Plate tectonics: Evidence of plate movement Plate Tectonics -- Evidence of plate movement
Plate tectonics: Evidence of plate movement
- Let's think a little bit about the different clues that have led us to conclude
- that we have these lithospheric plates moving relative to each other.
- Now, the first clue. This is something I think many students even in elemantary school first experience when they first learn about geography,
- it looks like the continents could kind of fit into each other.
- The most obvious one of these is when you look at this kind of little pointy part of South America,
- if you have a more detailed map, it really is amazing how well it seems to fit into the Nigerian basin in Africa.
- It looks like at one time this little pointy part was nudged into this part of Africa that they were actually connected.
- And if you're a little bit more creative, there are other parts of the world that you can kind of start to see
- how they might have fit in with each other in the past.
- That by itself is just a very small clue but kind of hints at,
- you know, if they at one time were fitting, were next to each other or were kind of connected,
- then they have had to move departed some time.
- Although it doesn't tell us that it is still moving or what might have caused the move.
- But it definitely doesn't tell us that they even moved.
- Maybe this is just a coincidence that this coast of South America looks very similar to this coast of Africa.
- Now, the next clues we really came over I would say by the last 60 or 70 years.
- The first clue is that OK, if you go to the Mid-Atlantic Ridge right here, so if you look at the Atlantic Ocean,
- let me look at this photograph right over here.
- So this is... you normally don't see the oceans highlighted like this, so let me make it very clear to you.
- This right here is South America.
- This right here is Africa.
- This right here is North America.
- And if you actually look at the elevations in the middle of the ocean,
- people noticed in the middle of the 20th century that there is a ridge in the middle of the Atlantic Ocean!
- There is kind of a mountain ridge that goes straight up the middle of the Atlantic Ocean.
- So, that by itself doesn't tell you that these plates are moving apart,
- but it is kind of a curious thing to look at.
- And that all means that at these ridges are a lot of a underwater vulcanic activity.
- You have magma and lava flowing into the water and it's kind of forming this ridge
- that really goes across the whole Atlantic Ocean.
- There are other ridges in the world like that - underwater ridges.
- You have one over here in the Pacific Ocean.
- You have these here in the Indian Ocean.
- So that by itself is just a little clue, but that by itself doesn't explain,
- doesn't tell you that these plates are actually moving apart at the ridge.
- The more conclusive, this is just the beginning of the clue, but what made this conclusive
- is the separate discovery. And this is what's interesting,
- because you had these separate discoveries in different domains that eventually led you to a pretty neat conclusion.
- So you've had a separate discovery that if you look at different errors of magnetic rock,
- or maybe I should say different magnetic rock from different periods in geologic time,
- and you can tell where they are in geologic time by why they are layered,
- so this would be newer rock,
- and then this would be a little bit older,
- and then this would be even older.
- Geologists noticed something interesting. If I would take magnetic rock and if it was molten lava,
- and remember, it's a magnetic rock, so it would align with the poles the same way a compass would.
- So if I had a bunch of mangetic.. so let's say this is a lava right here.
- And so the molecules can align themselves when it's liquid,
- they can align themselves, they go in a natural one align with the poles.
- So they naturally all want to align in one direction
- because of Earth's magnetic field.
- And so when that lava hardens into actual rock, that alignment would kind of be frozen.
- Now, if Earth's magnetic field was constat over time, then when you look at magnetic rocks from any period,
- you would expect them all to be aligned in the same direction.
- So it's as we are taking a cross-section of this rock,
- let's say an alignment towards the north pole looks like this.
- I draw it like that, that's an arrow pointing into our screen.
- Now, let say an alignment pointing to the south pole would look like this.
- This would be an arrow pointing out of our screen.
- So what you expect is that newer rock, that kind of the alignment of the rock would go into the screen.
- And then the older rock, it would still go into the screen.
- It looks like a top view just so let me draw like this as so I make sure every one is on the same pace.
- Let my just draw a cross-section like this.
- So we know what we are talking about.
- Let my draw a cross-section like this.
- So this is the surfcace appear.
- When I talk about going into the page that means that the magnetic rock would be aligned in that direction.
- And when I talk about going out of the page that means, so I would go to draw it like that,
- that means that the magnetic rock would be aligned in that direction.
- Now, like I said, if the magnetic field of Earth never changes,
- then lava that essentially turns into hard, cools down into non-lava rock, you could say it freezes into rock,
- it would all point into the same direction regardless of when it hardened.
- This would be the situation in a constatn magnetic field.
- But what we've seen is that that's not the case.
- When you look at older magnetic rock, then, depending on how old you go,
- you have the newer rock that's aligned with our current magnetick field,
- you go a little bit older, and right now we think it's about 780,000 years ago roughly,
- you have to find magnetic rock that hardened at that time,
- it's actually in the opposite direction.
- So actually the magnetick rock is hardened in a way as if the north pole was at the south pole now, the magnetic north pole.
- So it's aligned in the opposite direction. So it's kind of pointing out of the page here.
