Geology 200

Study Guide

Unit 12:
Structural Geology and Earthquakes

Overview

In Units 2-5, we discussed Earth materials (rocks and minerals) and the processes involved in the formation of rocks in each major category. In Units 6-11, we examined processes that operate on the surface of the Earth. In Units 12-15, we explore both the changes in bedrock that are driven by powerful forces originating deep within the Earth and the sources of those forces.

Unit 12 examines the deformation of the Earth’s crust and the features of rocks at the Earth’s surface that provide evidence for movement of the Earth’s crust. We also discuss earthquakes—phenomena that provide measurable evidence for movement of the crust. The unit begins with an examination of the types of forces that work on the crust, and then examines various features of the rocks—especially folds, faults, and unconformities—that are produced by tectonic forces. We cover earthquakes in Sections 3 and 4.

Objectives

After completing this unit, you should be able to

1. distinguish between stress and strain.
2. distinguish among compressive stress, tensional stress, and shear stress.
3. list and describe three types of strain or deformation.
4. explain the concepts of strike and dip.
5. draw diagrams of folds to illustrate a monocline, an anticline, a syncline, an overturned anticline, an overturned syncline, an axial plane, a hinge line, and a plunge; and indicate the relative ages of rocks in the centre of the folds.
6. distinguish between joints and faults.
7. describe and illustrate the three major types of faults.
8. define unconformity, and describe and illustrate the three types of unconformities: disconformity, angular unconformity, and nonconformity.
9. describe and interpret geological structures in three dimensions, using block diagrams, geologic maps, and geologic cross sections.
10. explain the elastic rebound theory of earthquakes.
11. describe the motion and velocity of the three major types of seismic waves.
12. describe how the location of an earthquake can be determined.
13. explain what the Richter scale measures, and identify the factors that control the intensity and magnitude of an earthquake.
14. describe four effects of earthquakes on the Earth’s surface.
15. define tsunami, and describe how tsunamis are generated.
16. describe the relationship between earthquakes and plate tectonics.

Section 1: Tectonic Forces

The reading assigned for this section discusses the effects of tectonic forces on rocks. Remember that stress is the force acting on a rock, and straining is the response of the rock to stress. The textbook identifies three types of stress: compressive, tensional, and shear; and three types of strain or deformity: plastic, elastic, and fracture. The type of deformity exhibited by a rock depends in part on the rock’s physical characteristics; for example, ductile rocks deform plastically, but brittle rocks deform by fracturing. However, more important factors in the types of deformation are the intensity and speed of the stress and the degree of confining pressure.

Reading Assignment

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

• Introduction to Chapter 15: Geologic Structures (pp. 361-362).
• “Tectonic Forces at Work” (pp. 362-364).

Study Questions

1. Explain the difference between stress and strain.
2. Draw a diagram to indicate the difference between compressive stress, tensional stress, and shear stress. Use arrows to show the direction of the force.
3. List the three types of strain or deformation, and explain the differences among them.
4. Explain the difference between ductile rocks and brittle rocks in response to stress.
5. Describe two ways in which sedimentary rocks could deform plastically.
6. What is a fault?
7. How fast is the San Andreas Fault moving at Hollister, California?

 Answer Key

Section 2: Geologic Structures

“Attitude” of Bedding

Geologic maps and cross sections are used by geologists to present information about the distribution and nature of rock units, and the occurrence of structural features. According to the principle of horizontality, layers of sedimentary rock begin as horizontal beds. Deformation of sedimentary rock can result in tilting or folding of the beds. The “attitude” (or positioning) of tilted layers of rock at an outcrop is recorded as the “strike” and the “dip” of the bed. The strike and the dip are recorded by a strike symbol and angle for a particular location on a geologic map. The information from a geologic map can be used to construct a geologic cross section, a vertical representation of a portion of the Earth that is used to help the observer visualize geology in three dimensions.

Reading Assignment

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

• “How Do We Record and Measure Geologic Structures” (pp. 364-366).

Study Questions

8. What is the principle of original horizontality?
9. Why can geologic structures sometimes be more easily understood from a geologic map of an area than from flying over the area?
10. Define and distinguish among strike, direction of dip, and angle of dip. Draw a diagram of an inclined plane, and label the strike and dip.
11. Does a horizontal bedding plane have a dip and strike?
12. What is a geologic cross section?

 Answer Key

Folds

The following reading describes the possible geometries of folds that can result from deformation of rock. The textbook defines anticline and syncline as types of folds. You may find it more useful to remember that the arch of a fold is called an anticline, while the trough is called a syncline. An observer cannot see the whole fold, as it is covered by overlying rock, soil, and vegetation; therefore, geologists use the relative ages of rock layers to determine whether an outcropping structure is a syncline or an anticline.

The textbook mentions plunging folds, overturned folds, and recumbent folds. It does not, however, discuss monoclonal folds. A monocline is a double flexure that connects strata at one level with the same strata at another level. In a monocline, horizontal layers are bent down and pass beneath younger horizontal strata.

Reading Assignment

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

• “Folds” (pp. 366-371).

