PlanetGeo: The Geology Podcast
PlanetGeo: The Geology Podcast
Whence Plate Tectonics? Q&A with Dr. Rachel Phillips aka GeoGirl
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Jesse joins forces with Dr. Rachel Phillips, AKA Geo Girl, to tackle viewer questions about Earth's oldest rocks. We cover a wide range of topics, from how Earth's age is calculated using meteorites to the intriguing details of zircon dating. We also discuss the beginnings of plate tectonics, early continents, and the much-debated timeline of Earth's geological history. This in-depth discussion is packed with scientific insights, humorous quips, and plenty of geologic puns to keep you entertained. Don't forget to check out our Camp Geo mobile app for a deep dive into Earth's ancient rocks!
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Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you. We've got a special one today, a discussion with a friend of Planet Geo, Dr. Rachel Phillips, aka GeoGirl. Rachel had us on her YouTube channel a while back to talk about Earth's oldest rocks, a topic we are deeply familiar with, and YouTube viewers had a lot of questions. So Rachel and I decided to sit down and try and answer them. It took us quite a while, so this is part one of our listener questions episode with Dr. Rachel Phillips. If you're interested in Earth's oldest rocks, please check out our Camp Geo mobile app. It's the first link in your show notes, and you can find out a ton more as we have a standalone audiobook on that very topic where we go deep into the weeds on this. I'll be doing more listener questions with Rachel in the future, so please hit us up with your thoughts and questions. Find us on all the social medias at Planet Geocast or shoot us an email via the contact us link on our website, planetgeocast.com. Let's go.
SPEAKER_01So getting right in to all the viewer questions, which sifted through like 800 of these comments, guys.
SPEAKER_00That is so many comments. That's impressive. I I'm amazed.
SPEAKER_01Again, it was really high engagement videos. I think it these discussions were really interesting to a lot of people and obviously to us because we went on for I don't know how many hours, but and probably will again today.
SPEAKER_00But had to had to had to cut ourselves off.
SPEAKER_01Yeah. So one of the main things we talked about was the cool fact that we don't actually date Earth with Earth rocks, we date Earth with meteorites. So a lot of people were wondering wouldn't the radiometric age of meteorites be affected by the heat and pressure of impact?
SPEAKER_00Ah, yes. This is so such a good question. So for meteorites, yes, absolutely. And for actually the rocks that the meteorite hits during an impact. And so it's kind of a question of scale. So think of what you're measuring, whether it's a rock or a mineral or a side a bunch of minerals or a bunch of rocks, and think about the sort of intro to chemistry. Is this a closed system or an open system? And so if what you're measuring, if you're taking one mineral grain and your data point is measuring that one mineral grain, then yeah, elements could have left that mineral grain or been reset and exchanged with the mineral grain right next to it, maybe during that impact process. That's the case with zircon sometimes. That's the case with a lot of other minerals. They they react individually. But if you dissolve the whole rock, if you take the whole rock, crush it up, dissolve it, you know, samarium and neodymium might not have moved far enough. They might have moved between the mineral grains, but the entire rock system has remained a closed system, in which case, if you dissolved your whole rock, that's a good data point still. So it kind of depends on the scale of your measurement and how much these elements, because we're really dealing with individual elements in these radioactive systems, how much they've moved. So it also depends on the type of system. So samarium and neodymium, those minerals, or excuse me, those elements don't move in water very easily. They do a little bit, but not a lot, whereas rubidium moves a ton. And so the rubidium strontium system could be reset very easily, whereas samarium neodymium system might not be reset. And so by kind of comparing and contrasting the different systems, you can work out what has moved and what hasn't. So that's what people do with, for instance, like lunar rocks or meteorites is take one sample, whether it's rock or mineral, and make multiple measurements of that using different systems. So does the rubidium strontium system agree with the samarium neodymium system? Does that agree with the uranium-led system? If they all agree, great. That gives us confidence. It's not reset. If they don't agree, then maybe rubidium strontium is actually recording the resetting age and things like that. So it's my instinctual reaction to that. But I mean, what what is your take on this question?
SPEAKER_01No, that's I mean, based on our last conversation, what I've taken about geologic dating is that it's this like multi, multi-proxy approach. Just like with my research where I was trying to reconstruct past climates, like we use multiple elemental systems, isotopic systems, and we know how each one behaves and what each one reconstructs, and how each one is altered over time by different processes. And some are more robust and under these alteration processes, and some are more robust under different alteration processes. So then we can utilize all of them to kind of see if they're all telling us the same thing and like corroborate our findings that way. And I feel like it's very similar in geologic dating, where like, you know, maybe zircon and uranium lead dating is good for this type of rock that's been under this kind of stress, and then, you know, other isotopic systems or other minerals or the entire rock bulk composition is better for this kind of dating, depending on what that rock has been through.
SPEAKER_00Yeah, that's right. And I think that that's a really good analogy because I think people often, and maybe part of this question is people think that affected or altered or you know, disturbed is a negative thing or throws uncertainty into our geochronology when actually the disturbance, it's actually really interesting to know the disturbance. So actually, sometimes we would target a system we know is disturbed because we're trying to get the age of disturbance, which then can help inform our other data that is less disturbed. So if we know the age of the shock metamorphism from a rubidium-stranium isochron from the meteorite, that can give us more confidence in one of the more well-preserved isochrons like a summer neodymium system or uranium lead system. So it's actually more tools in our toolkit rather than than damaged, like people think of it as like, oh, you broke you broke your screwdriver because it doesn't work. When actually it's like, no, it's a different kind of screwdriver, actually, now. Yeah. And so it's another tool instead of uh a broken tool, I guess.
SPEAKER_01Yeah. Um yeah, I feel like a lot of times too, people conflate. I mean, everybody hears carbon dating or like and we talked about this in the last we talked about this already, but everybody thinks there's just one way we do this. And it's like, no, the reason it works so well is because we have all of these things, totally all of these different isotope systems that we can use to make sure one isotope system makes sense with the other one, and it's exactly right.
SPEAKER_00And actually, like the age of the meteorite, for instance, in meteorite literature, sometimes we'll be interested in when did the rock form, the meteorite rock form? We'd also be interested in when did it get disturbed in many instances. So did that meteorite float around, or or was it part of maybe it's a lunar meteorite, so it's part of the moon, and then you know it got hit and ejected, or it got altered in some way, shape, or form. Then when did it come to Earth? When did it land on Earth? Like all these things are interesting data points. And so we're actually the ability to use disturbed quote unquote systems is actually really powerful for understanding these processes, understanding the solar system processes, I would say.
SPEAKER_01So that's really cool. I I also fell under that, I guess, you know, misconception as well of just thinking the more altered, the harder to date. Yeah, and the you know, less you can figure out about the age and stuff.
SPEAKER_00Yeah, yeah. It it is, you know, there are many systems that are altered and they're just a mess. And we actually can't extract more information. Like our research group is working on altered zircons a lot now. And there are some of these things that we just can't make heads or tails of the ages. Like it's just an altered mess. We can't make any sense out of the confusion. So yeah, it's I don't want to present it as like it's a perfect system. We always get age information. Sometimes we do not get age information after a bunch of work.