- And if you get even older rock, it's more aligned with our traditional direction. So it's more aligned than that.
- And so the only reasonable conclusion that we can draw from this
- is that Earth's magnetic field has actually fluctuated over time.
- Magnetic field fluctuates.
- Now you probably are thinking: 'Sal, how was this relevant to plate tectonics?'
- Well, once you accept that magnetic field fluctuates over the history of the Earth,
- there is another interesting observation you can make about the rock that's kind of at the basin of the ocean floor.
- So not only do have this Mid-Atlanic Ridge have these volcanoes spuming kind of new rock into the ocean
- creating this kind of underwater mountain ridge,
- but it also turns out that the rock that forms the seafloor also contains a lot of magnetit which is magnetic.
- And what's really interesting about that - so let me draw.
- So we have a top-view just like we have over here.
- So let's say this is the Mid-Atlantic Ridge right here.
- And this is really cool. When they looked at rocks that are very close to the Mid-Atlantic Ridge,
- and once again, we're looking at rocks that are at the floor of the ocean,
- they're aligned in the way that you would expect with the current magnetic field.
- They're aligned just like that, the way you would expect when you look at the magnetic rock that's close to the ridge.
- But if you go a little bit further, when I say a little bit, I'm talking about thousands of miles,
- but if you go further out of that, you have stripes of other magnetic rock that is going the opposite direction.
- It's going like this.
- And what's even cooler than the idea that it switches directions depending on how far you've gone from the rift,
- is that there's a symetric stripe of magnetic rock on exactly the same distance,
- or roughly the same distance away from the rift that's also pointing in that same direction.
- And you go a little bit further out and you'll find some rock that's pointing in the original direction.
- And even better, you go on the symetric other side of the actual ridge,
- and you find another set of rocks that's doing the exact same thing.
- So if you accept that Earth's magnetic field has kind of been flip-flopping over time,
- the only reasonable conlusion that at least I can think of or geologists can think of,
- is that all of this was formed at a similar period in time.
- This came out as lava, magnetic lava, and the all aligned with Earth's magnetic field and that's why it looks similar.
- You fast-forward in time some, or actually let's not fast-forward in time.
- The only way that these could have formed and they could have been so similar,
- so if we rewind in time, the only way that these purple magnetic rocks could have aligned this exactly same way in the same distances,
- is that in some point they were much closer to each other and were actually connected.
- So if we rewind in time maybe at the Mid-Atlantic Rift you had all of the purple rock
- coming out from those underwater volcanoes and at that time Earth's magnetic field was the opposite as it's right now.
- And then of course you had this blue rock that is looking like that.
- And so this seems like a reasonable explanation. This rock and this rock were at some point touching,
- they were actually formed at the exact place and at the same time.
- And so if this is the case that one point this purple rock was all together and they formed at the same time at the Mid-Atlantic Rift,
- we're assuming all the rock was, well, we don't have to make that assumption,
- but if you assume that they formed at the same time, and based on the pattern it does look like they do,
- it's a symetric distace away from that rift, that the only reasonable conclusion I can think of
- is that the rift has had to move apart from this period to that period.
- And there was a time when all this blue rock was together.
- So that by itself frankly is the most affinitive evidence in the 1960s
- when it kind of became conclusive that you did have these plate that were moving away from each other.
- Now, obviously the plates are moving away from each other at some point,
- and that means that based on how the map looks, at some point they're also going to move into each other,
- we can talk more about that in future videos, but at certain point they're actually moving on,
- one plate is moving under another. We'll talk about how that might partially explain,
- I won't talk about all of the explanations of why we think the plates might actually be moving.
- But if we fast-forward the more present time as now we have the GPS satellites and all the rest,
- we can actually measure the movement of the plates.
- This is an image from NASA showing the vectors of the movements at different points on the surface of the planet.
- And you could see we've got a lot of vectors from the U.S., it's hard to read, it's so chock-full of vectors.
- We can see right over here why the Pacific Plate at that point is moving in this northwest direction. And it's measured by GPS satellites.
- And I wanna make clear - this movements is relatively slow, it's roughly the speed in which fingernails grow.
- But if you do it over millions of years, that actually amounts to thousands of miles.
- We're talking on the order of about a centimetre a year for most of the plates,
- some of the plates might be moving a little bit faster, maybe close to 10 or 15 cm,
- but most of them are moving about a centimetre a year, at the same rate your fingernails are growing.
- But this is fantastic 'cause you can actually measure it because GPS is so accurate.
- We can see that the North-American Plate is kind of rotating generally in that direction.
- The Nazca Plate right here is moving roughly in that direction into the South-American Plate.
- I'll leave you there right now... actually before I leave you there,
- this is another thing that I though to recap that shows that same magnetic strenght it's maybe a slightly neater drawn,
- I don't know which one might be more helpful for you.
- But I'll leave you there in this video. In the next video we'll think about some of the theories.
- We know now that the plates are moving. Let's about some of the theories as to why they might actually be moving.
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At 5:31, how is the moon large enough to block the sun? Isn't the sun way larger?
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