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Classification of Metamorphic Rocks: Texture & Parent Video

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  • 0:07 Metamorphic Rocks
  • 0:30 Foliated vs. Non-Foliated
  • 1:00 Foliated Rocks and…
  • 2:19 Protoliths of Foliated Rock
  • 4:39 Non-Foliated Rocks and…
  • 7:06 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Metamorphic rocks are classified by their texture, which is either foliated or non-foliated, and according to their parent rock. Learn about the different textures and parent rocks of metamorphic rocks.

Metamorphic Rocks

Did you know that metamorphic rocks have parents? Now, they're not parents like your mom and dad, but they do come from parent rocks. These parent rocks can be any type of rock, meaning they can be igneous, sedimentary or even other metamorphic rocks. Knowing the parent rock helps us classify these rocks, and we can further classify them by their texture.

Foliated vs. Non-Foliated

When we classify by texture, what we're really taking a look at is the mineral crystals within the rocks. The texture of a metamorphic rock can be either foliated and appear layered or banded, or non-foliated and appear uniform in texture without banding. Foliated rocks contain many different kinds of minerals, but non-foliated rocks contain only one main mineral, which contributes to their more uniform appearance, as we will learn later.

Foliated Rocks and Regional Metamorphism

Now we can recall that metamorphic rocks form because they are subjected to intense heat and pressure. Under the influence of these forces, the mineral crystals within the rocks recrystallize, or reorganize inside the rock.

When we look at how these rocks are classified, it helps us to gain a basic understanding of the two types of metamorphism that these rocks can undergo. The first is regional metamorphism, which is a type of metamorphism where rock minerals and texture are changed by heat and pressure over a wide area or region. This is a fairly easy term to recall if you realize that 'regional' refers to the fact that this takes place over a wide region, like a mountain range.

With this understanding of regional metamorphism, we can talk about the formation of foliated rocks. The intense heat and pressure that takes place with regional metamorphism causes mineral crystals within rocks to align into rows, giving the rocks a patterned or banded appearance. It's almost as if the multiple kinds of crystals found within foliated rocks are called into formation, like tiny soldiers forced to stand in rows. In some of these rocks, these crystal bands will be very clear and easily seen with the naked eye, while in others, the bands may be microscopic.

Protoliths of Foliated Rocks

Foliated rocks can be ordered by increasing exposure to heat and pressure, and this is where the parent rock becomes important. Now, the term we use for parent rock or the original, unmetamorphosed rock is protolith. So, the protolith can be thought of as a prototype of the rock yet to be formed. And starting a discussion on metamorphism often starts with the protolith, shale.

Shale is a sedimentary rock that can be metamorphosed by heat and pressure into slate. Slate is a foliated rock, so there is some alignment of minerals taking place, but the mineral crystals are very small, so you will not see distinct bands when you look at a piece of slate with your naked eye.

But we can add more heat and pressure by maybe going deeper underground. When we do this, the slate turns into phyllite. So we see that we are getting an ordering to our metamorphic rocks as we're adding more heat and pressure. For instance, shale was the protolith for slate, but then slate became the protolith for phyllite. With phyllite, we have foliation, but like the slate, the mineral crystals are still small and hard to see.

But our process of ordering continues as we add more heat and pressure to the phyllite. With this added exposure, phyllite becomes the protolith for schist. With schist, the mineral alignment becomes more noticeable, and you can start to see the banding with your naked eye.

With more heat and pressure, we get schist metamorphosing into gneiss. Gneiss rock has very nice banding, or foliation, which we see here:

Gneiss rock has banding, or foliation.
example of gneiss banding

What we are looking at is the different minerals within the rock that have aligned themselves into the distinct stripes. Gneiss rock is rock that is exposed to very intense heat. In fact, if more heat were to be added to gneiss, this rock would most likely melt, making it an igneous rock and ending our discussion on metamorphism.

You can recall this order of metamorphism by using the acronym S-S-P-S-G, or Striped Stones Pressed So GNice. Just don't forget the silent 'g' for the gneiss stone.

Non-Foliated Rocks and Contact Metamorphism

Earlier, we noted that there are two types of metamorphism that these rocks can undergo. We just talked about regional metamorphism. Let's take a look at the second type, contact metamorphism. Contact metamorphism is a type of metamorphism where rock minerals and texture are changed, mainly by heat, due to contact with magma. So, with this form of metamorphism, we see that the rock actually comes in contact with the heat source, magma. This heat is not enough to melt the rock, but it is going to bake the rock and morph it into a new rock.

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