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What is a Continent? Part 1 - The Composition · PlanetGeo: The Geology Podcast

0:00
SPEAKER_00: The continents are not granite though.
0:02
SPEAKER_00: The continental average is something in between, but we don't actually know it very well, Chris.
0:07
SPEAKER_00: And this is why I mean you know I you know I I love this stuff when it's like a fundamental thing that we don't understand as a scientific discipline.
0:15
SPEAKER_00: I l Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
0:30
SPEAKER_02: Yeah, let's do it, Jesse.
0:32
SPEAKER_02: I uh like I'm just gonna say right from the outset, I have a massive head cold.
0:37
SPEAKER_02: Um and so there's a frog in Chris's throat.
0:41
SPEAKER_00: It's croaking away, too.
0:44
SPEAKER_02: It is.
0:45
SPEAKER_02: So I I will definitely crack uh during this episode.
0:48
SPEAKER_02: So I apologize in advance.
0:49
SPEAKER_00: You'll have the old uh teenager squeaky voice cracks every once in a while, won't you?
0:53
SPEAKER_02: You know, you know, Jesse, it's when I get excited and I start to talk a lot, that's when it cracks the most and it and it goes, so I gotta try to stay calm today.
1:02
SPEAKER_00: Staying calm might be tough on this topic here.
1:05
SPEAKER_00: I know, it is tough.
1:06
SPEAKER_00: This is uh this is part uh one of probably a couple, and we're talking about oh, we're on my home turf again, Chris.
1:14
SPEAKER_00: I'm I feel sorry for you and for the listener here.
1:16
SPEAKER_00: Um, but we're we're talking about the continent crust.
1:18
SPEAKER_00: What is continents?
1:19
SPEAKER_00: How do how do we make it?
1:20
SPEAKER_00: You had a good catchphrase in the intro.
1:21
SPEAKER_00: What was it?
1:22
SPEAKER_00: How how to make a continent?
1:23
SPEAKER_00: Is that what you said?
1:24
SPEAKER_00: Is that how you um or how to how to stabilize a continent?
1:27
SPEAKER_00: How to stabilize a continent.
1:28
SPEAKER_00: Yeah, yeah, yeah.
1:29
SPEAKER_00: Yeah, exactly.
1:30
SPEAKER_02: Oh right.
1:30
SPEAKER_02: It's well you've alluded to this a number of times, and we've never really we never really dove into it.
1:36
SPEAKER_02: And uh the other thing, Jesse, is that this is gonna what we're gonna talk about today in some to some extent is gonna contradict what most people know about the composition of the crust of of like continental crust.
1:48
SPEAKER_00: Yeah, that's sort of intra.
1:50
SPEAKER_02: Especially especially at a rudimentary level, at an intro-level you know, physical geology class, what you've learned is wrong.
1:56
SPEAKER_00: So, what do you mean by that?
1:57
SPEAKER_00: What what do you learn at an intro-level class about the continental crust uh that's wrong?
2:04
SPEAKER_02: Okay, um I'll for be the first to say that I'm guilty of this.
2:08
SPEAKER_02: Oh, yeah, we are that yeah, for sure.
2:11
SPEAKER_02: And I think we have to be, and I think it's okay.
2:13
SPEAKER_02: But continental crust is felsic, and and so we say that the continental crust is granitic, and you know, that's the part that's not entirely true.
2:22
SPEAKER_00: It's only partly true.
2:23
SPEAKER_00: It's only partly true.
2:24
SPEAKER_00: That's right.
2:25
SPEAKER_00: And if we look at the rocks, we've talked about this before.
2:28
SPEAKER_00: We look at the rocks on on continents, what we what we see, we see a lot of basalt, we see a lot of granite.
2:34
SPEAKER_00: Um, and there's reasons for that.
2:36
SPEAKER_00: The you know, granite is sticky magma that kind of sticks beneath the surface, it does not erupt as much.
2:41
SPEAKER_00: And when it does erupt, it erupts in huge volcanic eruptions.
2:45
SPEAKER_00: Basalt is is much more running, it can make its way to the surface.
2:49
SPEAKER_00: So we see those two rock types very commonly, but we see also sediments.
2:53
SPEAKER_00: Sediments are very felsic in our terminology here.
2:56
SPEAKER_00: They're very rich in silica, aluminum, um, they have less magnesium, less iron in them.
3:01
SPEAKER_00: And so those sediments are also felsic in our mines.
3:04
SPEAKER_00: We're gonna put felsic and uh in one core category, which is granite, sediments, etc.
3:09
SPEAKER_00: We're gonna put mafic in another category, which is basalts, these are gabbros, this is oceanic crust typically, but the average of those two things is not felsic, the average of those is somewhere in the middle, is intermediate.
3:23
SPEAKER_00: And we often think of like granite as as really felsic.
3:26
SPEAKER_00: The continents are not granite, though.
3:28
SPEAKER_00: The continental average is something in between, but we don't actually know it very well, Chris.
3:34
SPEAKER_00: And this is why I mean, you know, I you know, I I love this stuff when it's like a fundamental thing that we don't understand as a scientific discipline.
3:41
SPEAKER_00: I I I I love that type of stuff, and it makes you a little uncomfortable sometimes about that.
3:46
SPEAKER_00: Um but how does this one make you uncomfortable?
3:50
SPEAKER_02: Um well, uh just a little bit.
3:52
SPEAKER_02: I'm getting better with it, and we've talked about this before.
3:55
SPEAKER_02: I get a little uncomfortable when I I don't like to say that we don't know.
4:00
SPEAKER_02: I I it bothers me, and and that's what we're we really don't know.
4:04
SPEAKER_02: Um, you know, like so, Jesse, the deepest borehole is this cola borehole that we we the the Russians drilled down 12.2 kilometers.
4:13
SPEAKER_02: That's as deep as we've actually been able to directly get to, which is sad.
4:19
SPEAKER_02: Yeah, it's so sad.
