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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.

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Sedimentary rocks
The Ordovician period
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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.

What are molecules?

“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
What is travertine?

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
What are igneous rocks?
More about volcanoes

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
What are carbon atoms?

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 

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Learn by doing: finding different kinds of rocks
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Bibliography and further reading about rocks:

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