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Metamorphic Rocks
Unit 10
Metamorphic rocks form from older rocks.
You have already learned about two types of rocks: igneous and sedimentary rocks. You learned that all rocks are put into groups based on how they form (their origin). A rock that forms from molten rock cooling is an igneous rock. A rock that is made of particle of other rocks, minerals, or organic material is a sedimentary rock. You also learned that rocks that form from evaporation (like rock salt) are sedimentary rocks. The last rock group are the metamorphic rocks. How do they form?
Metamorphic rocks form from changes to other rocks. This includes igneous rocks, sedimentary rocks, and other metamorphic rocks. Because each metamorphic rock comes from another rock, we say that metamorphic rocks have a parent rock. The parent rock (Figure 2) is the rock the metamorphic rock came from. The parent rock literally turns into the metamorphic rock. Once the parent rock changes into the metamorphic rock, the parent rock no longer exists! It now exists as the new metamorphic rock. Fig. 1: Gneiss (pronounced nice) Figure 3 shows how this happens to many rocks over is a metamorphic rock made time.
from granite.
Metamorphic rocks form as a result of change. We call this change metamorphism. Metamorphism is the change of one type of rock into a metamorphic rock. Metamorphism can cause changes to the rock’s mineral composition, texture (grain size, shape, and pattern), and even the chemical composition of the parent rock. These changes can only happen deep within the earth! There are two things that cause changes in rocks: heat and pressure. (For the super science-­‐y: there is a third way called chemically
active Nluids. You’re welcome to read about it at home!)
Pressure
Fig. 2: (Left) A sample of shale is the parent rock of (right) slate. Tremendous pressure is applied to the shale which compacts it into the new metamorphic rock slate. Slate is an example of a foliated metamorphic rock because it easily breaks apart into layers.
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Fig. 3: This diagram shows how one metamorphic rock can change into another. The parent rock is shale. It turns into slate under lots of pressure. Notice how recrystallization occurs. Slate changes into phyllite. Slate is the parent rock of phyllite. This process continues until gneiss is formed. If gneiss is heated further, it melts and becomes magma. It is no longer a metamorphic rock.
Metamorphism can be caused by heat.
When we studied minerals, we learned that heat and pressure can cause graphite to turn into diamond. The same thing can cause one rock to change into another type of rock.
Heat can cause a variety of changes in rocks. Heat causes the recrystallization of minerals. Remember, crystallization is when the atoms arrange into a three-­‐dimensional, orderly, repeating pattern. Recrystallization is when the atoms arrange into a different three-­‐dimension, orderly, repeating pattern. Two things can happen in recrystallization: the crystals grow larger and new minerals form (Figure 4). For example, think again how graphite turns into diamond. A new mineral forms and the crystal gets much bigger! Why does this happen?
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Recrystallization occurs because the old mineral is unstable with all the heat. It’s like trying to wear a winter coat in July. It’s too hot so you take it off to be more comfortable. The same thing happens to minerals: too much heat causes the atoms to change how they are arranged.
The heat that causes these changes comes from deep within the earth. (For the super science-­‐y students: the heat is from radioactive decay and thermal energy.) The deeper you go into earth, the higher the temperature becomes. The higher the temperature, the greater the change will be.
If the temperature becomes too high the rock melts. Once the rock melts, it is no longer considered metamorphic rock! In fact, now it’s magma and it will become igneous rock. Rock never melts to become metamorphic rock.
Fig. 4: An example of recrystallization forming new minerals. Notice the large crystals that formed in this metamorphic rock. (For the super science-­‐y: the crystals you see are called porphyroblasts. They get this name because they are large mineral crystals that grow on a background of smaller crystals. Remind you of porphyritic texture? It should! That’s how they get their name!)
Metamorphism is caused by pressure.
Pressure also can cause a lot of changes in rocks. First, it can cause the rock to become more dense. Why? Remember, density is how much matter is in a certain space. When you add pressure to a rock, it causes more matter to be in the same amount of space. Therefore, it increases density. Second, it can cause the minerals in the rock to recrystallize (grow larger or change into new minerals). Third, pressure can cause the rock to Nlatten or fold. This causes its texture to change.
The only place we can Nind enough pressure to cause all of this change is deep within the earth. If we tried to add that much pressure on the earth’s surface, the rock would simply shatter or break.
A common feature of metamorphic rocks is foliation.
You have learned heat and pressure cause a lot of changes in metamorphic rock. Usually, this makes the rock looked smushed. It can also cause the rock’s minerals to form into separate layers. This is called foliation (Figure 5).
A foliated rock is when the minerals are re-­‐arranged into layers. Foliation occurs only when rocks are put under pressure (not because of heat). Foliation can be very easy to see! If you see different colored layers, the rock is foliated. However, you cannot also see it. So, how do you know if a rock is foliated if you can’t see it? Break it! If the rock breaks apart in layers, there is a good chance it is foliated.
Foliation only occurs in metamorphic rocks. This is because foliation is a result of pressure causing change. This is different from being “banded.” Banding occurs when particles are put into layers before the rock forms. Foliation occurs when pressure forces the minerals to get into layers. This usually happens when there is a lot of pressure over a Unit 10 | Metamorphic Rocks Reading!
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large area (many kilometers) of rock. For example, you can Nind this in mountain ranges and deep, deep within the earth.
Metamorphic rocks can also be non-­‐foliated. A non-­‐foliated rock is when the minerals are left in a random arrangement. This occurs when there isn’t enough pressure or the rock is only made of one mineral (and you can separate one mineral into different layers).
Fig. 5: This is an example of foliation in metamorphic rocks. (A) the minerals are randomly arranged. After metamorphism (B) the minerals are arranged in layers. This is caused by a tremendous amount of pressure!
Metamorphic rocks have many uses.
Metamorphic rocks are usually used for building materials. Metamorphic rocks are usually very durable and strong. So, they can be used in some buildings. Some metamorphic rocks break apart into Nlat sheets (Figure 6). These can be used in rooNing, Nloor tiles, and even chalkboards!
Fig. 6: Slate breaks apart easily as shown in this picture. This is a result of foliation. This makes it break into Plat sheets. These sheets are cut into shapes to be used for chalkboard, rooPing, and Ploor tiles.
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