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The Theory of Plate Tectonics
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Plate Tectonics
In this course, Dr Rebecca Bell (Imperial College London) explores plate tectonics. We begin with a module explaining how the theory of plate tectonics developed, and outlining the world’s major plates and types of plate boundary. The second module then looks at one of these types – divergent boundaries – in terms of how they develop and the hazards they generate, as well as covering fault formation. In the third module we turn to convergent margins, examining why continental and oceanic plates converge differently, and what volcanic and seismic activity takes place along them. The fourth module then considers the final main type of plate boundary, conservative margins, and mantle plumes (also referred to as volcanic hotspots). In the fifth module, we conclude by looking over the main proposed drivers of plate tectonics and their relative importance, and exploring how current plate movements might reshape the Earth’s surface in the future.
The Theory of Plate Tectonics
In this module, we think about the theory of plate tectonics and how it developed. We focus on: (i) the location of the major tectonic plates, the main types of plate boundary (convergent, divergent, and conservative), and the internal structure of the upper layers of the Earth – the lithosphere and asthenosphere; (ii) Alfred Wegener’s theory of continental drift, and his evidence that the continents once comprised a single landmass; (iii) the key discoveries which led to the eventual acceptance of Wegener’s theory, most notably seafloor spreading as a mechanism for continental movement; (iv) how the discovery of plate subduction, alongside seafloor spreading, produced the theory of plate tectonics.
Hello.
00:00:05My name is Dr Rebecca Bell,
00:00:06and I'm a senior lecturer in geology and geophysics from Imperial College, London.
00:00:08Today we're going to talk about plate tectonics.
00:00:11Plate tectonics is often described as the
00:00:14great unifying theory in the geosciences.
00:00:16Once you accept plate tectonics,
00:00:19lots of observations about the world around us suddenly makes sense.
00:00:21It explains why we have mountains,
00:00:24why we have the deep ocean and why we have earthquakes and volcanoes where we do
00:00:26Plate Tectonics says that the outer part of the earth
00:00:30is divided up into a number of rigid plates,
00:00:33and those rigid plates are moving and interacting in different ways.
00:00:36We have divergent plate boundaries where plates are moving apart,
00:00:39and these are locations where new ocean crust forms.
00:00:44And that's happening very famously in the Atlantic Ocean at the mid Atlantic ridge.
00:00:47In other places, the plates are colliding together, and when that happens,
00:00:52that gives us very large faults,
00:00:55which can allow us to have very large earthquakes like the 2011
00:00:57Japan magnitude nine earthquake that produced the devastating tsunami in 2011.
00:01:01In other places, the plates are moving side by side,
00:01:07forming conservative plate boundaries and strike slip faults.
00:01:10And probably the most famous example of that is the
00:01:14San Andreas fault on the western coast of North America.
00:01:16And we're gonna be talking about all these
00:01:19types of plate boundaries in this lecture.
00:01:21Those plates are not only made up of the crust,
00:01:23they're also made up of the upper part of the mantle, which is cooler.
00:01:26The 1300 degrees Celsius
00:01:30in that part of the mantle behaves in a rigid way,
00:01:32so the crust and the upper mantle together
00:01:35form something which we call the little sphere.
00:01:38And that's what the plates are made of.
00:01:40Those rigid litmus Feyerick plates float on top of the hotter part of the mantle,
00:01:42which is called the S Thinness fear,
00:01:47and that flows in a more duck tile plastic way.
00:01:49A very common misconception that's still in lots of a
00:01:53level textbooks is that the mantle is entirely molten,
00:01:56and that's not the case.
00:01:59The mantle is predominantly solid,
00:02:01but it can flow where it's hotter the 1300 degrees Celsius, but very, very slowly.
00:02:03If the mantle really was molten everywhere, we'd have volcanoes popping up.
00:02:11You know in Central London, which we don't, we simply don't see that.
00:02:14So the mantle only melts in very specific locations.
00:02:17And we're gonna be talking about those over the course of this lecture.
00:02:21So these rigid plates have the same behaviour as a biscuit.
