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Plate tectonics: Geological features of divergent plate boundaries

Plate Tectonics -- Geological Features of Divergent Plate Boundaries. Created by Sal Khan.

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  • hopper happy style avatar for user wknehrboss
    Why does the magma melt through the lithosphere and thin out the crust why doesn't it just melt a path all the way through the crust?
    (18 votes)
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    • piceratops ultimate style avatar for user Daniel Westberg
      Due to the cooling effects, the magma simply cannot penetrate the crust without crystalising while rising towards the surface. The effect is twofold. Firstly, temperature increases as preassure increases and preassure increases as depht increases. This means that the closer to the crust, the cooler it will be. Secondly, due to heat transfer to surrounding rock, you eventually loose so much energy that the magma crystalises (freezes). As a side note, magma is not that hot, just a couple of thousand degree C. The oceanic plates that are pushed down into the earth at subduction zones can sink all the way down to the outer core before they melt completely.
      (19 votes)
  • spunky sam blue style avatar for user THE STIG
    if gravity can pull anything down, how is the magma going up?
    (8 votes)
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  • aqualine ultimate style avatar for user Tim Lawrence
    how dos the land get thinner? shouldn't it form a mountain or something if the bubble is going up?
    (11 votes)
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    • piceratops ultimate style avatar for user Nicolas Posunko
      The way I understand it, the bubble isn't actually going up at first, in the sense of hot magma erupting. First it creates the pressure in the process of kind of "trying to come out", which results in a big crack. This crack is what leads to land getting thinner, because where the crack's formed there's less rock left, since it's cracking open. Eventually, when hot magma does start to come out along the ridge mountains do form indeed. The combination of the cracking motion and the formation of the mountain range out of solidified magma in the centre leads to land being thinned out on the whole, but with an uplifted mountain range in the centre.
      (7 votes)
  • leaf orange style avatar for user Mohamed Saleh
    Could this be happening anywhere around the world, or is it limited to the borders of the plates? Like can a divergant plate be formed say in the middle of the South American plate? Why/Why not?
    (5 votes)
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  • piceratops seed style avatar for user bitterminion1
    What would happpen is the plates stop moving what would happen to socity and the food chain?
    (4 votes)
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    • duskpin ultimate style avatar for user Betty :)
      Well on one hand there'd be less earthquakes and volcanic eruptions, which would decrease the amount of life and money lost through damaged buildings and lahar, lava and pyroclastic flows. This would be beneficial especially to South - East Asian countries and the nations along the ring of fire, which experience many earthquakes and significant volcanic activity due to their location on a subducting plate boundary.
      However, a lack of geological activity would also result in a lack of soil fertility. Take Australia for example. Part of Western Australia, especially the Pilbara granite craton, which has been geologically inactive since the Archean eon, has incredibly low soil fertility. The significant amounts of weathering and erosion that takes place over billions of years has swept away any useful nutrients or minerals plants might need to grow. The lack of healthy topsoil is what's responsible for the high soil erosion in that area too, because there is a lack of vegetation (their roots hold and clump together soil, decreasing the rate of soil erosion) due to dry and low-nutrient soils. The billions of years of weathering has also meant that Australia has a relatively low relief on the Western side. This results in low rainfall as clouds don't often precipitate over the central Western side of Australia, and thus- no rivers, or if any- slow running streams that are inefficient and ineffective at removing excess salts from the soil. This results in a relatively high salt content within the soil, and contributes to the lack of soil fertility there. From this it is clear that volcanic activity would greatly benefit this dry, parched side of country, as condensed ash and igneous rock formed after a volcanic eruption erode and weather quickly into the soil, returning nutrients and valuable minerals to the soil. Basalt-rich soil developed from Eastern Australia's hotspot activity in the past 20 million years has seen a spike in fertility in comparison to the West side.
      Ultimately, a complete lack of tectonic activity would be very unlikely on Earth, as it's what has been occurring for billions of years since the formation of the lithosphere. But if it were to happen, the above hypotheses may paint a picture for you.
      (5 votes)
  • duskpin ultimate style avatar for user Anisha
    How can the land be stretched out if the Earth isn't getting bigger? Or is it getting bigger?
    (3 votes)
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  • blobby green style avatar for user Usman Sharif
    why doesn't this process rips apart our existing plates into two ? e.g. why isn't the african plate ripped into two due to the magma below ? cos if the plates were first formed by fracturing of the lithosphere then surely more fracturing should happen even now !
