Learning about rocks and the rock cycle in your classroom this year? You may already know that there are three main types of rocks: igneous, sedimentary, and metamorphic. In this post, we will take a closer look at metamorphic rocks. Get ready to explore the formation, subtypes, properties, and examples of metamorphic rocks!
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How do metamorphic rocks form?
Metamorphic rocks form deep below the Earth’s surface. Metamorphic rocks form when other rocks experience lots of heat and pressure underground. The heat comes from the Earth’s interior and the pressure comes from the weight of overlying layers of rock and sediment.
Every metamorphic rock was once a different rock, called the parent rock, or protolith. The protolith can be sedimentary, igneous, or even a different metamorphic rock. For example, when the sedimentary rock sandstone is exposed to intense heat and pressure, it can become the metamorphic rock quartzite. This transformation, called metamorphism, forces the minerals in the parent rock to reorganize into a more compact structure, making metamorphic rocks quite sturdy and dense.
Metamorphism can only happen when the amount of heat and pressure are just right. If there is too little heat and pressure, metamorphism will not happen. If the heat and pressure grow too high, the rock will melt into magma instead.
Properties of Metamorphic Rocks
There are many different metamorphic rocks, and they do not all look the same. Shimmering schist, sturdy slate, and glimmering marble are all metamorphic! Their varied appearances are influenced by several properties including their mineral composition, grain (crystal) size, and foliation. Let’s learn some more about each of these properties…
Composition
As you now know, metamorphic rocks all share one important thing in common: they formed under heat and pressure. However, each rock has its own special blend of minerals. Metamorphic rocks are made of one or more minerals. If you can identify the mineral(s) in a rock, it will help you identify the rock.
Grain Size
Metamorphic rocks are made of crystals of minerals. Crystals come in different sizes. Rocks with large crystals can appear sparkly or shimmery, like schist or marble. But not all crystals are big. In fact, some rocks are made of crystals so tiny that you need a microscope to see them. Rocks made of microscopic crystals may appear more dull.
In general, bigger crystals form under higher heat and pressure. A rock with large, visible crystals formed under higher heat and pressure than a rock made of tiny crystals.
Foliation
Some (but not all) metamorphic rocks have foliation – which means banding or layering. Foliation presents in several different ways, depending on the conditions while the rock is formed and the mineral crystals in the rock. A foliated rock with large crystals formed under high heat and pressure. This rock may appear to have bands or stripes of different colored mineral crystals. An example of this is gneiss, which appears to have alternating bands of light and dark crystals. However, a foliated rock with microscopic crystals formed under lower heat and pressure. This rock may appear to be made of layers or plates, like slate.
Metamorphic Subtypes
There are two types of metamorphic rocks: foliated and nonfoliated. With a little practice, you can learn to tell them apart. Let’s learn a little more about each.
Metamorphic Foliated
Metamorphic foliated rocks have bands or layers. Some, like gneiss, have alternating bands of light and dark crystals. Others, like phyllite and slate, look like they are made of sheets or layers. Foliation forms when a rock is being squeezed hardest in a particular direction rather than from all sides. This may happen when a rock is buried deep underground. The directed pressure forces the minerals to line up in the same direction, creating the bands or layers.
Metamorphic Nonfoliated
Metamorphic nonfoliated rocks form when the pressure comes from all sides or if the minerals do not line up. Consequently, nonfoliated rocks do not have bands or layers. Instead, nonfoliated rocks are usually a single solid color and made of interlocking mineral crystals. These crystals can be quite large or appear fine-grained. Some nonfoliated rocks, like marble, are prized for their strength and beauty, which makes them popular in construction and fine arts.
Examples of metamorphic rocks
Here are some examples of common metamorphic rocks. If you’re studying rock identification or creating a rock collection, you’ll want to make sure to examine the following rocks:
Foliated Rocks
- Gneiss
- Schist
- Phyllite
- Slate
Nonfoliated Rocks
- Marble
- Quartzite
- Hornfels
What’s Next?
For everyone: there’s more to learn about rocks! Take a closer look at igneous rocks or sedimentary rocks. Or, continue reading more about the rock cycle or the difference between rocks and minerals.
For educators: check out my posts on how to set up a rock ID lab activity, directions for a fun rock ID review bingo game, and a list of rock and mineral activity ideas that you can try in your classroom!
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References and Further Reading
- Earle, S. (2019). Physical Geology. BCcampus Open Education. (Chapters 2-7). Available: https://opentextbc.ca/physicalgeology2ed/
- Johnson, C., Affolter, M.D., Inkenbrandt, P., & Mosher, C. (2017). An Introduction to Geology. Salt Lake Community College. (Chapters 3-6). Available: https://opengeology.org/textbook/
- Jones, C. E. (n.d.). Igneous Textures. University of Pittsburg. Department of Geology and Planetary Science. Available: https://sites.pitt.edu/~cejones/GeoImages/2IgneousRocks/IgneousTextures.html
- Plummer C.C., Carlson D.H., & Hammersley L. (2019). Physical Geology. McGraw-Hill Education. (Chapters 1-7).