Some Rocks You SHOULD Know - Metamorphism and Plate Tectonics
[00:00:00]
Let's do it. Are, are you, uh, you doing video too?
Dr. Jesse Reimink: yes, got to straighten up. we go. We've got, we've got a zipped up tight Bolhuis here. Oh,[00:00:30]
Chris Bolhuis: always.
Dr. Jesse Reimink: Um,
Chris Bolhuis: Why, why, why would you lead in that
Dr. Jesse Reimink: I'm leading in that way because I'm just, the wheels, they are spinning in this episode. I mean, I, I don't know my, I'm just excited.
I don't know where this is going to go. So, it's an interesting topic and it's kind of something that I haven't thought that deeply about really, and I don't know what do you think?
Chris Bolhuis: Um,
Dr. Jesse Reimink: What kind of Bolhuis do I have this morning? Let me, there, what kind of Bolhuis do I have today?
Chris Bolhuis: so that's an interesting question. You have, [00:01:00] ah, what kind of Chris do you have today? Um, happy Chris, of course. Okay. Always. But this episode is about, um. It's rocks that we really want to teach because they're freaking awesome, but we don't. And a little bit about like why that is, but we need to, I think, to understand that there's this discussion about pedagogy with the teaching of, you know, metamorphic rocks,
Dr. Jesse Reimink: Yes, [00:01:30] totally. Maybe Chris, would it be okay if we sort of put out a call if you are, if you're listening to this and you're a teacher and you've taught geology or you are teaching geology, we really want some feedback on this episode, because I don't know where this is going to go.
It might end in a proposal to sort of change, fundamentally change how we teach Metamorphic rocks in the intro level classes. Do you feel it get that sense, Chris, or not? Cause this is something I've thought about a little bit and I'll be honest. I've tried to do this slightly in my class, but it's a really, I do a hacked job of it.
Like I [00:02:00] don't do it. It doesn't tease the through line. Like it really should. If I kind of went wholesale, in this direction, I suppose, if that makes sense.
Chris Bolhuis: It does make sense. I don't think it will. It won't for me. I've thought about this a lot, actually, and especially getting ready for this episode, but when you talk about these niche kind of metamorphic rocks, which is what this episode is really about, things like blue schist and eclogite and green schist and amphibolite, and, these are Really awesome [00:02:30] rocks.
And the thing is that I love about them. And I know you're the same way is that they tell us really specific information. What was the tectonic setting for the formation of these rocks? The problem is, is that they're. In some cases, and in most of the cases, incredibly rare. So that's why I'm not going to change whole scale.
I can't in an intro level class. Like I can't, I can't do that. [00:03:00] And I don't think that that's our job to be honest
Dr. Jesse Reimink: well, I, let me do like just a brief introduction, Chris, and then we can kind of pitch what, where we're thinking or sort of clarify what we're, what this discussion is going to be like. And what we're talking about here is a suite of metamorphic rocks that you've taken an intro level class or, or really, if you've taken a second level, like physical,
I mean, really rocks and minerals class, you would have seen these things.
These are the, the rocks that are, make up the phase diagram or [00:03:30] the facies diagram. Many people would call this the facies or facies diagram for metamorphism. And what that means is the metamorphic petrologists back in the day established that there's this pressure temperature space.
If you can imagine pressure on the y axis and temperature on the x axis. The whole range of earth conditions are on that plot. And if you take basalt, take your average run of the mill basalt, put it at different places in that diagram, so you heat it up to 100 degrees and, and, you know, [00:04:00] 2 kilometers deep in the earth, or you heat it up to 600 degrees and 8 or 20 kilometers deep in the earth, that rock, that basalt will melt.
Change its composition, change its minerals to be a new rock. And what that new rock is, the minerals in that new rock are named the rocks we're going to talk about mostly. And we name the field, we name the area in the diagram after that new rock, if that makes sense.
Chris Bolhuis: that does make sense. But can you really quick, cause you threw out some terms. You threw out these. facies, or facies as [00:04:30] you sometimes intellectually call them.
Dr. Jesse Reimink: Yeah. I'm just trying to appease our British listeners by calling it Fassies.
Chris Bolhuis: Love it. Let's go ahead, Jesse. Let's define exactly what that means because any, if you get into metamorphic petrology at all, you're going to get into facies. So what does that mean? Refer to, is this a, a suite of minerals that go along with a set of pressure temperature conditions? Like, what are we talking
Dr. Jesse Reimink: it's, it's, it, [00:05:00] it is that that's, um, it can mean a couple different things depending on the context we're talking about it. But it, it fundamentally is a region of pressure temperature space where a certain suite of minerals are stable. And what that means is that For this context in the rocks that we're going to talk about is we're going to start all these rocks We're going to take a basalt is the starting rock.
So that's our starting recipe we're going to take a basalt and if I heat it up to 400 degrees centigrade and Put it about 12 kilometers deep in the earth. It's gonna change into what's called [00:05:30] a green schist So we're gonna get the minerals that are gonna form are gonna be stable It's the same recipe basalt But it has new minerals in it and it looks totally different and it's gonna be a green schist and so that's We call the facies the region in pressure temperature space where green schist is stable, where those minerals are stable with the recipe called basalt.
