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Earth’s Crust Formation
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0:00
so I got a comment not too long ago relating the beginning of plate tectonics on Earth with crust formation on Earth but actually there's a big difference between tectonic plates and Earth's crust so in this video I'm going to be talking about how Earth's crust formed and the difference between tectonic plates and Earth's crust so we'll talk first about Earth's crust and the difference between Oceanic and continental crust then we'll move on to how Earth formed and then how crust
0:29
formed on Earth and how it differentiated into Oceanic and continental crust and then we'll talk about the difference between crust and tectonic plates so I talked about this a little bit in my recent Earth age or dating Earth video where I talk about the oldest oceanic crust versus the oldest continental crust basically oceanic crust is formed at mid-ocean ridges or Oceanic spreading
0:52
centers where new crust comes out from the mantle because the tectonic plates are being pulled apart allowing that to happen uh but because this new crust that forms oceanic crust is more dense than the buoyant continental crust it becomes subducted under continental crust over and over again and recycled back into the mantle and then blooms back up through mid-ocean ridges and because of this constant recycling the oldest oceanic crust is only about 260 and I've also even seen
1:22
200 Millions years old the point is that it's much younger than the earth which is billions of years old whereas continental crust is preserved for a lot longer because it's too buoyant to subduct the oldest Continental material ever dated is about 4.4 billion years old but still this is not even as old as the Earth which is about 4.55 billion years old why does the oldest continental crust
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which doesn't subduct not equal Earth's age well Earth wasn't born with crust in other words it didn't accrete and immediately have crust it took a while for crust to form and cool and also to differentiate into Oceanic and continental crust of very different material Shannon crust is very dense and more mafic meaning just contains more dense elements like iron and magnesium whereas continental crust is a lot more buoyant and more felsic containing a lot more silica and less of
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those dense materials and so it's very different material and this took a long time to not only form but also differentiate into these two major types of crust that we have on Earth today if you want to know more about how we actually got the 4.55 billion year estimate for Earth's age you can watch my how we dated Earth video but for this video let's talk a little bit about how Earth
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formed or accreted and then how the crust finally formed so like I've mentioned in a few videos the solar system formed after a supernova or Star exploded and you had this nebula of materials spinning around in space which then started to accrete or material from that nebula started to accrete to form the planets as well as the sun in the center of it and on the rocky planets or the inner planets like Earth you had material with pretty heavy elements like iron magnesium
3:11
and silicon Collide and accrete and the accretion process was held together because of gravity after it got to a certain size and then more and more accretion happened because it had more gravity and it pulled more material into it at that point Earth was very hot and molten because all of those impacts were providing heat and energy so it wasn't like a very cool or solid material it was very molten and active and still having much bombardment on it during this time in the Haitian
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Eon which went from around 4.55 to 4 billion years ago and three main factors contributed to the heat of Earth at this time these three factors included of course impacts or bombardment as well as gravitational compression and Radioactive K of Isotopes so I've talked in the earth age video as well as my stable versus radioactive isotope video about the decay of radioactive isotopes or unstable isotopes that also releases heat to Earth through time and so during this point it
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was a lot of Decay now that we've gotten you know 4.55 billion years in to Earth's lifetime there's less radioactive decay but it's still going on because some radioactive elements or Isotopes Decay much slower than others however it did eventually start to cool down and as it did it differentiated into different layers if you've ever seen a cut of Earth kind of like this one here you've seen that Earth has layers we know Earth has a solid inner core liquid outer core a
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mantle a crust and those layers formed because the dense elements as it was very molten at the time sank to the core and the less dense elements Rose to the surface or just toward the outer layers of of Earth and this is what led to the layers we see today so we see less dense silica Rich material towards Earth crust and upper mantle and more dense iron-rich material in Earth's core and the
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formation of Earth's liquid iron outer core led to the magnetosphere which has protected Earth from harmful solar radiation and solar wind throughout its lifetime and this has allowed you know things to live on earth without the magnetosphere it would be an extremely harsh environment to be on earth so how did Earth's crust actually form and differentiate into both oceanic crust and continental crust because if you think about it there's not a special mantle providing material
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to the ocean versus the continents they're very different materials but they came from the same source so how did that happen well first on Earth the very initial crustal formation on Earth was very heterogeneous oceanic crust however plate tecton Onyx got its start early and was faster on early Earth because Earth was so much hotter now I want to put a little disclaimer here that there have been debates of how fast plate tectonics really was on early Earth was it really faster
6:13
but in general we would expect that due to the intense Heat and the mantle at the time we would have expected intense convection and therefore very intense and Rapid plate tectonics but you know just disclaimer that there's research going into pretty much everything I say on my channel so don't think that I'm just saying that this is the way it was there's plenty of different hypotheses
