Plate tectonics: Geological features of divergent plate boundaries Plate Tectonics -- Geological Features of Divergent Plate Boundaries
Plate tectonics: Geological features of divergent plate boundaries
- 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,
- 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 be going into the page
- and so they should've had these Xs here.
- Now, with that out of the way, let's draw a little diagram
- of what happens at the early stages of these divergent plate boundaries.
- So you might have your... here you have just your crust,
- and maybe it's continental crust...
- 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, the superhot 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, this right here we consider the mantle.
- Now, there's some debate, and we'll talk about this in the next video,
- of how hotspots 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 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 in in another color... pink.
- So this is particularly hot magma here.
- And we know, or maybe we don't know, you'll learn to know 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,
- 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'll be able to break through the lithosphere because it's so hot it can melt its way through.
- So let's fast forward this a little bit,
- so this is Step 1 up here.
- Now Step 2 - this hot magma is rising out through the lithosphere,
- and so it's going to create a hotspot, a dome in the lithospere 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 is the original lithosphere,
- and it's now kind of been broken into by this hotspot.
- So the lithosphere is now broken into, or about to be broken into by this hotspot.
- All of this is still the lithosphere, I just write 'litho' for short,
- this up here is the crust.
- And if you take any rigid material, and the crust and the lithisphere they are rigid,
- and you push outward on that, it won't stretch nicely like a nice elastic balloon,
- it will start to crack, it have to be pulled apart on order to kind of take the pushing from below.
- So this crust is going to start to crack.
- Actually, the best example where you see this is actually in like sourdough bread
- that has really hard shell around it.
- You see sourdough bread, let me see if I can draw a loaf of sourdough bread.
- It has these cracks on the surface
- and that's because the outer layer, the outer shell of the bread is really rigid
- and so if the inside heats up and the surface really has to expand,
- these kind of rift form in the bread to allow that kind of rigid shell to actually expand.
- The exact same thing would happend to the crust, or acutally the entire lithosphere.
- So let me draw this hotspot again.
- I'll do it in the pink color.
- Now the hotspot has got this far.
- This is the hot magma right over here.
- And if we fast-forward a little bit more, the crust starts to be fully pulled apart.
- So you fast-forward a little bit more,
- the bottom boundary of the lithosphere might be now start 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 the hotspot right over here.
- It's got that far now.
- And all the crust on top, what we normally see,
- has now been pulled apart to kind of have to cover this new surface area.
- So now it looks something like this.
- 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's almost exactly as the bread analogy.
- When you look at bread like this, the rift, the depression which is expanding most vigorously,
- those parts of the bread are actually thinner and they're not as hard as the parts that moved away.
- And you the exact same thing happening with the land.
- And all of this stuff is going to start getting pushed, continuously getting pushed outwards,
- essentially to kind of make space for this hotspot.
- Now, this step right over here you might have a volcano or two,
- but more importantly, you're going to have what's called a rift valley.
- Right now we're assuming that we're not below the 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 become a little valley in between higher land.
- You actually see that on Earth.
- And the most famous is the African Great Rift Valley.
- That's right about this region.
- Actually, I've got a better diagram that depicts the African Rift Valley right over here.
- This whole region of Africa is actually a big valley created by a hotspot right over there.
- Now, as a hotspot kind of keeps maturing,
- eventually some of the rift
- will become so depressed that'll actually be below the sea level.
- Remember, all of this land appear 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 comes in contact with what we view 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 super-thin on top, and remember it's keep on getting pulled apart,
- from this bubble of hot magma that's coming up from below.
- Let me draw it like this.
- This is all solid rock here.
- What I do 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 peak through every now and then and create a volcano there,
- but in generall it's keep pushing the land up and outwards.
- And so this land, even though you say: 'hey, it's been 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,
- it get low enough, below sea level actually, it comes in contact with the 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 hotspot.
- 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 flown into it, the way the Red Sea has just yet,
- but if it kept capping, eventually it's get low enough so that the water will flow into it.
- So the Red Sea it exactly that.
- You essentially have the Indian Ocean flowing into this rift that formed from this hotspot.
- And the if you fast-forward a bunch so that finally the magma can kind of surface,
- so let's fast-forward from this point even more,
- and let's say now the land has been pushed a good bit apart.
- Now the hotspot is actually surfaced.
- Now the crust might look something like this.
- So it's been pushed apart a good bit at this point,
- you know, now we're talking about on the order of hundreds or tens of thousands of years,
- for example, the land that was here might now be out here,
- and this part of the land might now be out here.
- What's going to happen is that this hotspot is going to continue fuel,
- and we're assuming everything is under water at this point,
- since this depression that was created it now so low,
- the crust was stretched thin,
- We assume that all of this under water.
- The hotspot is essentially going to come out of underwater volcanoes
- and start creating what's now in this body of water that's got large enough
- that we can call it a mid-oceanic ridge.
- And so it'll actually start creating, I'll set a different color,
- it'll start creating an actual ridge with volcanoes in the centre.
- So that's why One: we see things like the the Rift Valley in Africa,
- we see things like the Red Sea,
- and maybe more importantly, we see something like The Mid-Atlantic Rift in the middle of the Atlantic Ocean.
- Where you have all this depressed land, that was essentially analogous to that Rift Valley,
- but is at a much later stage and that's why it's able to collect water,
- because the when the land was pushed out,
- it stretched thin, water was able to flow into it,
- going back to the bread analogy, it's essentially when this bread was backing
- in this part the crust pushed outwards, you have this rift formed,
- and then if there was some water on the bread or it was raining
- or it was in kind of body of water, water would eventually flow into here.
- And if that bread kept growing, this rift would have kept growing
- eventually in the size of the Atlantic Ocean, our theoretical bread,
- and so that's why you have this huge depressed area where an ocean can form
- but in the middle of it you have this submerged mountain chain,
- this submerged chain of vulcanoes, this submerged rift
- 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 a little confusing to me,
- why you see uplifted land and everithing around the uplifted land
- is much lower, while the whole thing is submerged as it's moving away.
- So hopefully that cleared things up a little bit.
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