Study Questions

13. Draw a cross-section showing four layers of sediments that have been folded to form an anticline and syncline. Label the axial plane of the anticline and the axial plane of the syncline. Now extend the diagram into three dimensions to show the map view. Label the hinge line or axis in three dimensions to show the map view. Label the hinge line or axis of the anticline and the hinge line of the syncline. Label the oldest bed “1,” the next oldest “2,” the next “3,” and the youngest “4.” Do the older beds fall along the hinge line of the syncline or along the hinge line of the anticline?
14. What is a plunging fold? Draw a diagram to show the map view of a plunging anticline and a plunging syncline. Label the anticline and the syncline, and show the strike and dip of a bed in each. Label the oldest and youngest formation in your diagram.
15. Of the rocks exposed at the surface of a structural dome, where are the oldest found? —at the centre of the circular structure or at the rim of the structure?
16. Would an open fold or an isoclinal fold be the product of a more intense comprehensive force?
17. Explain and sketch the distinction between overturned folds and recumbent folds.

 Answer Key

Fractures

The reading for this subsection deals with the two main types of fractures: joints and faults. As the textbook points out, joints are fractures along which no movement (essentially) has occurred; faults are fractures along which movement has occurred.

The relative direction of movement of the blocks on either side of a fault determines the type of fault; for example, in dip-slip faults movement is parallel to the dip of the fault surface, and in strike-slip faults the blocks are displaced to the left or right.

The textbook also mentions grabens and horsts. A graben is a block of bedrock that has subsided when blocks on either side of it have moved apart through tensional force; a horst is a block of material that has been thrust upward as the blocks on either side have moved together through comprehensive force (see Figure 15.23 in the textbook).

Reading Assignment

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

• “Fractures in Rock” (pp. 371-380).

Study Questions

18. Define the terms joint and fault, and distinguish between these types of fractures.
19. Draw diagrams to illustrate the relative movement of blocks in a normal fault, in a reverse fault, and in a strike-slip fault. What kinds of forces result in normal faults? —in reverse faults?
20. What is the difference between a horst and a graben?

 Answer Key

Unconformities

Unconformities are contacts or surfaces that represent a break in the geologic record; that is, the rock unit above the contact is considerably younger than the rock unit below the contact. Unconformities are sometimes very obvious in geologic cross sections. The reading below describes the distinctions among disconformities, angular unconformities, and nonconformities.

Reading Assignment

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

• “Unconformities” (pp. 185-187).

Study Question

21. Define the term unconformity, and describe and illustrate (by a cross-section sketch) the three types of unconformities.

 Answer Key

Section 3: Earthquakes

Most earthquakes are caused by the sudden rupture of Earth materials that are being distorted beyond the limit of their strength. Study of California’s San Andreas Fault has led to the development of the “elastic rebound hypothesis,” summarized below (see Figure 16.1 in the textbook).

• A rock becomes progressively more strained as stress is applied to it. Imagine a line passing through a fault zone at right angles. Such a line becomes more and more warped as movement takes place along the fault. The degree of warping indicates the stress on the rock.
• When the relative movement on either side of the fault reaches a certain distance, the distortion exceeds the strength of the rock, and the rock ruptures.
• As ruptures take place, the blocks on either side of the fault, driven by the stored elastic energy, snap back into unstrained positions.
• The line across the fault is broken to form two separate lines, one displaced a distance from the other along the fault.

The sudden tearing apart of the rock and the snapping back of the blocks into unstrained positions is the mechanism associated with the generation of earthquakes: seismic waves—the waves of energy released when a rock breaks—cause the ground to tremble and shake (Judson, Kauffman, & Leet, 1987, pp. 183-184).

The intensity of an earthquake at a particular location depends on

• the total amount of energy released,
• the distance of the location from the epicentre of the earthquake, and
• the type of rock and its degree of consolidation (Hamblin, 1989, p. 385).

Reading Assignment

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

• Introduction to Chapter 16: Earthquakes (pp. 382-383).
• “Causes of Earthquakes” (pp. 383).
• “Seismic Waves” (pp. 383-386).
• “Locating and Measuring Earthquakes” (pp. 386-392).
• “Earthquake-related Hazards” (pp. 392-399).

Study Questions

22. Explain the elastic rebound hypothesis.
23. Define the terms focus and epicentre.
24. Describe the motion and velocity of each of the three major types of seismic waves.
25. How is the location at which an earthquake has occurred determined?
26. What does the Richter scale measure? At what magnitude on the Richter scale does damage begin?
27. What factors control the intensity and magnitude of an earthquake?
28. List four effects of an earthquake on the Earth’s surface.
29. What are tsunamis? How are they generated?

 Answer Key

Section 4: Earthquakes and Plate Tectonics

According to plate tectonic theory, most earthquakes occur along the boundaries of plates. The depth of the earthquake’s focus and the width of the earthquake zone depend on the type of plate boundary involved. The following reading describes the distribution of earthquakes at divergent, transform, and convergent plate boundaries.

Reading Assignment

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

• “World Distribution of Earthquakes” (pp. 399-401).
• “Earthquakes and Plate Tectonics” (pp. 403-404).

Study Questions

30. What is a Benioff zone?
31. With what type of faults are earthquakes associated at divergent plate boundaries? Are earthquakes associated with such boundaries deep or shallow? —wide or narrow?
32. Describe the distribution of earthquakes at transform boundaries.
33. Describe the three different processes that result in Earthquakes as oceanic floor is subducted under a continent.

 Answer Key

Unit 12 Self Test

You have now finished Unit 12, 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.