    (4 votes)
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  • blobby green style avatar for user s1927210
    what makes volcanos to stop erruptinng
    (5 votes)
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  • aqualine ultimate style avatar for user Gracie Jarrell
    what makes the volcano erupt?
    (5 votes)
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  • starky tree style avatar for user DiemLe
    When mountains form because of the plates, does the mountains become new land?
    (4 votes)
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Video transcript

Before we go into possible theories as to why plates actually move, what I want to do in this video is think a little bit about why we see the geological features we do see at plate boundaries. And in particular, I want to focus on the features we see at divergent plate boundaries, where the plates are moving away from each other, or where new land is being created like we saw in the mid-oceanic ridges, where we see new land being created right in the center and moving outwards from them. So to do that, let's think about the different layers. And actually I want to make one quick correction on the last video. Over here I had drawn these arrows going in that direction. And based on how I defined them they should have been going into the page. And so they should have had these X's there. Now, with that out of the way, let's draw a little diagram of what happens in the early stages of these divergent plate boundaries. So you might have your just your crust, and maybe it's continental crust. So this right here is the Earth's crust. And then you have the solid part of the mantle, and the combination of them is the lithosphere. And then you have the liquid part, or the super hot part of the mantle. So this down here is magma. It hasn't solidified. It's hot enough to be in the liquid state. And all of this combined, so this right here we consider the mantle. Now, there's some debate, and we'll talk about this in the next video, of how hot spots actually form. It could be these mantle plumes that start at the border between the mantle and the core. It could be some type of convection currents in the actual mantle. We'll talk more about that in the next video, or maybe a few videos from now. But let's take it for granted that hot spots form in the mantle. So let's say we have an area of magma right here that is particularly hot. Let me do this in another color. I'll do it in pink. So this is particularly hot magma here. And we know, or maybe we don't know, well, you'll learn right now, if you take the same material and you make it hotter it's going to become less dense, because the particles essentially are going to bump into each other with more kinetic energy and have more space in between them. And so this really hot part of the magma, or this really hot part of the mantle, it is going to move upwards, because it is less dense. It will have buoyancy. And as it moves upwards, it will heat up the things around it. And it will eventually make its way into the lithosphere. And it'll kind of be able to break through the lithosphere because it's so hot it can kind of melt its way through. So let's fast forward this. Let's fast forward this a little bit. So this is step one up here. Now step two. This hot magma is rising now through the lithosphere, and so it's going to create a hot spot. It's going to create a dome in the lithosphere and actually on the crust. And so it might look like this. So the crust is now going to have a dome in it. And this was the original lithosphere. And it's now kind of been broken in two by this hot spot. So the lithosphere is now broken in two, or it's about to be broken in two by this hot spot. So all of this is still the lithosphere. I'll just write litho for short. This up here is the crust. And if you take any rigid material, and the crust and the lithosphere for that matter, they're rigid, and you push outward on it, it won't stretch nicely like a nice elastic balloon. It'll start to crack and have to be pulled apart in order to kind of take the pushing from below. So this crust is going to start to crack. And actually, the best example where you see this is actually in like sourdough bread that has really hard shells around it. You see sourdough bread. Let me see if I can draw a roll of sourdough bread. It has all of these cracks in the surface. And that's because the outer layer, the outer shell of the bread, is really rigid. And so the inside heats up and the surface area has to expand. These kind of rifts form in the bread to allow that kind of rigid shell to actually expand. And that exact same thing would happen to the crust, or actually the entire lithosphere. So let me draw this hot spot again. Let me do it in that pink color. Now the hot spot has gotten this far. This is the hot magma right over here. And if we fast forward even a little bit more, then you could actually have the crust starting to be fully pulled apart. So you fast forward a little bit more, the bottom boundary of the lithosphere maybe now starts to look something like this. The magma has kind of broken through the hardened part, the rigid part of the mantle. So maybe it looks like this right now. You have your hot spot right over here. It's gotten that far now. And the crust on top, the actual what we would kind of normally see has now been pulled apart to kind of have to cover this new surface area. So now it kind of looks something like this. So it's been pushed apart. Let me see how well I can draw this. So now it's been pushed apart, and as it gets pushed apart it kind of thins out a little bit as you can imagine it doing. It's almost exactly as the bread analogy. When you look at bread like this the rift, the depressions where it was expanding most vigorously, those parts of