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Chapter Overview

This chapter on metamorphic rocks, the third major category of rocks in the rock cycle, completes our description of earth materials (rocks and minerals). The information on igneous and sedimentary processes in previous chapters should help you understand metamorphic rocks, which form from pre-existing rocks.

After reading the chapter on weathering, you know how rocks are altered when exposed at Earth's surface. Metamorphism (a word from Latin and Greek that means literally "changing of form") also involves alterations, but the changes are due to deep burial, tectonic forces, and/or high temperature rather than surface conditions.

As you study this chapter, try to keep clearly in mind how the chemical composition of a rock and the temperature, pressure, and water present each contribute to the metamorphic process and the resultant metamorphic rock.

We also discuss the hydrothermally deposited rocks and minerals, which are usually found in association with both igneous and metamorphic rocks. Hydrothermal ore deposits, while not volumetrically significant, are of great importance to the world's supply of metals.

Because nearly all metamorphic rocks form deep within the earth's crust, they provide geologists with many clues about conditions at depth. Therefore, understanding metamorphism will help you when we consider geologic processes involving Earth's internal forces. Metamorphic rocks are a feature of the oldest exposed rocks of the continents and of major mountain belts. They are especially important in providing evidence of what happens during subduction and plate convergence.

Learning Objectives

1. Metamorphism changes the texture or mineralogy (or both) of its parent rock usually in response to high temperature and pressure within the earth's interior and under conditions that produce ductile (plastic) strain.

2. The chemical composition of the parent rock controls that of the metamorphic rock, although mineralogy may change. Minerals are stable within a particular temperature range, but that range varies with pressure and presence of other substances. The upper limits of metamorphism may overlap partial melting.

3. New metamorphic minerals crystallize under high confining pressure and tend to be denser than their low-pressure counterparts. Differential stress (compression and/or shearing) produces foliated textures described as slaty, schistose, or gneissic. Water triggers metamorphism and promotes new mineral formation. Metamorphism is a slow process taking place over millions of years.

4. The classification of metamorphic rocks is based on texture (foliated versus nonfoliated) and mineralogy (chemical composition is controlled by parent rock).

5. Metamorphism is either contact (high temperatures but low confining pressure) or regional (high-temperatures and high confining pressure). Metamorphic rocks produced in a contact aureole ("baked zone" of Chapter 3) include hornfels (shale parent rock), marble (limestone parent rock), and quartzite (quartz sandstone parent rock). All of these rocks have nonfoliated textures.

6. Regional metamorphism usually produces foliated rocks such as greenschist (basalt parent rock) and amphibole schist (basalt parent rock), although marble and quartzite also form, if their appropriate parent is present. Progressive metamorphism of shale can produce slate, phyllite, schist or gneiss as temperature and pressure increase. Partial melting produces migmatites. Retrogressive metamorphism reflect the effects of water movement that allows recrystallization under conditions below the peak of metamorphism.

7. Plate tectonics, particularly subduction, explain differential stress and temperature variations, which increase toward the continents because of rising magma from melting at depth.

8. Water plays an important role in metamorphism. Metasomatism involves hot water transporting ions from outside the rock, and form significant ore deposits. Hydrothermal rocks are formed by crystallization from hot water, most commonly quartz veins and disseminated ore deposits.

9. The water involved in metamorphism originates as either ground water or water trapped in descending oceanic crust in a subduction zone.

Related Readings

Blatt, H., and R. C. Tracy. 1996. Petrology: Igneous, Sedimentary, and Metamorphic. 2nd ed. New York: W. H. Freeman.

Hibbard, M. J. 1995. Petrography to Petrogenesis. Englewood Cliffs, NJ: Prentice-Hall.

Kerrick, D. M., ed. 1991. Contact Metamorphism. Reviews in Mineralogy, vol. 26. Washington, D.C.: Mineralogical Society of America.

Mason, R. 1989. Petrology of the Metamorphic Rocks. 2nd ed. London: Unwin Hyman.

Yardley, B. W. D. 1989. An Introduction to Metamorphic Petrology. Essex, England: Longman.

Answers to EOC Questions

Following are answers to the End of Chapter Questions for Chapter 7:

7.D, 8.D, 9.B, 10.A, 11.C, 12.B, 13.C, 14.C, 15.D, 16.C, 17.A, 18.D

Boxed Readings

This chapter contains the following boxed readings:

Environmental Geology
Box 7.4: The World's Largest Human-Made Hole - The Bingham Canyon Copper Mine

In Greater Depth
Box 7.2: Index Materials
Box 7.3: Metamorphic Facies and the Relationship to Plate Tectonics

Planetary Geology
Box 7.1: Impact Craters and Shock Metamorphism






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