rocks which make up the mantle that carry heat away from the earth's core and towards the surface. This is thought to be the primary underlying driving force of plate technonics. So basically, as I mentioned before, you have upwellings of magma around the mid-oceanic ridges, which push apart the oceanic crust that exists there, forming new parts to the crust and pushing the plates away from each other.

It's thought that that is driven ultimately by these very large, very slow convection currents, which exist in the mantle as a whole. And so the convection current kind of pushes up at the medeceanic ridges and then pulls along. So if you imagine it going to either side, pushing the tectonic plates to either side of each other, and then the current sort of falls back down, goes back into the all the way down to the outer core, and then comes up again. So it forms a kind of a loop on either side. And these currents are not, as I said, not very well understood and quite complicated,

but it's thought to be this very slow convection current in the mantle. Even though it's made of rock, but it's not fully brittle rock. Remember, it's only the lithosphere that is sort of what we think of a solid brittle rock. The lithosphere consisting of, once again, the crust, and then the uppermost part of the mantle. And it's the lithosphere that forms the tectonic plates as being like different bits of the lithosphere, and all of that sits on top of the sinosphere, which is the plastic part of the mantle. And so think of the lithosphere sort of floating on top of the sinosphere, driven by currents in the embaltosphere, driven by currents in the mantle underlying and including the astinosphere.

So that's the underlying mechanism, mantle convection, that's thought to explain continental drift and why they move. But I also did mention slab pull, which is thought to be a contributing factor as well. And this is the basic idea that once you have one portion of oceanic crust that starts to subduct under either another region of oceanic crust or possibly continental crust, then the weight of that is it's being pulled down,

well pushed down, but also then stuff. to be pulled down by gravity. That kind of end that's sort of falling down towards the Earth's core begins to drag the rest of the oceanic crust down with it. So that's an added sort of force that's pulling the rest of the plate down. Now it never pulls the whole thing down, or at least it usually doesn't, because in most cases the plates being added to it at a mid-oceanic ridge or through other processes. But it can ultimately pull the whole plate down and cause its material to be recycled, because plates do come and go over very long periods of time. But so it's basically a combination here of the convection currents in the mantle getting everything going, but then it's augmented

by these forces of gravity sort of pulling plates down once they start to subduct. Before we finish this episode out, I wanted to present some of the evidence for continental drift. You may recall that I talked about how the theory of continental drift has existed for a long time, but only since around the 1960s has it become widely accepted, or the modern variant of continental drift called plate tectonics. I've mentioned some of the evidence for it as we've gone throughout the episode, but I thought I'd talk about it all here at the end, just so you can understand how we know these things, particularly, as I said, because we don't have any direct access to the mantle or the core to see what's happening there.

The single strongest piece of evidence is the magnetic striping on either side of the mid-oceanic ridges, which is evidence for C4 spreading, so the process of the new oceanic crust being produced at the middle of the boundary of tectonic plates at regions where they have oceanic crust at the middle of the oceans. that is extremely strong evidence in favour of plate tectonics, because there's no other way you can get this banded pattern of oldest material at the edge of the ocean, and then increasing the newer materials you go towards the middle, where the very newest material is being graded.

So that's kind of a smoking gun, if you like, but it's not the only evidence we have in favor of plate technics slash continental drift. The very first evidence in favor of the hypothesis was actually just the apparent geological fit of the continents between each other. The clearest example of this is South America and Africa. If you kind of push them together, they kind of fit into each other. Indeed, you kind of bring in, rotate North America around a bit, and it kind of fits on top of the northwestern part of Africa. You can fit other continents together as well, so you can push Australia down to part of Antarctica.

This is not very rigorous evidence, but it was some of the earliest evidence that got people thinking, even from centuries ago. And it's now known that this is not coincidence. It is because these continents were once joined together. They broke apart and drifted away, and their coast had changed somewhat, but they still resemble, show a pattern resembling how they were originally fitted together. Understanding how the continents used to be fitted together is critical because it relates to some of the other key lines of evidence in favor of the theory of plate tectonics. In particular, evidence from biogeography and glaciation.

So what you find is that there are similar patterns of fossils found in certain regions of the earth. For example, some regions of South America have similar fossil patterns to some regions of Africa. Likewise, some regions of Antarctica and Australia or some regions of Africa and India, even though today these places are very widely geographically separated, often by very large oceans. So it's hard to explain why this would happen unless you have a situation in which once upon a time, those regions were close to each other, and they now moved further apart. The evidence from glaciation is similar.

