Lea Abbott: The next speaker is Dr. Skip Stoddard. He got his AB from Amherst College and his Ph.D. from UCLA. He's presently at NC State University and he is going to talk to us a little bit more about the petrology.

Dr. Stoddard: Well, I'm going to start with a few overheads and do some very basic mineralogy and petrology. I hope I don't insult anybody, but this is just the way I want to organize things. This is pretty basic. Rocks are composed of different minerals and the way we classify rocks is based on two factors: the composition of the rock and the texture. For all rocks, no matter what kind of rock they are, and no matter how they formed originally, these are the two main considerations in classification.

Rock texture refers to the sizes and shapes of grains that are in the rock, and how they fit together. This is something that's sometimes hard for beginning students to figure out. When we say texture, they feel a rock and think that that's showing them the texture. But that's not what we mean by texture. The three basic kinds of textures that are different for each of the three major groups of rocks are:

Sedimentary rocks are characterized mainly by what we call clastic texture, broken pieces of pre-existing material that are dumped together and then cemented by some agent later on that holds the grains together.

Igneous rocks have an interlocking texture. The grains crystallize from a hot liquid, from a magma or lava, and they fit perfectly together like the pieces of a jigsaw puzzle fit together. There's no need for any glue because they fit exactly together and these igneous rocks are divided into two groups. Those that cool slowly, deep underground, are plutonic or intrusive rocks. They tend to have large crystals. The volcanic rocks that cool on the surface of the ground very quickly are either completely fine-grained, so that the grains are so small you can't see them with your naked eye, or they have some crystals that started to grow, maybe at depth, and then the rest of the rock crystallized quickly around it, so you have a bimodal distribution of grain sizes.

And then finally, metamorphic rocks have what we call a foliated texture where they have been affected deep in the earth by being squeezed under pretty great pressure and relatively elevated temperature. This causes grains to grow in particular orientations so you develop a platiness to the rock and that's called foliation.

Life is never totally that simple, so igneous rocks, which we are particularly interested in here, have a variety of details we can look at with different kinds of texture. Coarse-grained rocks, the ones that cool slowly, we call them phaneritic. Phaneritic (coarse-grained) rocks include rocks like granite and gabbro. Aphanitic rocks, or fine-grained rocks, typically include volcanic rocks. Rhyolite and basalt would be the ones that would be of most interest here, and andesite, too. And porphyritic rocks are those that have bimodal grain size. The big crystals are called phenocrysts and then the fine-grained material, the matrix between phenocrysts, is called groundmass. And this is of major importance in looking at the volcanic rocks in the Slate Belt, when we are looking at mainly porphyritic rocks. Volcanic rocks also may be vesicular. When the lava crystallizes, gas bubbles escape from the lava and leave holes in the lava and so you have these round or elliptical holes in the rock. Later on, those holes can be filled in by mineral material and it causes a particular texture called amygdaloidal. The little round elliptical material is called amygdules and there are a lot of those in the Slate Belt. Volcanic rocks can be glassy. Obsidian is the best example of that although some glassy rocks have some phenocrysts in them too. And the thing about glass is that volcanic glass over time devitrifies. This means it begins to crystallize and is no longer glassy after a long period of time. So most of the good glassy volcanic rocks that exist are very young volcanic rocks.

OK, so that's a look at texture. Rock composition is the other aspect of naming a rock and basically when we talk about the composition of a rock, we are talking about the minerals that are in the rock. A determination of what the minerals are and their percentages is the most commonly used basis for determining the composition of a rock. So you've got to be able to identify the minerals and estimate their percentages.

You can base the determination of a rock's composition on a chemical analysis. And the way that geologists do this is to report a chemical analysis for an entire rock. You take the rock and grind it up and have it analyzed and report that analysis, usually in oxides. Igneous rocks are silica rich, so the SiO₂ rich rocks, like rhyolites, have 75% silica. Basalts have 50% silica, more or less, and that's the range of most igneous rocks, 50-75% SiO₂. But then all the other oxides are important. We classify igneous rocks based on these compositional groups and based on the textures that we talked about before.

There are two things I want to mention about that right now. One is sort of a gross chemical grouping of igneous rocks. Igneous rocks are grouped into what we call felsic, intermediate mafic and ultramafic groups. When you pick up an igneous rock, felsic is very light colored and ultramafic is very dark colored. Ultramafic rock is composed entirely of dark minerals, iron and magnesium rich minerals.