SPEAKER_01Yes, yes. Well, there's plenty of rocks to work with.
SPEAKER_00So yeah, yeah, absolutely, absolutely.
SPEAKER_01That brings me to another question that was very commonly asked in our comments from the last couple videos. And that is we talked a lot about zircons and dating, using zircons and specifically uranium-led system to date the rocks, um, especially when it came to the oldest rocks on earth, like Acosta Nice.
SPEAKER_00Yes, nicely.
SPEAKER_01I got my piece. If anybody hasn't seen the videos we made, go watch the videos wherever you do. He sent me this ice. But um, but yeah, this is this is over four million.
SPEAKER_00That's not just sitting, you know, next to your garbage can. It's hopefully sitting somewhere very precise.
SPEAKER_01Next to my head at night. This is on my bedside table. And I look at it every night and I'm like, oh, so old. And I'm going to Iceland next year to get my youngest rock so I can do the oldest, the youngest.
SPEAKER_00Very cool, very cool. That's awesome.
SPEAKER_01But even though this is super old, we know the oldest ones are the meteorites. So we talked about how we use zircons for these old earth rocks, but do we also use them for meteorites?
SPEAKER_00Uh, yes, in certain instances. Zircons are not common in meteorites, not super common in meteorites. And you got to think of like what types of rocks meteorites come from. So we have rocks from Mars, meteorites from Mars. Those have zircons in them, and they're actually some of the oldest zircons that have ever been measured. They're 4.45 billion year old zircons, and they're incredibly pristine. This uranium lead data from them is spectacular. It's actually concordant. I think we talked about that briefly, but it's like really, really good data. We have rocks from the moon that also have zircons in them. Those are often shocked and have had all this whole impact history, but they're really useful as well. There are some zircons in other classes of meteorites, although they're pretty uncommon. And the reason is that most of the meteorites we have are either basalt, which basalt just compositionally does not crystallize zircon primarily, or they're iron, they're iron meteorites or some combination of those two, iron meteorites from like a the core of a planetesimal, and that will not have zircon crystallizing in it either. It just has a stable mineral phase. So the answer is yes, we do, and some of the oldest zircons we've ever measured as a species come from meteorites, but they're not common because the composition of meteorites, meteorites often reflect a undifferentiated or uh an undistilled planetary body. So the ones who come from more distilled bodies like Mars and in the moon have some zircons. Yeah, well.
SPEAKER_01I remember, yeah. And I remember us talking last time about you know how as Earth differentiated, the zircons would typically concentrate in the more silicious, more fractionated, differentiated material. And so it makes sense that really primitive, non-differentiated meteorites are not gonna have a lot of zircons.
SPEAKER_00So yeah, right. Exactly. And this kind of this is a nice question that builds on the the previous one about alteration, is that you know, one of the big outstanding questions in planetary, well, one of the many in planetary science is what is the impact history on the moon? Because the impact history of the moon is used to kind of calibrate the entire impact history of the solar system, the inner solar system. So knowing when craters formed, what the ages of the big craters are is a big question because the moon is the one planetary body that we can look at, count the craters, figure out the relative order of all the craters, and then we we have some chance of getting the ages of those craters, the real ages of those craters. So then we could plot the impact rate versus time of the in of the solar system. And so one thing people do is they look at altered zircon from the moon because the alteration is often an impact dating event. The zircon is altered by. So that's one way where where people kind of look for that. And there's some clear like textural things people can look at in zircon that say this is a shocked zircon. This thing got hit by a meteorite. Maybe not the zircon, but a the shockwave from a meteorite impact passed through this zircon grain. Actually, one of the best acronyms, it's called Friggin. Friggin Zircon.
SPEAKER_01Oh my god, I just saw a whole talk about this at GSA or somewhere. Yeah.
SPEAKER_00Oh yeah, yeah. It's uh readite in Granoblastic Zircon. It's great. Friggin' Oh, it was one of your students.
SPEAKER_01That's why. Yeah.
SPEAKER_00One of my yeah, well, one of my students found found uh found a paper that outlined what Friggin Zircon was. I love it. It's such a good acronym. It's one of the few, like the seismology community is great at acronyms. The volcanology community is awesome at acronyms. This is one of the best like petrology acronyms I've ever seen.
SPEAKER_01So yeah, geology acronyms are pretty good. There's some, yeah, there's some really fun ones out there.
SPEAKER_00Yeah, yeah, yeah. That's amazing.
SPEAKER_01Okay, so we will definitely get way more into moon stuff in the next video. But for the kind of follow-up to this question on zircons and if we use them in meteorites, is you know, we talked last time about how useful zircons are. So a lot of people were wondering why exactly are zircons like the top at the top of dating material? Like, why do we use them so much and why are they so great for that?
SPEAKER_00It's sort of like the perfect mineral for the perfect decay system. So we have all these decay systems, these duchronology decay systems, rubidium decays to strontium, or one isotope of rubidium decays to one isotope of strontium, one isotope of samarium decays to one isotope of neodymium. Uranium and lead have two have a paired system. So 235 uranium decays to 207 lead, 238 uranium decays to 206 lead. And so that means we can measure one measurement of uranium and lead in a minograin or a rock gives us two ages that we can compare and contrast with one another. So they're kind of like two different sand dials that you flip over and they run at different rates. And if you know the rate, you can compare how well the sand dial is looking. So we can kind of intercalibrate the two. And zircon is an amazing mineral for this because it takes in uranium when it grows from a magma, but it excludes lead. Lead is a huge ion with the wrong charge to fit into the zircon structure. And so it'll take in uranium, excludes lead, gets us away mostly away from the initial daughter problem with isotope geochemistry, but it also is really robust to later alteration or later disturbance, later metamorphism. It'll survive in a weathering cycle. It'll survive most metamorphism. And so it's it's really great for that. There is a small but growing interest in zircon alteration events. So it is a weird mineral because it's it's really resistant. So you can imagine the crystal lattice is really rigid. It's like a steel building with all the eye beams going everywhere. It's really rigid, but the radioactive decay blasts apart little parts of that structure, and it's a very rigid crystal lattice, so it doesn't sort of self-anneal, it doesn't heal itself very well, which other minerals sometimes do. And that's great for preserving the initial age, but it means that at low temperatures, fluids can sometimes get in that radiation damage bits and remove lead and disturb the system. But again, back to our earlier like conversation, the disturbance is interesting, and the fact that it is disturbed, we can often extract age information from that. So that's why things like Zircon might be really good at recording the shock event, the deformation alteration event on the moon in addition to the crystallization.
SPEAKER_01Oh, wow. Yeah, Zircon's be they're pretty cool. Well, I think it's really cool too. I think it's awesome that they're like this this thing that survives all of this, you know, later metamorphism, but within like a matrix that, you know, of other minerals that might show those later events, so that you can get this entire history of the rock where the zircons give you the oldest history, and then you see the rest of all the events and the other minerals and other isotope systems.
