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

If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked.

Main content
Current time:0:00Total duration:11:58

Plate tectonics: Geological features of divergent plate boundaries

AP.ENVSCI:
ERT‑4 (EU)
,
ERT‑4.A (LO)
,
ERT‑4.A.2 (EK)

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 the features we see at divergent plate boundaries where the plates are moving away from each other where new land is being created like we saw in the mid-oceanic ridges where we see a new land being created right in the center and moving outwards from them so to do that let's think about the different layers 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 define them they should be 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 at kind of at the early stages of these divergent plate boundaries so you might have your you might here you have just your crust and maybe it's continental crust so this right here is the Earth's the Earth's crust and then you have the solid you have the solid part of the mantle and the combination of them is the lithosphere 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 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 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 hotspots form in the mantle so let's say we have an area of magma right here that is particularly hot let me draw it let me do this in another color in pink so this is particularly hot magma here and we know or maybe we don't know where you learn known right now if you take the same material and you make it hotter it's going to become dents 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 it is going to move upwards because it is less dense it will have buoyancy and as it moves upwards it'll heat up the things around it and it'll eventually make its way into the lithosphere and it will kind of be able to break through the liquid 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 this hot magma is rising now through the lithosphere and so it's going to create a hotspot 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 this is the original lithosphere and it's now kind of been broken into by this hotspot so the lithosphere the lithosphere is now broken in to or it's about to be broken into by this hotspot 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 the pushing from below so this crust 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 it has these cracks in the surface and that's because the outer layer the outer shell of the bread is really rigid and so when the inside heats up and the surface area has to expand these kind of rifts form these rifts form in the bread to allow that kind of rigid shell to actually to actually expand and the exact same thing would happen to the crust or actually the entire lithosphere so let me draw this hot spot again so now the hot spot I want to 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 a little bit 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 about the bottom boundary of the lithosphere maybe now starts to look something like this the the the meant the magma has kind of broken through the the hardened part the rigid part of the mantle so maybe it looks like this maybe it looks like this right now you have your hot you have your the hot spot right over here it's gotten that far now it's gotten that far and the crust on top the actual what we would kind of you know what we 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 parts that 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 you see that exact same thing happening in land and all of this stuff is going to start getting pushed 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 on a 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 on earth and the most famous is the great is the African Rift Valley that's right about right about this region here actually 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 hotspot created by a hot spot over there now as the hotspot kind of keeps maturing eventually eventually some of the rift eventually some of the rift will become so depressed that will actually be below sea level remember all of this land appears being stretched 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 is now maybe below sea level and this next step and it comes in contact with the movie an ocean or a sea and so now it might look like this so now the land is super thin on top do my best to draw it so it's super thin on top and remember it's it's kind of keeps getting pulled apart it keeps getting pulled apart from this bubble of hot of hot magma that's essentially that's coming up from below let me draw it like this this is all solid rock here what I drew an orange is the crust this is kind of the rocky part and actually I shouldn't draw it this is the rocky part of the of the mantle so the combination is the lithosphere and now you have the hot hot magma coming up like this and it might peek through every now and then and create a volcano there maybe it'll peek through and create a volcano there but it in general is going to keep is keep 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 and below sea level actually and it comes in contact with a body of water or even if it doesn't actually water will start to gather over there water will start to gather and once again we actually see that we actually see that in the rift forming 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 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 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 is actually surfaced now the crust might look something like this the crust might look something like this so it's been pushed part a good bit at this point you know now we're talking about in 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 not might now be out here what's going to happen is that this hot spot this hot spot is going to continue to fuel and we're assuming everything's under water 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 under water the hot spot is essentially going to is essentially going to come out of underwater volcanoes it's going to come out of underwater volcanoes and start creating and start creating what's now this body of water has got large enough that we could call it a mid-oceanic ridge and so it'll actually start creating we do this in a different color it'll start creating an actual Ridge it'll actually create a ridge with volcanoes in the center and so that's why that's why one we see things like we see things like the after the 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 what 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 it's essentially when this bread was baking and this part of the the crust pushed outwards you had this rift form and then if there was some water on the bread if it was raining or if it was connected to a body of water water would have eventually flowed would have actually flowed in here and that bread kept growing this rift would have kept growing eventually to the size of the Atlantic Ocean and in our 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 submerged mountain chain this is the 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 cleared things up a little bit