Lesson Objectives

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

Introduction

In the large outcrop of metamorphic rocks in Figure below, the rocks’ platy appearance is a result of the process of 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.
Example of platy layers in metamorphic rock.
Example of platy layers in metamorphic 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 rocks 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 the rock melting 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 to 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 the chart below. Extreme pressure may also lead to foliation, flat layers in rocks that form as the rocks are squeezed by pressure. Foliation normally forms when pressure is exerted in only one direction.
A foliated metamorphic rock.
A foliated metamorphic rock.

A foliated metamorphic rock.
Metamorphic rocks may also be nonfoliated. Quartzite and limestone, shown below, are nonfoliated. Both of the two main types of metamorphism are 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.
Common Metamorphic Rocks|| Picture || Rock Name || Type of Metamorphic Rock || Comments ||
external image images?id=309213
Slate
Foliated
Metamorphism of shale
external image images?id=309197
Phyllite
Foliated
Metamorphism of slate, but under greater heat and pressure than slate
external image images?id=315471
Schist
Foliated
Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite
external image images?id=309209
Gneiss
Foliated
Metamorphism of various different rocks, under extreme conditions of heat and pressure
external image images?id=309180
Hornfels
Non-foliated
Contact metamorphism of various different rock types
external image images?id=315476
Quartzite
Non-foliated
Metamorphism of sandstone
external image images?id=315475
Marble
Non-foliated
Metamorphism of limestone
external image images?id=315469
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 the Figure below). Ground up marble is also a component of toothpaste, plastics, and paper.
A decorative marble vase.
A decorative marble vase.

Marble is used for decorative items and in art.
Quartzite is very hard and is often crushed and used in building railroad tracks (see the 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.
Showing crushed quartzite that is used under railroad tracks because of its hardness and durability.
Showing crushed quartzite that is used under railroad tracks because of its hardness and durability.

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 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

Vocabulary

regional metamorphism Changes in rock that occur because of high pressure over a large area. foliation Flat layers in rocks due to squeezing by pressure. contact metamorphism Changes in a rock that result from temperature increases when a body of magma contacts a cooler existing rock.

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?

Metamorphic Rocks

Lesson Objectives

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

Introduction

In the large outcrop of metamorphic rocks in Figure below, the rocks’ platy appearance is a result of the process of 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.
Example of platy layers in metamorphic rock.
Example of platy layers in metamorphic 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 rocks 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 the rock melting 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 to 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 the chart below. Extreme pressure may also lead to foliation, flat layers in rocks that form as the rocks are squeezed by pressure. Foliation normally forms when pressure is exerted in only one direction.
A foliated metamorphic rock.
A foliated metamorphic rock.

A foliated metamorphic rock.
Metamorphic rocks may also be nonfoliated. Quartzite and limestone, shown below, are nonfoliated. Both of the two main types of metamorphism are 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.
Common Metamorphic Rocks|| Picture || Rock Name || Type of Metamorphic Rock || Comments ||
external image images?id=309213
Slate
Foliated
Metamorphism of shale
external image images?id=309197
Phyllite
Foliated
Metamorphism of slate, but under greater heat and pressure than slate
external image images?id=315471
Schist
Foliated
Often derived from metamorphism of claystone or shale; metamorphosed under more heat and pressure than phyllite
external image images?id=309209
Gneiss
Foliated
Metamorphism of various different rocks, under extreme conditions of heat and pressure
external image images?id=309180
Hornfels
Non-foliated
Contact metamorphism of various different rock types
external image images?id=315476
Quartzite
Non-foliated
Metamorphism of sandstone
external image images?id=315475
Marble
Non-foliated
Metamorphism of limestone
external image images?id=315469
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 the Figure below). Ground up marble is also a component of toothpaste, plastics, and paper.
A decorative marble vase.
A decorative marble vase.

Marble is used for decorative items and in art.
Quartzite is very hard and is often crushed and used in building railroad tracks (see the 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.
Showing crushed quartzite that is used under railroad tracks because of its hardness and durability.
Showing crushed quartzite that is used under railroad tracks because of its hardness and durability.

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 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

Vocabulary

regional metamorphism Changes in rock that occur because of high pressure over a large area. foliation Flat layers in rocks due to squeezing by pressure. contact metamorphism Changes in a rock that result from temperature increases when a body of magma contacts a cooler existing rock.

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?