4:20
SPEAKER_02: When you think about the you know, in in relation, it's a major accomplishment, don't get me wrong, but when you think about the thickness of the crust, you know, 35 to what 75, and even thicker if you're talking about mountainous areas uh with deep, deep roots, 12.2 kilometers is it.
4:37
SPEAKER_02: It's not very much, that's right.
4:38
SPEAKER_02: We haven't even it's not very much.
4:39
SPEAKER_00: I mean we we haven't even gotten into the bottom of the upper crust at that point, the upper continental crust.
4:46
SPEAKER_00: So all these models for continental crust, you you know, you said, well, okay, you said 35 to 75, I think is what you said, which is 75 is kind of or 70 is really the thickest that we see of the continental crust, the crustal thickness.
4:58
SPEAKER_00: So the surface of the earth to the moho, you that range.
5:01
SPEAKER_00: The average is probably closer to 35, a lot closer to 35.
5:06
SPEAKER_00: It's somewhere in that 35 to 40 range.
5:08
SPEAKER_00: Um, but Chris, here's a I this this episode's gonna be a lot full of a lot of little thought exercises, I think, because it's an important way to think about this and uh and to I think to kind of plug into the science of it.
5:18
SPEAKER_00: So listeners should try and go through these thought exercises as we're talking about them.
5:24
SPEAKER_00: Like, think about the question.
5:26
SPEAKER_00: Our drill holes have only gone down to 12 kilometers when we're talking about 35 kilometers on average thickness of crust.
5:34
SPEAKER_00: How would we measure?
5:36
SPEAKER_00: How would we get information about the middle and the lower crust or the lower crust that the total package?
5:43
SPEAKER_00: Some people divide it into three, think about the continental crust divided into three layers, some think about it divided into two layers.
5:49
SPEAKER_00: How would we get information about those bottom one or two layers?
5:52
SPEAKER_00: Like, where would you go on earth?
5:55
SPEAKER_00: What rocks would you try and pick up to find that stuff?
5:59
SPEAKER_00: Or what geophysical techniques would you use to try and understand what is going on down beneath our feet?
6:05
SPEAKER_00: It's a really interesting problem, a really interesting question to ask.
6:08
SPEAKER_00: And uh, what do you think the average student of yours would would say to this?
6:13
SPEAKER_00: Because I've never done this exercise, I've never asked intro-level students this question.
6:17
SPEAKER_00: It's a very common question in like the the PhD qualifying exam or the candidacy exam committee.
6:24
SPEAKER_00: Like, think about you know, what do we know about the lower cross?
6:26
SPEAKER_00: How do we know that?
6:27
SPEAKER_00: That's a very common thread.
6:28
SPEAKER_00: But I'm curious what you think your the intro-level students would say to this.
6:32
SPEAKER_02: I don't know.
6:33
SPEAKER_02: I I don't know.
6:34
SPEAKER_02: Um I think that my students that uh are seniors and have gone out west with me and and seen the routes to mountain ranges essentially, maybe would be able to come up with some reasonable hypothesis.
6:52
SPEAKER_00: I think you think I do.
6:54
SPEAKER_02: I'm not sure.
6:54
SPEAKER_02: I don't know because I've never really asked this either.
6:56
SPEAKER_02: Yeah, yeah.
6:57
SPEAKER_02: And I'm going to.
6:58
SPEAKER_02: I'm gonna start asking this question because it's a really it is a good it's a good thought exercise.
7:03
SPEAKER_02: I mean, uh you and I have talked, I think, ex pretty extensively on this in our you know, our audio um podcast series on the Tetons or the the podcast series on on the Grand Canyon.
7:16
SPEAKER_02: I think that we've we didn't like directly say this, but we indirectly did, yeah, right?
7:22
SPEAKER_00: No, that's right.
7:22
SPEAKER_00: That's right.
7:23
SPEAKER_02: We talked about uh minerals that that we see, right?
7:26
SPEAKER_02: You know, you know, in the Vishnu Shis, for instance, we talked about this uh these garnets that that formed 20 miles beneath the surface of the earth, and here they are now exposed at the surface.
7:38
SPEAKER_02: And so I think just like walking through well, how did that happen?
7:41
SPEAKER_02: Those rocks, they they're now at the surface, but they formed moderately deep into the crust.
7:48
SPEAKER_02: So they give us that glimpse, right?
7:50
SPEAKER_00: They give you a glimpse, that's exactly right.
7:52
SPEAKER_00: And and this comes into the question.
7:54
SPEAKER_00: So you you're touching on exhumed terrains, is one answer to the question where would we go to look at the the middle to lower crust?
8:03
SPEAKER_00: And you'd go to places where there was big mountain belts formerly that have been eroded, and the roots of those mountain belts are now exposed on the surface.
8:12
SPEAKER_00: And there's a bunch of locations on Earth.
8:14
SPEAKER_00: We think of these, especially for the lower crust, like the lower part of the continent crust, we call these granulite fasces.
8:22
SPEAKER_00: That's a metamorphic um grade.
8:23
SPEAKER_00: It's it's really high temperature, really high pressure.
8:26
SPEAKER_00: We've covered this in our CampGeo on our app.
8:29
SPEAKER_00: On the Camp Geo app, we talk about metamorphic rocks and metamorphic fasces or facies, and and granulite is the really high temperature stuff, the stuff that has been melted.
8:37
SPEAKER_00: So we have rocks on the surface.
8:39
SPEAKER_00: Southern India has a big granulite terrain.
8:42
SPEAKER_00: There's a big uh uh a big exposure in uh in the middle of Europe, that's called the variskan, and and there's places in the United States that have these lower crustal components, the big, a big block of lower crust that's kind of come up in some way, shape, or form.
8:56
SPEAKER_00: These metamorphic core complexes are exposed at the surface, usually, as you said, in in like former mountain belt collisional uh areas.
9:04
SPEAKER_00: So we call those exhumed terrains, and and that's lower crust stuff that used to be down at really high pressures that's been thrust up in some way, shape, or form.
9:14
SPEAKER_00: Okay.
9:14
SPEAKER_00: And we can walk across those, we can map them, we can look at them.
9:17
SPEAKER_00: Tons of people do that.