I'll show you a simplified igneous rock classification that shows those groupings [Slide 1]. Felsic, intermediate mafic and ultramafic--light colored rocks on the left, dark colored rocks on the right, high silica on the left, low silica on the right, sodium and potassium increase toward the left, iron and magnesium increase toward the right, and a few other elements have trends across this chart. And this chart only has seven basic rock names on there. The top row is the coarse-grained rocks, granite, diorite, gabbro, peridotite, and then the fine-grained varieties are the volcanic rocks, rhyolite, andesite, basalt. So this is a simplified igneous rock classification. This is the one we use in our introductory geology class, but it is very handy, something you can use in the field.

The more accepted igneous rock classification that has been adopted and is used pretty much universally now is called the IUGS classification of igneous rocks. This is based on the percentages of the minerals quartz, alkali feldspar, and plagioclase (QAP). These are the most important minerals in determining an igneous rock's name. And there are separate sets of names for the coarse-grained or phaneritic rocks and the fine-grained volcanic rocks, aphanitic rocks.

Here's that QAP classification. The top diagram here is for coarse-grained or phaneritic rocks [Slide 2]. These are the proportions of quartz, alkali feldspar, which is potassium feldspar, orthoclase, sanidine, microcline, and over here, plagioclase feldspar. And for the coarse-grained rocks, there they are, granite being the biggest one you are familiar with. Granite has to have a large proportion of each of those three minerals.

But what you're probably more interested in is this one here. This is for the volcanic rocks [Slide 3]. This is the QAP classification triangle for volcanic rocks and this is the one most geologists use now. It has quartz up here, alkali feldspar and plagioclase again and the names of the rocks. You'll see here's rhyolite in this area and I want to point out there is a dashed line going down through the middle of this field. Many people will divide this rhyolite portion of the triangle into two regions and call this rhyolite and call the other side of that line rhyodacite. OK. So we have dacite, rhyodacite and rhyolite across here. Down at the bottom you have rocks that have mostly feldspars and very little quartz phenocrysts. So that's the basic classification that we use for volcanic rocks.

Volcanic rocks pose special additional problems. The first thing is that they're generally too fine-grained to determine what all the minerals are in them. So you have a couple of choices. You can either look at the phenocrysts that you can identify, if they are large enough to see, and base your name only on the phenocrysts. That's what you have to do when you're out in the field.

The other thing you can do is collect samples and get the chemical analyses. With the chemical analyses, what we do with volcanic rocks is use the chemical composition of the entire rock to calculate hypothetical minerals. So in other words, if a rock has a very fine-grained aphanitic groundmass and phenocrysts of quartz in it, we can base the name on the fact that it has quartz phenocrysts. But we really don't know where it falls on that triangle, because the magma didn't have enough time to grow the minerals big enough to identify. But if we crush it up and get a chemical analysis, we can calculate the hypothetical minerals and it will tell us what the percentage, what the relative proportions of those three minerals-quartz, alkali feldspar, and plagioclase-would be if the rock had had a chance to cool in a different way.

And using that technique, what we can do is eliminate the differences between volcanic rocks that are the result of differences in cooling history. And so we're really looking at what the composition of the magma was that way. And these are called normative minerals or norms. You may see that term if you look in the geological literature. You also may see a chemical analysis at the top where they have SiO₂ and Al₂O₃ and underneath it, it'll say CIPW norms and it'll have a list of abbreviated mineral names: Q, Ab, Or, An, and so forth. And those add up to one hundred. Those are the hypothetical minerals. And that to me is a better way to classify many of these volcanic rocks.

There are other problems with volcanic rocks. One can have either lava flows or pyroclastic deposits, as Jim talked about. In the Slate Belt, many of the rocks are pyroclastic. They don't trend when you map them. They don't tend to have a great lateral extent. They are very localized and they pinch out. They may be very thick and pinch right down, as opposed to say sedimentary units which tend to have great lateral extent and be a fairly consistent thickness.

Pyroclastic material that comes out of a volcano is looked at in terms of its size. You have ash and lapilli, and then larger chunks, bombs and blocks, and we have those in the Slate Belt, too. In pyroclastic rock, some of these are vitric, which means they are glass, chunks of glass. Crystal fragments are phenocrysts that may be blown up in the air out of the volcano but crystallized, and fallen down. Lithic fragments are chunks of other rocks, usually made of more than one kind of mineral, whatever made up the pre-existing rocks.