That's a facies. And you can use that same term facies if you change the composition. So if you put a shale into the recipe book and [00:06:00] you do the same thing, you can get like garnet or, you know, salamite facies because salamite is stable in this pressure temperature regime with this starting composition called shale.
So,
Chris Bolhuis: Okay. So how come, Jesse, the rocks that we're going to talk about, Bluchist, eclogite, greenschist, amphibolite, granulite, all these rocks. I guess the common denominator with them is the protolith. the rock that they came from. [00:06:30] so these rocks are not as common as their other regionally metamorphosed rocks.
I mean, a lot of these are schists technically, but things like slate and phyllite and schist and
gneiss, but they, a lot of times they have a different protolith, right? They have a different parent rock that they existed as before the metamorphism
Dr. Jesse Reimink: Let me just jump in there, Chris, because we talked about these, the previous episode, the episode where we kind of started this conversation about like teaching these, this suite [00:07:00] of metamorphic rocks, we talked about schist and phyllite and slate in the 25 rocks you need to know episodes. So you can go back and listen to that, or even better yet, you can go to our Camp Geo app, first link in your show notes.
We have a whole chapter on metamorphic rocks, and we talk about both of these types of compositions there. We talk about all the rocks we're going Talk about today. And we also on there talk about all the rocks we talked about last episode.
so that's what you're referring to is, is those things if you put shale in as the recipe. Now, it's a really, really important question. [00:07:30] so the reason that the basaltic recipe version of the rocks we're going to talk about here in a minute are less common than the shale Versions or the sediment versions is you kind of got to think about how plate tectonics works, right?
sediments are on the continents, on the edges of the continents, and they're in the continents. And so if you deform and metamorphose continents, you're going to metamorphose a bunch of sediments. Where do you get basalt? Mostly. Sure, there's some on the continents, but you get a lot in the oceanic [00:08:00] crust.
What happens to the oceanic crust? Well, it gets recycled into the mantle. So it's getting, you
know, overprinted Through subduction.
It's getting subducted away and, like, completely destroyed. So, at least on the modern Earth, sediments are, just trapped on the continents, which is the point.
continents are where geologists walk around. If you had geologists walking around on the ocean floor, you'd see more, you know, zeolite and green shis kicking around there. but we don't. So that's, that's one of the reasons at least. So
Chris Bolhuis: Yeah. Which gets [00:08:30] to the heart of the matter, right? It was in terms of like, why, my rock sets have phyllite and slate and schist and gneiss as opposed to blue schist and eclogite in these rocks that like most beginning students have never heard of
Dr. Jesse Reimink: that, yeah, I agree. I agree. And the question kind of is why, so maybe Chris, let's just. Let's go through these rocks real quickly, and then we can kind of frame the discussion after that, or have the discussion, or debate, maybe even, where should we
Chris Bolhuis: All right. Well, I think, I think it makes sense [00:09:00] to start with blue schist.
to me, this logically makes the most sense because to me, I wanna work from like lower grade to higher grade, and I wanna work my way through these
tectonic settings, if
that, does that make
Dr. Jesse Reimink: I agree with you completely. Let me interrupt real quick and just define grade. You said grade. Metamorphic grade is really it's the temperature, but it's the intensity of metamorphism. So low grade is low intensity. High grade is high intensity and high temperature.
Chris Bolhuis: So, Jesse, let's start off by talking about [00:09:30] Bluishist because to me, it just makes sense to make our way through the tectonic processes.
So it's a regionally metamorphic rock. We talked about that in our last episode. This is one that involves, Significant amounts of heat and pressure. so it's a subduction zone related rock where you have oceanic crust basalt, which is that important protolith that we discussed just a second ago, but it has a relatively low geothermal gradient.
In other [00:10:00] words, what that means is that the temperature is not increasing at a high rate with depth. it's the opposite of
that, actually. Okay. And so what we get then is that kind of faces that metamorphic faces that you described earlier, we get relatively high pressure, but lower temperatures. And we get these rare minerals that like, I don't know, Jesse, I, minerals like lozenite and aragonite, these are not in jadeite, these are not [00:10:30] common. Minerals at all.
Dr. Jesse Reimink: no. Belusius though is just such a pretty rock. It's It's very, very vibrant blue. it's like undeniably blue. I think it's just a really, really pretty rock and it's a schist. So it has this kind of these undulations in it. It's really, it's quite fine grain, but it has a foliation. So it's just, it's a blue schist.
you pick it up and you'd be like, if I know what a schist looks like, this is a blue schist, right? It's very easy to identify. And you're right, these minerals are [00:11:00] not common. They're pretty rare. really only found in Bluchist. And here's the reason. The reason they're found in Bluchist is because Bluchist is formed in a really specific pressure temperature range, as you alluded to, right?
It's high pressure, but low temperature, relatively low temperature. And so it gets Thrust down really deep before it has time to heat up. So it goes down quicker than it can heat up really in the subduction zone system. And it turns into this really unique set of minerals that you [00:11:30] really only find in blue schist.
And it's really powerful that way. If you walk across, if you see a blue schist, you know, you're near a subduction zone or what used to be a subduction zone.
Chris Bolhuis: Why would this kind of rock then be so rare if you just think about that for a second? Why would a blue schist be rare?