9:18
SPEAKER_00: Colleagues of mine at Penn State do that uh a lot and try and understand their composition, what happened to them, how did they melt, etc.
9:25
SPEAKER_02: What do you think?
9:25
SPEAKER_02: And this is just this.
9:27
SPEAKER_02: I'm I was just gonna say that this is that really rinse and repeat process of you know, you have this uplift, and then you have gigantic belt grinder from above, weather and erosion stripping rocks away, removing mass, bringing it down to the valleys, it gets mass wasted away, and then more uplift, and and so you just gradually bringing these deep seeded rocks to the surface.
9:50
SPEAKER_02: Um Jesse, one thing.
9:52
SPEAKER_02: Yeah, go ahead.
9:53
SPEAKER_02: One thing that I do ask my classes though is you know, all right, well, we've talked about how granite forms.
10:01
SPEAKER_02: The classical idea of what a magma chamber looks like and it's slow, relentless cooling.
10:07
SPEAKER_02: Well, okay, well, then if that's it, this is really, really deep deep-seated stuff, then how does it make its way to the surface?
10:13
SPEAKER_02: Why is granite such a common rock at the surface?
10:15
SPEAKER_02: So I do ask them that.
10:17
SPEAKER_02: And it's an interesting thing because most of them never really thought about it.
10:21
SPEAKER_02: Wait a minute, you're right.
10:22
SPEAKER_02: Wait, why how how is that so common?
10:25
SPEAKER_00: Yeah, yeah, yeah.
10:25
SPEAKER_02: It's it's really this process, just not not as extreme.
10:29
SPEAKER_00: That's right, that's right.
10:30
SPEAKER_00: So basically, what we can see when we look at these exhumed terrains, they have a fair amount of felsic material in them.
10:38
SPEAKER_00: So these are a lot of sediments that have been shoved down by tectonic processes, shoved down into the deep crust, have been metamorphosed, they've usually been melted, and melting is distillation, and so they've usually been partially melted, and then at some level they've been tectonically brought back up.
10:55
SPEAKER_00: So the residual stuff, the stuff that was left behind after melting, has been brought back up to the surface.
11:00
SPEAKER_00: And we can look at that stuff and see that it has lost.
11:03
SPEAKER_00: And each time this happens.
11:05
SPEAKER_00: And each time that happens, but in a tectonic That's right, and each time this happens, it gets more and more felsic, right?
11:11
SPEAKER_00: It gets more and more felsic.
11:13
SPEAKER_00: Um, however, it's kind of locked by granite, like you can't get more felsic than granite just because of the thermodynamics of the system.
11:19
SPEAKER_00: And this is a really important part.
11:20
SPEAKER_00: So the upper part of the continental crust, we think we have that kind of nailed the composition.
11:26
SPEAKER_00: This upper 10-15 kilometers is a lot of sediment, there's a lot of granite.
11:30
SPEAKER_00: It is granitic in composition, that upper part, and we have really good estimates of the even even down to sort of the uranium and thorium concentration on average of the upper part of the continental crust.
11:40
SPEAKER_00: It's when we start to get lower and talk about the bulk continental crust, is that we don't really know too much about that.
11:45
SPEAKER_00: Or we have some estimates, but we have there's there's not much agreement about on the composition of that.
11:50
SPEAKER_00: So another place, Chris, which is less intuitive than these exhumed terrains, another place where we could go and pick up a rock and say, ooh, this came from the lower crust.
11:59
SPEAKER_00: What do you think students would get to this?
12:01
SPEAKER_00: Like, hey, this is uh an area the the xenoliths would would could record this record, or no?
12:08
SPEAKER_00: It depends if you've talked about diamonds first or not, I think.
12:11
SPEAKER_00: Like if we've talked about diamonds and how they get out.
12:14
SPEAKER_02: No, I definitely have not.
12:16
SPEAKER_02: Yeah, I definitely have not.
12:18
SPEAKER_02: Diamonds is is gonna be a conversation that just comes up because it's an organic conversation.
12:24
SPEAKER_02: They want to know, they'll ask a question about diamonds, and then okay, well, here we go.
12:28
SPEAKER_02: We're talking about Kimberlites, and you know, we're gonna we're gonna talk about these pipes that come up from deep, deep down.
12:34
SPEAKER_02: But there is a way that they can maybe arrive at this, and that would be through their knowledge of a porphyritic igneous rock, like a porphyritic andesite or something like that, right?
12:45
SPEAKER_02: Which isn't this, but it's close.
12:48
SPEAKER_00: Yeah, okay.
12:48
SPEAKER_00: In what way?
12:49
SPEAKER_00: What do you mean by that?
12:51
SPEAKER_02: Xenoliths, we we we think of that as the you know, chunks of rock, country rock that get broken off and and not really consumed you know in the magma that is bringing this kind of slush up to the surface.
13:05
SPEAKER_02: Porphyritic rocks, um, it's similar to that in that you have these, you know, a deeper seated magma chamber, you have these early formed crystals, and then when the volcano erupts, it brings those slushy crystals with it to the surface.
13:19
SPEAKER_02: So they're coming from deeper down, if that does that make sense or something.
13:23
SPEAKER_02: No, that's exactly right.
13:23
SPEAKER_00: Yeah, they're coming from deeper down.
13:25
SPEAKER_00: In this case, they're coming from far deeper down.
13:27
SPEAKER_00: So we can go to places like the basin in range, for instance, where there's all this extension, the the continental crust has been thinned, it's been cut in half by it's been stretched twice, so you know, twice as long.
13:39
SPEAKER_00: So it's half as thick, and there's basalt erupting through there.
13:43
SPEAKER_00: It's a mantle melt, it's forming at the base of the continental crust, and it's erupting through the continental crust.
13:49
SPEAKER_00: And sometimes the stuff that it picks up at the bottom, in the middle, in the top, is carried up with the basaltic lava.
13:55
SPEAKER_00: We find these as xenoliths.
13:56
SPEAKER_00: So we can look at these and measure these things, and people have done this for a very long time.
