1. Metamorphic Rocks
S E C T I O N 5 - 5

2. Objectives Under what conditions do metamorphic rocks form?
How do geologists classify metamorphic
rocks?

3. Every metamorphic rock is a rock that has changed its form.

4. In fact, the word metamorphic comes from the Greek words meta, meaning “change,” and morphos, meaning “form.”

5. Heat and pressure deep beneath Earth’s surface can change any rock into metamorphic rock.

6. When rock changes into metamorphic rock, its appearance, texture, crystal structure, and mineral content change.

7. Metamorphic rock can form out of igneous, sedimentary, or other metamorphic rock.

8. Collisions between Earth’s plates can push rocks down toward the heat of the mantle.

9. Magma rising through the crust can also provide heat to produce metamorphic rocks.

10. Rock buried deep in the crust is under pressure hundreds of thousands of times greater than at Earth’s surface, and that pressure can change rock into metamorphic rock.

11. When metamorphic rock is forming, high temperatures can change the size and shape of grains in the rock.

12. Tremendous pressure can also squeeze rock so tightly that the mineral grains may line up in flat, parallel layers.

13. Geologists classify metamorphic rocks by the arrangement of the grains that make up the rocks.

14. Metamorphic rocks that have their grains arranged in parallel layers or bands are said to be foliated.

15. Foliated rocks—including slate, schist, and gneiss—may split apart along these bands.

16. One common foliated rock is slate, which is basically a denser, more compact version of shale.

17. Sometimes metamorphic rocks are nonfoliated.
The mineral grains in these rocks are arranged randomly.

18. Nonfoliated metamorphic rocks— including marble and quartzite—do not split into layers.

19. Quartzite forms out of sandstone.
The weakly cemented quartz particles in the sandstone recrystallize to form quartzite, which is extremely hard.

20. Marble and slate are two of the most useful metamorphic rocks.

21. Marble is relatively easy to cut into thin slabs, and it can be polished so that its surface is smooth and mirrorlike.

22. These qualities have led architects and sculptors to use marble for many buildings and statues.

23. One of the most beautiful buildings in the world, India’s Taj Mahal, is made of gleaming white marble.

24. Slate, because it is foliated, splits easily into flat pieces that can be used for flooring, roofing, outdoor walkways, or chalkboards.

25. Like marble, slate comes in a variety of colors, including gray, black, red, and purple, so it has been used as trim for stone buildings.

26. End