This is this [00:12:00] Intermediate kind of subduction
Dr. Jesse Reimink: Chris, let me interrupt. It. you know, the starting recipe is basalt, one of the most common rocks on the surface of the earth. Like, why is Blucious so rare? Great question. would you answer to somebody who came up and said that to you?
Chris Bolhuis: Well
Dr. Jesse Reimink: Bolhuis! Mr. Bolhuis! why is Blucious so rare?
Chris Bolhuis: Well, what happens to this is that it's involved in a subduction zone. So if it continues to subduct, it continues to go through more [00:12:30] pressure and more heat. Then that's where Bluishist then will transition into a different.
It'll transition into the next rock we're going to talk about, which is an eclogite, which has a different metamorphic pressure, temperature, faces,
so in order to get a blue schist, something has to interrupt this whole process, Something has to interrupt the subduction process kind of exhumed this toward the
Dr. Jesse Reimink: bring it up to the surface quite quickly [00:13:00] so that it doesn't what we call retrogress react back along the pathway because metamorphic rocks, can do that they can they can transition between different types of metamorphic rocks right as it kind of comes back to the surface.
Chris Bolhuis: So Jesse, I have a question for you then. So have you seen blue schist in the field before?
Dr. Jesse Reimink: yes, I've been to the, the sort of type locality for subduction zone metamorphism, where like a lot of the details were worked out, which is called the Franciscan complex, which as the name implies is
north of San [00:13:30] Francisco. It is a, it's an exceptional, package of rocks, because it's basically all of these metamorph well, these Bluchist and Eclogite, these subduction zone metamorphism rocks are everywhere.
And it's just really beautiful to see. You can see some of these Bluchists with, with little tiny garnets in them. Some of them have bigger garnets. They're really, really beautiful rocks. and they represent this totally cool process. basalt went down, and we're not talking a little ways down.
We're talking like, 30 [00:14:00] kilometers down into the earth in a subduction zone or even deeper and then came back up, you know, went down 30, 40 kilometers and then came back up and is now exposed on the surface, kind of bobbed back up the subduction zone system. And so,
I mean, just,
Chris Bolhuis: what did that? Like what caused that? Like I've been, I know what you're talking about. I've seen the Franciscan complex too. And there we're going to allude to this actually few more times because we're going to see other host rocks here. But what [00:14:30] interrupted this process? Do you
Dr. Jesse Reimink: It's such, it's such a good question. and it's a topic of quite a lot of debate, I think, how these things ultimately get what's called exhumed on the surface and the same thing goes for eclogite, which we'll talk about next, but a lot of people call this a, it's a French word. So we'll talk about that.
forgive us for using a French word here, but melange, the subduction
zone
melange, which is just this mess of stuff that comes up in a subduction zone system, and some people think that these things, it's kind of like a washing machine, they kind of get stuck in [00:15:00] this bobbing around cycle where it's kind of tumbling around and around
and around,
Chris Bolhuis: of convective
cycle then a little bit like
yeah,
Dr. Jesse Reimink: gets caught by the down going, Exactly. Gets dragged by the down going slab, but then, you know, due to density, kind of percolates its way back up and then gets caught and brought back down again, and some people would argue that there's sort of cycles to these, but it's kind of in
the, in the early phases of the subduction zone system, because remember, the oceanic crust sits on it.
Oceanic mantle, and that's the [00:15:30] tectonic plate that's going down. So the oceanic crust can get scraped off the top by the overriding continental plate, basically, depending upon the tectonics of the region. So, sometimes it can kind of get, just get scraped off in the trench system.
Anyway,
Chris Bolhuis: Well, let's, can we move on then to the next
Dr. Jesse Reimink: I was just going to say, we're, we're at risk of getting really long winded on these cool
Chris Bolhuis: Well, I know, I know, I know, but there, so I want to talk about the next one, which I think is the next one in the proper sequence, which is called [00:16:00] Eclogite. And to me, raved about the beauty of Bluchist and it is, but I love, I don't know, I guess green is my favorite color.
And so
Dr. Jesse Reimink: Is it
the
Chris Bolhuis: me, this is an absolute no.
Dr. Jesse Reimink: Oh, you should have answered that with some more confidence.
Chris Bolhuis: Well, um, no, she, Jenny has bi colored eyes, so she's got some brown and some
green. So that's why
Dr. Jesse Reimink: Sure.
Chris Bolhuis: it on me. I know what I'm [00:16:30] talking about.
Dr. Jesse Reimink: Joyce is gonna get
Chris Bolhuis: Oh,
Dr. Jesse Reimink: and give you a hard
Chris Bolhuis: Joyce is sleeping. She's been sleeping for the last five
Dr. Jesse Reimink: Oh yeah, as soon as you said
Bolhuis, she's out.
Chris Bolhuis: So take blue schist, shove it down more, and now you're at higher pressures and higher temperatures, and the facies turns into the rock that will form eclogite.
And this has a higher concentration than of garnets and minerals like a [00:17:00] clinopyroxene, so it's a different host of minerals that are here, but it is absolutely beautiful because it has these, like, it's got these other minerals, Jesse, minerals that I love, minerals like kyanite and rutile. And then it does have some quartz in it too.