SPEAKER_00So I just think that's really absolutely it's you can piece together in that that rock you're holding in your hand. People have, I mean, it's one of the more famous old rocks, and people have looked at all basically all the mineral grains in that rock to for geochronology. So you can piece together like a really amazing story. The sort of full life life events of that rock are pretty recorded, and you know, zircon starting out, and then all the you know, well, I remember you talking about like there was a metamorphic event at 1.8 billion years ago, and then 1.3, and then one point, and it's like, how do we know all this?
SPEAKER_01It's just so all this stuff. Amazing. It's amazing. Oh my gosh.
SPEAKER_00Yeah, yeah, yeah. Lots of work, lots of PhD theses working on it on this particular suite of rocks. Yes, lots of effort. Yeah.
SPEAKER_01Well, and speaking of kind of our precision now with geologic dating, a lot of people were really shocked at this the precision of this number that you mentioned in our previous videos, which is that we think the earth is about 4.5682 billion years old. So the earth and the solar system based on like our meteorite dates, which is like so 4 by 5682. I mean, that's so that seems like so, you know, just precise. And so a lot of people were wondering, what are we actually, what is that actually dating? Like what defines the birth of Earth? Which I think was a really I mean, this is almost we're getting to like the philosophical range of things again. But um, yeah, what defines the birth of Earth? And a lot, and also another person asked, what does this number actually mean? Does it mean when Earth reached a certain size or when Earth's crust solidified or something else?
SPEAKER_00Okay, that is such a great question. I mean, and Chris and I, we have a whole podcast episode basically about this question, about this exact thing. Like, what does that number mean? Because it was something that Chris in his astronomy class had like come students had come with a question about you know, what is the age of the earth? What does it mean? What does the age of the solar system mean? Is that the same as the earth? Is it different than the earth? Like, how does this work? Short answer is that age, 4.5682 billion years or 4568.2 million, is the age of the condensation of the first solids of our solar system. So the first things that kind of condensed from our early solar system disk, that's time zero. So we kind of say that number 4.5682 billion years ago is the start of solids in the solar system. It's not the age of the earth because we have to, in order to build a planet, we have to take all this gas and dust and aggregate it together.
SPEAKER_01So, well, okay, before you go any further, what exactly solid? Okay, but like solid. What s like does dust count?
SPEAKER_00Well, dust is yes, but not in a way it's hard to date that stuff. Okay. Um, and there is some dust that's from before the solar system as well that we can find. But these are what these are what are called CAIs, calcium aluminum rich inclusions. So there's some petrology that just goes into these things, but it's it's kind of gas and dust accreting in this really hot disk of stuff. And so they're getting together and it's being kind of melted together and then forms this little inclusion. So we find these as inclusions in primitive meteorites.
SPEAKER_01So those inclusions are what give us that five point, what is it?
SPEAKER_004.56, yeah, exactly. Yep. And those are these are like uranium-led ages in those in those in those those uh particular meteorites. So yeah, that kind of sets sets time zero. And then from there we work forward. And basically the first few million years are formation of chondrules. Some people might have heard of chondritic meteorites, these are things that have chondrules in them that give um ages that are like 4.565 instead of 4.568, you know. So a few million years things are starting to coalesce, but forming a planet is a totally different ball game. Like small dust runs into dust and it makes bigger stuff, and then you know, bigger stuff has more gravity and so it attracts all the other stuff. And so this kind of starts to grow and starts to grow as a feedback positive feedback loop there. And these planets are getting hot, there's all sorts of radioactive elements around, so they're heating up, they're internally melting, and the big ones get bigger because they start to attract other stuff. And that process, well, I don't know, when is a planet a planet? Like, what is the event? Is there a single event that that like marks that in time?
SPEAKER_01I don't know. Um, I mean, I like the idea, like the one person that said, is it when it reaches a certain size, but also at the same time, I mean, we have planets of very, very different sizes, so I don't know what that would be. Yeah, I don't know enough about the early soul system because then you got asteroids versus planets and like how they all start to like orbit. I don't know.
SPEAKER_00It's a it's a tough one, and I think there's no uh to me, people focus on what they can date rather than any genetic definition for what a planet is, like I don't know, any size. So we focus on stuff we can date. Yeah, and one thing we can date broadly is when the core formed. So when the planets are getting hot, so even small planets at that time will have gotten pretty hot because there's a bunch of elements around that are rapidly decaying away, generating a ton of heat. Aluminum has an isotope that was decaying away like crazy, and so it'll heat up and it'll melt. And as soon as some planetes will melts, then the the heavy stuff can drop to the bottom. The think of it that way, the core, the iron core will form. And so we can date that process on planetary bodies. So on Mars, it's you know, kind of around 10 million years when the core formed or when the planet was mostly accreted, and we can use a a short-lived system called the hafnium-tungsten system to date that anyway. So that's the the kind of time frame for forming a planet, sort of tens of millions of years for getting these planets kind of formed. And then there's a whole bunch of physicists who do the simulations, they kind of simulate solar system formation and they can kind of see this physically a planet that has uh a molten core, iron core, iron nickel core, and then silicate stuff above it. But that silicate stuff is almost certainly molten for a while. And so we have a mag what's called a magma ocean planet now, where the mantle's not solid, the mantle's all liquid, and there's probably some like crust on the surface, some you know, thin rind of stuff that's that's crystallized. And then the question is when does that mantle crystallize, fully crystallize, or separate out into chemical parts? And and that's a whole nother question, but that's like a maybe a 50 million year to a hundred million year kind of process there.
SPEAKER_01Yeah, no, that is a good way to explain that kind of the time zero thing. But yeah, I feel like I never thought too deeply about like what defines the birth of Earth. I mean, I always just took our like 4.5682 billion year ago time point as like the beginning of when planets started to form, and then every planet obviously took its took its time doing that.
SPEAKER_00But um yeah, that's there's so there's a great um there's a great paper, one of the early papers in geochronology. It's from nineteen fifty six by Claire a guy called Claire Patterson. And the title is just Age of the Earth. And it's an amazing kind of first look at uranium lead isotope systems from a whole bunch of different rocks, ore deposits, clay, these Weird iron nodules and basically created what's called a planetary isochron for it. So the age of the entire planet from looking at all the different chemical rocks on the planet comes up with an age broadly of 4.55 billion years ago, which is amazing to think about because I just described an evolution from like 4.568 and then 10 million, maybe 50 million years later. And that's that's the number.
SPEAKER_01We have a lot more data, but the fact that it corroborates that still is crazy. Exactly.
SPEAKER_00From that work, it's just like such a cool, uh transformative initial data set. It's amazing. Amazing. Oh my god, amazing.
SPEAKER_01And yes, in the next video, we will talk about the moon forming impact and how that reset things. But the next question for this one, and you hit on this a little bit in your answer to that one, but have we ever dated rocks older than our solar system? And if so, where did they come from and how do we know?
