Geology 200

Study Guide

Unit 5:
Metamorphic Rocks

Overview

In Unit 5 we examine the third major group of rocks: metamorphic rocks. Metamorphic rocks derive from the alteration of sedimentary rocks, igneous rocks, or even previously metamorphosed rocks, at high temperature and pressures. Section 1 addresses the process of metamorphism and the characteristics of the rocks it produces, while Section 2 outlines the classification of metamorphic rocks and types of metamorphism. Section 3 discusses plate movements as a factor in metamorphism, and Section 4 deals with hydrothermal processes in metamorphism. This unit concludes our examination of the various minerals and rock types.

Objectives

After completing this unit, you should be able to

1. list and describe the four most important factors that determine the characteristics of metamorphic rocks.
2. describe the metamorphic textures produced as a result of different kinds of stresses.
3. describe the effect of pressure that confines growing minerals.
4. describe the evidence for the importance of the presence of water during metamorphism.
5. explain how metamorphic rocks are classified.
6. describe where contact metamorphism occurs, identify the dominant factors involved, and describe the main types of rocks produced.
7. explain the origin of the high temperature, high confining pressure, and high differential stress characteristic of regional metamorphism.
8. name and describe the different rock types formed as shale undergoes progressive metamorphism.
9. define migmatite.
10. explain the concepts of metamorphic facies and index minerals.
11. explain (with the aid of a diagram) how temperatures and geothermal gradients vary across a subduction zone, and describe the effect this variation has on the distribution of metamorphic facies.
12. explain how veins and metasomatic rocks form.
13. identify the major metamorphic rock types, and discuss the significance of their texture and composition.
14. explain the contributions that the chemical composition of a rock and the temperature, pressure, and amount of water present make to the metamorphic process and the resultant metamorphic rock.
15. draw a diagram illustrating the complete rock cycle, describe how each component of the cycle differs from the others, and explain how they are related.

Section 1: Metamorphism and Metamorphic Rocks

This section introduces the concept of metamorphism, and describes the factors that determine the types of metamorphic rocks that will develop under various sets of conditions. The textbook lists four factors that control the characteristics of metamorphic rock. However, the third factor—the effects of tectonic forces—is not addressed directly in the reading. Tectonic forces apply differential stress to rocks, which results in the development of foliation.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• Introduction to Chapter 7: Metamorphism and Metamorphic Rocks (pp. 157-159).
• “Factors Controlling the Characteristics of Metamorphic Rocks” (pp. 159-163).

Study Questions

1. Define metamorphism.
2. What is the difference between weathering and metamorphism?
3. What evidence indicates that metamorphic rocks form from other rocks by a solid-state transformation rather than by crystallization of a melt?
4. What four factors are most important in determining the characteristics of metamorphic rocks?
5. Briefly explain the concept of mineral stability, and describe how it is applied in studies of metamorphic rocks.
6. How does an increase in temperature affect chemical reactions?
7. For a granite under high pressure, what is the maximum temperature under which metamorphism and the production of a metamorphic rock, rather than melting and the production of an igneous rock, can occur?
8. Draw figures to illustrate the difference between differential stress and confining pressure. Use arrows to show the direction and intensity of pressure.
9. Describe the effect of confining pressure on growing minerals.
10. Explain, using a diagram, the difference between compressive stress and shearing.
11. What is foliation?
12. Distinguish among slaty, schistose, and gneissic foliation.
13. What role does water play during metamorphism?

 Answer Key

Section 2: Classification of Metamorphic Rocks and Types of Metamorphism

Section 2 examines how metamorphic rocks are classified and named, first by texture (foliated or nonfoliated), and then by composition.

The textbook describes three types of metamorphism: shock, contact, and regional, of which the last two are the most common. It does not discuss cataclastic metamorphism, which is a purely mechanical effect of grinding related to faulting and folding, which results in no changes to mineral chemistry. In cataclastic metamorphism, minerals and rock are broken or pulverized to produce coarse-grained fault breccias or finer-grained mylonites.