And you know, I've collected kyanite and rutile, actually. I collected rutile in, Arkansas
long, long
Dr. Jesse Reimink: Oh, yeah, yeah, totally cool. Not out of a, uh, eclid right there in Arkansas, probably, but [00:17:30] the,
the,
Chris Bolhuis: of a riverbed,
Dr. Jesse Reimink: oh, cool. Oh, that's, but that, I'm sure somewhere nearby, exposed nearby, maybe there was a, cause, cause rutile's quite common in these things. It's the Christmas tree rock, right? It's deep, deep, beautiful red garnets, and really bright green sodium rich clinopyroxenes.
It's just an unbelievably beautiful rock. And Chris, we talked about this a lot in the Camp Geo app on, Plate Tectonics, this is the most important rock. on earth probably for operating plate tectonics. Eclogite as [00:18:00] soon as it transforms to eclogite, usually there's melting. So you, between blue schist and eclogite, you lose some melt.
And as soon as you transformed to eclogite, the garnet and the composition of the clinopyroxene is really dense. That rock is actually more dense than the mantle. And so it sinks down into the mantle and it drags, it's like an anchor. dragging the entire subducting plate with it. So it kind
of
people think of like, you
Chris Bolhuis: It lends to the mechanism of plate tectonics. It
lends to this kind of [00:18:30] slab
Dr. Jesse Reimink: slab pull. That's exactly right. People think of plate tectonics is stuff shoving continents around. Actually, it's eclogite pulling the oceanic crust down. That's the main driving force. So an eclogite, eclogites, that's the one that does it. And it's because the Peculiar mineral, arrangement.
It's more dense than, than the mantle around it. So anyway, we're, we're belaboring this, but I've
seen it in the Franciscan complex. You get it in these areas. You get big cobbles, like big, big boulders. They call the blocks, blocks of [00:19:00] eclogite kind of you've seen them, Chris, you know, in a sea of blue schist, you'll get a big block of eclogite.
And
Chris Bolhuis: And they're beautiful. if you think about how rare a blue schist is, then think about how rare an eclogite is, because this is deeper. the exhumation process is much more robust. It has to be to bring this to the surface. Something had to interrupt this. And like, it's, it's a really rare rock at the surface of the earth, but it is so, so important in
telling us what's going
Dr. Jesse Reimink: and so [00:19:30] diagnostic. If you see an eclogite in the field, you know. you're at an ancient subduction zone system where the Eclogite has come back up so these rocks, Bluishist and Eclogite, it's hard to really overemphasize how valuable they are for tracing. Subduction zone systems. I mean, there are people who would argue that go back and you find the oldest blue schist on earth and you've identified the start of plate tectonics on earth because they're so diagnostic of just plate tectonics in subduction zone systems. [00:20:00] So they're really, really important.
Sorry, go ahead.
Chris Bolhuis: No, no, no. I just, you're, you're absolutely right. But I want to transition then into the next host of rocks
that we're going to talk about,
which are Greenschist. Yeah. Yeah, they are. And I want to talk about why. So we're going to talk about Greenschist, amphibolite, and then a rock called granulite in that
order. So let's start, Jesse,
Dr. Jesse Reimink: Let me, Chris, can I, can I just frame this
really quickly just to set this out? These
rocks form at the same pressure [00:20:30] conditions as blue schist, eclogite do, but they're hotter. if you go down 30 kilometers and you're at 200 to 300 degrees, you have a blue schist. If you go down 30 kilometers and you're at 400, 450 degrees, you're gonna form a green schist.
And
so,
it's higher temperature,
Chris Bolhuis: I want to, double tap on what you just said and then I'm going to have a, I have a question for you, but you said, look, these rocks. Greenschist, amphibolite, and granulite. They format roughly the same pressures, but they're hotter.
So what is [00:21:00] the difference in the geoscience setting or the tectonic setting in which these happen?
Dr.
Dr. Jesse Reimink: This, this is I think it's valuable to go really slowly and let the listener ponder that for a minute. Because, yeah, what would cause that difference?
It's so important, right? What would cause these rocks to, at the same depth, be higher temperature? And, yeah. I think it's best to think what makes Bluishist and [00:21:30] Ecloget unique is not that they go down deep.
I mean, they do go down deep. It's that they go down deep and cool. And they go down deep and cool because of subductions on system. They're actively being carried down. So they start cold at the ocean level, right near the surface, and then they get carried down and they go deeper. faster than they can heat up. Every other type of tectonic setting, you know, if you take, stuff in the continents and you smash it together, it's going deep, but it's going deep slower, and so it's [00:22:00] heating up as well as going down. I mean, we've talked about the geothermal gradient before a lot. And why is the interior of the earth hot?
The interior of the earth is hot. So if you move rocks down slowly, they're going to heat up. and get to higher pressure. so basically they're not subduction zone system rocks. They're more like continental collision zone metamorphism,
green schist,
Chris Bolhuis: Modern day analogies would be things like where the Himalayas are being actively built today, where you have the subcontinent of India colliding with the continent of [00:22:30] Eurasia or back further in time, the eastern part of the United States when, Pangea came together and it formed the Appalachian mountains.
Another perfect example for this kind of setting where you had massively thick continental crust, which means you have a lot of pressure
and then the geothermal gradient provides the heat.
you get the ingredients then for these kinds of
rocks.