SPEAKER_00Right. I would say uh have we ever dated rocks older? No, we've dated fragments of things, and these are um and again, I'm not the expert on these pre-solar grains, is what they're called. And the name tells you a lot. They're grains from before our solar system, pre-solar grains. They're crazy. Where I did my postdoc uh work at the Carnegie Institute of Washington, they have a bunch of people who are experts on these things and spent careers studying presolar grains, but they are a lot of silicon carbide, they're basically incredibly resistant dust, really resistant dust that's left behind in certain meteorites that have not been melted and they're in primitive meteorites, so people can extract them by doing this like really aggressive acid leaching, and these things are what's left behind, basically. And we know that they're from not from our solar system because they have incredibly crazy isotopic compositions. So just things that we do, they're they're orders of magnitude different in a bunch of isotopes than anything in our solar system. So they they come from outside our solar system and could we ever track solar grains?
SPEAKER_01What solar system that did they come from the solar system that was here before our solar system?
SPEAKER_00Like how do we it's uh yeah, you're you're you're you're pushing my knowledge a little bit.
SPEAKER_01We're getting into philosophy.
SPEAKER_00Well, what they do talk about is they talk about star types of types of um star systems. Oh, yeah. So, you know, is it a red giant or I'm gonna get this wrong, the astronomers will hate me.
SPEAKER_02Right, right. But different different star types, yeah.
SPEAKER_00Types of of uh supernova, different classes of supernova generated these things, or different parts of a stellar life cycle would generate, you know, this particular grain, but it so they kind of categorize them in different classes without talking about you know, it came from the one just before us or something like that.
SPEAKER_01Okay, yeah, I do want to make the disclaimer that we are GeoGirl and Planet Geo, not Astro Girl and Planet Astro. So we uh if we get any astronomy things wrong, don't come at us, just let us know in the comments very nicely.
SPEAKER_00Yeah, yeah, yeah. That's right. I just want to go look at rocks.
SPEAKER_01We are answering your questions.
SPEAKER_00Yeah, I'm not too worried about astronomy things. So, yes, that's I like that.
SPEAKER_01I've had to say that before. I am geo girl, guys.
SPEAKER_00This is geo. Emphasis on the geo.
SPEAKER_01Yes.
SPEAKER_00Stick to what you know. Yeah, I think that that that so these things are they're really crazy. I remember I might get the details of the story wrong, but there was somebody who told me at when I was working at Carnegie, they they do this work in Antarctica a lot, and you can get these like big dust vacuums that are basically sucking dust out of the air, and the circulation on Earth says that most of the dust there, or some not most, but much of the dust in this area will be from outside of the planet. It's not planetary dust, it's like coming in from space, it's a lot of space dust. So they suck these things in, put them through some filters, and these we're talking about these presolar grains are like a micron. These are thousands, you know, millionths of a meter across, they're tiny, tiny things. And then they'll look at the filters and there'll be some larger fraction of presolar grains in there. And so these things in Antarctica and a lot of the Antarctic meteorite collections have have presolar grains in them, but we've kind of reached my knowledge of them. But some of them are very old. The model ages for them can be like up to two or three billion years older than the solar system itself. That is so we're going back six or seven billion years. Yeah, it's hurts to think about.
SPEAKER_01Wow.
SPEAKER_00To be honest with you.
SPEAKER_01Yeah, that is just I thought that was a really, really intriguing question. I wish we knew more about it, but yeah, the answer is yes. And we we can tell because of isotopes, and that's all we got.
SPEAKER_00That's about it. Yeah, that's all we got. I don't think you can leave it there.
SPEAKER_01Yeah, the rest of the questions are very much more geocentric.
SPEAKER_00Okay, perfect.
SPEAKER_01Um, so now we gotta want to switch gears into early continents, cratons, and plate tectonics. A lot of the questions were about kind of the beginning of plate tectonics and stuff. Very interesting stuff. So the first question, just to kind of kick us off, is what is the difference between if there is a craton and a continent?
SPEAKER_00Ah, okay. So craton, that's a great question. Kraton, think of cratons as the core, the thick, thick cores, basically. They're usually ancient. So part of the definition, and these are both like sticky definitions or true or sort of squishy definitions, maybe, but cratons are often stable for billions of years. So they often have a lot of old rocks on the surface. They also have really thick roots. So the lithosphere, if you think about what up, what is a tectonic plate, what makes up a tectonic plate? It's the crust and the mantle lithosphere part beneath it. So the mantle that's that's kind of riding along with the crust that's cool and rigid. Kratons often have very thick lithospheric roots, so more than 150 kilometers thick, maybe even down to 250, 300 in some instances. We call those things keels. Like think of a uh a sailboat that has this keel going down that's kind of stabilizing the thing. Kratons are smaller than continents. Continents are made up of a bunch of cratons. And so continents, again, squishy definition, depends kind of what type of scientist you're talking to, but they're they're above sea level, they're seismically thick, so the crust is thicker, the crust is more like 30 kilometers thick, whereas oceanic crust is seven kilometers thick, and it's usually more evolved, more felsic, more solicit, more silica, um, more potassium, sodium, etc. It's not oceanic crust. The continental crust is is not oceanic crust, right?
SPEAKER_01Like continents, what are they? They're not oceans. That's they're not oceans.
SPEAKER_00The other part of the bimodal curve, they're the you know, not the mode that is oceans. So um that's kind of the general definition. Kratons are really interesting because they're very important resources. They have most of the world's extracted gold and platinum are in cratons, and along the margins where the cratonic core is kind of remaining solid, but the margins between them gets deformed and has this history of deformation, that's where a lot of metal deposits end up occurring. So it's not just a sort of an arm wavy ambiguous definition. Like the definition kind of matters a little bit here.
SPEAKER_01So yeah. Yeah, no, I love that. I read a thing that said cratons are like the continental seeds that continents grow around, which I think is pretty good. But you're right, it depends on the scientists. Like continents, I mean, of course, they have a certain thickness, they're a different composition than oceanic crust, but then there's tons of places where they're a lot thinner or they're a lot thicker, or they're more basaltic, but they're not still not oceanic crust. So then it's like, you know, because there's so much variation, but overall, they're the you know, thick, silicious, less dense stuff.
SPEAKER_00Oh, that's a okay, yeah. I like that. I like that. That's a very good uh I like that. It's a good one. Yeah.
SPEAKER_01Okay, so we talked about last time how the continents came later. There was oceanic crust or some kind of big mixture of like crust on Earth, and then continents differentiated out later, and we'll get into differentiation in the next question. So we didn't don't need to go into that right now. But the first thing we that people were wondering is what was the first continent and or Kraton? Like, was it there's I know there's hypothesis hypothesized early supercontinents like Valbara or Kenorland? I I don't know how confident we are around those or what our evidence is even.