00:02:25So if I take this biscuit and I snap it in half,
00:02:30who wants to imagine that each side of the biscuit
00:02:33is a tectonic plate and I rub the plates together?
00:02:35You can see all of the crumbling and the cracking
00:02:38which is happening is that the boundary of those biscuits,
00:02:41the interior of the biscuit, is staying relatively deformation free.
00:02:44And exactly the same happens with the tectonic plates of the Earth, the crumbling,
00:02:48the cracking, the development of fault lines,
00:02:52which is where we get earthquakes and most volcanoes.
00:02:55All of those happen at the plate boundary.
00:02:58Sometimes that's called the plate margins.
00:03:01You might hear me kind of using those two terms interchangeably.
00:03:03The middle of the plate stays relatively defamation free.
00:03:06So the U K.
00:03:09Where I'm speaking to you from at the moment,
00:03:10we are right now in the middle of one of these tectonic plates,
00:03:12which is why we don't experience many earthquakes.
00:03:15Firstly, though,
00:03:18it's useful to step back and have a bit of a history
00:03:20lesson and think about how the theory of plate tectonics evolved.
00:03:22And the person who's often coined as the
00:03:26grandfather of plate tectonics is Alfred Wagner,
00:03:28who is a meteorologist.
00:03:31In 1912, he put forward a radical new theory called Continental Drift,
00:03:33and his idea was that although the continents are, you know, in the position,
00:03:38they are the current day,
00:03:43that wasn't where they've been throughout geological history.
00:03:44And in fact, he suggested that 250 million years ago,
00:03:47the continents were all grouped together as a supercontinent,
00:03:50which he called Panja.
00:03:54And over time they all drifted into their current configuration.
00:03:55And he had several lines of evidence to propose that
00:03:59Firstly, as any young child will tell you when they first look at a map of the world,
00:04:02some of the continent's looked like they could fit together like jigsaw.
00:04:06Puzzle piece is
00:04:10the clearest example of that is South America and Africa.
00:04:11They just looked like they can fit together really quite nicely,
00:04:14so that was one line of evidence.
00:04:17Another line of evidence was that fossil
00:04:20plant in animal distributions across multiple continents
00:04:22use exactly the same plant and animal
00:04:26species over very widely distributed continents.
00:04:28So it's impossible that those plants and animals can travel thousands of
00:04:31kilometres across the ocean to get from one continent to the other.
00:04:36It's also improbable that exactly the same species
00:04:39of plant or animal evolved on different continents.
00:04:43Alfred Vagana proposed that if you reconstruct the
00:04:47continents to form the supercontinent of Pangea,
00:04:50those plant and animal distributions nicely all line up together.
00:04:53So it really helps to explain those, um, that those observations,
00:04:57another line of evidence which was very close
00:05:02to Alfred Vega's heart as a meteorologist,
00:05:04was that 300 million years ago we have
00:05:06evidence of glaciation in the Perma Carboniferous,
00:05:09and evidence for this glaciation was
00:05:12found over very widely distributed continents.
00:05:14It was found on Antarctica, South America, Africa, Madagascar, India, Australia,
00:05:17places that are now very far away, and it's very different latitudes,
00:05:23and in particular for India.
00:05:27It's at the equator,
00:05:28so it's hard to imagine why glaciation would continue all the
00:05:29way from the South Pole right up to the equator.
00:05:33In some places.
00:05:35Alfred Reagan are also made the observation that when you have glaciation,
00:05:37they produce scratches in the rocks.
00:05:42And if you reconstruct the continents to form this landmass of Pangaea,
00:05:44those striations form kind of radial patterns emanating
00:05:50away from that prehistoric South pole position.
00:05:54So that's again all really good evidence that
00:05:58the continents were at one time together.
00:06:00So you might think that all of these lines of evidence they're all quite
00:06:04compelling to suggest that the continent's really
00:06:06do come together and drift apart.
00:06:09But Alfred Wagner was ridiculed when he put forward this idea in 1912
00:06:11and really continued to be ridiculed right up until his death in 1930.