There is a widespread distribution of certain types of glacial sediments, which can be identified as belonging to a particular time and place, but there are commonalities between the types of glacial deposits found in certain reefs. like, for example, Antarctica and India, or Antarctica and parts of Africa, or South Africa. And again, if you sort of put these together, it turns out that this is explained by the fact that the glaciers used to be continuous with each other and part of one large glacial region when the continents were together and the whole region sat over the, or much closer to the South Pole. Another piece of evidence in favor of the theory of tectonic plates is the apparent polar wandering. We can use patterns of magnetic domains that are frozen into old mineral.

deposits at the time those minerals crystallized, and they will be directed either towards the north or the south. And what we can do there is not only observe the flipping of the polarity of the magnetic field, as I mentioned before, but you can also see what direction the north or south pole was relative to those minerals when the minerals were laid down. And what we find is that the poles apparently wandered over time. Again, we're not just talking about the flipping of the polarity here. We're talking about the fact that they seem to move all over the earth, even more than the pole already wanders today. And the best explanation of this is that, in fact, it's not the poles that have wandered. It's not like the north or the south magnetic pole was ever on the equator or something

like that. Rather, the tectonic plates have moved. So they were laid down at a place where it made sense, you know, for the minerals to be oriented in a particular way. So they were originally pointing towards north, essentially. But then they've since moved, and now they point in a completely different direction. And so, again, if you piece all these together and work it out, it is consistent with the pattern of distribution of the continents at different points in the past that's indicated by the glaciation and the biogeography and the geological fit. So all of these different pieces of evidence together, the glacial evidence, the biogeography, geological fit, the paleomagnetism, and the sea floor spreading, all point towards the idea that the continents do move relative to each other, and they have,

in the past, existed at different locations relative to where they are today, and one of the major mechanisms for this is the production of new material at the middle, of bottom of the oceans through the mechanism of seafloor spreading. So this forms the basic ideas of the fuse of plate tectonics, which, as I said, at the start of the episode is kind of the underpinning of really much of geology today. So that concludes what I wanted to talk about in this episode. I hope you found that interesting. If you would like to support the show, you can jump onto Facebook and type in The Science

of Everything podcast and give the page there we have a like. It's a good way to stay updated on what's happening and maybe get some extra visual material that I post there from time to time. You can also give the podcast a review on the aggregator side of your choice, such as iTunes or wherever else you download your episodes. If you'd like to get in touch with me, you can send me an email. My address is Fods12 at gmail.com. That's FODDS1.2 at gmail.com. I always love to hear questions, feedback or suggestions for future shows.

If you would like to be particularly generous and financially support the show, you can click on the link that should be in the description to either become a Patreon supporter or give a one-time donation via PayPal. I've committed to keep ads off the show. I don't like ads much and I think they're distracting. So I always appreciate anyone who is able to make a small contribution to help cover costs and also allow me to spend more time getting content out to you guys. And I appreciate all my existing patrons. Finally, before finishing up, I wanted to kind of add a little new segment to the show that I'll do occasionally at least.

It's been suggested that I should mention what I'm going to be doing in upcoming episodes. And I've never really done this before unless I'm running a longer series. but I think it's a good way for people to kind of be invested and excited about what's coming up. So what usually happens is that I've got a few different projects in the works and kind of depending on how things go and what materials I have and how much time I have and what interests are particularly burning at the moment, I'll get one or more of those out. So what I can tell you is not necessarily what the next episode will be, but I can tell you some of the projects that are in the works.

So I have mentioned that I do intend to do another earthquakes episode following up from this. So that will happen at some point, but probably not for a little while. In terms of next episodes, probably the next episode will be an introduction to microbiology. I've been wanting to do that for quite some time. It's an area that we haven't really covered before, and it's somewhat topical as well at the moment, although I generally don't do topical episodes, but nevertheless, this is one that's been on the list. So look forward to that. We'll be talking about bacteria and viruses and all that stuff.

In terms of what else is on the agenda, I am working on an episode on computational chemistry, in which we'll talk about how we estimate the structure and properties of molecules and more complicated atoms and how we solve the stranger equation for multi-electron atoms and the difficulties there. That's more of a niche topic, but given the feedback I've received, I'm pretty sure there's a good fraction of you who really enjoy that kind of stuff. So keep an eye up for that. I also want to do a couple of episodes on the cell membrane, including the external cell membrane and the membranous structures of exist inside the cell. There's a lot of interesting stuff there relating to structure, functions, cell signaling, protein folding, and movement of materials around the cell. So that's another one that is on the agenda. So that gives you a few things to hopefully wet your appetite. And

as these projects come to fruition, I'll make periodic announcements about other things that you can look forward to in future episodes. So that's enough for now. Thanks again for listening, and I'll talk to you next time.