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but this is you know one of the major ones and because of this rapid and intense plate tectonics at the time Crystal differentiation occurred and so what does this mean well partial melting of this heterogeneous material that had formed on early Earth led to the purification of oceanic crust and the formation of the first continent middle crust about 4.4 billion years ago so what does this mean what is partial melting and how did it differentiate these two types of crust
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well partial melting is when only part of a solid is melted so that makes total sense part of the solid melts and part stays behind this has been happening on Earth for a very long time in Earth's mantle and that leads to the melted part of the material the solid from Earth's interior Rising through Earth's crust coming out of volcanoes and actually forming new material at Earth's surface
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the stuff that doesn't melt fully stays behind in the mantle and doesn't come and form stuff at or Surface this is important because felsic material melts at lower temperatures than mafic material in other words if we look at this figure to the left we see that this heterogeneous material on the left of the figure begins to melt and because of partial melting or because felsic material the pink and white material melts at lower temperatures than the mavic material it
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will melt and rise and exit the system or exit the mantle whereas the mavic material will stay behind as residue because temperatures don't get high enough or typically don't get high enough to melt all of the material that's in that particular magma chamber or whatever it might be so some very mafic material is always left behind and the Melt is always more felsic than its source that leads
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to over time more and more felsic material being what comes out at Earth's surface and so now we can see why at present day at mid-ocean ridges kind of mafic dark basaltic or gabroic material is formed at these ridges to form oceanic crust because the Melt is coming from a more ultramafic source and ultramafic if you don't know what that means it's just something more mapic than mavic so basically because the source is ultramafic the material coming out of the mid-ocean ridges
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is mafic whereas when you have a subduction zone the plate that's subducting is an oceanic plate it brings down water with it because that material was hydrated when it was at a surface being oceanic crust and it brought down that water with it causing the melting temperature of that material to lower and then it causes melting when it gets to a certain depth that reaches a certain temperature and the melting of that material because it's mavic instead of ultramafic it causes
9:38
more felsic material to form and over time this has caused more and more felsic material to come out of volcanoes at subduction zones and that has caused continental crust to be much more felsic than what comes out at mid-ocean ridges which is straight from the ultrametic mantle Source but this is is a present day scenario how did this initially happen how did continental crust even start to form from the heterogeneous monolithic crust that was initially formed on Earth so step
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by step here we're going to go through basically everything that was happening on early Earth that caused the buildup of continental crust over time first and foremost we obviously had oceanic crust forming at mid-ocean ridges that Rose from mantle material just like we have today and we had re-milting of this crust at subduction zones now initially the plates that had formed were kind
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of like ocean and ocean plates and at ocean ocean convergent zones or zones where ocean plates are pushing together there can be subduction of one oceanic plate under the other so that's kind of what was going on in this heterogeneous monolithic time and you had the subduction and when you had the seduction you got the partial melting that caused a little bit more felsic material every time you had seduction and melting of that subducted plate because that subducted plate
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represented already partially melted and kind of fractionated material that was slightly more felsic than the mantle source and so that melting caused more felsic material to come out and then that felsic material was again partially melted because the next time something subducted under it it went through that material and again went through partial melting through that zone as we can see in this part labeled a here in the figure so everything going through that zone
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of more felsic material is again becoming more felsic as it partially melts that material and then eventually comes out at Earth's surface again everything becoming more felsic over time this led to the purification of the initial heterogeneous oceanic crust because it was releasing all of its felsic material at these subduction zones forming these masses or PODS of initial continental crust
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that started to not only form but treat onto each other because you had these PODS of continental crust that then headed toward each other and as they head toward each other they closed Oceanic basins because the oceanic crust would subduct underneath them because they were much more felsic and more buoyant and so as the oceanic crust subducted beneath them they then met up with each other as we could see in the bottom part of the figure here where they met and accreted
12:23
onto one another and that just formed a larger pod or mass of continental crust these masses of continental crust were forming throughout early Earth's history and by around 2.5 billion years ago 80 percent of continental crust that is on Earth today had formed since then the last 20 percent has been forming Slowly by subduction and accretion over time so in short partial melting or the formation of more felsic material at subduction zones led to the differentiation of
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Earth's crust into both Oceanic and continental crust just to round out the pig picture a little bit so you know what else was forming on early Earth at this time you had the atmosphere being developed by outgassing or gases being extruded from volcanism at the time because again volcanism was rampant because things were so hot especially right after Earth had it created and so because
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you had this volcanic outgassing they were pouring out these gases into the atmosphere or forming the atmosphere with these Gases such as nitrogen carbon dioxide methane water vapor oxygen was something that came later due to the evolution of photosynthesis and because it outgassed so much water vapor from the hydrated rocks that had accreted to form Earth this water vapor then eventually cooled and condensed After Earth had cooled below the boiling point of water to form