SPEAKER_00Yeah. Oh, that's a great question. So if we look at the the rock record, the preserved rock record, like what are the ages of the rocks that we see on the ground today? We have less going back in time. So by like two billion years ago, the rocks that we see on Earth that are older than two billion years are tens of percent. So a couple, you know, 20%, 25%, or something like that. So what that means is by about 2.5, 2.7 billion years, it means that there's less rocks, but there are still rocks there. So we can start to kind of do things, look at those 2.5 billion year old rock packages, 2.7 billion year old rock packages, and we can kind of compare and contrast them to each other. And people have done this, and there's kind of two camps. If you go back to that time period, 2.7 billion years ago, roughly, some people would argue the kind of classic argument is that it was all one supercontinent called Kenorland. Um that's kind of the the big supercontinent model. The other model is that it wasn't one big supercontinent, they were all they were they were disaggregated into maybe three, then one's Valbara, one's Sclavia, one's Superia, like that they're different little uh microcontinents or or aggregate cratons where where multiple cratons were together. And so um in those cratons, think of them as breaking and forming, breaking and going back together over time. And so if you break the puzzle apart, put it back together, but you mix up the pieces, then you break it apart again, put it together, mix up the pieces, you can imagine stuff that used to be next to each other is now caught up in different continents. And so that's kind of as far back as we can go with any degree of confidence. We get back 3.4 billion years ago, and like there are rock suites on earth that are 3.4 billion years old, and some of them have some similarities to each other, but it's really hard to connect them together and say, was it one continent 3.4 billion years ago? How big was it? We have no idea. Were they all connected, or was it a bunch of different little disaggregate ones? Again, no idea. It's really hard to do that kind of work because the further back in time, the less rocks we have and the less diverse rocks we have. When we go back to 3.4 billion years ago, we're stuck with like metamorphic gneisses. That's pretty much all we have. We don't have sediments, we don't have basalts with some, but not many. Yeah, so it's really hard to kind of link things up in that way. Well, speaking of gneisses, exactly.
SPEAKER_01Um, I know we talked about in the last video about how these the oldest rocks on earth, things like Acostanis, have zircon grains that you know suggest that their earliest formation was over four billion years ago, but that predates all of the earliest Kratons and continents that we're talking about. So what exactly, because we know we talked about how zirgons tend to form in more silicious material, more continental crust-like material. So, what are these earliest zircons telling us that there was true continents back then, or was it kind of like the earliest start of differentiation that these are dating?
SPEAKER_00The answer is we don't know. We really have no idea because we just have so few locations with zircons that old. There's there's a couple locations with zircon grains that old. And you can form, we can look at the modern earth and we can say we we can form zircon-bearing rocks in many different ways, right? You meteorites have zircons, Mars has zircons, the moon has zircons, Iceland has zircons, plate tectonic subduction zones have zircons, continental collision zones have a lot of zircons. So zircon itself forms in more felsic, more silica-rich rocks, but it doesn't necessarily mean that we know the tectonics of that production. Yeah. And it depends on who you talk to and depends on which grains you're looking at. There's some fairly convincing data from detrital zircons in the Jack Hills that those were related to plate margin type sequences. Wow. I would argue that they're more like iceland, Iceland-like processes, not necessarily plate subduction going on, but but more like uh what people call intracrustal working or kind of a thick plateau that's internally melting itself.
SPEAKER_01Right. Okay. So which is kind of more similar to the early tectonics that I've heard about. But when you mention something like plate margin, it kind of suggests plate tectonics, which brings me to the next question.
SPEAKER_00Okay, yes, all right.
SPEAKER_01Because a lot of people were wondering. I mean, I know we talked about how the beginning of plate tectonics on earth, global plate tectonics, is still very uncertain. Um, the timing of that. But you mentioned Jack Hill zircons in Australia are what 4.4 billion years old or something. 4.35. Yep. Yeah.
SPEAKER_02Yep.
SPEAKER_01So that's way before a lot of the estimates of when plate tectonics are proposed to have begun, like 2.5-ish billion years ago. So there were a lot of questions around like when did plate tectonics begin and what is some of our evidence. So I'm going to read off some of these. Um I find their wording very intriguing.
SPEAKER_02And also the questions that you mean. Yeah.
SPEAKER_01So somebody asked, how did early continents form on a pre-play tectonic earth? One person said, How can people even doubt that there were plate tectonics if there was land or continent crust? And then I guess we had talked about subduction. So somebody said, You say that subduction drove crustal differentiation and thus continent formation, but subduction typically occurs when oceanic crust subducts beneath continent crust. So how did the first continents form? Was it subduction of oceanic crust under other oceanic crust? And lastly, would this have been classified as subduction, or would it have been more like drip tectonics, or some people have called it in the literature sagduction tectonics?
SPEAKER_02Sagduction, which I love.
SPEAKER_01Um, so all of these questions I think are getting at one, we kind of need to talk a little bit about differentiation before we kind of answer them, but they're getting at, yeah, when did plate tectonics begin? And how can we even say that continental crust was forming before that? So can you talk a little bit about before we get into those? Like, can you just tell us what is why are these questions even a thing? Like, why do we try and necessitate plate tectonics for continental crust formation? And I think that needs to be kind of a conversation around what is partial melting, what is differentiation, and how does how does silica rich crust form?
SPEAKER_00Gotcha. Okay, so yeah, the first thing distillation is a great word. I love that word. It's um, or differentiation, kind of the same thing. How do we get continents that are more rich in silica, potassium, sodium, etc., and are buoyant and are thicker? And on Earth, we can look around and say subduction zone is a good way to do that. A subduction zone system is kind of taking oceanic crust, it's shoving it down into the mantle. That oceanic crust dewaters sponge that gets squeezed out, the water goes into the mantle, melts the mantle, and then that water laden basaltic stuff sits there and fractionates and creates evolved stuff at that up at the top. So that's what happens on the modern earth. It's a good way to do it. You can also do it by smashing continents together, and stuff that's caught up in the middle gets melted and internally differentiates itself. To create ultra-distilled crust and therefore ultra stable crust, we often need really high temperatures and continental collision and maybe even continental collision zones with sediments involved. Because basically you need to smash stuff together, heat it up, and do partial melting, not full melting, but partial melting, which is distillation. Yeah. Did I answer? Did is that enough to go into the questions or yeah.
SPEAKER_01My like main thing with this, because this when I learned about this, it like blew my mind. Because at the beginning of my geological journey, I was like, How did we get these two very different end member types of crust? How did we get these really buoyant silica-rich continental crusts and this really dense basaltic oceanic crust? And how did that differentiate on early Earth and then start moving around like plates? And when I learned this, it like made everything so clear for me. But the the point I want to drive home is when you said partial melting, essentially anytime, and correct me if I'm wrong, but anytime we have rocks in the lithosphere or you know, crust or anytime in the interior of Earth rocks melt, that melt is more felsic than the original, like what's melting, as long as it's partially melting and not all melting. So if you have this, you know, set of basaltic rocks and you raise the temperature enough to melt some of it out, but not all of it, that melt, and then whatever resolidifies from that will be more felsic than the basalt you started with because the felsic stuff melts first. And so everything in the crust, when we have these collisions and stuff in between these two plates starts to melt, not everything melts, just some of it. So everything is essentially at plate boundaries a partial melt, and whatever melt is melting is gonna be more felsic, it's gonna be more differentiated than the original material. So every single time on early Earth when things started to, you know, solidify and then we had partial melts form, and then things ran into each other and partially melted, that partially melted material, and then again and again and again it became more and more silica-rich every single time till we got continental crust, and then a bunch of continental pods smashed into each other to form these big continental land masses, and now we have this big, you know, very end-member two types of crust on Earth. And so that's kind of the way I think about it is like is just this differentiation process started small and then built upon itself. But like you said, on modern Earth, the biggest kind of regional process of partial melting where this happens, where we have continental crust material forming, is at subduction zones at plate boundaries that are going under continental crust and melting that already morphsic material. And that's key because that brings up the question of if that's how it happens today, how did it happen before continents were a thing? Yeah, yeah, yeah.
