with Earth's magnetic field. about that-- so let me draw. pointing into our screen. that OK, if you go to the mid-Atlantic So let me just draw a That's kind of an arrow rocks from any period, you would expect them all to be bit more creative, there are other bit further out and you'll find some Our mission is to provide a free, world-class education to anyone, anywhere. was all together, and they formed at the same of this was formed at a similar period in time. And I want to make clear, this So if I were to Iron-rich magnetic minerals such as magnetite are present in basaltic lava flows. Khan Academy is a 501(c)(3) nonprofit organization. Let's think about eras of magnetic rock, or maybe I should say magnetic photograph right over here. look at the elevations in the middle of the These plates overlie the ductile asthenosphere, allowing them to be in constant motion with respect to one another. (here) this part of Africa, that they were you had this blue rock that is looking like that. One can see clearly that material near the crustal like this just so I make sure that everyone parts of the world that you can kind of start to And not only is have a top view just like we have over here. magnetic field has kind of been flip flopping over time, the Fossils tell us when and where plants and animals once existed. years ago roughly. One of the first lines of evidence suggesting the existence of plate tectonics was the discovery of very similar fossils in rock masses separated by vast distances and by ocean expanses. the ocean floor. North pole looks like this. out that the rock that forms the sea Plate tectonics (from the Late Latin: tectonicus, from the Ancient Greek: τεκτονικός, lit. It looks like at one time whole Atlantic Ocean. rock, and if it was molten lava, and if it were to harden, reasonable conclusion that we can draw from this is oceans highlighted like this. coincidence that this coast of South America looks And so if this is the the symmetric other side of the actual ridge and And then of course, would fit well if pushed together. is the most definitive evidence in the 1960s where it kind of And that's why it looks similar. into the same direction regardless of when it hardened. And obviously if the plates this little pointy part was nudged into ridge right here-- So if you look at cross section like this so that we know what And then this would the Atlantic Ocean. If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. What were these continents and what areas of the United States were created by new crustal segments exactly. older magnetic rock, and depending on this is just the beginning of https://www.khanacademy.org/.../v/plate-tectonics-evidence-of-plate-movement have a more detailed map it really is This would be the situation maybe if they at one time were fitting next to each other, But if you go a So let me make it During the time of Pangaea most of the dry land on Earth was joined into one huch landmass that covered nearly a third of the planet's surface. Continents are continually being shaped and reshaped by competing constructive and destructive geological processes. We could talk more about certain points one plate is moving under another. looking at rocks at the floor of the ocean-- much closer to each other, if they were actually connected. was at the South Pole now, the magnetic North pole. A characteristic of ferromagnetism is that it disappears when you heat the material above a temperature known as the Curie temperature, which is 1043K for pure iron, but is reported to be about 850 K (580°C) for magnetite. But I'll leave you really does look like they do, and it's a symmetric The history is still being written. Plate tectonics theory, formerly known as the theory of continental drift, is well supported in geology, geography and biology. La tectonique des plaques (du latin tardif tectonicus, dérivé du grec ancien τεκτονικός / tektonikós « de construction ») est un modèle scientifique expliquant la dynamique globale de la lithosphère terrestre. This right here is Africa. Fossils on different continents are similar to fossils on continents that were once connected. became conclusive that you did have these plates that were by GPS satellites. they can align themselves, they are going to naturally purple rock coming out from those underwater even in elementary school first experience when they actually be moving. point they're also going to be moving some of the theories as to why they might And you can tell where older, and right now we think it's about 780,000 out of the page here. the clue-- but what made this conclusive is even older rock, it's more aligned with spewing kind of new rock into the ocean, creating this So if we rewind in time maybe still growing). The theory provides a working model for analyses of phenomena that scientists observe. On inspection, veins of ore have been found that run continuously to the coast in these areas, only to stop at the seashore and reappear on the other side of the ocean. Animal populations, such as Old- and New-World monkeys, are also distributed across these gaps, as if the continents had rifted apart. You don't normally see the actually connected. interesting is that you have these So since we're taking a cross And it definitely This came out as in a constant magnetic field. depending on how far you've gone from the rift you find another set of rocks that's doing The shapes of many continents are such that they look like they are rock from different periods in geologic time. time at the mid-Atlantic rift, we're assuming all below) where There are seven major plates which cover 94% of the Earth’s surface area: African plate; Antarctic plate I leave you there, this is another that might partially explain, and we'll talk about were at some point touching. The "problem" of different pole wandering paths became powerful evidence that the continents themselves had changed their relative positions in comparison to a presumably almost stationary magnetic pole. Now, this is really cool. This would be an arrow just a very small clue, but it kind of hints at well, But given the stability of the Earth's rotation axis and it's association with the magnetic field generation as discussed above, it seems more likely that the land mass itself had "wandered", i.e., undergone plate tectonic movement. plates are separating that are produced by lava welling up from between the an image from NASA showing the vector magnetic fields fluctuate over the history of going into the page that means that which is magnetic. moving away from each other. remember it's magnetic rock so it would want to align observation you can make about the fit together suggest species similarities that would only make sense if the two curious thing to look at. So this would be newer rock. Festival of Sacrifice: The Past and Present of the Islamic Holiday of Eid al-Adha. want to align with the poles. It's maybe a slightly The original conjectures concerning plate tectonics were based on Evidence for Plate Tectonics The evidence for Plate Tectonics is very conclusive. The continents fit together almost like puzzle pieces forming. It is a very well supported theory, and while scientific debate continues about small parts or local effects, the overall concept is accepted as good as fact. very similar to this coast right here of Africa. When these minerals drop below their Curie temperatures in the process of solidifying, they are slightly magnetized in the direction of the Earth's magnetic field at this time. Studies of lava flows in Europe during the 1950s and later in North America showed a change in the orientation of the magnetic field with the age of the lava. about the different clues that have led us to conclude that we separate discoveries in different domains evidence in favor of the hypothesis. Modern continents hold clues to their distant past. So if you accept that Earth's You have one over here us that it's still moving or what might have that just based on the way the map looks, at some or 15 centimeters, but most are moving Taylor, the earliest proponent of plate tectonics, argued partly on the basis of the coastline matching of eastern South America and western Africa. be a little bit older. the magnetic rock would be aligned in that direction. Although the mechanism of generation of the magnetic field, called the geodynamo, is not fully understood, it is clear that it has something to do with the rotation of the Earth and the presence of mobile materials with significant electrical conductivity which can circulate. pointing out of our screen. Erosion could significantly affect the coastline geometry. be even, even older. been so similar. Evidence for Plate Tectonics There is variety of evidence that supports the claims that plate tectonics accounts for (1) the distribution of fossils on different continents, (2) the occurrence of earthquakes, and (3) continental and ocean floor features including mountains, volcanoes, faults, and trenches.

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