Dr. Jesse Reimink: And I think let's go through the rocks real quick. Cause that's a perfect segue. And we [00:23:00] can talk about like specific examples here, but green schist, it's the lowest temperature and pressure version of this, but it's actually a really big one. It's a really big field in pressure temperature space.
So
green schist is really common. um, it's it's very, it is the most common of these rocks that we're discussing today. they're green because they have chlorite mainly and serpentinite as minerals and some epidote. There's some beautiful epidotes around here in just in like Northern Maryland, Pennsylvania border.
There's the South Mountain Bapholith, which has some, it's all [00:23:30] green schist, faeces, metamorphism of basalts. Epidote everywhere. It's really pretty stuff. But those are all green. Chlorite, Serpentine, Group Minerals, and Epidote. They're all really green.
And
Chris Bolhuis: Can I jump in a second, Jesse, on this? The green schist that you're describing is different than an eclogite.
In terms of, you know, the mineralogy is really different, right? the green schist doesn't necessarily have a lot of garnets in it.
Um, it doesn't have clino pyroxene. to me, Jesse, is [00:24:00] this kind of, it resembles a smearing of this beautiful
green
color in an otherwise black
Dr. Jesse Reimink: that's a really good word.
because green schists have that greasy. They look greasy, I think because of that, serpentine group minerals, chloride, epidode even is a little bit greasy and they're, they're more of a dark green, like a dark forest green, whereas Eclogite is a really bright, vibrant green, like spring leaf green, kind of like really, really vibrant [00:24:30] green.
So you're exactly right. Greenschist has this kind of greasy feel to it. Now, I mean, greenschist can be found in subduction zone systems for sure, but it's much more common in sort of continental collision or regional metamorphic environments where you don't have this low temperature metamorphism.
Chris Bolhuis: Think about what, how we described slate and phyllite with those regionally metamorphosed rocks that are super common. I would put green schist a little bit higher in terms of [00:25:00] metamorphic grade than that, but then lower than the next rock that we're going to talk about, which is amphibolite.
Dr. Jesse Reimink: Amphibolite. Amphibolite is such a fun one. Can we talk about amphibolite now?
Chris Bolhuis: Yeah, let's go ahead. Oh, first of green schist, like you said, can be found all over the place. in fact, you know, in terms of the way my logic goes, I should probably add green schist to my rocks
Dr. Jesse Reimink: And maybe amphibolite. Just as far as metamorphic rocks they're going to come
across. Green
schist certainly not [00:25:30] uncommon. They're not
Chris Bolhuis: So I'll tell you this, Jesse. I actually do what I do on, in my class is I'll lay out my metamorphic rocks, my regionally metamorphic rocks in an order that makes sense on a big lab table. I'll start with like shale or basalt as the proli, and then I'll go from that to slate and then fill light.
And then I have my whole host of rocks that are schist because you have so many different index minerals
and I'll start [00:26:00] with what I call often a metabasalt, which is a It's a green
Dr. Jesse Reimink: It's a green schist,
Yeah, that remember this term, you know, the famous pillows behind the Menards were metabasalts they're no longer there, but they're green schist, right? They're metamorphosed to green schist faces. And so they're, they're really quite common. I mean, I, I would argue that if you find a basalt that's older than a couple hundred million years old, it's pretty rare that it's a basalt.
It's most likely going to be a green schist. It's most likely going to be slightly metamorphosed and have that greenish kind of color to it.
Chris Bolhuis: but it's [00:26:30] hard to teach that and here's, here's,
why I, I find this difficult
Dr. Jesse Reimink: agree.
Chris Bolhuis: because, in, in terms of pedagogy, we do minerals before we do rocks because they have to be able to identify minerals in order to identify rocks, but finding chlorite as a standalone mineral is very difficult to do,
Dr. Jesse Reimink: Totally.
Chris Bolhuis: which, and that's the primary mineral of a green
Dr. Jesse Reimink: And it's not important to teach it as a mineral because it's only present you know, green, just rock. Like it's,
it's
not the, it's not,
yeah, I mean, it's not that [00:27:00] sort of important. let's cover amphibolite and granulite because amphibolite is another
pretty common one.
And Chris, you and I have seen probably the most spectacular amphibolites there are, which is the garnet amphibolites in the Gore Mountain Mine. I mean, huge garnets, right? So Amphibolite is temperature and higher pressure than green schist, but it's mainly amphibolite and plagioclase.
That's the mineralogy of the rock. It's black, it's salt and pepper. It's black minerals and
plagioclase, white minerals,
Chris Bolhuis: That's right. [00:27:30] Can I interrupt you one second, Jesse, because like, I want to just say that when, when you say amphibolite, I think most people associate a more common mineral with that as hornblende.
Dr. Jesse Reimink: sorry. I, yep. O'Horn
Chris Bolhuis: No, no, no. That's, that's, that's totally fine. Yeah. hornblende is a type. of amphibolite.
Amphibolite is a mineral group. It's kind of like muscovite and biotite are part of the mineral group, the micas. Hornblende is the same as that
for amphibolite.
Dr. Jesse Reimink: that's a good point. No, no, no, no, that, that's a really, that's a [00:28:00] really good point. I mean, it's mostly, most often it's very much hornblende and plagiclase, very little quartz in it because again, you started with basalt. There's not much like free quartz around or free silica around there. Chris, we've seen these with massive garnets in them.
