If you're seeing this message, it means we're having trouble loading external resources on our website.

如果你被网页过滤器挡住,请确保域名*.kastatic.org*.kasandbox.org 没有被阻止.



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 that I think many students even in elementary school first experience when they first learn about geography is it looks like the continents could kind of fit into each other and the most obvious one of these is when you look at this kind of little pointy part of South America and if you have a more detailed map it really is amazing how well it seems to fit into this into the Nigerian Basin right here 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 to fit in with each other in the past and that you know that by itself that's just a very small clue but it it kind of hints at well maybe the you know if they want at one time we're fitting were next to each other or if this was kind of connected then they've had to moved apart at some time although it doesn't tell us that it's still moving or what might have caused the movement and definitely doesn't definitively tell us that that they even moved maybe this is just a coincidence that this coast of South America is looks very similar to this coast right here of Africa now the next clues rich truly came over I would say about the last 60 or 70 years the first clue is that okay 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 it's a little you don't normally see the oceans highlighted like this so let me make it very clear to you this right here is South America this is South America this right here is Africa this right here is North America and so if you look at if you actually look at the elevations in the middle of the ocean people noticed in the middle of the 20th century that gee there's a ridge in the middle of the Atlantic Ocean there's kind of a mountain rage that goes straight up the middle of the Atlantic Ocean so that by itself doesn't tell you that that D that you have these plates that are moving apart but it is kind of a curious thing to look at and not only is there's ridge there's a lot of underwater volcanic activity you have magma flowing out 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 you have these here in the Indian Ocean so that by itself that's just a little bit a little clue but that by itself doesn't explain doesn't tell you that these that these plates are actually moving apart at the ridge the the more conclusive you know this is just the beginning of the clue but what made this conclusive is one the separate discovery and this is what's interesting is that you have these separate discoveries in different domains that eventually let you kind of come 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 how they're layered so this would be newer newer rock this would be newer and then this would be a little bit older and then this would be even even older even older geologists noticed something interesting if I were to take magnetic rock and if it was if it was if it was molten lava and if it were too hard and remember it's magnetic rock so it would want to align with the poles the same way a compass would so if I had a bunch of if I had a bunch of magnetic so let's say this is some lava right here and so you know the the the the molecules can align themselves since its when it's liquid and they can align themselves they're going to naturally want to align with the poles so they'll naturally want to they'll naturally all want to align in one direction they'll naturally all want to align in one direction because of Earth's magnetic field and so when that lava hardens into actual rock that that that alignment will kind of be frozen now if Earth's magnetic field was constant 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 if I let me so since we're taking a cross-section of rock here let's say an alignment and alignment tour the North Pole looks like this and what I draw it like that that's kind of an arrow pointing into our screen and let's 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 would expect is the newer rock that kind of the alignment the the field the the alignment of the rock would go into the screen and then the older rock it would still go older into the screen let me so if I were to draw top view let me draw it like this just so I make sure that everyone is on the same page let me address draw a cross section like this so what we know we're talking about let me draw a cross section like this and so this is the surface up here this up here is the surface when I talk about going into the page so when I talk about 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 it means so if I were 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 changed then lava that essentially turns into hard cools down into non lava rock or you can say freezes into rock it would all point into the same direction regardless regardless of when it hardened regardless of when it Harvin hardened this it would be the situation in a constant magnetic field but what we've seen is that that's not the case when you look at older magnetic rock older magnetic rock and depending on how old you go you have the newer rock that's aligned with our current magnetic field you go a little bit older and right now we think it's about 780,000 years ago roughly you have to find rock of that age magnetic rock that hardened at that time it's actually in the opposite direction so the so it's actually the magnetic rock has hardened in a way so that it's pointing it's as if the is it's 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 conclusion that the only reasonable conclusion that we can draw from this is that Earth's magnetic field has actually fluctuated over time or its so magnetic field magnetic field fluctuates magnetic field fluctuates now you're probably thinking Sal how is this relevant to plate tectonics well once you accept that