Welcome back. And now that I know that you all understand and can identify any mineral you want, you've probably been walking around looking for minerals all over the place. Now we're going to put that to good use and be able to take our mineral knowledge and be able to to now identify rocks, so you'll have even more things to identify outside. What we're going to start off with here is the rock cycle. And before we started off with minerals, we had that really tight description of what a mineral is, having chemistry.
and form and everything else. Now what we're going to do is we're going to talk about something where you just shove together anything you find, whether it be mineral matter, whether it be organic, whether it be inorganic, minerals. As long as it's naturally occurring, it makes a rock. And what we have with rocks is what we call a rock cycle.
And the rock cycle relates all the different varieties of rocks together. We have three different types of rocks. We have igneous rocks, we have sedimentary rocks, and we have metamorphic rocks.
This week we're going to start off by looking at igneous rocks. Igneous rocks came up from under the ground. They had to have started off as a liquid, so they started off as melted rock.
Then they came up and were either placed within the ground or came to the surface as a volcano. That then makes an igneous rock. The next phase in our rock cycle is a sedimentary rock. If you erode down those igneous rocks, weather them, break them down into a solid, then you in the little pieces or dissolve them and carry them away, deposit them someplace else, lithify them into a rock, that makes a sedimentary rock. And so that's our second phase and we'll do that next week.
The third is if we take those sedimentary rocks, bury them down to a certain depth, they will change their mineral chemistry based on pressure and temperature changes. And so therefore then they become metamorphic rocks. Now the rock cycle then if you, supposedly you keep heating them up, those rocks will make melt and then you turn them back into igneous rocks again and that's why you have this cycle. Now however the cycle can have a couple of diversions to it.
Metamorphic rocks can be eroded into sedimentary rocks. Igneous rocks can go straight into metamorphic rocks. The only thing you can't have happen is to get sedimentary rocks into igneous rocks.
Why is that? Because in order to melt the sedimentary rock first you have to heat it up and pressurize it thus turning it into a metamorphic rock. That's the only part of the cycle. at where you really can't go. Otherwise that forms what we call a rock cycle.
There are two ways in which you classify all rocks whether they be sedimentary, igneous, or metamorphic. And the two ways are texture and composition. Okay so what we're going to look at now is we're going to take a look at different texture types for igneous rocks and then we'll take a look at composition.
So to start off with the easiest way to deal with this is whether a rock Crystallizes underground or comes to the surface as a volcano if it crystallizes underground is what we call Plutonic okay, so Pluton rocks form these big blobs of Liquid underground that cools very slowly Because they cool slowly the minerals are allowed to grow very big If the liquid comes all the way to the surface hits the air it's much cooler cooler at the surface, they come out of the volcanoes, and when they come out of volcanoes they tend to cool very quickly. And so therefore the grains are very small, if they have grains at all. They may be just pure glass.
So things that come on the surface are going to be our volcanic rocks, and where they have crystals that are very fine grain, they are what are called aphanitic. If they cool underground and you get coarse grains, you get what is called phaneritic. So that's what we'll take a look at here.
This is a phaneritic rock, and you can see individual grains. Notice each one of these mineral grains are identifiable as an individual mineral. Every single grain can be seen. This is an affinitic rock. Notice that you can't see any of the grains.
They're all way too fine to see. You will not be able to identify any individual mineral grains in an affinitic rock. Some rocks can be quite porphyritic, so therefore you have to be very careful to look.
In cases like this, you can see that we have a lot of crystals in it with very small amount of matrix around them where you can't tell what the mineral is. This is what is called a porphyritic affinitic rock. You have to look closely before you decide whether it's a porphyritic rock or a phaneritic rock.
One last texture we can talk about for... coarse-grained rocks is what we call a pegmatitic texture. As you can see in here, we have one mineral which is shown in this dark gray, but all the rest is a single mineral. So with pegmatitic texture, we get huge mineral grains in a single rock.
These rocks, not even were they cooled slowly, but they were cooled with a lot of water in place, and therefore it allowed the minerals to grow very big. In some cases, these minerals can be 40 feet long. This is an example of a pegmat... metitic texture.
In addition to these textures where the minerals were actually formed, there are also a series of volcanic textures. And these are textures that are mostly glassy, so where the liquid came out but didn't even have a chance to crystallize at all. And so that made this series of textures where you can't even identify, there's not even a question to identify minerals.
And there are several different types of these. The first one we can look at is what is called obsidian. And obsidian is volcanic glass. It can come in several different colors. Usually it's black or brown.
But the easy way to tell this is it looks like glass and it has... It has a fracture surface on it that looks like a broken piece of glass. This is what is called conchoidal fracture. You can see it in quartz as well. And by looking at that you can definitely guarantee that this is what is called obsidian.
In addition to that, you also have a lot of gases escaping when a volcano goes off. All of that smoke and ash you see coming that goes straight up in the air, that's all escaping from the gas. a lot of escaping gases in these rocks. And so what we have is when the gases escape, they go through the liquid rock and leave bubble trails behind them. And so therefore you have a whole series of rocks that are made up of kind of bubbling through the liquid.