Contact metamorphism occurs adjacent to hot bodies of igneous rocks. Since there is no directed pressure, the resulting rocks are nonfoliated. Regional metamorphism is associated with mountain building and high temperatures, and with confining pressure and differential stress. Rocks resulting from regional metamorphism are foliated. Contact and regional metamorphism each produces characteristic rocks; in regional metamorphism the type of rock depends on the intensity of metamorphism. The assigned reading below describes how shale changes to a slate, a phyllite, a schist, a gneiss, and finally a migmatite, based on intensity of metaphorphism.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Classification of Metamorphic Rocks” (pp. 163-166).
• “Types of Metamorphism” (pp. 166-169).
• “Metamorphic Grade” (pp. 170-172).

Study Questions

14. List the factors that the impact of a meteorite can have on rocks.
15. Explain one theory for the extinction of dinosaurs.
16. What is the dominant factor in contact metamorphism? What effect does this factor have on the texture of the resulting metamorphic rocks?
17. Give the name of the metamorphic rock and the dominant mineral produced by contact metamorphism of each of the rocks listed below.
    1. shale
    2. basalt
    3. limestone
    4. sandstone
18. What is the most common heat source for contact metamorphism?
19. What is the origin of the high heat, confining pressure, and differential stress that cause regional metamorphism?
20. What must occur before we can see regional metamorphic rocks?
21. Describe what happens to a shale as it undergoes progressive regional metamorphism, and name the rock types at each of the six main stages.

 Answer Key

Section 3: Plate Tectonics and Metamorphism

Regional metamorphism is associated with convergent plate boundaries and the accompanying mountain building events. Figure 7.21 in the textbook is a diagram of a convergent plate boundary, showing the zones where intense shearing and intense compressive directed pressure occur, and the variation in temperature (at the same depth) at various locations. Study this diagram and the box titled “Metamorphic Facies and Its Relationship to Plate Tectonics” carefully.

The concept of metamorphic facies is described in Web Box 7.3. In general, a facies can be defined as “an assemblage of mineral or rock (or fossil) features reflecting the environment in which the rock was formed” (Judson, Kauffman, & Leet, 1987, p. 117). Metamorphic facies are collections of minerals that reached equilibrium during metamorphism under a specific set of temperature and pressure conditions. An index mineral is a mineral that forms over a narrow range of temperature and is thus characteristic of a particular degree of metamorphism. Table 5.1, below, shows typical mineral assemblages for metamorphism of a shale and of a basalt.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Plate Tectonics and Metamorphism” (pp. 172-176).

Study Questions

22. Draw a diagram of a convergent plate boundary, and indicate the zones where intense shearing and intense compressive directed pressure occur.
23. Study Figure 7.17 in the textbook. Would you expect the intensity of metamorphism to be uniform along the base of the continental crust? Explain your answer.
24. What is the sequence of metamorphic facies for rocks exposed to increasingly higher pressures at relatively low temperatures such as along subducting oceanic crust?
25. What is the sequence of metamorphic facies with increasing depth at a volcanic plutonic complex?
26. What facies occurs
    1. where pressure is low but temperature is high?
    2. where pressure is high but temperature is relatively low? In terms of plate tectonics, where would you expect this facies of metamorphism to occur?

 Answer Key

Section 4: Hydrothermal Processes

The reading assigned for this section describes rocks that are formed from the precipitation of ions from solutions of hot water and rocks that are altered in the presence of significant amounts of water. These waters import ions into and remove ions from the rocks that are being metamorphosed, resulting in metasomatism.

Reading Assignment

Plummer, C. C., Carlson, D. H., & Hammersley, L. Physical Geology (15th ed.).

• “Hydrothermal Metamorphism” (pp. 167-169).
• “Hydrothermal Metamorphism and Plate Tectonics” (pp. 175-176).

Study Questions

27. What is a hydrothermal rock?
28. What is the difference between metamorphism and metasomatism?
29. How is a contact metasomatic ore deposit formed? (You may use a diagram in your explanation.)
30. Why do quartz veins form?
31. Why are hydrothermal veins important economically?
32. List the possible sources for the water involved in metamorphism.

 Answer Key

Unit 5 Self Test

You have now finished Unit 5, so please complete and submit the associated laboratory exercise and assignment. Instructions can be found in the Assignment Drop Boxes section of the course homepage.