Deformation: Modification of Rocks by Folding and Fracturing

Transcription

1 CHAPTER 7 Deformation: Modification of Rocks by Folding and Fracturing Chapter Summary A geologic map is a scientific model of rock formations that are exposed on the Earth s surface showing outcrops, fault traces, and significant boundaries between rock bodies and geologic structures. The orientation of formations is indicated on geologic maps by angles of strike and dip (see Figure 7.3). A geologic cross section shows diagram features that would be visible if a vertical slice were made through the crust. All rocks may bend (behave ductilely) and break (behave brittlely) in response to the application of forces. Laboratory experiments have revealed that whether a rock exhibits ductile or brittle behavior depends on its composition, temperature, depth of burial (confining pressure), and rate with which tectonic processes apply force. In the field and on geologic maps, strike and dip of a formation shows its orientation at a particular place. Ductile behavior is more likely when a rock is exposed to higher temperatures, deeper burial, slower application of tectonic forces, and is a sedimentary rock. Brittle behavior is favored when rocks are cooler, closer to the Earth s surface, exposed to more rapid application of tectonic forces, and is an igneous or highgrade metamorphic rock. Folding is a result of ductile deformation. From the type of fold and its orientation geologists can interpret the orientation of the tectonic forces and characteristics of the rock layers during deformation. Faulting and jointing are a result of brittle deformation. Jointing occurs when a rock fractures but there is little movement along the fracture planes. Faults are fractures along which there is appreciable movement (offset). The type and orientation of faults and joints provides valuable information about the tectonic forces and the characteristics of the rock layers at the time of deformation. The type of fold or fault provides a basis for geologists to interpret the type of tectonic force acting on the rock during deformation. Tectonic forces can be of three types: compressive, tensional, and shearing forces. These same kinds of forces are active at all three types of plate tectonic boundaries: compressive forces dominate at convergent boundaries, where plates collide or subduct; tensional forces dominate at divergent boundaries, where plates are pulled apart; and shearing forces dominate at transform faults where plates slide horizontally past each other. 91

2 92 PART II CHAPTER 7 Geologic structures such as folds, faults, and joints occur on all scales from microscopic to the size of a mountainside. Geologists deduce the geologic history of a region in part by unraveling the history of deformation, thereby reconstructing what the rock units looked like before deformation. Regional deformational fabrics can help geologists decipher the plate tectonic history for the region. 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 the factors that determine whether a rock breaks or bends. Understand what geologic structures result from compressional, tensional, and shear stresses. Identify the major types of folds and faults. Know what styles of deformation are characteristic of each tectonic plate boundary. Geology Skills/Applications/Attitudes Measure and record a strike and dip of a planar rock feature. Given information about geologic structures in an outcrop or series of outcrops, interpret the geologic circumstances responsible for the deformation. General Education Skills Write an interesting piece for your local paper on a local example of deformation. Freshman Survival Skills Encourage Students to preview Chapter 7 by awarding credit for restating in their own words acceptable answers to all Chapter Preview Questions and turning in their answers before the lecture. (previewing/textbook reading) 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. Chapter 7: Deformation Modification of Rocks by Folding and Fracturing Measuring strike and dip Constructing a geologic map and cross section

3 Deformation: Modification of Rocks by Folding and Fracturing 93 Forces types of stress Compressive Tensional Shearing Behavior (plastic, elastic, brittle) of rocks depends on Composition Temperature Confining pressure Rate at which stress is applied (time) Small amounts of fluids Deformation response to force Brittle Deformation Joints Faults normal, reverse, thrust, strike-slip, rift valley (horst and graben) Folds Symmetrical and Asymmetrical Monocline Syncline Anticline Overturned Plunging Domes Basins 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. Practice Exercises 1 and 2 from Chapter 7 of the Student Study Guide (available in the Understanding Earth e-book) work well as a Think/Pair/Share. Present the exercise on the board or as an overhead, then ask student teams to fill in the blanks. Coop Exercise 1: Geologic Structures For each of the following five illustrations of deformed rocks, name (a) the geologic structure, e.g., normal fault, syncline; (b) the type of force, e.g., compressional, tensional, shearing force, responsible for producing each geologic structure; and (c) the plate tectonic boundary, e.g., convergent, divergent, or shear, with which the geologic structure is commonly associated. A. Geo structure B. Type of force C. Commonly associated plate tectonic boundary

4 94 PART II CHAPTER 7 A. Geo structure B. Type of force C. Commonly associated plate tectonic boundary D. Geo structure E. Type of force F. Commonly associated plate tectonic boundary G. Geo structure H. Type of force I. Commonly associated plate tectonic boundary J. Geo structure K. Type of force L. Commonly associated plate tectonic boundary M. Geo structure N. Type of force O. Commonly associated plate tectonic boundary

5 Deformation: Modification of Rocks by Folding and Fracturing 95 Coop Exercise 2: Silly Putty Silly Putty is a popular teaching aid (and toy) with geologists because it exhibits at room temperature all three kinds of deformation characteristic of solids. If you pull on the putty quickly, it will snap into two pieces. It is easy to bend and mold the putty into many shapes. Plus, if you throw a ball of it on the floor, the ball will bounce. Compare the properties of Silly Putty with the behavior of rocks by completing the table below. Behavior of Geologic Structure Produced Silly Putty Behavior of Rock Type of Force by This Style of Deformation snaps into pieces tensional bends ductile bounces Elastic rocks do Compressional The ball NOTE exhibit elastic behavior; of putty is compressed by Earthquakes are attributed more on this when we the impact with the floor. to the elastic properties of rocks. study earthquakes. Freshman Survival Skills Assignment If you encouraged your students to preview chapters before attending lecture it is a good idea to reinforce this idea from time to time. One good approach would be to preview Chapter 7 for extra credit. Tell the students they must write a brief paragraph in their own words for each of the Chapter Preview Questions. Credit should be given only for answers turned in before the lecture occurs. (previewing/textbook reading) Topics for Class Discussion Compare the behavior of rocks with that of Silly Putty. Discuss. Refer to Coop Exercise 2 above. Rocks behave (deform) in three different ways in the Earth s crust characterize each type of behavior. What factors influence how a rock deforms? What is the field evidence for each style of deformation? What kind of deformation occurs when a tall building sways in the wind? 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. Note the relationship between cataclastic and mylonitic metamorphic rocks.

6 96 PART II CHAPTER 7 High Cataclastic (brittle) Rate of Strain Mylonite Ductile (plastic) Brittle = high rate of strain, low T & P Ductile = low rate of strain, high T & P Low Low Rate of Recrystallization High Strain Rate vs. Behavior of Solids Low strain rate elastic behavior Slow strain rate ductile behavior Fast strain rate brittle behavior 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 Part II, Chapter 7: Deformation Modification of Rocks by Folding and Fracturing Before Lecture: Study Tip Preview Questions and Brief Answers During Lecture: Note Taking Tip After Lecture: Check Your Notes Intensive Study Session Exam Prep: Chapter Summary Practice Exercises: Silly Putty Geologic Structures Anticline vs. Syncline Identifying Geologic Structures Review questions