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4.4: Metamorphic Rocks

Created by: CK-12

Lesson Objectives

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

Vocabulary

  • contact metamorphism
  • foliation
  • regional metamorphism
  • stable

Introduction

Metamorphism changes rocks by heat and pressure. These agents create an entirely new type of rock. Metamorphism changes rocks physically and/or chemically.

Metamorphism

Metamorphic rocks start off as some kind of rock. The starting rock can be igneous, sedimentary or even another metamorphic rock. Heat and/or pressure then change the rock’s physical or chemical makeup.

During metamorphism a rock may change chemically. Ions move and new minerals form. The new minerals are more stable in the new environment. Extreme pressure may lead to physical changes like foliation. Foliation forms as the rocks are squeezed. If pressure is exerted from one direction, the rock forms layers. This is foliation. If pressure is exerted from all directions, the rock usually does not show foliation.

There are two main types of metamorphism:

1. Contact metamorphism results when magma contacts a rock, changing it by extreme heat (Figure below).

(A) Hornfels is a rock that is created by contact metamorphism. (B) Hornfels is so hard that it can create peaks like the Matterhorn.

2. Regional metamorphism occurs over a wide area. Great masses of rock are exposed to pressure from rock and sediment layers on top of it. The rock may also be compressed by other geological processes.

(A) Regional metamorphic rocks often display layering called foliation. (B) Regional metamorphism with high pressures and low temperatures can result in blue schist.

Metamorphism does not cause a rock to melt completely. It only causes the minerals to change by heat or pressure.

Hornfels is a rock with alternating bands of dark and light crystals. Hornfels is a good example of how minerals rearrange themselves during metamorphism (Figure above). The minerals in hornfels separate by density. The result is that the rock becomes banded. Gneiss forms by regional metamorphism from extremely high temperature and pressure.

Uses of Metamorphic Rocks

Quartzite and marble are the most commonly used metamorphic rocks. They are frequently chosen for building materials and artwork. Marble is used for statues and decorative items like vases (Figure below). Quartzite is very hard and is often crushed and used in building railroad tracks. Schist and slate are sometimes used as building and landscape materials.

(A) Marble is a beautiful rock that is commonly used for buildings. (B) Many of the great statues of the Renaissance were carved from marble. Michelangelo created this Moses between 1513 and 1515.

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 tectonic forces.

Lesson Review Questions

Recall

1. Why do the minerals in a rock sometimes rearrange themselves when exposed to heat or pressure?

2. List and describe the two main types of metamorphism.

Apply Concepts

3. How does layering form in metamorphic rocks?

4. What clues in metamorphic rocks tell you how they were formed?

Think Critically

5. Suppose a phyllite sample was exposed to even more heat and pressure. What metamorphic rock would form?

Points to Consider

  • What type of plate boundary would produce the most intense metamorphism of rock?
  • Do you think new minerals could form when an existing rock is metamorphosed?

For Table above,

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