Metamorphism: Alteration of Rocks by Temperature and Pressure

Transcription

1 CHAPTER 6 Metamorphism: Alteration of Rocks by Temperature and Pressure Chapter Summary Metamorphism is the alteration in the solid state of preexisting rocks, including older metamorphic rocks. Increases in temperature and pressure and reactions with chemical-bearing fluids cause metamorphism. Metamorphism typically involves a rearrangement (recrystallization) of the chemical components within the parent rock. Rearrangement of components within minerals is facilitated by: higher temperatures, which increase ion mobility within the solid state; higher confining pressure compacts the rock; directed pressure associated with tectonic activity can cause the rock to shear (smear), which orients mineral grains and generates a foliation; and chemical reactions with migrating fluids may remove or add materials and induce the growth of new minerals. The two major types of metamorphism are regional metamorphism, associated with orogenic processes that build mountains, contact metamorphism, caused by the heat from an intruding body of magma, and seafloor metamorphism, also known as metasomatism. Other less common kinds of metamorphism are: burial metamorphism, associated with subsiding regions on continents, high-pressure metamorphism, occurring deep within subduction zones and upper mantle, and shock metamorphism due to meteor impact; refer to Figure 6.4. Metamorphic rocks fall into two major textural classes: the foliated (displaying a preferred orientation of minerals, analogous to the grain within wood) and granoblastic (granular). The composition of the parent rock and the grade of metamorphism are the most important factors controlling the mineralogy of the metamorphic rock. Metamorphism usually causes little to no change in the bulk composition of the rock. The kinds of minerals and their orientation do change. Mineral assemblages within metamorphic rocks are used by geoscientists as a guide to the original composition of the parent rock and the conditions during metamorphism. Metamorphic rocks are characteristically formed in subduction zones, continental collisions, oceanic spreading centers, and deeply subsiding regions on the continents. 84

2 Learning Objectives In this section we provide a sampling of possible objectives for this chapter. No class could or should try to accomplish all of these objectives. Choose objectives based on your analysis of your class. Refer to Chapter 1: Learning Objectives How to Define Your Goals for Your Course in the Instructional Design section of this manual for thoughts and ideas about how to go about such an analysis. Knowledge Know and understand the causes of metamorphism. Know the various kinds of metamorphism. Understand how metamorphic rocks can be used to reconstruct past geologic events. Understand how metamorphic rocks can be used as geothermometers and geobarometers. Know the chief types of metamorphic rocks and how they are classified. Understand how metamorphism is linked to plate tectonics. Understand how metamorphism is associated with the formation of mineral deposits. Metamorphism: Alteration of Rocks by Temperature and Pressure 85 Geology Skills/Applications/Attitudes Given a suite of metamorphic rocks exposed on the Earth s surface today, interpret possible associations with ancient plate boundaries. General Education Skills Using online resources, write a short and interesting newspaper article on one of the gemstones, such as garnets, emeralds, or diamonds that result from metamorphic processes. Freshman Survival Skills Reinforce the importance of promptly filling in gaps in your notes by stopping about halfway through the lecture on metamorphic rocks and telling students to check your notes with the student sitting next to you. (note review) Sample Lecture Outline Sample lecture outlines highlight the important topics and concepts covered in the text. We suggest that you customize it to your own lecture before handing it out to students. At the end of each chapter outline consider adding a selection of review questions that represent a range of thinking levels.

3 86 PART II CHAPTER 6 Chapter 6: Metamorphism Modification of Rocks by Temperature and Pressure Causes of Metamorphism Heat Confining pressure due to burial Directed pressure due to tectonics Fluids chemical alteration Kinds of Metamorphism Regional Contact Seafloor (metasomatism) Burial High-pressure and ultra-high-pressure Shock Textures and Types Foliation preferred orientation of crystals; slaty cleavage; slate, phyllite, schist, gneiss, migmatite Granoblastic (non-foliated) granular; no preferred orientation of crystals; hornfels, marble, greenstone, amphibolite; porphyroblasts, e.g., garnet and staurolite Metamorphic Grade Regional Metamorphism Temperature and pressure Parent rock composition Metamorphic facies Plate Tectonics and Metamorphism Metamorphic P T paths prograde, retrograde Spreading centers meta-basalts Subduction zones mélange and blue schists Continent-ocean convergence Continent-continent collision ophiolites exhumation links between the plate tectonic and climate