14:00
SPEAKER_00: There's a bunch of people who've who've uh spent a long time categorizing this and trying to answer this question of what is the continental crust, and they looked at the xenolith record.
14:09
SPEAKER_00: So these are little pieces of the lower crust, middle crust that have been carried up in lavas.
14:15
SPEAKER_00: And if we look at those, they have a broadly mafic composition or a dominantly mafic composition.
14:22
SPEAKER_00: Those things are very heavily weighted towards basaltic, they're they're metamorphosed basalt at granulite facies conditions.
14:30
SPEAKER_00: So you take a basalt and it metamorphose it to really high pressures and temperatures, you know, 700, 800 degrees centigrade and 30, 40 kilometers thick, 40 kilometers deep, that's going to be a granulite fascies rock.
14:42
SPEAKER_00: That's what these things dominantly look like.
14:44
SPEAKER_00: There's hints of sediment, there's like 10, 15% sediment in the xenolith record, but it's heavily weighted towards mafic stuff.
14:51
SPEAKER_00: And so that data is uh is a large um reason why people have argued traditionally for a three-layer crust, a three-layer cake for the Gano crust where you have really felsic stuff up top, you have intermediate stuff in the middle, this is more like andesitic composition stuff in the middle, and then the bottom would be mafic, would be basaltic in composition.
15:12
SPEAKER_00: That three-layer approach kind of comes from the xenolith record.
15:16
SPEAKER_02: Okay, that's interesting.
15:17
SPEAKER_02: I want to come back to that comment in a second, but before we do that, so these are in in contrast, does it maybe mean that the exhumed terrains you know, through isostic isotic adjustment and this gravitational resetting, if you will, maybe that's not as deep-seated as where these xenoliths come from?
15:38
SPEAKER_00: That's a good question.
15:39
SPEAKER_00: It could be it could be true, however, we do have very deep-seated stuff.
15:44
SPEAKER_00: We can, you know, in if you take a shale and you metamorphose it, we have all these cool minerals that form.
15:49
SPEAKER_00: We have garnet, we have kinite, we have slumonite, all these really cool minerals that form that give us a pretty accurate pressure-temperature data point.
15:57
SPEAKER_00: So we can say, oh, this this mineral grew at really high pressures.
16:01
SPEAKER_00: And so we know really with a good deal of confidence that some of these sediments were down in the lower crust at least.
16:08
SPEAKER_00: But I think you're you're bringing up a really important question, which is okay, we've got this exhumed terrain thing, we've got xenoliths.
16:16
SPEAKER_02: Are either of the appear to contrast each other?
16:18
SPEAKER_02: Yeah, they're kind of different.
16:19
SPEAKER_00: And to the exhumed terrains kind of have a similar composition broadly to the continental crust.
16:25
SPEAKER_00: There's like a big peak in the mafic category, and there's a big peak in the felsic category.
16:30
SPEAKER_00: So, like a big um population of them that's mafic, and then a big population of them that is kind of granitic and sedimentary.
16:37
SPEAKER_00: So you get kind of this bimodal two modes, two groups in the in the compositions here, in the terrains, in the exhumed terrains.
16:45
SPEAKER_00: And the question we have to ask is like, are either of these, one or the other of these, biased in any way, shape, or form to kind of preferentially give us a view of maybe too felsic or maybe too mafic when we're thinking about what gets brought up to the surface and is our surface exposure.
17:04
SPEAKER_00: We know rocks in the surface are biased compared to what's down in that depth, but how much is kind of the question here.
17:10
SPEAKER_02: Well, it's an interesting thing, Jesse, because andesite's missing from both of these.
17:16
SPEAKER_00: Right.
17:17
SPEAKER_02: And if we had a three-layered cake, that wouldn't be the case.
17:20
SPEAKER_02: I mean, that's not to say that we don't have andesite in the continental crust, but it's a niche setting.
17:26
SPEAKER_02: It's this is the you know, subduction zone-related partial melting, this kind of tectonic, very specific setting where you get andesite, but we don't see it in either one of these situations.
17:37
SPEAKER_02: We don't see xenoliths that are bringing up andesite, we don't see exhumed terrains that have enough andesite to really speak of either.
17:46
SPEAKER_02: So it's a it's an anomaly to me.
17:48
SPEAKER_00: That's right.
17:49
SPEAKER_00: It is, it is, and that kind of alludes to a third category where there are a few locations on Earth, they're more recent, they're more modern settings, but there are places where an entire section, uh, entire subduction zone system has been tilted on edge, and we get a window going from the upper crust all the way down into the lower crust of a former arc.
18:11
SPEAKER_00: And so the Sierra Nevadas have this.
18:13
SPEAKER_00: If you go from Tuala Yumni, if you go from Yosemite National Park and you head south, you basically get down into the roots of a volcanic arc, deep down into the roots.
18:23
SPEAKER_00: There's a famous one called the Coast An Arc Batholith that's kind of caught up in the in the Himalaya collision there, and it's another one of these that's been kind of tilted on edge.
18:31
SPEAKER_00: There's one up in Alaska, Talkitna.
18:34
SPEAKER_00: And so there are arc terrains where we can get this like cross-section through a continental arc, but that's that's a relatively modern.
18:43
SPEAKER_00: Well, it depends when you believe it's subduction started on Earth, but that's a that's a one particular tectonic setting, and it's not a collisional zone setting.
18:51
SPEAKER_00: It's it's exposed and tilted in a collision, but it was formed in a juvenile arc setting.
18:56
SPEAKER_00: So it's not necessarily totally representative of the continental crust necessarily.
19:01
SPEAKER_00: In all these terrains, let's let's go back to the xenolith one, they could all be biased, or they almost certainly are biased in some way, shape, or form.
19:08
SPEAKER_00: So exhumed terrains.
19:10
SPEAKER_00: Well, maybe sediments are more likely to get exhumed.
19:13
SPEAKER_00: Like that would make sense.
19:14
SPEAKER_00: They're a little bit more buoyant.
19:16
SPEAKER_00: Maybe in a big continent collision, you smash stuff together, you erode off the top of the of the of the huge mountain belt that's formed, and everything doesn't rise back up to the surface.
