Back To CourseEarth Science 101: Earth Science
24 chapters | 168 lessons | 16 flashcard sets
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Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.
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.
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.
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.
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:
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.
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.
With contact metamorphism, we see non-foliated rocks being formed, and this is mainly because the rocks transform due to heat more so than pressure. Without the pressure, the mineral crystals within the rocks do not align themselves and give the banded appearance. We also know from earlier in this lesson that non-foliated rocks tend to be made up of only one mineral, which further adds to the fact that these rocks do not look striped or banded.
Hornfels is a metamorphic rock that is solely formed through contact metamorphism. Hornfels might sound like a funny name for a rock, but its name comes from the fact that its toughness and texture reminded those who named it of an animal's horn. Hornfels can come from a variety of rocks, but the most common parent rock is shale.
Other non-foliated rocks can form from either contact or regional metamorphism. An example is seen when we take the parent rock limestone, which is a sedimentary rock, and expose it to heat and pressure to make marble.
Marble is a very pretty stone, and it can be used in construction for things like flooring. Marble is made up of calcite, which is also what makes up the parent rock, limestone. This illustrates how non-foliated rocks contain only one main mineral. In this case, the calcite crystals in the limestone metamorphose to form the larger calcite crystals in the marble.
We also see the sedimentary rock sandstone, which is high in quartz, can be morphed into quartzite, which from its name you might imagine is composed essentially of quartz. And we see that the rock bituminous coal can be metamorphosed into anthracite coal, which you might use for heating because it burns nice and slow and gives off an intense heat.
Let's review. Metamorphic rocks are classified by their protolith, which is the parent rock or the original, unmetamorphosed rock, and by texture. 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 are made up of many different kinds of minerals that can be ordered by increasing exposure to heat and pressure. As these forces increase, we see metamorphic rocks form and then become parent rocks for the new metamorphic rocks. Shale, which is a sedimentary rock, can be metamorphosed into the foliated rock, slate. This ordering then continues as slate is metamorphosed into phyllite, which changes to schist and finally to gneiss, which has the nicest bands. You can use the acronym S-S-P-S-G, or Striped Stones Pressed So GNice, to help you recall this sequence of metamorphism.
Non-foliated rocks are typically made up of only one mineral and can be metamorphosed due to contact metamorphism, which is mainly through contact with a heat source. Hornfels is a metamorphic rock that is solely formed through contact metamorphism. Hornfels' main parent is shale. Non-foliated rocks can also change through regional metamorphism, which involves both heat and pressure over a wide region.
Some examples of non-foliated rocks that can change due to contact or regional metamorphism are limestone, which is mainly calcite changing into marble, which has larger calcite crystals. And we also see that sandstone, which is high in quartz, can be morphed into quartzite, and bituminous coal can be metamorphosed into anthracite coal.
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Back To CourseEarth Science 101: Earth Science
24 chapters | 168 lessons | 16 flashcard sets