# 12: Crustal Deformation

Learning Objectives

After completing this chapter, you should be able to:

• Understand the different types of stress that rocks undergo, and their responses to stress
• Demonstrate an understanding of the concepts of strike and dip
• Recognize the different types of folds and faults, and the forces that create them
• Use block diagrams to display geologic features
• Create a geologic cross-section

• 12.1: Introduction
The Earth is an active planet shaped by dynamic forces. Such forces can build mountains and crumple and fold rocks. As rocks respond to these forces, they undergo deformation, which results in changes in shape and/or volume of the rocks. The resulting features are termed geologic structures. This deformation can produce dramatic and beautiful scenery which shows the deformation of originally flat (horizontal) rock layers.
• 12.2: Stress and Strain
Rocks change as they undergo stress, which is just a force applied to a given area. Since stress is a function of area, changing the area to which stress is applied makes a difference. For example, imagine the stress that is created both at the tip of high heeled shoes and the bottom of athletic shoes. In the high heeled shoe, the area is very small, so that stress is concentrated at that point, while the stress is more spread out in an athletic shoe.
• 12.3: Strike and Dip
To learn many of the concepts associated with structural geology, it is useful to use block diagrams. As you examine these blocks, note the different ways that you can view them. If you look at a block from along the side, you are seeing the cross-section view (like what you see along roads that have been cut through the mountains). If you look at the block from directly above it, you are looking at a map view.
• 12.4: Lab Exercise (Part A)
The following questions concern the concept of "strike" and "dip" discussed in the previous section (Section 12.3).
• 12.5: Geologic Structures Created by Plastic and Brittle Deformation
Folds are geologic structures created by plastic deformation of the Earth’s crust. To understand how folds are generated, take a piece of paper and hold it up with a hand on each end. Apply compressional forces (push the ends towards each other). You have just created a fold (bent rock layers). Depending on how your paper moved, you created one of the three main fold types.
• 12.6: Geologic Maps and Cross-Sections
A geologic map uses lines, symbols, and colors, to include information about the nature and distribution of rock units within an area. It includes a base map, over which information about geologic contacts and strikes and dips are included. Geologists make these maps by careful field observations at numerous outcrops throughout the mapping area. At each outcrop, geologists record information such as rock type, strike and dip of the rock layers, and relative age data.
• 12.7: Practice with Block Diagrams
For practice, complete the questions below about block diagrams. A key is provided after this section to check your work. For each diagram, draw in the geological contacts on each side of the block. Add strike and dip symbols, and other symbols to document geologic features (like direction of movement on faults). Also, state the name of the geology feature in the diagram.
• 12.8: Answers to Practice with Block Diagrams