19:26
SPEAKER_00: It's kind of like we use this this uh cargo ship analogy a lot when we're talking about this process, Chris.
19:30
SPEAKER_00: Like uh you it's a good analogy.
19:32
SPEAKER_00: It's a great analogy.
19:33
SPEAKER_00: The cargo ship, you start to unload the top layer and the whole the whole rest of the ship kind of rises up, right?
19:39
SPEAKER_00: It floats higher.
19:40
SPEAKER_00: And so by the time you you're getting to the bottom layer of the cargo, that stuff is riding a lot higher than it used to.
19:46
SPEAKER_00: But that's not actually how it works.
19:47
SPEAKER_00: So you don't just unload all the layers all at once.
19:49
SPEAKER_00: There's more of a feedback loop.
19:50
SPEAKER_00: Like you could unload one side, it starts to float a little higher, which on Earth, that would mean that it makes it easier to erode that because it's a little bit higher.
19:57
SPEAKER_00: So then you keep there's feedback loops in how this stuff erodes.
20:01
SPEAKER_00: So and if the lower part is more buoyant and the ships are already tilted to begin with, a fully loaded ship has all the light stuff up front, all the heavy stuff in the back, it's a little bit more tilted.
20:12
SPEAKER_00: You you might start to erode the stuff in the front quicker, which means the front nose would float higher quicker.
20:18
SPEAKER_00: Is that if that makes sense?
20:19
SPEAKER_00: Like yeah, it does.
20:20
SPEAKER_02: Yeah, so so that's the way to kind of think about it.
20:22
SPEAKER_02: Let's ask the question then about the the xenoliths then and why they are predominantly more mafic.
20:29
SPEAKER_02: And and just this is again another maybe thought experiment.
20:33
SPEAKER_02: Well, how might that be biased?
20:35
SPEAKER_02: Could there be felsic xenoliths that were there on the way up?
20:40
SPEAKER_02: Right.
20:40
SPEAKER_02: Well, what happens, Chris?
20:42
SPEAKER_00: What happens when you take a 1200 degree centigrade basaltic melt and you flush it through a bunch of sediment?
20:48
SPEAKER_00: What happens to that sediment?
20:49
SPEAKER_00: You you melt it.
20:50
SPEAKER_00: You melt it, it's going to be assimilated, right?
20:51
SPEAKER_00: So maybe there was a whole lower temperatures.
20:55
SPEAKER_02: Lower melting temperatures.
20:56
SPEAKER_02: Lower points.
20:56
SPEAKER_02: Lower melting points.
20:57
SPEAKER_02: Yep, absolutely.
20:58
SPEAKER_00: Felsic minerals have low melting points.
20:59
SPEAKER_00: You could totally melt the granite into the basalt and lose that xenolith record, and only the mafic stuff survives, right?
21:07
SPEAKER_00: In in a way.
21:08
SPEAKER_00: Um, we often see this process in reverse.
21:10
SPEAKER_00: We see really low temperature granites with mafic xenoliths in them because the low temperature felsic granite does not have the thermal power to actually melt the mafic xenolith that's intruded in it.
21:21
SPEAKER_00: But you do that in reverse, and the the basalt has a lot of energy and can melt that thing.
21:25
SPEAKER_00: So that's one example of a way that this record could be potentially biased.
21:31
SPEAKER_00: And so all of these are potentially biased.
21:34
SPEAKER_00: You know, the subduction zone, the tilted subduction zone sections, those could be biased towards a tectonic setting.
21:40
SPEAKER_00: They could be just only giving us a view of a modern subduction zone system or relatively modern hundred million year old subduction zone system, which is not necessarily indicative of how the entire continental crust formed over three or four billion years.
21:54
SPEAKER_00: So we're kind of at a conundrum.
21:56
SPEAKER_00: I mean, I think I there's not a ton of agreement on this three.
22:00
SPEAKER_00: Layer two model.
22:00
SPEAKER_00: So if I just go back to this three-layer two-layer model, basically there's two camps like there's a couple different models for a three-layer model.
22:09
SPEAKER_00: It's felsic up top, middle crust is is from 12 kilometers to 25 kilometers is is intermediate, and then from 25 kilometers to 35 or 40 kilometers is mafic.
22:19
SPEAKER_00: That's kind of the the one model.
22:21
SPEAKER_00: The other model, the two-layer model, says the upper crust is felsic for sure.
22:25
SPEAKER_00: It's it's really granitic, uh almost close to very granitic up top.
22:29
SPEAKER_00: And then the lower crust is intermediate, and that is their model.
22:33
SPEAKER_00: So from 15 to sort of 40 or 35 kilometers is intermediate.
22:37
SPEAKER_00: Then it's a two-layer model, and we'll come back to that that theme later on, which which is what happened to the the bottom layer, I guess, in that model.
22:47
SPEAKER_00: But it I don't know, it's uh it's a it's an interesting thing.
22:51
SPEAKER_00: Like I one of these things in graduate school for me where you sort of realize, wait, we don't understand a lot about the earth.
22:58
SPEAKER_00: We don't understand when continents formed, we don't understand actually what the composition of the continents are underneath of our feet.
23:03
SPEAKER_00: What is the average continental crust?
23:06
SPEAKER_00: It's interesting that we don't know that very well.
23:08
SPEAKER_00: I mean, we have some pretty good estimates, but we don't we don't have it nailed, which is interesting.
23:12
SPEAKER_02: What so what do you subscribe to three-layer model or two-layer model?
23:18
SPEAKER_00: I'm not really sure.
23:19
SPEAKER_00: Uh that's a tough question.
23:21
SPEAKER_00: I think the two-layer of only because it's just it's a little bit simpler.
23:26
SPEAKER_02: Um yeah, but but it's not it's not just that though, Jesse.
23:30
SPEAKER_02: It do you have a lack of diorite and intermediate uh rocks?
23:36
SPEAKER_00: They're just not that common.
23:37
SPEAKER_00: Right.
23:38
SPEAKER_00: Yeah, that's right.
