Lesson Objectives

• Describe how metamorphic rocks form.
• Describe the properties of some common metamorphic rocks.
• Relate some common uses of metamorphic rocks.

Vocabulary

• contact metamorphism
• foliation
• regional metamorphism

Introduction

In the large outcrop of metamorphic rocks in Figure below, the rocks’ platy appearance is a result of the process metamorphism. Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.

The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism.

Metamorphism

Any type of rock – igneous, sedimentary, or metamorphic – can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in Table below.

Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (Figure below). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and limestone, shown in Table below, are nonfoliated.

A foliated metamorphic rock.

The two main types of metamorphism are both related to heat within Earth:

1. Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures.
2. Contact metamorphism: Changes in a rock that is in contact with magma because of the magma’s extreme heat.

Table below shows some common metamorphic rocks and their original parent rock.

Picture	Rock Name	Type of Metamorphic Rock	Comments
	Slate	Foliated	Metamorphism of shale
	Phyllite	Foliated	Metamorphism of slate, but under greater heat and pressure than slate
	Schist	Foliated	Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite
	Gneiss	Foliated	Metamorphism of various different rocks, under extreme conditions of heat and pressure
	Hornfels	Non-foliated	Contact metamorphism of various different rock types
	Quartzite	Non-foliated	Metamorphism of sandstone
	Marble	Non-foliated	Metamorphism of limestone
	Metaconglomerate	Non-foliated	Metamorphism of conglomerate

Uses of Metamorphic Rocks

Quartzite and marble are commonly used for building materials and artwork. Marble is beautiful for statues and decorative items such as vases (see an example in Figure below). Ground up marble is also a component of toothpaste, plastics, and paper.

Marble is used for decorative items and in art.

Quartzite is very hard and is often crushed and used in building railroad tracks (see Figure below). Schist and slate are sometimes used as building and landscape materials. Graphite, the “lead” in pencils, is a mineral commonly found in metamorphic rocks.

Crushed quartzite is sometimes placed under railroad tracks because it is very hard and durable.

Lesson Summary

• Metamorphic rocks form when heat and pressure transform an existing rock into a new rock.
• Contact metamorphism occurs when hot magma transforms the rock that it contacts.
• Regional metamorphism transforms large areas of existing rocks under the tremendous heat and pressure created by geologic processes.

Review Questions

1. Why do minerals change composition as they undergo metamorphism?
2. Describe the process by which minerals in a rock rearrange to become different minerals when exposed to heat or pressure.
3. Describe the conditions that lead to foliated versus non-foliated metamorphic rocks.
4. List and describe the two main types of metamorphism.
5. What can geologists look at in a metamorphic rock to understand that rock’s history?
6. Suppose a phyllite sample was metamorphosed again. How might it look different after this second round of metamorphism?

Further Reading / Supplemental Links

• Film: The Living Rock: Earth’s Continental Crust.

Points to Consider

• What type of rock forms if an existing rock heats up so much that it melts completely and then forms a different rock?
• What clues can a rock give about its history if it was so altered by metamorphism that it is unrecognizable?

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