the bread are actually thinner, like these parts of the bread are actually thinner, and they're not as hard as the parts that moved away. And you see that exact same thing happening with the land. And all of this stuff is continuously getting pushed outward, essentially to kind of make space for this hot spot. Now, this step right over here, you might have a volcano or two, but more important, you're going to have what's called a rift valley. Right now we're assuming that we're not below sea level yet, or we're assuming that this kind of depression in the land that you see here hasn't come in contact with another body of water. And so it'll just kind of become a little valley in between higher land. And you actually see that on Earth. And the most famous is the African Rift Valley. That's right about this region here. Actually, I have a better diagram that depicts the African Rift Valley right over here. It's this whole region of Africa is actually kind of a big valley created by a hot spot right over there. Now, as the hot spot kind of keeps maturing eventually some of the rift will become so depressed that it will actually be below sea level. Remember, all this land up here is being stretched apart. So let me go to the next step. The next step will be right over here. The land on top is now maybe below sea level in this next step, and it comes in contact with maybe an ocean or a sea. And so now it might look like this. So now the land is super thin on top. I'll do my best to draw it. So it's super thin on top, and remember it kind of keeps getting pulled apart from this bubble of hot magma that's essentially coming up from below. Let me draw it like this. This is all solid rock here. What I drew in orange is the crust. This is kind of the rocky part of the mantle. So the combination is the lithosphere. And now you have the hot magma coming up like this. And it might peek through every now and then and create a volcano there. Maybe it'll peak through and create a volcano there. But in general, it's going to keep pushing the land up and outwards. And so this land, even though you're saying hey, it's being pushed up, because of the outward motion, this land over here is going to be lower than the land around it, like the loaf of bread. If it gets low enough and comes in below sea level actually and comes in contact with a body of water, or even if it doesn't actually, water will start to gather over there. And once again, we actually see that in the rift forming between the African and Arabian plates. The Red Sea is actually an example of exactly that. The Arabian Plate is moving away from the African Plate because of this hot spot. This is pushing all of the land up and out right over here. And so this is going out. That is going out. It's moving outward in every direction. And so it creates these depressions where water can flow inwards. The Rift Valley hasn't had water flow into it the way the Red Sea has just yet, but if it kept happening eventually it's going to get low enough so that the water will flow into it. So the Red Sea is exactly that. You essentially have the Indian Ocean flowing into this rift that formed from this hot spot. And then if you fast forward a bunch so that finally the magma can kind of surface. So let's fast forward from even this point even more. So let's fast forward even more, and let's say now the land has been pushed a good bit apart. Now the hot spot has actually surfaced. Now the crust might look something like this. So it's been pushed apart a good bit at this point. Now we're talking about on the order of hundreds of thousands of years or tens of thousands of years. So the land, for example, the land that was here, this part of the land might now be out here. And this part of the land might now be out here. What's going to happen is that this hot spot is going to continue to fuel, and we're assuming everything's underwater at this point. Since this depression that was created is now so low the crust was stretched thin. We're going to assume that all of this is underwater. The hot spot is essentially going to come out of underwater volcanoes and start creating what's now-- this body of water's gotten large enough that we can call it a mid-oceanic ridge. And so it'll actually start creating an actual ridge with volcanoes in the center. So that's why one, we see things like the Rift Valley in Africa, we see things like the Red Sea. And maybe even more importantly, that's why we see something like the mid-Atlantic rift in the middle of the Atlantic Ocean, where you have all of this depressed land that was essentially analogous to that Rift Valley but it's at a much later stage. And that's why it's able to collect water, because when the land was pushed out and stretched thin water was able to flow into it, going back to the bread analogy, essentially when this bread was baking and this part of the crust pushed outwards, you had this rift form, and then if there was some water on the bread, or if it was raining, or if it was connected to a body of water, water would've eventually flowed in here. And if that bread kept growing this rift would have kept growing, eventually to the size of the Atlantic Ocean in our theoretical bread. And so that's why you have this huge depressed area where the ocean can form, but in the middle of it you kind of have this you have this submersed you have this actual submersed mountain chain, this submersed chain of volcanoes, this submersed ridge where the land actually does go up a little bit because of all that magma flowing directly out of it. So hopefully that clears up a little bit. That was always confusing to me why you see uplifted land but then everything around the uplifted land is much lower, and why the whole thing is submersed as it's moving away. So hopefully that clears things up a little bit.