35 Introduction

Module 11

Structural Geology

A road cutting through a mountain. The cut part of the mountain is U shaped showing different layers fo rock.
Figure 1. A syncline in a roadcut in Sideling Hill, in Maryland. Synclines are folds that most often exhibit a concave shape, where the youngest rocks are the in the middle of the fold and the oldest rocks on the outside of the fold.

Overview

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, as evidenced in the figure of above, which shows the deformation of originally horizontal rock layers.

Why is it important to study deformation within the crust? Such studies can provide us with a record of the past and the forces that operated then. The correct interpretation of features created during deformation is critical in the petroleum and mining industry. It is also essential for engineering. Understanding the behavior of deformed rocks is necessary to create and maintain safe engineering structures. When proper geological planning is not considered in engineering, disasters can strike.

For example, the Vajont Dam was constructed at Monte Toc, Italy in the early 1960s. The site was a poor choice for a dam because the valley was narrow, thorough geological tests were not performed, and the area surrounding the dam was prone to large landslides. The steep canyon walls were composed of limestone with solution cavities, not known for its stability, and shifting and fracturing of rock that occurred during the filling of the reservoir was ignored.

An old photo showing a dam filled with dirt and debris.
Figure 2. An image of the Vajont reservoir shortly after the massive landslide (you can see the scar from the landslide on the right, and the dam is located in the foreground on the left).

In 1963, a massive landslide in the area displaced much of the water in the dam, causing it to override the top of the dam and flood the many villages downstream, resulting in the deaths of almost 2,000 people.

Figure 3. View of the village of Longarone, which was below the dam, showing the extent of the damage after the ‘wave of death’ had passed through.

 

Select an image to view larger

 

A close up of rock in a roadcut that curves like a U, showing the rock layers. Trees are interspersed in the rock.
Figure 4. Close up view of Sideling Hill syncline.

 

Figure 5. Diagram of different types of folds. Think of a stack of blankets on a table top that you push together so that they wrinkle up. Now imagine what would happen if you sliced the peaks of the folds so that the height was the same.

 

A rock formation with folds that zig zag in chevron shapes
Figure 6. Chevron folds with flat-lying axial planes. Millook, North Cornwal, UK.
Rshort cliff of vertically banded rocks in dark grays, surrounded by grass
Figure 7. Migmatite in an exposed roadcut, showing tight plastic folds resulting from shearing and stretching in the migmatite. Adirondack Lowlands, New York.

 

a roadcut showing tilted rocks indicating a fault
Figure 8. Copper Creek Thrust Fault. The prominent linear feature from upper left to lower right is the Copper Creek Thrust Fault in Tennessee. Thrust faults are low-angle reverse faults, formed by compressional stress. They are common in mountain belts formed by tectonic collision – this example is in the Appalachian Mountains (= result of a collision between Africa and North America during the Pennsylvanian). The folded rocks above the fault are part of the Rome Formation (Middle Cambrian). The tilted rocks below the fault at lower left are Moccasin Formation limestones (Middle Ordovician).

 

A rock formation showing layers of rock offset from each other indicating faults
Figure 9. Normal Faults. Five faults totaling 12 meters in offset. Honaker Trail Formation. In the footwall of the Moab Fault Zone.
A rock formation showing layers of rock offset from each other indicating faults
Figure 10. Reverse Fault in Eagle Ford outcrop, West of Del Rio Texas. This part of the Eagle Ford, closer to the Sierra Madre Oriental, has been subjected to Laramide-era contraction.

Module Objectives

At the completion of this module you will be able to:

  1. Describe the types of stresses that exist within the Earth’s crust.
  2. Explain how rocks respond to those stresses by brittle, elastic, or plastic deformation, or by fracturing.
  3. Summarize how rocks become folded and know the terms used to describe the features of folds.
  4. Describe the conditions under which rocks fracture.
  5. Summarize the different types of faults, including normal, reverse, thrust, and strike-slip.
  6. Understand how measurements of strike and dip of a geological feature are determined.

Activities Overview

See the Schedule of Work for dates of availability and due dates.

Be sure to read through the directions for all of this module’s activities before getting started so that you can plan your time accordingly. You are expected to work on this course throughout the week.

Read

Physical Geology by Steven Earle

  • Chapter 12 (Geological Structures)

Module 11 Assignment: Identifying Structural Features in a Geological Landscape

15 points

After you complete the reading, you can start working on Module 11 Assignment – Identifying Structural Features in a Geological Landscape

Module 11 Quiz

10 points

Module 11 Quiz has 10 multiple-choice questions and is based on the content of the Module 11 readings and Assignment 11.

The quiz is worth a total of 10 points (1 points per question). You will have only 10 minutes to complete the quiz, and you may take this quiz only once.
Note: that is not enough time to look up the answers!

Make sure that you fully understand all of the concepts presented and study for this quiz as though it were going to be proctored in a classroom, or you will likely find yourself running out of time.

Keep track of the time, and be sure to look over your full quiz results after you have submitted it for a grade.

Your Questions and Concerns…

Please contact me if you have any questions or concerns.

General course questions: If your question is of a general nature such that other students would benefit from the answer, then go to the discussions area and post it as a question thread in the “General course questions” discussion area.

Personal questions: If your question is personal, (e.g. regarding my comments to you specifically), then send me an email from within this course.

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