So we have rocks that are made up of this material called volcaniclastic rocks, but then as Jim also pointed out, sometimes that material is reworked by sedimentary processes and this is the area where volcanic and sedimentary processes merge. A lot of the rocks in the Slate Belt are like that--formed by explosive eruptions and these pyroclastic materials. Epiclastic rocks are those that have been dominated by sedimentary processes. So we might talk about a sandstone, a volcaniclastic sandstone or something like that.

And here are some of the names of these kinds. They have their own classification types. This is a triangle that shows a classification of volcaniclastic rocks. So these would be volcanic rocks that are made up of the fragments. Fine-grained stuff is ash that makes a rock called tuff. The coarsest-grained stuff is volcanic breccia. A lapilli stone has medium-sized grains and we use these terms for rocks that are composed of some of each of these types of material.

I like that question about the Uwharries being the most difficult geologic area in the world--in many ways it is. I mean, I've had so many times when somebody would bring in a rock for me to identify and I just cannot identify it, because many of these rocks started out as volcanic rocks, were reworked by sedimentary processes, and then subjected to metamorphism later. So this person asks me, "Is this an igneous rock or a sedimentary rock or a metamorphic rock?" And all I can say is, "Yes. It's all three."

So sometimes rocks are difficult to name. So you have to kind of look at what aspects of the rock dominate its characteristics. If its main features are metamorphic, then you should probably give it a metamorphic rock name. But if you can still see the original sedimentary or volcanic features, as is the case in most of the Slate Belt rocks, then we tend to give it a sedimentary or igneous rock name. If we want to be precise, we should put the prefix "meta" in front of that and say "meta-rhyolite," "meta-andesite," or whatever.

This last point here is something that is probably not going to be too popular. For years and years we have called the felsic volcanic rocks in the Slate Belt "rhyolites." They are really not rhyolite. Rhyolite is supposed to have alkali feldspar phenocrysts and quartz phenocrysts and almost all of the felsic volcanic rocks in the Slate Belt have plagioclase feldspar phenocrysts and quartz phenocrysts and they really should be dacite. Now if we grind them up and do that normative thing I was talking about, some of them end up being rhyodacites but this is something to look at, in that most of them are really dacites.

OK. I have some slides I can show now.

Question: [about flow-banding]

Answer: Yes, the kind of volcanic structure that Jim Hibbard was talking about. Domes very commonly have really intricate banding. They have to have cooled shallowly in the earth's crust to get that. Because if it were very deep, it wouldn't have much of that flow-banding, but very shallow intrusives, a shallow dike or a dome can have banding similar to a lava flow.

OK, I'm just going to show you some slides here of some different rock features. This is one way that we study rocks. Cut them on a rock saw, and maybe polish them and that way we get a better indication of the details of the texture, grain size of the different minerals, and especially the percentages of the different minerals. And this is coarse-grained granite. You can see we don't have too many of those in the Slate Belt. Here's a glassy volcanic rock from New Mexico, or I guess, Arizona. It also has phenocrysts in it. The little white crystals there are crystals that grew in the magma before it was erupted and cooled really rapidly to make it glassy.

This is vesicular volcanic rock from out West. Those are those holes where gases escaped. I've got a couple pictures here from Mount St. Helens to show you what life might have been like around here if you'd been here 560 million years ago or so. Ash--you can see how it's going to be moved around by water and wind, but it might eventually end up becoming a rock and it would be a tuff.

This is a large block. You see the two people standing at the base of it there. This was blown out of the volcano. So blocks can be the size of a football or up to that size. And there are blocks, huge blocks like that around the Slate Belt as well.

Let me show you some thin sections. This is what's called a "vitrophyre." It's a glassy volcanic rock with phenocrysts of earlier formed crystals. Now most of what you see in the microscope here is volcanic glass. The brown, light brown material and the white, most of the white background, is glass and when we look at it under cross-polarized light, that will all be black. But the large white areas are phenocrysts. So you can see all of that black stuff is isotropic which means that it's glass.

Here is basalt. I'm showing you pictures of young volcanic rocks first and then we'll look at some Slate Belt rocks. That big curved area in the middle is a vesicle. So that's a hole in the rock. The phenocryst on the left is olivine and up on the right is plagioclase. And the smaller white crystals are plagioclase, too, but the background or the groundmass of this rock shows the interlocking nature of the igneous texture. There it is with crossed-polarized light.