23:39
SPEAKER_00: And I think it goes I would what do you what do you what do you uh what's the crystal heist?
23:45
SPEAKER_00: Yeah, I agree, I agree with you.
23:46
SPEAKER_02: The two layer just because it's my gut is just because like it's just to me, I just I cannot get my mind past the lack of those types of intermediate rocks at the surface.
24:00
SPEAKER_02: Um I I just think that we would have more of it.
24:03
SPEAKER_02: I but yeah, I I don't know.
24:05
SPEAKER_02: I mean, we don't know.
24:07
SPEAKER_02: Which leads us to the to the next segment here, Jesse, is you know, why do we care?
24:13
SPEAKER_02: Then why are we talking about this?
24:14
SPEAKER_02: Why is this important?
24:15
SPEAKER_00: There's a lot we don't know about the earth, but why do we care, right?
24:17
SPEAKER_02: Yeah, right.
24:18
SPEAKER_02: It did get me thinking though that Jesse, you you threw out some words that are near and dear to my heart, um, places that you and I could have a lot of fun walking through Toolumney, you know, absolutely going to cathedral lakes and and you know, up by clouds rest and just you know hiking through the heart of the heart of Yosemite.
24:39
SPEAKER_00: I mean, what what more could you want in a vacation?
24:42
SPEAKER_00: I mean, my goodness.
24:43
SPEAKER_00: So why do why do we care what the composition of continental crust is?
24:46
SPEAKER_00: Well, the composition tells us a lot.
24:49
SPEAKER_00: It tells us how it formed, really.
24:51
SPEAKER_00: It tells us the process by which it formed, because we we know that different igneous rocks, different compositions of fig of igneous rocks form in different ways.
25:00
SPEAKER_00: The granites or the rhyolites that are erupted on Iceland are incredibly different from the granites and rhyolites that are erupted in Yellowstone or the ones that are erupted on any subduction zone system on Earth.
25:12
SPEAKER_00: And so that tells us a lot.
25:14
SPEAKER_00: If the average continental crust looks like a subduction zone system or looks like yellowstone or looks like Iceland, it tells us that maybe that's how the cottonhole crust actually formed.
25:24
SPEAKER_00: The process by which the cottonal crust got stabilized.
25:27
SPEAKER_00: How do you build a stable continent?
25:28
SPEAKER_00: So the composition matters a lot.
25:30
SPEAKER_00: And I think if we wanted average composition of the cottonhole crust, what if there's one rock type, Chris?
25:39
SPEAKER_00: This is a little this is a little quiz.
25:41
SPEAKER_00: If you could measure the composition, geochemical composition of one rock type to estimate the average of the continental crust, what would you look at?
25:52
SPEAKER_02: Of continental crust or just the composition of the earth?
25:57
SPEAKER_02: Composition of the continental crust.
25:58
SPEAKER_00: Well, two two two parter.
26:00
SPEAKER_00: Okay, what what uh the composition of the earth first and then composition of continental crust two?
26:06
SPEAKER_02: Um well I think for the composition of the earth, I would want to know the composition of meteorites.
26:14
SPEAKER_00: Okay, good.
26:15
SPEAKER_00: Non-intuitive though, why?
26:17
SPEAKER_00: Why meteorites?
26:18
SPEAKER_02: Because that well, because because that's gonna tell us what the interior of the earth is that we definitely can't see.
26:24
SPEAKER_02: We're never gonna get to see.
26:26
SPEAKER_00: It's the building blocks of earth, right?
26:27
SPEAKER_00: Yeah, exactly.
26:28
SPEAKER_00: Right.
26:28
SPEAKER_02: So okay, that okay, great.
26:30
SPEAKER_02: Now that's a that's easier, I think, than the other question.
26:34
SPEAKER_02: Yeah.
26:35
SPEAKER_02: This is seriously.
26:36
SPEAKER_02: Um so what you're asking me, uh let me just I want to be straight on this, because we didn't talk about this.
26:42
SPEAKER_02: No, no, this is impromptu time.
26:44
SPEAKER_02: Yeah, I know, I know.
26:45
SPEAKER_02: Um, you want to know what rock I would want to know a geochemical analysis of to help me understand the chemical composition of the crust.
26:57
SPEAKER_00: Let's say.
26:59
SPEAKER_00: The upper crust.
27:00
SPEAKER_00: Which is kind of a hint.
27:00
SPEAKER_02: Oh well, I would say granite.
27:04
SPEAKER_00: Ooh, okay.
27:04
SPEAKER_00: Why granite?
27:07
SPEAKER_02: Because that's primarily oh, I don't know.
27:11
SPEAKER_02: I mean, we do have a lot of sedimentary rocks in the upper crust too, but like closer, warmer, warmer.
27:19
SPEAKER_00: Warmer.
27:20
SPEAKER_00: Um what is it, what sediment best represents the continental, the average continental crust?
27:26
SPEAKER_00: If you could take a big ass sample.
27:30
SPEAKER_02: A big ass sample of it?
27:32
SPEAKER_00: Shale?
27:33
SPEAKER_00: Shale is the classic one.
27:35
SPEAKER_00: Shale, if you had a a big a big sample of shale, people have used this as like average continental crust in some way, short.
27:42
SPEAKER_00: There's one that's you that's more more recent people have been focusing on, which is diamictite, which is take a huge sample of glacial till and go back in time and say, oh, there's old diamictites, there's old till samples.
27:55
SPEAKER_00: Uh a till is just glacial kitty litter as you describe it.
27:59
SPEAKER_00: That's a pretty good estimate for average because it's just unsorted, there's big boulders in there, there's little fine-grained clay particles, and and people have looked at diamictite back.
28:08
SPEAKER_02: I don't feel that's a fair okay, you that's not fair.
28:13
SPEAKER_00: Yeah, I didn't say it was fair.
28:16
SPEAKER_00: But but shale, you you got it right.
28:18
SPEAKER_00: Shale is the one that people have used for a long time as a good estimate for uppercotinal crust composition.