SPEAKER_00Yeah.
SPEAKER_01Maybe before plate tectonics was a thing.
SPEAKER_00Before plate tectonics was a thing. Yeah, for sure. You're right that there's multiple processes that can generate the same type of rock. Like we can generate a granodiorite in a bunch of different ways on the modern earth. So that the rock type itself is not necessarily indicative of a plate of a tectonic regime or a tectonic process. On the modern earth, we generate a lot of andesite and granodiurite at subduction zones. And subduction zones are great because it's a conveyor belt of stuff. Like it's it's bringing water down in the mantle, the mantle's recycling, it's melting over and over and over and over and over. So we can do huge volumes that way. It's a really good way to grow continents by doing that process.
SPEAKER_02Yeah.
SPEAKER_00So onto the like the continental or the plate tectonics question is such a sticky one. There is a bit of um, I wouldn't say consensus, but like a majority view that plate tectonics as we know it today kind of started around 2.5 or 3 billion years ago. I don't buy that. I'm I don't I'm not a believer in that. I I think that's not accurate.
SPEAKER_01I think there's when do you think it was?
SPEAKER_00I think it's earlier. I think and and then the way uh uh one of my colleagues it I think does a nice way of framing it. It it's not plate tectonics or non-plate tectonics. He breaks it up into mobile lid, meaning the lithosphere, the rigid outer part of the of the planet is breaking and stuff on the surface is making its way down into Earth. That's a mobile lid. Okay, and then he puts it in stagnant, the stagnant lid regime where the lid is not breaking. Like Mars is a stagnant lid. There's one tectonic plate, it's not breaking and diving down into the mantle in any way, shape, or form. And that kind of gets you away from what is the driving force of plate tectonics and what is a seduction zone. You can kind of remain that's like a higher level categorization. Yeah. And so my view is that something mobile lid tectonics, I think we have some evidence for it happening back to 3.8 billion years in certain locations. Yeah. And then, you know, something similar, so a mobile lid regime of some kind was operating since that time and just progressively growing to us.
SPEAKER_01But I think you're right. It's kind of like what defines the birth of Earth. I think there's are we talking one subduction zone on Earth? Are we talking globally operating mobile lid plate tectonics? That's I think two different time points that probably took millions, if not billions of years.
SPEAKER_00That's a really good point. And I think there's a there what something seems to be a kind of a thing that comes out of the modelers, the people who try and model these processes, is that if you go back in time, you heat up the mantle. So you go back in time, the mantle's almost certainly some bit hotter. And so what happens, you it's harder to get a long-lived, like hundreds of millions of years subduction zone system that's kind of a plate diving down as a rigid block. What seems more likely to happen is as the mantle heats up, that rigid plate that's diving down heats up quicker, and therefore it can break. So it kind of gets squishy and breaks off, which kind of locks up the subduction zone. So we kind of get this stop start, stop, start, stop, start sort of subduction zone system, which is mobile lid, but it's certainly not plate tectonics as we know it today. It's not like modern styles of plate tectonics. So it's more local, short-lived, punctuated tectonics, maybe, or punctuated um plate tectonics.
SPEAKER_01So yeah, well, I think that's also similar, I think, to like the sag duction or maybe a transition from sag duction, which I think was more just like things dripping down rather than subducting down. I I had Steve Bauman on to talk about this, who has a channel, is great. So go check him out or go check out, I'll link that video down below for you guys. But he kind of explained it as it was a more vertical process as opposed to a laterally moving plate process. So today things are moving laterally, plates go down and they, you know, sometimes subduct pretty shallowly, but then you, you know, as things were hotter in the mantle, it would just kind of sink down and drip and and then melt and then re recycle. So I think it was a lot more, you know, things like hot spot plumes and then dripping down and sacduction in the dense areas and that. led to you know these partial melts and the beginnings of differentiation when it wasn't a full blown plate boundary. But that's why I found your description of the Jack Hill Zircons as you know potentially being indicative more of a kind of plate margin type tectonics very interesting. And it makes sense why now you know you say that your stance is that plate tectonics started earlier because the things like that.
SPEAKER_00There's and it and these are really complicated geochemical arguments that are probably not worth going into because it's suffice to say there's debate about what they mean. But the subduction I kind of break the the mobile lid regime horizontal mobile lid tectonics of some way where the lid's breaking and you're kind of thrusting stuff underneath of other stuff is a really good way to do a lot of partial melting because you're taking water or hydrated stuff at the surface and shoving it down and water is just a lubricant for melting. Yeah. And so water will induce a lot more melting. The problem with that I have with the the the vertical tectonic arguments, the sagduction, the kind of drip tectonics heat pipe volcanism is that you're you're starting with dry stuff and you're moving that dry stuff up and it's pretty hard to get wet stuff, what uh hydrated rocks and water down deeper.
SPEAKER_02Yeah.
SPEAKER_00That's my it's hard to do a lot of melting in a vertical kind of process, a vertical tectonic system in some way, shape or form. So you know you'll talk to 10 other people and you get eight other opinions about this.
SPEAKER_01Well again like you said the further we go back the less we know but it makes it super exciting. The other thing or the other part of that question I want to answer or try to attempt an answer at is um so we mentioned how you know subduction of oceanic plates under continental plates is a really great process for getting partial melt after partial melt and really really felsic material. But we had the question of like how did this happen before continents like what was the first subduction zone if oceanic plates subduct under continents? And the short answer is yes, oceanic plates can subduct under other oceanic plates because oceanic plates aren't all the same exact density and sometimes you do have that happen even on modern earth. But that is to my understanding kind of how everything began maybe not in a full plate subduction mode of tectonics but different densities of this heterogeneous mixture of oceanic crusts started to cause the downward movement and the upward movement of different types of rocks until you had and even we see even today like it obviously didn't have to start with like full blown felsic material because even these zircons that like we said we can find these at places like Iceland where it is basaltic. It is not felsic it is mafic. So like oceanic crust you know can create zircons but yeah it's my understanding that it would have started with essentially very slight changes in oceanic crust density to this ocean I don't know.