So you can have garnet. You can
Chris Bolhuis: Size of a baby's
head, Jesse. The size of a baby's We have garnets that are that
large. They're
Dr. Jesse Reimink: unbelievably big. so you can have garnet and think of if you just change the recipe of the basalt, so you just, [00:28:30] some basalts have more iron in them, that iron will go into forming garnet.
So you just change the recipe of the basalt slightly and you can get garnets or not garnets, in this amphib light. And so in FTE's, a really, It's quite a common metamorphic rock. At least it's not as rare as eclodrite or blue schist again, but it's
kind of, you know, this is in the five to 700 degree range and, pretty deep, sort of the, the center of the continental crust.
And then we get into, if we go continue higher pressure and really higher [00:29:00] temperature, we're going to get to a rock that I really love called granulite, uh, which is just a very cool rock.
Chris Bolhuis: Yeah. This is a high grade regionally metamorphosed rock, which what I mean by that again, is I mean high temperature and relatively really high pressure also. And it's called granulite is kind of a, like, I love it when geology makes sense. And to me, this term makes sense, you it's called granulite because the, the mineral grains, they're [00:29:30] usually.
granular, they're usually the same size, which is not true of amphibolite. So they're usually the same size and they're equidimensional. And it's very like, I think, symmetrical in that kind of
way, if that makes sense.
Dr. Jesse Reimink: That's totally right. And the way that you get grain sizes like that, where often we think of this as having what we call 6120 connections, where basically three minerals
come together to form a triple junction, that's kind of the most
stable.
Chris Bolhuis: columnar joints and so on.
Dr. Jesse Reimink: [00:30:00] It's the most stable configuration, thermodynamically stable.
The way that you get that is by having melt around Then removed. And so granite is a rock that has had melt removed from it. So you've lost melt. It's been melted. it's the sponge after it's been squeezed out.
Chris Bolhuis: and this is on the precipice of, you know, gosh, this is almost igneous, you know, it's on that kind of squishy boundary,
if you
Dr. Jesse Reimink: It's what's left behind after you've made an [00:30:30] igneous rock is really what it is. And granite is just such a cool rock. Because it's usually, this is what's in the deep crust. If you drill down, let's say the continental crust is 40 kilometers thick, the bottom 10 kilometers are almost certainly all going to be granulite or the vast majority of it's going to be granulite because it's so deep, it's so hot, it's had all the good stuff removed.
All the goodies have been removed, squeezed out of it. And all you're left with is this really stable, What we call residual stuff that's everything, [00:31:00] all the good stuff's been melted out and it's just what's left behind.
it's just a cool rock.
Chris Bolhuis: So Jesse, correct me if I'm wrong here, but this is not going to be really uncommon rock because it's a crustal rock. although deep in the crust, it's still a crustal rock. And so you can think about it. How does it get to the surface in a manner similar to the way that granite can get to the
Dr. Jesse Reimink: yeah, it's less like [00:31:30] Shouldering its way through the crust like magma will do. But basically you get a package of crust that can get tilted. Like basically think of India, the subcontinent of India. The bottom half of India is granulate for all intents and purposes.
Like it's
been tilted. The whole package of the continent has been tilted on its side. Exposed it's legs, it's calves, up above, bobbed up, it's like Chris Bolhuis going swimming, and those big ol calves kind of float to the surface,
they, [00:32:00] they,
Chris Bolhuis: what you're saying right now? Did you really just, oh my gosh,
Dr. Jesse Reimink: it's hard, it's,
Chris Bolhuis: a really, Not a good image right
Dr. Jesse Reimink: I'm just thinking Chris Bolhuis in his bathtub, gets in, lays back, and his legs just kind of float, outside
of his control.
Chris Bolhuis: All
right.
Well, Jesse, uh, do you know,
I'm going to try to move on as quickly as we
can.
Um, so question where have you, what's your favorite place [00:32:30] that you've seen granulite?
Dr. Jesse Reimink: You know, I saw it was a really, um, transformational experience for me. But as, as an undergrad, we went and did some mapping in Southwest Sweden in a big metamorphic terrain.
And we were walking across granulites. Uh, it,
yep. Dr. Ed Hansen from Hope. And,
um, he's, an expert in these sort of, this type of metamorphism.
and we were walking across mapping them and it was just, you walk across and you're like, holy crap, this, this rock has literally lost 30 percent of its volume. to a [00:33:00] magma, like it is, it's the other half of a granite, this is the
other part of it, like, this is
So, cool.
Chris Bolhuis: that's really interesting. so another follow up question, did you come across this at all in your Northwest studies, Northwest territories of, Canada? Did you come
Dr. Jesse Reimink: So I haven't seen, not in that part of the Northwest Territories, where I have seen them up there is they get carried up in the, the kimberlite pipes that bring diamonds up the sort of volcanic eruptions, they'll, because it takes samples all the way through as it's erupting up through the crust, it'll sample [00:33:30] granulites from there.
And actually, there's a PhD student who, was working on granulites at Penn State. And actually one of our listeners has asked about these. But, um, from the little, basalt eruptions, uh, southwest United States in the, in the sort of basin and range environment,
these little small volume magmas can bring up pieces of granulite from the lower crust there.