And the first one and the most common that everyone has seen is what is called pumice. Some people use it on their bunions and things. But pumice is a very light rock.
It's made made up of almost a froth that formed at the top of the lava chamber. It is so full of bubbles. It can actually float on water. And in fact, they think a lot of the species of plants were distributed around the Pacific because the seeds floated on pumice rafts. And so therefore, pumice is a very light rock.
You can always tell. It always has that, typically has that gray color. Sometimes you can also have it with a red color.
This is what is called a volcanic bomb. So it came out. as a single piece which was shot out of the volcano but nonetheless it's also very light and what is called pumice.
Now in addition to that we can have bubbles in rocks where they leave holes in the rock. Okay these pumice had a lot of holes but you can have single bubbles that go through rocks and leave little almost worm like passages through the rock and these make holes which are called vesicles. Vesicles would then be these kind of round wormy-like holes that go through rocks.
If you fill the vesicle, later some water can carry in minerals and put it inside of the holes, and it deposits a mineral within the hole. These filled vesicles are what are called amygdules. So amygdules are filled vesicles.
The last thing you can also have are what are called pyroclastic rocks. Clastic rocks are a term for sedimentary rocks. means fire. So these are almost sedimented rocks where you had ash or flows coming out of a volcano stacking up a rock.
And what you see is something that looks like this where it has a distinct layering to it. And this is what is called a tuff. Okay, so a tuff is made up of ash and chunks of volcanic debris that have fallen down in layers to make this ash. This would be what was called an ash flow.
flow tough or it could be an ash fall tough depending on whether it flowed down the side of the volcano or was dropped down out of the air. Okay, so we took care of the texture and we can say there's a phonetic or faneritic Affinitic being volcanic faneritic being plutonic what we're going to do now is take care of the composition I remember last time when we talked about minerals. I said don't use color. It's no good. Well, for rocks it is good.
So we're not going to do, we're going to do a general compositional breakdown that will go purely by color. So all you have to do is kind of look at the color of the rock and say, all right, it falls into one of these categories. There are four.
distinct categories. The first category is what we call felsic. Felsic rocks, that's a shortening for feldspar and a lot of silica, which is quartz. Okay, so felsic rocks are rich in quartz and feldspar.
Now contrast that to mafic. Mafic is rich in magnesium and iron. MA for magnesium, the F comes for iron, FE is the symbol for iron, and those are really the two parts that divide up. However, there's a whole category in the middle.
between the two which we call intermediate and even something that's more mafic than mafic we've been higher in iron magnesium and we call that ultramafic now these fall in kind of color categories so felsic rocks tend to be very light in color mafic rocks tend to be dark in color intermediate rocks are in the middle ultramafic rocks sometimes darker sometimes a little lighter than mafic we'll tell deal with those separately okay so let's take a look at what what we can do with some real rocks. Okay, first thing, we can look at a rock as something like that, very light in color, okay? If we look closely, we'll see that we can't see the grain size.
That one falls over in the felsic volcanic range. In other words, it's rhyolite, okay? Next thing, we can have another light-colored rock. This one we already said was granite.
It's felsic, and it falls over in here. Notice how the two tones of those are similar. Here's another example of granite. granite without the pink fels bar in it.
So it's a little whiter, but nonetheless very light in color. Contrast that to this rock, which we said was basalt. Basalt is a mafic volcanic rock.
It's too fine-grained to see. This is mafic, so it's very dark in color. We can put that there.
Here is a coarse-grained version of basalt known as gabbro. Okay, so the grains are now big enough to see, however very dark in color. Notice that They're very similar.
Contrast those to something kind of in the middle. So what do we call it? Intermediate, because it's in the middle.
Okay, and notice how it's got a gray something in between those two. This is a fine-grained rock. We'll call this an andesite, the volcanic version.
And we call it a diorite if it's the plutonic version. And so therefore you can see the breakdown that it's purely color-wise. So you can look at the rock, figure out the grain size, just look at how...
darker light it is and generally get the idea of what the rock is. An example of an ultramafic rock is this one. You could see it's really kind of lighter than these mafic rocks. Sometimes they're darker, sometimes they're lighter, but they don't have any quartz or feldspar whatsoever in it. That's what makes it ultramafic.
So that would go on the side. There are no volcanic ultramafic rocks, only plutonic, so we should put it there. And that means that's that puts our whole thing together. So you can only get volcanic Plutonic rocks in the range of mafic, intermediate, felsic, and plutonic rocks, mafic, intermediate, felsic, and ultramafic. What we've done so far then is we have a general classification for these rocks already.
And all you had to do was look at the texture and then generally put them into an order as to what their color is. And so we almost are there. The last thing we have to do is...
is kind of do the confirming and add any names to the rock that you might need. Okay, so we're going to confirm what our guesses are, and then we're going to add a name. So we're going to modify the name a little. What you will usually need then is to get...
get a hand lens. Now in some cases with the volcanic rocks, if it's porphyritic, you can actually look at any of the grains to make sure they fall into the right category. Now, you'll look in the book, there's a chart that tells you what minerals should appear in what rocks. So for example, let's take a look at this granite.
