What is metamorphic rock? Marble, slate, diamonds

Home » What is metamorphic rock? Marble, slate, diamonds
Print Friendly, PDF & Email
gray-brown rocks in long thin horizontal layers, with water running over them: Metamorphic rocks

What is metamorphic rock? Slate rocks in Cascadilla Gorge, Ithaca, New York

Metamorphic rocks form under pressure

About 450 million years ago, some of the sedimentary rocks began to transform into metamorphic rocks. When sedimentary rocks like limestone or shale were under water, the weight of the water pressed them down.

(Read more about sedimentary rocks)

Sometimes the weight of the water pressed them so hard that they actually changed the way their molecules locked together. They became a different kind of rock.

“Metamorphosis” means “changes form”, and that’s why we call these metamorphic rocks. These changes only happen when the rocks happen to be under a lot of pressure, so metamorphic rocks are much more rare than igneous or sedimentary rocks.

Sedimentary rocks that morph

A bright white cliff cut in rectangular shapes, with a blue sky behind it

What is metamorphic rock? A marble quarry in Italy

Each kind of sedimentary rock turns into a different kind of metamorphic rock. Limestone turns into travertine or marble.

(Read more about marble)

Shale turns into slate. Sandstone turns into quartzite. Even though slate is a metamorphic rock, it still looks a lot like a sedimentary rock – it is still in thin layers.

Diamonds - glittery and semi-transparent

More rocks formed under pressure: Diamonds

Igneous rocks that morph

Igneous rocks can also turn into metamorphic rocks. Granite, for example, changes into a rarer rock called gneiss (pronounced NICE). Also, some metamorphic rocks form out of plants.

(Read more about granite)

When the plants die, they decay and turn into peat. Then the peat can eventually turn into a sedimentary rock called coal (which is entirely made of carbon atoms).

(Read more about coal)

The graphite inside your pencil

The graphite inside your pencil

Finally, if the coal is under pressure, it can become the metamorphic rock graphite (also made of carbon atoms, but they’re arranged in a different way).

Carbon can also metamorphose into diamonds, but usually not from coal. That transformation happens deep down inside the earth, under tremendous pressure, while coal is on the surface.

Looking for a second source to cite? Check out this excellent article about the Rock Cycle 

Did you find out what you wanted to know about metamorphic rocks? Let us know in the comments!

Learn by doing: finding different kinds of rocks
More about jade

Bibliography and further reading about rocks:

More about different kinds of rocks
Quatr.us home

By |2018-05-28T16:21:09+00:00June 26th, 2017|Geology|0 Comments
Cite this page: Carr, K.E. What is metamorphic rock? Marble, slate, diamonds. Quatr.us Study Guides, June 26, 2017. Web. July 16, 2018.

About the Author:

Dr. Karen Carr is Associate Professor Emerita, Department of History, Portland State University. She holds a doctorate in Classical Art and Archaeology from the University of Michigan. Follow her on Instagram, Pinterest, or Facebook, or buy her book, Vandals to Visigoths.

Leave A Comment

This site uses Akismet to reduce spam. Learn how your comment data is processed.