SPEAKER_00It's a tough yeah it's a it's a tough so you're you're highlighting one of the difficulties here is that we don't really understand at a great deal of detail how you break a lithosphere. How do you create that initial break? I mean it's a lithosphere it's strong it is lithosphere because it's cool and strong and solid how do you break that initially and so there are groups of of scientists who publish a lot on meteorite impacts being a big something that a big meteorite impact could break the lithosphere and you could maybe start a subduction zone system that way. I'm again a little suspicious of those arguments I would say as being a major player. I just don't think there's too much evidence for that. But it it points to a a good it points to the problem is once you break the lithosphere once there's a break and you have a weakness there it's easy to reactivate that weakness we see this as faulting all the time you know a reverse fault will turn into a normal fault. Yeah and that the same fault will be active multiple times in over millions of years hundreds of millions of years and it's the same thing with a break in the lithosphere but how do you start that process is a tough one. And so I think it's adds a tough you're you're pointing to exactly like the tough geodynamic problem of how do you create that initial break. You probably have to have some density difference, some age difference, some little bit more felsic block over here. I kind of like the Iceland analogy and there's some modeling from the 80s that sort of suggest you know went back and said okay that if you create this little felsic protocontinent thing that could be an initiation point for breaking the lithosphere and creating a subduction zone that then like maybe it shuts off but then it has a it has a fault in there a fault a break in the lithosphere that could be activated again over time. I wonder I mean isn't that kind of how like hot spots today can lead to these you know kind of Y sections or triple junctions of like faults that go off and then that can eventually lead to this just full blown divergent plate boundary right I mean we have these rift zones that come that start from like a hot spot splitting apart the continent and then that forms like I learned this process a long time ago I don't remember do you know what I was saying yeah yeah no I know I but it's also how do you break a supercontinent apart I the geodynamics that are kind of tough maybe the mantle just gets so hot under there it's like an insulating blanket that then has to punch out yeah the other thing that we have to think about here is that the continents are distilled which means that we have to have some you talked about the partial melting you described that really well what happens to the stuff that's left behind we have to think about that that has to go back into the mantle probably and so that means that lithosphere and we've seen this on actually the east coast of North America there's a big 150 by 100 kilometer block of lithosphere that's been removed that's not there anymore. It probably is related to some of the little volcanic plugs in Virginia um that we see but the lithosphere has dropped off into the mantle and that's a good way to start to break a continent right there's a weakness now there and if you push heat up into there and create melting and start to to create a fracture system that's put one potential way to kind of break a continent apart.
SPEAKER_01And then there's plate forces you know subduction zone systems and the Wilson cycle the classic cycle arguments I mean yeah that's the thing is like today's system is just so different because when you look at a a modern system where yes you can have a hotspot type you know volcanic plume cause this breakup of a continent and eventually turn into a divergent boundary that's laterally continuous but that is also driven on either end by subduction zone processes and pull slab pull and slab pull.
SPEAKER_00Yeah slab pulls the whole game and the slab pull is probably tough to do on the early earth.
SPEAKER_01Yeah exactly because the slab's breaking off so would that have turned into kind of this laterally continuous plate boundary on early earth? I don't know maybe not I don't know that I think out of the entire like going from heat pipe volcanism like kind of like Io type volcanism to mobile lid to this like melty vertical drip tectonic regime, stagduction regime regime to then mobile plate tectonics to then finally stagnant lid once earth cools off. The series of events makes sense when you think that Earth started really hot and it's been cooling this entire time the lithosphere is getting thicker. That's why we're seeing these changes to the tectonic regime becoming more rigid and hard and cold. But that transition between the drippy to the platey, the horizontal the vertical to the horizontal is the one that makes my brain like so confused. And it's also that we don't have any modern examples elsewhere in our solar system. Like Mars probably once had plate tectonic there are other Venus might have plate I don't know but like debate they don't now we don't I think that that's a really I think a really valuable point is that plate tectonics is rare when we look around our solar system.
SPEAKER_00And so going back we know I mean almost certainly after the moon forming impact there wasn't plate tectonics happening right then. So but but when does our our our mental model switch from being like plate tectonics is the default or is not the default to plate tectonics becomes the default because we go back a hundred million years ago plate tectonic was probably happening. So like you but by some point in that time we have to flip our mindset from like plate tectonic was probably happening or is the null hypothesis to be disproven to plate tectonics was probably not happening. And so you know we have to prove that it existed so it's a really tough question. It's a really tough one we have no data and the we've been debating this as a scientific community for decades and decades now.
SPEAKER_01And yeah you know I think the primary answer to a lot of these questions is we know we don't know fully we don't know that's right that's exactly right. Tukey's coming in to say enough on that question already.
SPEAKER_00That's right sick of this question, Tuki I mean me too.
SPEAKER_01I'm so done with this it's it's been forever and all you guys are saying is that we don't know. We don't know we don't know yeah exactly I really like because this one is also one I think that had me confused as a geology student for the longest time. But essentially it is if continental crust today on modern earth is not being produced at mid-ocean ridges it's not recycled at subduction zones like oceanic crust is you know oceanic crust is being produced it's being recycled it's got this whole cycle it makes sense but continental crust is kind of just sitting there it's riding above the oceanic crust that gets subducted underneath it. So does this mean that continents formed early on in these differentiation processes that we're talking about and then all the felsic material came out of whatever early crust it was and that's all tied up in continents now like have has Earth finished differentiating and is this all the continent crust that Earth can form and is it no longer forming or is it still forming?
SPEAKER_00I this is like I don't know this is oh my goodness this is so this is such a great question and I have I have thoughts but no answers. There's some answers so on the question of are we forming more continental crust it's a really good really open question because we don't know how much if any continental crust is getting recycled into the mantle at the point okay so we we know we're like getting more attached subduction zones right like accretion of our accretion right yeah right like you know there's an arc the coast and arc batholith in India that you know got caught up in the collision isn't was part of the mantle pretty recently now is part of the continent we know that that's happening we don't know how much is being recycled into the mantle so how much crustal volume cottonal crust volume is being put back into the mantle my suspicion is that it's not a lot and that it hasn't been happening for a very long time if there is some down there.
SPEAKER_01The hot spots record little hints of it happening yeah but I don't think there's a lot of cottonal crust that's been put back in the mantle that's my opinion so I mean it makes sense when you think about how continental crust works and that it doesn't it's too boy it doesn't go down it doesn't subduct that you know on earlier it formed faster and then it got slower it's still slowly slowly accreting and getting larger continents but then but they're not they're they're just ever growing slowly but they're ever growing and they're not recycling back into the mantle like oceanic crust does all the time.
SPEAKER_00Exactly the cottonal growth the the volume or the mass of cottonal crust is a big open-ended question and it depends a lot on how what you think the that recycling rate is if there's any and if so how much cottonal crust is going back in the mantle today. My opinion is that there's not a lot and that we've had kind of progressive cotinal growth through time which would say that in the future we will have continued growth of cottonal crust. That's kind of my default view which not everybody agrees with that at all.