magnetic fields fluctuate over the history of the earth there's another interesting observation you can make about the rock on the that's kind of at the basin of the ocean floor so not only do you have this do you have you know the middle this is this mid-atlantic ridge do you have these these volcanoes a spewing 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 that the seafloor is also contains a lot of magnetite which is magnetic and what's really interesting about that so let me draw so let's say that this is so where I have a top view just like we have over here so let's say this is this is the mid-atlantic ridge right here so mid mid-atlantic mid-atlantic or at lanta k-- ridge now this is really cool so when they look at rocks that are very close to the mid-atlantic ridge they're aligned and once again we're looking at rocks at the floor of the ocean they're aligned in a way that you would expect with the current magnetic field they are aligned just like that the way that you would expect when you're looking at the magnetic rock that's close to the ridge that's close to the ridge but if you go off a little bit further and let's say a little bit I'm talking about you know thousands of miles but when you go further out from that you have stripes of other you have other magnetic rock that is going in the opposite direction when it's going in the opposite direction it's going like this it's going like this and what's even cooler than the idea that it's switched directions depending on how far you've gone from the rift is that there's a symmetric there's a symmetric stripe of magnetic rock on exactly the same distance or you know roughly the same distance away from the rift that's also pointing in that same direction 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 that's pointing in the original direction and even better you go in the symmetric other side of the actual Ridge and you find another set of rocks that's doing the exact same that's doing the exact same thing so if you accept if you accept that Earth's magnetic field has kind of been flip-flopping over time the only reasonable conclusion at least that I could think of or and that the geologists can think of is that sure all of this all of this was formed all of this was formed at a similar period in time this came out as lava magnetic lava and then it all aligned with Earth's magnetic field and that's why it looks similar you fast forward in time some you fast forward in time some and the only way 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 way in exactly the same way exactly the same distance and at some point they were much closer to each other if they were actually connected so if we rewind in time maybe at the mid-atlantic rift you had all of the purple rock you had all of the purple rock coming out from those underwater volcanoes and at that time at that time Earth's magnetic field hat was the opposite as it is right now and then of course you had this blue rock you had the 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 in the exact at the same time and so if if this is the case if at 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've formed at the same time and that based on the pattern it really does look like they do and it's a symmetric distance away from that rift then the only reasonable conclusion I can think of is that the rift has had to move apart the wrist 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 that is the most definitive evidence in the 1960s where it kind of became conclusive that you did have these plates that were moving away from each other you did have these moot plates that were moving away from each other and obviously if the plates are moving away from each other at some point and that means that you know just based on the way the map looks at some points they're also going to be moving into each other we could talk more about that in future videos but you know right at certain points they're actually moving on one plate is moving another under another and we'll talk about how that might partially explain and we'll talk about all of the explanations for why we think the plates might actually be moving but now if we fast forward to more present times now that we have GPS satellites and all the rest we can actually measure the movement of the plates this is actually a this is actually an image from NASA showing the vector of the movement that's at different points on the surface of the planet and you could say we've you can see we've gotten a lot of vectors from the United States so it's almost it's hard to read since it's so chock-full of vectors you can see right over here in Hawaii the Pacific plate at that point is moving in this Northwest direction as measured by GPS satellites and I want to make clear this movement is relatively slow it's roughly the speed at which fingernails grow but if you do it over millions of years that actually amounts to thousands of miles so it's you know we're talking on the order of about a centimeter of year for most of the plate some of the plates might be moving a little bit faster maybe close to 10 or 15 centimeters but most are moving about a centimeter a year at the same rate your fingernails are going but this is fascinating so we can actually measure it because GPS is so accurate over here looks like the North American plate is kind of rotating generally in that direction we have you know I could you know the Nazca plate right here is moving is moving is moving roughly in that direction moving into the South American plate I'll leave you there right now and actually let me just actually before I leave you there this is another thing that I got off of Wikipedia that shows that same magnetic striping it's maybe a slightly neater drawing 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 think about some of the theories as to why they might actually be moving