28:24
SPEAKER_00: And it turns out that that composition looks in major elements in trace elements, it looks kind of like a subduction zone system.
28:34
SPEAKER_00: We have a depletion in the elements that subduction zone magmas are depleted in.
28:40
SPEAKER_00: There's enrichments in the fluid mobile ones that subduction zone systems are enriched in.
28:45
SPEAKER_00: We have more uranium than thorium, we have all these signatures that look kind of like an arc system that are preserved in the upper continental crust.
28:54
SPEAKER_00: And so that's just one indication of like why the composition matters, because it tells us something about the formation processes of the bulk of the continental crust that subduction or processes like subduction were probably important for the majority of the continental crust of construction of the majority of the continental crust.
29:12
SPEAKER_02: So but that does present a problem though, because when I think about a geochemical analysis of a rock, I don't just want to know the elements that are present there.
29:22
SPEAKER_02: And and to me, then shale makes sense because you know, sh the most common mineral at the surface is felds bar, and feldspar chemically breaks down into clay, and clay is what makes up shale and so on.
29:33
SPEAKER_02: That makes sense, but what you don't get out of that is a timestamp, really, or at least a good time stamp.
29:40
SPEAKER_02: We need to know age as well.
29:42
SPEAKER_02: That's right, and that that's a problem.
29:44
SPEAKER_02: You know, igneous rocks are more straightforward for that.
29:48
SPEAKER_00: Yeah, that is so that is a great that's why I was torn.
29:52
SPEAKER_00: That is a great, great comment, Chris.
29:54
SPEAKER_00: I'm very happy you said that because I think that leads I bet you are.
29:58
SPEAKER_00: I think that leads we don't I I I use a slide when I introduce a bunch of talks about earlier stuff, and everybody else who gives talks on the early earth use a very similar slide.
30:09
SPEAKER_00: And the slide shows the range of models that people have proposed for when continents grew on Earth.
30:17
SPEAKER_00: And the basically some people argue that we had the exact same volume of continental crust that we have today for 4.2 billion years ago, and some people argue we only had 50% of the continent crust a billion and a half years ago.
30:32
SPEAKER_00: There's a huge range.
30:34
SPEAKER_00: Oh my gosh, that's that's such a basically you look at this plot and the entire area of the plot is filled with model lines.
30:41
SPEAKER_00: Like people we just don't know.
30:43
SPEAKER_00: And so your point is exactly that.
30:46
SPEAKER_00: We even if we know the composition of the continent crust, it doesn't tell us much about time.
30:50
SPEAKER_00: And we can, there are blunt instruments we can use to understand the bulk composition of the conoic crust.
30:56
SPEAKER_00: So there are these things called model ages, like the meteorite record.
31:00
SPEAKER_00: We know the meteor, we look at meteorites, we can understand, we can look at those and we can measure the neodymium isotope composition, which is a samarium neodymium radioactive decay system.
31:10
SPEAKER_00: We can measure the lead isotope composition, which we've talked about before.
31:13
SPEAKER_00: Uranium decays to lead.
31:14
SPEAKER_00: There's two isotopes.
31:15
SPEAKER_00: There's a bunch of different isotopic systems that are constantly decaying away.
31:19
SPEAKER_00: And what that means is the average, the isotopic composition of, let's say, neodymium or lead in a bulk meteorite will be changing over time because of radioactive decay.
31:31
SPEAKER_00: So two billion years ago, there was less radioactive radiogenic lead than there is today.
31:36
SPEAKER_00: Um, so we can do the exact same thing based on a model of Earth.
31:40
SPEAKER_00: We can measure average neodymium isopomposition of continental crust or shales, let's say, or diametites.
31:47
SPEAKER_00: We can measure the lead isotope composition of those rocks, and we can say, what's the average age?
31:52
SPEAKER_00: And we say average age, it's a de we use this as like a depleted mantle model age, is the technical term, but we don't need to get into the weeds of that.
31:59
SPEAKER_00: It's just a a rough estimate of the age, it doesn't tell us when the rock formed, it tells us when the minerals, when the mass of the rock was extracted from the mantle on average.
32:10
SPEAKER_00: It sounds really vague, and it is really vague, but it's all we have.
32:15
SPEAKER_00: So when we do that for rocks like shale or dimictites, the average age of the the time of extraction of the cottonal crust is two billion years, around about 1.9, maybe 2.1, or something like that.
32:29
SPEAKER_02: Okay, that's interesting.
32:30
SPEAKER_02: Okay.
32:30
SPEAKER_02: I would have thought older.
32:32
SPEAKER_02: Okay.
32:34
SPEAKER_02: Because you're relying upon the date of the when those minerals formed, not when the rock was laid down.
32:42
SPEAKER_02: That's right.
32:43
SPEAKER_02: You know, which are two really, really different things.
32:46
SPEAKER_00: Totally different things, you're right.
32:48
SPEAKER_02: I would have thought it would have been an older age.
32:50
SPEAKER_02: Okay.
32:50
SPEAKER_02: So what you're suggesting then is continents are younger.
32:54
SPEAKER_00: Well, it's a good question.
32:55
SPEAKER_00: The average extraction of the mass of the continents from the mantle was around two billion years.
33:03
SPEAKER_00: But does that mean that continents were formed two billion years ago?
33:08
SPEAKER_00: No.
33:09
SPEAKER_00: Definitely doesn't mean that.
33:11
SPEAKER_00: Because two billion years is around about the the age of the earth, half the age of the earth, actually.
33:15
SPEAKER_00: So is it just two billion years?
33:18
SPEAKER_00: Let me correct you.
33:18
SPEAKER_02: You threw me for a loop there.
33:19
SPEAKER_02: I thought we were changing, I thought we were uh like Yeah, we're revising the age of the Earth here.
33:23
SPEAKER_00: Yeah, didn't you know this, Chris?
33:25
SPEAKER_02: Hey, it's our podcast.
33:26
SPEAKER_02: We can do what we want.
33:27
SPEAKER_00: With the edge of their earth is two billion years.
33:29
SPEAKER_00: No, sorry, I miss I misspoke there.
