地球的构造

what I want to do in this video is really make some clarifications and go a little bit more in detail about the different layers of the earth so let me draw let me draw a cross-section of the earth over here and I'll try to do it I won't be able to do it perfectly to scale but I'll try to do a little bit better job and giving you a little bit of a sense of how thick these layers are so let's say that this is the crust up here and I'm gonna make the continental crust a little bit thicker so let's say that that is continental crust and this is continental crust and then in between let me put some oceanic crust which is going to be thinner so this right here is actually within a different color let me do the oceanic crust in blue but this isn't water this is rock I'll do it in purple that's even better I don't want it to be that thick so let me draw the oceanic crust is thinner than the continental crust which I'm trying to depict right over here so this right over here is oceanic oceanic crust and up here is continental crust continental continental crust and the thickness or how deep you can go and still be in crust it depends on where you are and we know that near hot spots the oceanic crust can actually thin out a good bit but roughly when we talk about the crust we're talking about something that's thirty to sixty kilometers deep so thirty to sixty kilometers deep so if you are on a continent which I'm assuming you are and you dig for 20 kilometers you will still be in the crust 30 kilometers probably still in the crust if you dig for 70 kilometers or 100 kilometers you will probably reach the mantle and remember what we're describing here when we talk about the crust the mantle and the core we're talking about the chemical makeup let me make this clear we're talking about the chemical makeup the chemical makeup the crust is fundamentally different than the mantle based on the molecules that it is made up of made based on its composition so let's talk about the mantle now so the mantle layer like this and once again this is not to scale because the we're talking about 30 to 60 kilometers the mantle we're talking about on the order of about 2900 or 3000 kilometers thick so this right here is the entire mantle so that's the mantle and this is 2900 to 3000 3000 kilometers thick so this isn't even one thirtieth of that so I would have to draw it even narrower than the way I've drawn it over here and the mantle itself can be subdivided into the upper mantle and the lower mantle so let me draw this division we draw this division right over here the upper mantle and there's different ways to define the boundary but you could the upper mantle is roughly about 700 kilometers down so these are huge distances I mean this is going straight down so this is the upper mantle let me write it on the actual mantle here this is the upper mantle and this over here is the lower the lower mantle and just to be clear on things so the crust is a solid now when you go into the upper mantle the upper part of the up upper mantle and we'll talk about that a little bit more is cool enough to be solid so there is a solid portion there is a solid portion of the upper mantle so all of this up here is solid because it's cool enough it hasn't reached the melting point of of those of those rocks just yet and we learned in previous videos that the combination of the solid part the solid part of the upper mantle and the crust combined we call that the lithosphere and we talked about the lithosphere we're not talking about the mechanical make up we're not talking about what's solid and what's not solid so this is the lithosphere this is the lithosphere you go a little bit deeper right below the lithosphere now the temperatures are high enough for and I use the word liquid but that's not exactly right you can kind of think of it as kind of a deformable solid or a plastic solid or a magma and that's the asthenosphere so this area right over here this layer area right over here the liquid part actually I shouldn't use the word liquid kind of deformable if forms over long periods of time but it is it is it is more fluid than what we normally associate with rock magma would be a good way to think about it that's what we call the asthenosphere it is fluid just not as fluid as water it is more viscous than something like water so this is the asthenosphere stano's sphere now the upper mantle is able it's hot enough for the the rock to melt and be fluid and the pressure is low enough for two not four to still be able to kind of move past it itself to distill to have to be somewhat fluid but then once you get even deeper into the lower mantle into the lower mantle you even hot you have higher pressure higher pressure and so it has actually it's still fluid but it's less fluid it's less it's it's it's kind of thicker I guess it's the best way to think about it in the lower mantle it's thicker so this whole area over here you could kind of think of it as melted rock it's it's fluid but the upper part of the melted rock can it's more fluid it's able to move easier because there's less pressure and the pressure is just from all of the rock that's above it remember gravity is pulling down on everything gravity every every molecule here wants to go downward because of gravity so it's it's applying pressure downwards so the deeper you go the more pressure you get now when we get even deeper than that we get to the core and the core is divided between the outer core and the inner core so the our outer core outer core outer core and then of course you have the inner core the inner inner core and just so we have a sense for distances the width or the thickness of the outer core is approximately 2,300 kilometers so these are huge distances when you think about thickness you could go down another 2300 kilometers and you're in the or once you go through the mantle you can go 2,300 kilometers through the outer core and then you're in the inner core and that essentially takes you to the rest that's essentially the center of the earth and the inner core and maybe I should draw it maybe I should draw the boundaries a little bit more to scale let me do it this way it should actually look a little bit more like a little bit a little bit more like this because the outer core is thicker than the inner core so the outer core is as I said let me write rewrite it outer core is on the order it's about 2,300 kilometers thick and then you have your inner core you have your I shouldn't do it in blue I should do it in the hot color so the inner core right over here is just kind of takes us to the center of the earth and that's a little over a color that's over a little over a thousand kilometers thick so this is the inner core inner core the number I have is about 1200 1200 kilometers thick and both the the entire core when you're both the outer core and the inner core is mainly nickel and iron and think about when the earth was forming what happens is when this whole earth was super hot and it was kind of in a fluid state the heavier elements were allowed to the heavier elements were allowed to sink down when everything was fluid the things that are in between would kind of were the things that were lighter would go up and then the gaseous things that would naturally be in the gaseous state would kind of bubble up would kind of bubble up through that fluid you know kind of the way the way actually you know carbon bubbles up in a soda it would eventually bubble out of the fluid and it would actually form the atmosphere so that's why when you look at the composition of the earth you have the densest the heaviest elements in the center and then the lightest elements are forming the atmosphere and the outer core in the inner core they are made up predominantly of nickel and iron nickel and iron and their makeup is actually very similar so this division chemically they are they have a very similar composition what's different about them is at the outer core you have temperatures high enough that nickel and iron can melt but the pressures are low enough that they can still be in a fluid state so this is our liquid this is our liquid outer core and this has a pretty low viscosity especially even relative to the mantle so that's why people kind of consider this in kind of a more traditional liquid state but as you get deeper and deeper and deeper the pressure becomes so huge as you get to the inner core remember all of the weight of every of all of the rock above you have these thousands of miles of Rock you is all pushing down on the rock below it so the inner core even though the temperature is really really really hot the pressure is so big that the molecules can't flow past each other they can't be liquid they're kind of jammed packed and so the inner core because of the high pressure despite the high temperature is solid it's solid so the difference here is actually a mechanical one between the outer core and the inner core they're made up of the same things roughly the same chemical makeup it's just slightly lower lower pressure on the outside so you can actually be in a fluid state so hopefully that clarifies and gives you a little bit of depth on on the makeup of the earth