4 Teaching Tips Cooperative/Collaborative Exercises and In-Class Activities Refer to Chapter 4: Cooperative Learning Teaching Strategies in the Instructional Design section of this manual for general ideas about conducting cooperative learning exercises in your classroom. Coop Exercise 1: Classification of Metamorphic Rocks Based on Texture Practice Exercise 1 from the Student Study Guide is good for a brief Think/Pair/Share activity in lecture. Students often do not realize that metamorphism affects ALL rock types including preexisting metamorphic rocks. It seems that a common misconception is that metamorphic rocks are somehow immune from further metamorphism. Review retrograde metamorphism and metamorphic facies when you do this exercise. Directions: Fill in the table below. Bullets mark where to write your answers. Hint: Refer to the Metamorphic Textures section of your textbook and Table 6.1. Metamorphism: Alteration of Rocks by Temperature and Pressure 87 Texture Parent Rock Metamorphic Rock (foliated/granoblastic) shale foliated quartz-rich sandstone granulite granite limestone hornfels amphibolites and greenstones migmatite Coop Exercise 2: Five-Minute Write The Five-Minute Write is done during the last five minutes of lecture. Ask students to put their names on a sheet of paper and then address the three questions on the board; see adjacent sample. Start the next lecture by discussing the answers to some of the questions students had about the previous lecture. Sample Exercises Making comparisons and contrasts between things can enhance learning. With the completion of Chapter 6, your students can now compare igneous, sedimentary, and metamorphic rocks. Sample exercises involving students with making comparisons between the major rock types is provided below (Homework/Exercise 2).

5 88 PART II CHAPTER 6 Homework (Exercise 2): Comparing igneous, sedimentary, and metamorphic rocks Complete the table by filling in the blank spaces. Note that there may be more than one reasonable answer for some blanks. Some answers are provided as examples. Rock Type Rock Name Major Mineral igneous, sedimentary, e.g., granite, Composition Texture metamorphic sandstone, marble calcium carbonate quartz, K and Na feldspar, mica, and amphibole clay granoblastic phaneritic fine-grained clastic pyroxene, calcium feldspar, and olivine basalt quartz granoblastic pebbles and cobbles of a variety of rock types fragments of seashells and fine mud quartz, muscovite, chlorite, and garnet sedimentary schist Topics for Class Discussion Patterned fabrics in clothlike corduroy are a good analog for foliated fabrics in metamorphic rocks. Use a paper towel as a quick lecture demonstration to illustrate a preferred orientation within a material. Most paper towels will tear apart much easier in one direction than the other. Ask students to come up with an explanation for this characteristic of paper towels, then ask them to compare how the towel rips with how a foliated metamorphic rock will break. Because marble is commonly used as a facing stone for buildings and tombstones, I find students think it is a tough rock that will last for a long time. Since marble is made of calcite with a hardness of 3 on the Mohs hardness scale, it is easily sculptured, reacts with acid, and susceptible to weathering (dissolution), especially in regions where the acidity of rainfall is enhanced by pollution.

6 Metamorphism: Alteration of Rocks by Temperature and Pressure 89 Some students find the concept of metamorphic facies difficult. Consider a careful review that includes Thought Questions 4, 5, and 7 as an assignment. Inform the students they will need Figures 6.9, 6.10, 6.11, 6.12, and 6.13 to answer these questions. Deformation and metamorphism: Rock deformation and metamorphism are closely linked. The following conceptual graph illustrates the relationship between rate of strain (deformation) and rate of (re-)crystallization (metamorphism). How a rock behaves (brittlely or plastically) under stress will depend in part on how quickly minerals within the rock can recrystallize, which depends on temperature, pressure and the rock s composition. As temperature increases, ion mobility increases, and rates of recrystallization increase. High Cataclastic (brittle) Rate of Strain Mylonite Ductile (plastic) Low Low Rate of Recrystallization High Brittle = high rate of strain, low T & P Ductile = low rate of strain, high T & P Teaching Resources Student Study Guide Highlights (part of the Understanding Earth e-book) In Part I, chapters provide strategies for learning geology. Ideally, students would read these chapters early in the course. Chapter 1: Brief Preview of the Student Study Guide for Understanding Earth Chapter 2: Meet the Authors Chapter 3: How to Be Successful in Geology

7 90 PART II CHAPTER 6 In Part II, Chapter 6: Metamorphism Modification of Rocks by Temperature and Pressure Before Lecture: Preview Questions and Brief Answers During Lecture: Note Taking Tip Abbreviations After Lecture: Check Your Notes Intensive Study Session Exam Prep: Chapter Summary Practice Exercises: Classification of metamorphic rocks based on texture Comparing igneous, sedimentary, and metamorphic rocks Review questions