This is andesite. It's got phenocrysts of biotite and plagioclase and pyroxene as well, and again, a fine-grained interlocking groundmass. There aren't too many andesites in the Slate Belt, mostly the felsic rocks, what we tend to call rhyolites or dacites and some basalts.

This is from a Salisbury granite to show what a coarse-grained plutonic igneous rock looks like in thin section. The big grain on the left is alkali feldspar-microcline-and then the other feldspars with the parallel stripes are plagioclase.

This is a felsic crystal tuff from the Slate Belt. When you look at these thin sections in plane polarized light the feldspars and the quartz phenocrysts can be distinguished because the quartz phenocrysts are very clean-looking, and the feldspar phenocrysts are kind of "ratty-looking." They have straight edges. They're euhedral. They're nice crystals that have crystallized from the magma, but the quartz ones are clean and when I cross the polars, you'll see the characteristic stripes in the plagioclase. When you look at the top crystal there, you'll see that it's a hexagonal crystal. It's not a complete hexagon but you can see 3 or 4 of the 6 faces on it. These tend to be fairly well formed hexagonal dipyramid crystals, but they always were modified so they're never perfect. You know, you see 2 or 3 good crystal faces and that's enough. When you look at a crystal in any particular direction, you may see a diamond-shaped crystal if it's perfectly formed or hexagonal crystal if it's perfectly formed, or a modification of that.

Another thing you'll see is that some of these quartz crystals, and I don't think there are any in this picture, but many quartz phenocrysts in the Slate Belt rocks have had a reaction with the magma after they crystallized. So it may have been a well-formed crystal and then part of the crystal was melted back into the magma in the late stages, during the eruption perhaps. And you'll see a little indentation or "embayment" in the crystal. That's a reaction between the crystal and the magma.

This is an argillite and you can see it's composed of much, much finer grains. The texture is not very clear here but you've got bedding-up at the top is a finer-grained material. You are beginning to get a metamorphic feature called cleavage in this rock. You see that the beds are running just about horizontal in this picture. There's a formation of these dark lines that are at an oblique angle and that's the beginning of the metamorphic feature called cleavage. If the metamorphism went to higher grades, then you would actually form mica.

This is an epiclastic sandstone. So this is a rock from the Slate Belt that has sand-sized grains that you can tell have been rounded. They have a matrix around them which is finer grained, maybe ash.

Here is a lithic tuff or a crystal lithic tuff that's mafic in composition made of fragments of preexisting rocks. You can see the different fragments and the fine-grained groundmass between them. This one's mafic. With a mafic rock, when you cross the polars you tend to see more colors but this one doesn't have too much.

Here's a very crystal rich volcanic rock. There it is under crossed polars--feldspars and quartz--and that one may have some alkali feldspar in it.

This is a metamorphosed basalt-metabasalt. It's been changed. It's not as well preserved as that young basalt I showed earlier but this is a Slate Belt metabasalt. Many of the minerals have been modified and changed to their metamorphic counterparts but the texture is still an igneous texture. When you cross the polars, you see it a little better. You can see the interlocking nature of those plagioclase crystals that we call "laths, " in random orientation.

This is an amygdule in the Slate Belt metamorphosed basalt. That big elliptical thing is filled with the mineral called epidote that's characteristic of these metamorphosed basalts.

Here's a thin section of the Montgomery County agate material and under plane polarized light it's pretty boring. But when I cross the polars you'll see the feature in the lower right is a piece of veined quartz. That's good crystalline quartz. The rest of the slide is very, very fine-grained micro to cryptocrystalline silicia that sometimes is called agate. Sometimes people call it chalcedony, and it's very similar to chert in fact. All of these are varieties of quartz that are very, very finely crystalline, probably formed as a result of recrystallization during low temperature hydrothermal alteration. The feature of this stuff that is very distinctive is its fibrous-radial fibrous-aggregates. The next picture shows a banded example, more of the chalcedony type. The fine-grained stuff that's away from the bands is more like a "cherty" variety.

This right here is a rock from up near Mt. Rogers which actually turned out to be a dolomite, a sedimentary rock, not a volcanic rock at all. And it has little fossils in it. It's made of the mineral dolomite.

I have other thin sections I'll be happy to share with you at the workshop later on. Does anybody have any quick questions?

Question: …are metamorphosed basalts supposed to be greenstone?