SPEAKER_01So do we know when most of continent crust form like the big cratons like when they were done forming by because I I in undergrad and like reading textbooks, you see a lot of like by the end of the Archean most of like 80% of continental landmass had been created and then since then it's just been like slow accretion and stuff. But that said that's still before this like 2.5 billion year number of like potential play tectosy and so yeah that's a great question.
SPEAKER_00From I I was going to go back to the rock ages through time. And if we go back a billion years then 50% over 50% of the continental crust has ages older than a billion years. So we know that we at least a billion years ago had 50% at the very least we had 50% of the current volume.
SPEAKER_01I was always wondering how that was done.
SPEAKER_00That makes a lot of sense yeah I mean that's like the minimum right we go back three billion years and there's maybe five percent of the continental crust has ages that are older than three billion years. So again we're kind of roughly estimating here but that's a minimum because we could look at those rocks and say oh yeah you know five percent of the continents are thro over three billion years old so therefore they must have been there. So the rest is in model land. And some people would say that you know go back three billion years and we had a hundred percent of the volume of the continent crust we had the same volume of continents as we do today.
SPEAKER_02Really?
SPEAKER_00Yeah I I again I'm a little suspicious of those models. I think it's more like 25% of the continents three billion years ago or something like that. But these are all these are all geochemical models basically in some way they are geochemical models that people use to go to go kind of back in time.
SPEAKER_01And it comes back to that recycling versus reworking thing how much continental crust has been put back in the mantle over time versus how much of it is just recycled or uh reworked overprinted like remelted you know taking a taking a granite at the surface eroding it into sediments taking those sediments back into a continental collision zone and melting them to form new granites that's just reworking that's the same volume of content crust just getting reprocessed over and over yeah so I've okay so on that back to this kind of like is Earth done differentiating I am curious because we're not losing like you said continents are not losing material to the mantle they're not being recycled but they still can be formed and like partial melting happens all the time at subduction zones and plate material oceanic plate material is being partially melted and turned into more felsic stuff so is there a potential future of Earth where all like the oceanic crust has differentiated into continental crust and we're just this little continent or is that yeah is that zero science that's a really good question.
SPEAKER_00Um I've never really thought about it in that way I guess I think it depends on your rates it depends a little bit on when the earth kind of becomes a dead planet and dead means heat the heat inside is getting out easy enough like it it doesn't there's no more heat that needs to get out. Right. That's when a planet kind of dies um stops being dynamic. Yeah exactly and so I don't know exactly what the heat loss like curves look like um for Earth necessarily projecting that far like could we say oh yeah there's continents are growing at some rate and heat is being lost at some rate and so there's a crossover point when we'll have a lot of continents and heat needs to get out still um that's or is it kind of the planet dies before that happens thermally it's a tough one but the continents are getting more distilled like in and in different in in different ways than they did two billion years ago.
SPEAKER_01Right.
SPEAKER_00Kind of referred to land plants and weathering cycles on earth where there's a a a growing um emphasis on focusing on that in the the the weathering environments change the granite compositions and change how continents are actually distilled and how distilled they are if you look at the continent crust like how enriched is the top compared to the bottom that's distillation really right um and so so how that happens has changed over earth history. Yeah the differentiation so it's still happening and things are still getting more and more I guess felsic over time just slowly just it's slowly in felsic is yeah felsic this is in the weeds of granite so it's hard to get more felsic necessarily than a granite but you can get enriched in other stuff. You can put all the other goodies in there still just because melting is controlled by major minerals. So it's harder felsic than quartz and two felt spars.
SPEAKER_01So what happens if granite partially melts what comes out of that just more granite?
SPEAKER_00Yeah basically you get more granite I mean you get what's called the leukogranite which has like no biotite in it. The answer is it doesn't really partially melt very much it's weathering. So you weather the granite which is kind of a distillation step then you melt the sediment again which gives you uh a granite that's more enriched in aluminum for instance so you weathering is kind of a distillation uh step in the interesting system in a way so and correct me if I'm wrong I hope I'm interpreting as right but granites on early earth and you know millions and millions of years ago were more mafic and have become more and more felsic to the point that they're so felsic they can't become any more felsic that's kind of that's kind of right. Yes the the advent and we don't think of it as mafic and felsic we think of it as like potassium rich or aluminum rich or not. And so early earth rocks are very sodium rich sodic and those are kind of quote unquote juvenile those format subduction zone type settings on the modern earth the the potassium rich and aluminum rich granites really start about 2.6 2.8 billion years ago. Why? And those I think it's because sedimentation started and you started to you started to continents emerged above sea level at some time you had a weathering cycle and then you form granites from kind of progressive you could also do it just by progressive distillation. Like if you take the sodium rich granites granitoid rocks I should say from the early earth and you partially melt those you're gonna get potassium rich granites too. So there's multiple ways to kind of achieve the same goals here. But yes over time the continent crust has become more potassium rich more aluminum rich at least the upper parts of the crust have god crazy okay really cool yeah um and you kind of hit on this already I think you said like five or ten percent but um the last question was how much granite oh and that I guess that creates a different question but how much granitic continental land was exposed by three billion years ago uh great question so I'm gonna say granitic meaning continental crust like yeah is the answer different if it was just continental land yeah compared to granite okay yeah yeah I I would say I would probably we have no idea if it's granite specifically but um continental land well we don't really know we know there's some because there's sedimentary sequences that are depos that we are preserved in the rock record going back to 2.9 three billion years ago we started to get thick packages of sediment so that you had to have land exposed yeah then we don't really know how much and it depends how much you think continental mass there was actually my view is it's a little bit less we had smaller and submerged continents before three billion years ago and then after that we started to have larger and more emergent continent crust exposed to the atmosphere basically above sea level but above sea level speaking of sea level this is this is where we're kind of finishing out this video but next follow-up video we will get into when was the first oceans that was there a global ocean and did the moon forming impact was there an ocean before the moon forming impact and did the moon forming impact completely vaporize it again so there's a lot of questions surrounding early oceans on earth life on earth early life on earth the moon forming impact if the moon forming impact played a role in play tectonics early tectonics on earth so all of those questions will be in our second follow-up because we talk too much and we cannot keep these that short so it's impossible to not talk too much with you yes I know well we get into such great topics but anyway we will have a second video coming hopefully coming out in like a couple weeks max after this one uh but we shall see so um yeah make sure to stick around for that and thank you Jesse for coming on oh thanks Rachel it's always a pleasure to talk to to you about these topics it's you got such great questions on your yes thank you guys for asking these oh my god so many and so interesting and such deep ones actually such deep ones I was really impressed with all of the questions so thank you guys for those of you who asked questions in the comments I'm sorry it took us eight months or whatever to get back yeah absolutely hey that's wrap thanks for listening part two will come out next week so stay tuned for more of that you can find Rachel at her geogirl channel on YouTube. You can also find us on YouTube at Planet GeoCast. Our audiobook series on Earth's oldest rocks can be found at our Camp GeoMobile app first link in your show notes and remember hit us up with questions Rachel and I will be doing more of these so please let us know your questions, comments, thoughts we have an email contact us link at our website planetgeocast.com or hit us up on all the social medias at Planet GeoCast.