And if we look closely at it with the hand lens, we can use the same techniques we used last time to identify the minerals, but this time we'll look at them through this hand lens. We can do a hardness test on them. We can do streak We can do anything we did before to check that identity of that mineral So in this case we would look through here and see by using the streak test and everything else that the pink mineral is feldspar In here we would look and we would check and find that the gray mineral is quartz Okay, feldspar and quartz make felsic rocks And so therefore we have now confirmed that this is a felsic rock in addition And there are other minerals in here which do not go with the name. All you need is quartz and feldspar to make it a felsic rock.
So in this case we will look and we see little black spots and we check the little black spots and maybe scratch them off and see that they flake off. And when they flake off these are biotite. So the absolute name of this rock then would be a biotite granite.
Any additional mineral besides the quartz and feldspar. must be put into the name. Whether it be hornblende, biotite, muscovite, garnet, anything else that might occur with it would also show up in the name. Now the minerals that show up in here are going to be the ones that are with the felsic minerals.
So you will not get olivine, you will not get pyroxene, in most cases with the granite. You might get the hornblende, biotite, muscovite, garnet, things like that are more common. We do the same with all of these.
these rocks. We're going to look at them and confirm that identity. Okay, so for example, in this rock, which we said was a diorite, a diorite is going to be mostly made out of feldspar, plagioclase feldspar. And the plagioclase in a diorite is going to be white in color.
So you look and you see this salt and pepper type pattern. You confirm that that's white colored feldspar, plagioclase, and it can have some of the mafic minerals mixed in with it. Okay, and that would give us a diorite.
If this has anything else in it, in it like muscovite or biotite or anything else, you would put that in as a modifier. It should have hornblende, pyroxene, plagioclase. Anything else modifies the name.
And so that's a second one. Last one we might take a look at this. Now in these you'll notice these are very dark in color. And the dark color means that the pyroxene and the feldspar are about the same color.
What you would have to do is look closely at these and you will see those knife marks on the faces. of the plagioclase, that tells you that it's plagioclase. So in this case, it's to tell the difference, say, between something that's ultramafic, which would have no plagioclase, is to look for those knife marks on the cleavage planes, and that will tell you that it's plagioclase.
The same holds true with any of these other minerals in the volcanic rocks. Again, you need to identify the mineral, if you have a porphyric rock, to be able to tell which it is. So, Now the one thing you might want to deal with these porphyritic rocks is a porphyritic rock will have square crystals on it. The crystals will have sharp edges.
If you're dealing with amygdala rocks, they tend to be round. And so therefore, the easy way to tell the difference between if something is in a volcanic rock is if it's a squared edge, then it's a porphyritic rock. If it's rounded, then it's amygdala. And that's how you can tell the difference between the two.
Another form to fill out? I thought I did this last week. Alright, I guess we have to do it.
Okay, let's get started. Alright, here's my unknown. This one is one of the ones I have to do. I have a spot on my form that says sample number.
So I'm going to have to go with that sample. This is sample number one. And what I'm going to do with it is...
First thing is I have to fill out what's in question one. And box number one says texture. And texture is going to tell me whether it's an affinitic rock or a phaneritic rock. Now because I can see all of the grains in here, and they all look pretty big, I can see them without even using a hand lens, this rock is what I would call a phaneritic rock. So I will put down phaneritic.
The next thing it's going to ask me for are what visible minerals do I see? And obviously this is a light colored rock, and so we already know it's going to be in the felsic range. So the minerals I'm going to be mostly looking for, does it have quartz and feldspar? And then what else might it have? And what I will do is I'll take my hand lens and look through that and see what kind of minerals I have.
And sure enough, I have a pink feldspar and I have a gray quartz. And that is what it's asking me for here. And pink feldspar is typically a potassium feldspar, so I'll write case bar in the main box for feldspar. And I also have quartz, so I will write yes on quartz.
Other minerals I can see, I also see that there is a black mineral in here. I can look closely at it and see that it flakes up. I can also take the probe to it and scratch it.
And if it comes off in little flakes, then for sure. sure that mineral is biotite. And so biotite goes under my other category, biotite. Okay, color index. I should have done that first.
We already said that it was a light color. So the next category is light. And rock name, well, I already knew because it's light colored and that it is coarse grained that I would have guessed that it was a granite. Now with the quartz and feldspar in it, I know it's a granite. Now this one needs, I need.
a modifier though on it because I have biotite in and biotite is not there as required. So therefore I'm going to write in the rock name biotite granite. The last category is asking me whether it's intrusive or extrusive and why. And obviously the grain size on this is very coarse. And if the grain size is coarse it means it had to cool slowly.
And the only place it can cool slowly is in the soil. is in the ground, which means it is intrusive. So it is intrusive because it has coarse grains.
And therefore, this is, I can tell right now, this is a granite, it's a biotite granite, and I'll be able to identify this when the midterm exam comes up because I will not be able to use my notes on that either.