00:06:16One of the reasons for that is that he was
00:06:21very young when he proposed a theory in his early thirties
00:06:23and as a meteorologist.
00:06:26He also wasn't a classically trained geologist,
00:06:27so the professionals at the time really kind of looked at him with some suspicion.
00:06:30But the killer thing Alfred Vega couldn't do was to propose a hypothesis,
00:06:34a mechanism for why the continent's could drift through the oceans.
00:06:38He couldn't come up with any good mechanism why that could happen.
00:06:43So unfortunately, after his death in 1919 30 Alfred Reagan and never got to see that.
00:06:48Eventually,
00:06:54the scientific community did come to accept the theory of continental drift.
00:06:54In the 19 fifties,
00:07:00there were significant developments in our technology
00:07:01that allowed us to observe the Earth.
00:07:04Largely as a result of World War Two,
00:07:06we developed instruments that can allow us to very accurately determine the
00:07:08Earth's magnetic field and also enabled us to map the oceans.
00:07:13So in the 19 fifties, magnetometers were developed,
00:07:18which led to a new field of science called paleo magnetism,
00:07:21which is where scientists would go out and measure
00:07:24the magnetic field of rocks.
00:07:28As rocks fawn the magnetic minerals within them,
00:07:29which contain iron such as magnetite, the DYP,
00:07:33the magnetic die polls will align themselves with the earth's magnetic pole.
00:07:36What penny a magnetosphere found is that when they
00:07:40examined rocks of the same age across different continents,
00:07:43not only did the disciples not necessarily point to Earth's magnetic north pole,
00:07:47they all pointed in different directions,
00:07:52and as we only have one magnetic north pole,
00:07:54the only explanation for that is that the continents have drifted and
00:07:57moved into new positions so immediately Once this observation was made,
00:08:01the scientific community had to accept Alfred Vegas Theory of Continental Drift.
00:08:06We've now got hard, quantitative evidence that the continent's definitely moved,
00:08:10but that still doesn't move us closer to providing a mechanism.
00:08:15We didn't have to wait long until that mechanism came along.
00:08:18So in the 19 fifties,
00:08:22ships were going across the ocean mapping the depth to the sea floor using sonar.
00:08:23So so now involves sending out a high frequency sound wave.
00:08:29That wave travels through the water.
00:08:32When it hits the seabed, it gets reflected, basically echoes back,
00:08:34and we can detect how long it takes that sound pulse to be received.
00:08:38If we know the velocity of speed, the speed of sound in water, which we do,
00:08:43it's 1500 metres per second.
00:08:47We can use this simple equation.
00:08:49Speed equals distance over time that you all know
00:08:51from physics to calculate the depth to the seabed.
00:08:54Prior to sonar, we had to measure the depth of the ocean,
00:08:58basically using a lead weight on a rope and dropping it over the
00:09:01side of the ship and then measuring the length of the rope,
00:09:04which gave us very accurate measurements,
00:09:07but it was very slow and very one dimensional.
00:09:09With sonar, we can tow the sonar instruments at the back of a ship,
00:09:11steam across the Atlantic Ocean, for example,
00:09:16and collect two dimensional profiles of what the seabed looked like.
00:09:18So when that was done in the 19 fifties,
00:09:23a very interesting discovery was made that in the centre of the oceans,
00:09:25where you might expect the ocean floor to be at its deepest,
00:09:29it actually becomes quite shallow.
00:09:32And in the middle of all of the world's oceans, we have a mountain chain,
00:09:35and those mountains, in some cases, can be as high as two kilometres.
00:09:38And if we were to strip away all of the water in the oceans,
00:09:43they'd be the longest mountain chains anywhere on Earth,
00:09:45certainly longer than the Himalayas, For example.
00:09:47At the same time, people were making heat flow measurements across the oceans
00:09:51and finding that the greatest heat flow was coming
00:09:55out of where we had these mid ocean ridges.
00:09:57These mid ocean mountains
00:10:00magnetometers were also used offshore,
00:10:03as well as those instruments that can measure the magnetic field.
00:10:05And what they found is that in some parts of the ocean,
00:10:08the rocks that make up the oceanic crust, the oceanic sea floor.