And this is one way to really form granulite, is if the continent starts spreading apart, basically the mantle underneath is super hot and comes up to the [00:34:00] surface and heats up the bottom of the crust really quickly, and that will do this high temperature melting basically, that can produce granulite, so you can kind of So, you know, you can just bake the whole bottom half of the, of the continent with a mantle plume coming up.
We'll just bake the bottom half of the continent, form tons of granulite. so that's the other way you get granulite is in these Xenoliths that come up to the surface.
Chris Bolhuis: Okay. So there we go. With that different host of rocks from green schist to [00:34:30] amphibolite to granulate, which is increasing metamorphic grade, but in more of a continent to continent convergent boundary as opposed to subduction zone, where we talked about blue schist and eclogite.
So Jesse, let's round this episode out real quick. With the final rock here called zeolites.
I've collected some zeolites in Copper Harbor in really like as far north as you can go
[00:35:00] and still be in the state of Michigan,
Dr. Jesse Reimink: These are, um, these are low, really low grade, kind of hydrothermal metamorphism, I'd say, like really low pressure and temperature, and people, I think the gem collecting community will have heard of zeolite minerals, it's a mineral, zeolite is a mineral group, but if you take basalts, and if you have air pockets, what we call vesicular basalts, where there's little air pockets, slightly
Chris Bolhuis: and this is, can I interrupt you a second, Jesse? So basalt is, it [00:35:30] commonly has these air pockets because this is an extracevigneous rock. So it's degassing at the surface and as it's cooling, it will have these big air pockets that are preserved within the rock.
And that's where these zeolite minerals kind of
Dr. Jesse Reimink: That's exactly right. That's exactly right. So, if you take that, those zeolites, they just basically precipitate out of hot fluids circulating through the rock. And, it's kind of metamorphism. It's, it's, it's not
Chris Bolhuis: yeah, it
Dr. Jesse Reimink: it's not [00:36:00] like no, not really. It's just, It's just, been filled
with zeolites. No,
totally. So, but you're right. That's like the, the sort of lower end of the,
of the spectrum here.
Chris Bolhuis: Jesse, are these, are zeolites associated with petroleum at all?
Dr. Jesse Reimink: Oh, good question. the minerals sometimes are not in basalts, but in other types of rocks where you get, there's some mineral deposits, especially up in like the Yukon, where there's a lot of oil and
gas, you get, calcite precipitating with zeolite [00:36:30] minerals, and you also get a little, sort of hydrocarbon in these little pockets of, shales and, you know, Conglomerates and quartzites and stuff like that.
So yes, they can be certainly it's that it's that temperature range, like, oil maturations, like in the 100 to 200 degrees range. That's what we're talking about with zeolites is low temperature
stuff. Okay, we talked about
Chris Bolhuis: So we probably should have put zeolites up
top because it really didn't follow our flow, but
Dr. Jesse Reimink: but here's what it does do for us, Chris, it brings us full circle to talk about [00:37:00] why let's, let's talk about the pedagogy here for like five minutes.
Cause I think
it's a really interesting question. So we talked about, well, zeolites, the really low, low pressure, low temperature, we talked about blue schist eclogite, which are high pressure and relatively low temperature. Then we talked about green schist, amphibolite and granulite, which are medium pressure, medium temperature.
Let's put it that way.
Chris Bolhuis: different tectonic settings.
Dr. Jesse Reimink: All start with the same recipe. have different tectonic settings and dramatically different rock types that you get out of it. So let me pitch this [00:37:30] to you, Chris, and you can tell me what, where I'm, I'm sort of thinking about it wrong, right? you said, and every other, myself included, everybody else who teaches the intro to physical geology, like the basic geology class, teaches the rocks we talked about last episode, slate, phyllite, schist, gneiss, migmatite, right?
Like that's the sequence, which is just increasing metamorphic grade. This sequence here, we get to say the same recipe, but we also get to say tectonic [00:38:00] setting. And we get to explain how subduction zones have lower temperatures than other types of metamorphic settings. And recognize that rocks, Eclogite, Bluishist, Granulite, they're uncommon.
You're really not going to see them out in the field. But do you think that teaching, this sequence of rocks that we've just talked about, might give students a better way to understand the earth. Even though they're learning rare rock types or learn plate tectonics, maybe [00:38:30] even though it's rare rock types, or is it just too
deep?
I, I, like, I, I don't know. I, I, cause I, I,
I, cause I, I teach the basic ones, the ones we all teach. Right. but I also layer these on, I spend an extra lecture talking about, I say, okay, schist, phyllite, slate. That's if you have shale as a starting composition. If you change the recipe, you get green schist, amphibolite, granulite, but also weight, if you change pressure and temperature, you get blue schist, eclogite. And I don't, I kind of do a, frankly, a [00:39:00] half ass job of it and it doesn't quite land, I don't think. And I wonder what would happen if I did, you know, if I went the full way and just said, I'm only teaching these ones for
Chris Bolhuis: Yeah. Okay.
Dr. Jesse Reimink: thoughts.
Chris Bolhuis: so a first, my thought is that a lot of these are the same kind of rock where we talked about slate, fillet, schist, because a lot of these are schists. These are just parsing it out. Like let's get into the weeds now on our schists here at play and [00:39:30] these index minerals and these faces that exist within a schist.
