 Welcome to OpenGeology. This is a series of videos to share geology with everyone. This was created by a group of Salt Lake Community College professors and goes along with the book OpenGeology.org slash textbook where we have an entire online textbook available free to everyone. Today we're going to talk about metamorphic rock ID, identifying metamorphic rocks. And we're going to talk about foliated non-foliated rocks and how to identify those things. So when geologists are trying to put rocks into these different categories a lot of times they're really just trying to tell the story of how the rock formed. And that brings us to metamorphic rocks and how those are generally categorized. For the sake of my class I usually categorize metamorphic rocks based on two things. Something known as texture and something known as grade. And the first thing you want to decide is or some of the things you want to think about are was heat or pressure the dominant kind of player in metamorphosing this rock. Sometimes it's hard to distinguish and then then you can get into like how much pressure was applied in that case. What kind of minerals are present and the minerals are really going to be the true storytellers in these rocks. And when it comes to metamorphic rocks minerals are what is telling you about the heat and pressure conditions that generated that metamorphic rock. And it'll also give you some information and some clues about the protolith the rock that came before that rock. Texture is the is essentially a word in the case of metamorphic rocks that will tell you about the shape and arrangement of the mineral grains in the rock. And for the sake of my class it's knowing the difference between a non-foliated texture and a foliated texture. And we'll talk about foliation. Foliation is this idea I think. Foliage as in leafs on a tree right. This is the alignment of mineral grains just like an alignment of leafs. You get stacks and alignment of these plady layers. And this could be layers like in a slate but it can also be more visible mineral layers. Some examples that you could see are dark and light bands of minerals like in a nice. You can look for aligned plady minerals like micas in a schist which we'll look at pictures of or even elongated pebbles in areas where the rock cleaves. If the rock cleaves in a nice thin layers it's a good indicator that you're working with a rock that has foliation. Rocks that have lineation are also categorized under these foliated rocks. And these are rocks that have line aligned minerals in just one one line not in a plane like we're seeing with these cardboard boxes. And foliation can form in different ways during the process of metamorphism. You could be rotating plady minerals or recrystallizing old minerals into new minerals that are perpendicular into the direction of maximum stress. Or you can just be flattening some spherical shaped grains like you could see in a medical glomerate. Sometimes those pebbles are squished and that's from flattening. So all of these are common ways. Rotation, recrystallization, flattening. So let's look at metamorphic grade which is the second type of thing. And once you decide that you have a foliated rock you can decide what the metamorphic grade for that rock is. And this is essentially a measure of how much the amount that the rock has changed during metamorphism. It's a measure of the amount of metamorphism that the rock has undergone. You can go from a low-grade metamorphic rock which indicates a small amount or low metamorphism, smaller heat and pressure conditions to a high grade, a high metamorphism. And just as a reminder during metamorphism you're maintaining a solid rock. Essentially you're not turning it to a liquid. So here is the progression of grade from low to high and these are your different names up here of your different foliated metamorphic rocks. You have slate, the lowest grade, then a fillite, a schist, and a nice. Go beyond nice in terms of the pressure-temperature scenario and you get into the migratite which is a precursor for a completely melted magma. And then you're getting into the realm of igneous rocks here. So here's a picture of each. Some of the ways I remember how some of these look is slate. Used to be used for chalkboards. We have mostly white boards in our rooms now. But chalkboard, just imagine a chalkboard or a nice pool table. It cleaves along nice planes. It looks a lot like shale which is a sedimentary rock and a lot of times you can metamorphose a sedimentary rock. Shale to get slate. Slate has a little bit more of a clinky sound when you knock it together. Fillite is the next level up from slate and the good way to identify fillite is the sheen. It looks, I like to use the analogy of a catfish skin. Looks a lot like catfish skin. It's shiny and kind of wavy. The micas aren't quite developed enough to make out individual micro grains but you can see a nice shiny sheen. Like a smooth fish. A smooth fish like a catfish. The schist is the next step up and while we're using fish analogies we can say that schist looks like fish scales. So an alignment of fish scales or sequins is another good way to think about it. We have tiny to medium-sized micro grains lining up and kind of overlapping into a fully pattern. And then finally you have nice alternating light and dark bands of light and dark minerals. So let's look at a picture of each one. So here's some slate. Here's a nice slate roof in this picture. And some potential prototypes include shale, mudstone, or siltstone. And this is a fine-grained metamorphic rock. Here's a nice picture of phyllite and you can see that sheen, that catfish skin-like texture. And this is between a slate and a schist. It's kind of a load to medium-grade metamorphic rock. And the platey, micacious minerals are a little bit larger than what you'd see in a slate but not large enough to see without a petroscope. Here's a schist. There's a thumb for scale and you can see little individual micro grains glittering out there. Very much like a little sequins or glitter glittery rock. And these will often exhibit planar or layered structures so expressing their foliation. And finally here's a nice, this is you have high-grade metamorphic rock. You get migration of ions that results into segregation of light and dark minerals. And here is a organ nice. So it has these eye-shaped, organ is a German word for eye and it has these eye-shaped features in here which could form from shearing in some cases. Another common thing you could see in metamorphic rocks, especially schists, I've seen a lot in schists, are these porphyra blasts. And here's some garnets with a coin for scale. And oftentimes I'll be surrounded by smaller micas or something like that. This could help give you clues about the grade. These garnets generally indicate a higher grade of metamorphic rock than just a schist by itself. Then the non-foliate rocks are a little bit more straightforward. They don't have as distinguishable of these different grades. So some of the examples we like to look at are marble, quartzite, and hornfels. And a lot of times these are organized because they lack visible foliation. We organize them based on the minerals that they contain. So here's some marble. Marble, its protolith is limestone. So marble contains calcite that has been recrystallized and you can determine if it's marble. If it doesn't scratch glass and if it fizzes with hydrochloric acid, a good indicator that you are working with marble. And you've probably seen a lot of bathrooms have marble and it comes in a huge variety of colors. Here's another type of non-foliator rock. This is a quartzite. And a lot of times its parent rock, its protolith, is a quartz rich sandstone. And this is like marble. It has like a sugary appearance, a sugary texture, where the grains are fused together. Except unlike marble, a quartzite will scratch glass and it will not fizz with hydrochloric acid. And a hornfels, it could often look brick-like or even dark black. These are baked shales or clay rich rocks. And so it's essentially like a natural brick. A lot of times these are created by contact metamorphic, contact metamorphism conditions. So that is it for identifying metamorphic rocks. Something to note is that with the foliated metamorphic rocks, you can always skip grades. It doesn't necessarily always make this progression from slate to nice. Okay, I hope that was helpful in helping you identify metamorphic rocks. And please do follow the page if you're interested in learning more. Thanks.