13:47
Earth's oceans and we also got a lot of water from comets that contained water and impacted Earth but part of it was because of the volcanic outgassing of water vapor from the hydrated interior of ever so now that we've talked about how Earth formed how Earth's crust formed how Earth's crust differentiated into Oceanic and continental crust let's talk a little bit about the difference between crust and plates because from my discussion I mean even I use terminology
14:15
that makes it sound like crust and plates are one in the same because I say oceanic crust subducts under continental crust but it's really not crust that is what plates are made of it's actually lithosphere so if we look at this figure over here of this cross section of Earth we see the inner solid core of the outer liquid core the mantle which makes up all three of the next layers because we have like the lower the middle and the upper mantle and then we have the crust which is
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this brown and green stuff not to scale I should mention the crust is actually way thinner than for example this upper part of the mantle so keep in mind this isn't necessarily to scale I'm going to show you one in a second that should be a little bit more to scale but the gist of this figure is just to show that the crust is the outer part and the lithosphere actually includes both the crust
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and the uppermost part of the mantle this brown mantle layer here and that is what makes up the broken up plates that undergo tectonics on Earth so now if we look at this figure over here we can see a bit more of a clear explanation of what's going on here and a little bit more to scale as well we have the outermost part of continental and oceanic crust here again oceanic crust is much thinner than continental crust and denser which makes sense in this figure and then we
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can also see the lithosphere or the lighter brown part here is not part of the crust it's actually the upper most solid part of the mantle why do you ask do we not just make it simple and say lithosphere equals crust and just include all of the lithosphere and what we Define as crust that is because the boundary between the crust and the mantle is defined as the moho boundary it has a longer or official name that I'm not going to try and say because I've tried to say it in the
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past and it just doesn't work out but anyway the moho boundary is the boundary between crust and mantle uh pretty simple right so why does this matter why did we come up with this boundary it is because that position or that boundary marks a distinct change in velocity of seismic waves that indicate a distinct change in material density and so we have defined the density that we see above that boundary as crust material and the density we see below that boundary is mantle and
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so therefore the lithospheric part of the mantle must stay as being defined as mantle but it is the lithosphere that makes up the tectonic plates the point here is that lithosphere equals part Crest and Par mantle and because tectonic plates are lithospheric they can contain both continental and oceanic crust so another way that it's easy to tell that it's not just the green things on this
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map riding along the blue stuff it's actually all of that material broken up into deeper segmented plates and because tectonic plates have been moving around on Earth throughout Earth's history many super continents have formed as a result and some of the major supercontinents that have formed throughout Earth's history include Colombia which formed around 2.5 billion years ago and lasted to around 1.5 billion years ago rodinia which formed and lasted from around 1.1
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billion years ago to around 750 million years ago panettia which lasted from around 630 to 570 million years ago and of course the one we all know and recognize Pangea which was kind of during the dinosaur era or before it and then into the dinosaur era from around 300 to 180 million years ago and it greatly contributed to the greatest Extinction event of all time the great dying but supercontinents represent continental crust not lithosphere even though the lithospheric plates
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are moving around causing supercontinence to form so why then are supercontinence possible if it's not just continental crust that make up the plates well it's because if you remember lithospheric plates can contain both Continental material and oceanic material and when they contain both of those and two plates containing continental crust are moving toward each other the oceanic material would become subducted under one of those Continental masses and that will eventually close
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the ocean Basin completely and the continents will eventually meet and when they do you will have two continental masses right next to each other eventually if all of those plates containing continental crust come together and close their ocean basins and connect in some sort of way they will form a supercontinent with kind of a massive ocean around them uh and then in the same or equal but opposite process they can then separate and form ocean basins from new material coming out
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of a rift or an extensional tectonic boundary and I mentioned that supercontinence just like Pangea as well as rodinia and actually all of these supercontinents coincide with major climatic and even Extinction events such as Pangea which contributed to causing the largest mass extinction of all time but you might now be wondering why supercontinents have such an effect on climate or life and all of that so if you want to know more about how supercontinents have throughout Earth's
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history affected climate in life I talk about that in my Earth history playlist which I will pop up here on the screen for you at any moment so you can check it out my favorite instance of a supercontinent having major effects on climate is snowball Earth so I will link my snowball Earth video up here for you on the screen it's a bit old of a video on my channel so it wasn't the best production quality but I love that video and the information in it because I just find snowball
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Earth to be so so cool and I will see you guys in my next video thank you so much for watching bye
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