So we kinda are. I don't think it's fruitful for an intro level class to get these kinds of weeds. These are deep weeds.
These
are,
think they are, I think for a petrology class, it's totally appropriate. but I also think like the value of going out and seeing a hornblende schist or a biotite schist or, you know, things [00:40:00] like this is, you You can go almost anywhere to where you have mountain building in the Western U S and you can see the rocks that we talked
about in our previous episode, that's a powerful thing and you can talk about that formation in a very general sense But I think because we'd have to go Jesse We'd have to go everywhere. We'd have to go here and then there Thousands of
miles away to see a blue schist and an eclogite and so on unless we just spend our time at the franciscan
complex and
Dr. Jesse Reimink: no, it's a good point.
One of our, one of my [00:40:30] fellow faculty members described it just recently as, you know, we have all the good stuff. We have volcanoes. We have earthquakes. We have all the exciting, cool, important stuff. We just need to do a better job of selling it to our fellow human beings that need it.
We need to know this stuff because it's so important and so unique and it makes our planet, as you say, totally cool. so I just, I think that's part of the sales pitch for Geoscience that we just need to do a better job of, frankly. Highlighting the value of understanding plate tectonics because [00:41:00] it tells us where mineral deposits are going to be.
It's just so important. It's just as simple as that. so I totally agree. I totally agree with everything, everything you're saying there and, and in that light, you know, talking about the rocks that you're going to see most as you wander around the earth is, is, makes a lot of sense, you know, and people are very schist.
So even if it helps you understand subduction zones a bit better, I, see your point. It is second level. It's definitely deeper
than, have time to go through in one [00:41:30] semester class, for instance. So,
Chris Bolhuis: Yeah, I, I think so because it's hard enough with just understanding that We need to be able to identify rocks because it tells us a story, but we can't spend an inordinate amount of time on that
because we have other
important things to get to.
Dr. Jesse Reimink: glaciers, we gotta talk about streams, like all these other You're totally right. It's just, it's a
Chris Bolhuis: got to talk about climate. We got to talk about resources. We got to talk, These other things that the environmental [00:42:00] aspect,
Dr. Jesse Reimink: No, totally.
Chris Bolhuis: quality and groundwater contamination and, you know, things like this that are super important.
Dr. Jesse Reimink: Absolutely. Well, okay, Chris, I've got one, quite good story for you. and an episode proposal for you. actually before we were even talking about doing this episode, our good friend, friend of the pod, Jackie Faraday, who we've had on a couple of times,
who studies the
origins of the universe and all of humans, she was traveling and, was sending me photos of rocks.
And asking me what the heck, how did this form? And she was looking at green schist. She's looking at [00:42:30] green
schist in a place that has
subduction zones. And she's like, why are
all these rocks green? It's totally cool. And then there was a, she had a, like a quartz vein. And so I was explaining, I was just quickly, you know, text describing a quartz vein and you know what she said to me?
you know, I was mentioning quartz and chloride and things like this. And she goes, Oh, but I study quartz and enstatite and foresterite in the atmospheres of extrasolar objects. So I totally get it. And I was like, now I'm thinking we got to have her on to talk about this. What does that mean, Jackie?
And what did you think
about looking at your first green [00:43:00] schist out in the
field?
Chris Bolhuis: ha.
ha.
Oh, she'll let you know.
Dr. Jesse Reimink: Yeah. Oh,
yeah. But don't,
you think we should have her on talk about this?
Chris Bolhuis: You had me when you said Jackie Faherty. So,
Dr. Jesse Reimink: I
know. I mean, just the greatest. She's just the greatest.
Chris Bolhuis: absolutely. Yep. She is.
Dr. Jesse Reimink: see if she wants to come on. she can
tell us about her geologizing out in the world a little bit
too.
Chris Bolhuis: She'll let us
Dr. Jesse Reimink: Yeah. Totally. She's already, she said she's got some theories about how quartz
veins form. So we can, uh, we can hear
about those. It'd be great.
Chris Bolhuis: All right. I'd love to hear
Dr. Jesse Reimink: [00:43:30] Oh man. So
good. Hey, Chris, I don't know. What do you think? That a wrap?
Chris Bolhuis: that's a wrap.
Dr. Jesse Reimink: Cool. Hey, you can follow us on all the social medias. We're at PlanetGeocast. Go to our website, planetgeocast. com. There, you can choose one of two ways to support us. You can go to our website, planetgeocast. there's a support us link there.
Just click on that. You can donate to the cause also. You can go to our Camp Geo mobile app. First link in your show notes, download our mobile app. We've got tons of audio content on there with the images. So really, we think these images are, are great and, and provide a ton of [00:44:00] educational value. Just, you know, as you're driving in your car or you're going on a walk and we're describing things like Chris, this pressure temperature plot green, just where these things are in this plot.
We have a plot there on the Camp Geo app that shows it to you. Just, good graphics just add so much educational value. So head to our Camp Geo app. You can get all for free, the, all the content we made for the physical geology class And some audio books on the geology of very popular national parks as well.
So with that, send us an email, planetgeocast. gmail. com. If you have any questions,
[00:44:30] Cheers.