00:10:12The magnetic minerals in them show a very high magnetic field.
00:10:17But in other areas, though, the magnetic field measures much lower.
00:10:21And then you go to an area that measures higher again,
00:10:26and you end up with these stripes,
00:10:28where the magnetic field has a different strength.
00:10:30And those stripes are exactly mirrored. The other side of
00:10:34the mid ocean ridges, those mid ocean mountains,
00:10:37paleo magnetism,
00:10:41it's who were working on land at the same time had
00:10:42discovered that the Earth's magnetic field at certain times in history,
00:10:45has reversed.
00:10:49So at some point in history, the Earth's magnetic north pole is where it is today.
00:10:50But at other times,
00:10:55that magnetic North pole actually goes to the geographic South Pole.
00:10:57So the effect that has is that when we measure rocks in the ocean,
00:11:01when the dye polls of the magnetic minerals
00:11:05are pointing north to the magnetic north pole,
00:11:07today we see a high magnetic field strength when they reverse and point to the south.
00:11:09At times we've had a magnetic reversal.
00:11:15The magnetic field strength gets lower,
00:11:17so these stripes are actually showing us
00:11:19when the Earth's magnetic field has reversed.
00:11:21And because on land we can date
00:11:24the rocks where we've got those magnetic reversals.
00:11:27We've got a really nice way to date the ocean floor
00:11:30and the fact those stripes are exactly mirrored. Either side of the mid ocean ridges
00:11:33tells us that we've got crust of the same age either side of the ridge.
00:11:38So from those observations,
00:11:42it didn't take people very long to put them all together and realise
00:11:43that at these mid ocean ridges where we've got lots of heat flow,
00:11:47these are locations where the mantle is melting.
00:11:50It's erupting through the little sphere, and we're having new ocean crust forming,
00:11:53which is giving us that high heat flow
00:11:57at that mid Ocean ridge. We're getting divergence.
00:11:59We're having extension, the plates are moving apart,
00:12:02and that's causing the ocean crust, which was formed at one point in history.
00:12:05Let's say when the magnetic north pole was where it is today,
00:12:09that crust is being moved either side of the ridge.
00:12:12We might then have a reversal new crust forms,
00:12:15with the disciples pointing towards the South Pole that the
00:12:18crust then gets moved away and the process keeps happening,
00:12:21which gives us these stripes.
00:12:24So this led to the theory of sea floor spreading and sea
00:12:27floor spreading gives us a mechanism for how the continents can move
00:12:30across the surface of the Earth because new ocean crust is being
00:12:34made at those ridges that can allow the plates to move.
00:12:37One issue, though, is that if we're making new oceanic crust,
00:12:41that would suggest that the Earth's surface is
00:12:44actually getting bigger because we're making new crust
00:12:46and the earth isn't getting bigger, so we know that's not happening.
00:12:49Fortunately, there was an observation again made around the 19 fifties,
00:12:53that helped to show what was happening.
00:12:56So as well as having these ridges in the middle of the oceans
00:12:58in other parts of the oceans, we had deep ocean trenches,
00:13:02areas where the sea floor is really deep,
00:13:04so the one you've heard of is probably the Marianas Trench.
00:13:07So at the Marianas Trench, that's the location where the ocean is with us.
00:13:10Fear is actually being subjected. It's going down into the mantle.
00:13:13It's being recycled.
00:13:16So the theory of continental drift and sea floor
00:13:18spreading together led to the theory of plate tectonics,
00:13:21and this can be thought of as the world's largest recycling factory
00:13:24
Cite this Lecture
APA style
Bell, R. (2023, March 07). Plate Tectonics - The Theory of Plate Tectonics [Video]. MASSOLIT. https://massolit.io/courses/plate-tectonics/the-theory-of-plate-tectonics
MLA style
Bell, R. "Plate Tectonics – The Theory of Plate Tectonics." MASSOLIT, uploaded by MASSOLIT, 07 Mar 2023, https://massolit.io/courses/plate-tectonics/the-theory-of-plate-tectonics