Answer: Right. Greenstone is a more general name. That's the basaltic texture-that it had as an igneous rock. We tend to like to call it a metabasalt. That's what I was talking about-if the igneous features dominate, then we give it an igneous name and modify that. Once it becomes recrystallized to the point that it has a metamorphic texture, that is not igneous, but we still think it was a basalt originally then we'll start calling it a greenstone or something else.

Question from Stoddard: Has the aphanitic volcanic rock on top of Morrow Mt. been chemically analyzed?

Randy Daniel: Yes it has.

Stoddard: I was just wondering if we apply it to this rhyolite/rhyodacite continuum, we could see where that data falls. Perhaps we should be calling that a "metarhyodacite" instead of "rhyolite."

Randy: Bob [Butler] gently reminded me several times that just as the Carolina Slate Belt actually has no slate in it, that what we call "rhyolite" is really metarhyolite. This relates to a nomenclature issue which I'm going to raise in the Plenary Session tonight, and so I won't go into it. But it's something that we as archaeologists need to be aware of, that when we're talking about apples, we need to be talking about apples. There needs to be a certain archaeological and geological grounding in use of the terms. Bob gently reminded me of that from time to time because I would say something and he would say, "Well, no, not really." And I'd say, "But that's what we call it." And he'd shrug and smile.

Question from audience: Would the metamorphism have changed the chemical composition at all? Shifted from rhyolite more to rhyodacite?

Answer: That's very, very possible. The Slate Belt in general is sometimes referred to or characterized on the basis of the fact that it has relatively high sodium to potassium ratio, and when volcanic rocks interact, especially with seawater, they are known to lose potassium and gain sodium. However, even the most unaltered volcanic rocks in the Slate Belt tend to have pretty high sodium. So we think that's telling us something that's even more basic than that. But you're right; alteration is a factor, a definite factor.

Question: At archaeological meetings for several years we've been using various terms for tuffs-welded vitric tuff, vitric tuff, and I think early on we were under the impression that you could actually get incandescent ashfall that was so highly siliceous that it would form almost chert quality material. Is that the case, or what's our current thinking on how fine-grained tuffs form?

Answer: Maybe in an unmetamorphosed young volcanic area-I mean you're really talking about the process of glass formation, it sounds like. And I don't think there's any glass in the Slate Belt. We call Slate Belt rocks "vitric" but we don't expect it to look like real glass, like that young volcanic rock I showed. We expect it to be devitrified so that when we see it in the microscope it has very, very tiny little grains but it's crystalline. But nevertheless, because of its characteristics that might make it good for making tools, it impresses us as something that probably was glass originally. Technically though we really should reserve the term "vitric", I think, for something that is really glass.

Question: So, some volcanic tuffs can be fine-grained, almost to the point where they approximate chert?

Answer: Right, when it gets "cherty", that's the kind of thing I'm talking about. That may have been glass originally.

Question: I think south of Asheboro there's some material that's close to that.

Answer: Right, right.

Question: Since feldspar is a light color, the question I have is why are metavolcanics black?

Answer: The reason for the color is not obvious. I've seen very felsic rocks that are very dark because they have very tiny little iron oxide minerals in them, and it makes the whole thing black. It doesn't take much to do that. So I don't know what you can tell people. The way that it breaks, for that particular rock that you're talking about, the way that it breaks, tells you it's felsic immediately. So I think that's probably the way to go with that. I don't know of any mafic rock that has a conchoidal sort of fracture.

Question: Is that because the grains are too big or is it the lower silica content?

Answer: Probably both. I've never seen mafic obsidian. It must not occur.

Question: [indistinct] Many of the [archaeological] tools we find have a very weathered surface on them. Can you describe the process behind that weathering?

Answer: The main weathering process that occurs in these rocks is the feldspar being transformed to a clay mineral. And then the iron and magnesium bearing minerals, those that are there are also transformed. They all transform to softer things. I have never really done a detailed look at that, but mafic rocks have a weathering rind too and it tends to be a much more red, rusty colored. The felsic rocks' weathering rinds can be white or beige or sort of a tan color, maybe brownish but I don't think they're usually red. But you guys probably know more about this than me. It's usually a thinner weathering rind on the felsic rock-tends to be whiter. There are people, you guys probably know, there are some techniques that try to date how long something's been weathered on the basis of the rind?

Question: Has anybody done that with rhyolite?

Answer: I don't know.

Okay. Thank you.