33:31
SPEAKER_00: The two billion years is round about half the age of the earth, which means it could just be an average, it could be continents started forming four billion years ago, progressively grew till zero, and the average is two.
33:42
SPEAKER_00: So we don't really know how to interpret that.
33:45
SPEAKER_00: We know that it's not four and it's not zero, right?
33:48
SPEAKER_00: But it's somewhere in the middle, and it, like I said, it's a really blunt instrument.
33:51
SPEAKER_00: But I think that might be a place where we want to leave this episode, Chris, and come back to that because how continents like get reworked and overprinted and recycled is a really complicated question.
34:03
SPEAKER_00: And again, like I said before, there's very little clarity or very little agreement on this topic in the scientific uh literature on when continents grew, and there's some nuance into that.
34:15
SPEAKER_00: I think it's an interesting discussion, like what the various arguments are.
34:18
SPEAKER_02: I oh, for sure.
34:20
SPEAKER_02: This is great mental gymnastics for me.
34:22
SPEAKER_02: It is, it's fun to think about.
34:24
SPEAKER_00: I'm not sure how easy it is to make progress because it's a really intractable, really tough problem.
34:30
SPEAKER_00: It's a really tough problem.
34:31
SPEAKER_02: It is, but I don't feel good about ending right here, though.
34:35
SPEAKER_02: I I feel like we need to, for our listeners, we need to sum up like what's the takeaway for today.
34:42
SPEAKER_02: We've said a lot of we don't know, we don't know how continents really stabilized.
34:48
SPEAKER_02: We don't know when they really stabilized.
34:51
SPEAKER_02: There is a lot that we don't know.
34:54
SPEAKER_02: So uh what would you say the takeaway is for today's talk?
35:00
SPEAKER_00: I would say what have we done?
35:02
SPEAKER_00: I would say that we don't know a lot, but we do know some things.
35:06
SPEAKER_00: And we'll come back to the distillation thing.
35:08
SPEAKER_00: How do we how do we get how does it stabilize?
35:10
SPEAKER_00: We have a good sense of stabilization, perhaps.
35:12
SPEAKER_00: We'll come back to that.
35:13
SPEAKER_00: We'll come back to the the timing thing.
35:15
SPEAKER_00: There are models out there, there are ideas out there about when continents formed.
35:19
SPEAKER_00: I would say that the the takeaway, what do we know?
35:22
SPEAKER_00: We don't know whether there's two layers or three layers, but we know that the average upper continental crust has a lot of similarities to a subduction zone system.
35:30
SPEAKER_00: And so fluids and subduction-like processes maybe were part of the process that formed the bulk of the continental crust.
35:40
SPEAKER_00: So we kind of know water was involved.
35:41
SPEAKER_00: I think that's pretty a pretty safe thing to say.
35:45
SPEAKER_02: And there's this famous paper from 1983 that laid out Well, hold on though, Jesse, water in the water coming out of the magma, right?
35:53
SPEAKER_02: The distillation process.
35:56
SPEAKER_00: Yeah, water moving elements into the continental crust.
36:00
SPEAKER_00: And in the way that happens on the modern earth is the oceanic plate, you describe it as a sponge that soaks up a bunch of water when it goes down the subduction zone system, it gets squeezed out.
36:08
SPEAKER_00: When it gets squeezed out, uranium goes with it, whereas you know, thorium stays behind a little bit.
36:14
SPEAKER_00: Um, and so we have things like barium that are enriched in the cotton in the continental crust relative to other elements.
36:20
SPEAKER_00: So things like that are are um they kind of indicate that water was involved.
36:25
SPEAKER_00: And there's this famous paper from 1983 by Campbell and Taylor with a title.
36:29
SPEAKER_00: This is a great title.
36:30
SPEAKER_00: The title is No Water, No Granites, No Oceans, No Continents.
36:36
SPEAKER_00: And the point was that granites require water to be made on Earth, and the continents, the upper continents, have a lot of granite in them.
36:46
SPEAKER_00: So if we don't have water, then we don't have granites.
36:48
SPEAKER_00: If we don't have oceans, then we don't have continents.
36:51
SPEAKER_00: And that was the point that was made there.
36:54
SPEAKER_00: And uh I think it's a good one.
36:55
SPEAKER_00: And I think the data, you know, kind of backs that up that water and potentially subduction zone processes were kind of instrumental in forming uh the bulk of the continental crust at least.
37:07
SPEAKER_00: Well, there we have it.
37:08
SPEAKER_00: How do you feel about that?
37:10
SPEAKER_02: It's good, it's not done.
37:12
SPEAKER_02: It's not done.
37:13
SPEAKER_02: So this is that's this is part one of at least two, maybe part three.
37:19
SPEAKER_02: We don't know yet about that, but this is not done.
37:22
SPEAKER_02: So that's right.
37:23
SPEAKER_02: But it's a good, I think it's a good spot to end because then we're gonna move into next up, is gonna be really the stabilization of a continent, reworking it, recycling it, hence stabilization.
37:35
SPEAKER_02: What does stabilization mean?
37:36
SPEAKER_02: Something that you've referred to a ton, but we just never really explained it.
37:40
SPEAKER_02: We're gonna get into that.
37:41
SPEAKER_00: Sounds fun.
37:42
SPEAKER_00: I can't wait for it.
37:43
SPEAKER_00: All right.
37:43
SPEAKER_00: Hey, with that, that's a wrap.
37:44
SPEAKER_00: You can follow us on the social medias, Planet Geocast.
37:47
SPEAKER_00: Send us an email.
37:48
SPEAKER_00: There's a contact us link at our website, planetgeocast.com, and uh download the Camp Geo Mobile app.
37:52
SPEAKER_00: We talk about this and a bunch of other stuff.
37:55
SPEAKER_00: Intro to geoscience is there on the app and with a bunch of other audiobooks going to the bottom of the Grand Canyon, right, Chris, where we have some of these sediments exposed that that that made it to great depth.
38:06
SPEAKER_00: So check that out.
38:07
SPEAKER_00: Um, feel free to send us an email.