3.1: Introduction

Module 3

Plate Tectonics

Figure 1. Mt. Denali, Alaska. Because of active plate tectonics, earthquakes are frequent in the Denali area. It is estimated that there are some 600 seismic events per year. This seismic activity at the root of Denali suggests that uplift of the mountain continues to this day. Numerous faults, including the Denali fault (a major fault system), demonstrate a long history of active plate tectonics and associated earthquake activity. (National Park Service)

Introduction

In this module, you will be introduced to a fundamental theory in geology: Plate Tectonics. You will learn about the important hypotheses that eventually led to the development and acceptance of the plate tectonics theory over the last century, including Alfred Wegener’s hypothesis of continental drift and Harry Hess’ hypothesis of sea floor spreading. You will also learn about the key drivers of plate movement and the tectonic processes at work at plate margins. We will revisit this topic many times during the semester.

A theory is a well-supported explanation for a natural phenomenon that still cannot be completely proven. A Grand Unifying Theory is a set of ideas that is central and essential to a field of study such as the theory of gravity in physics or the theory of evolution in biology. The Grand Unifying Theory of geology is the theory of Plate Tectonics, which defines the outer portion of the earth as a brittle outer layer that is broken into moving pieces called tectonic plates. This theory is supported by many lines of evidence, including the shape of the continents, the distribution of fossils and rocks, the distribution of environmental indicators, as well as the location of mountains, volcanoes, trenches, and earthquakes. The movement of plates can be observed on human timescales and easily measured using GPS satellites.

Plate tectonics is integral to the study of geology because it aids in reconstructing earth’s history. This theory helps to explain how the first continents were built, how oceans formed, and it even helps inform hypotheses for the origin of life. The theory also helps explain the geographic distribution of geologic features such as mountains, volcanoes, rift valleys, and trenches. Finally, it helps us assess the potential risks of geologic catastrophes such as earthquakes and volcanoes across the earth. The power of this theory lies in its ability to create testable hypotheses regarding Earth’s history, as well as predictions regarding its future.

Select an image to view larger

Figure 2. Satellite view of the Himalayas, on the border between southeastern China and northern India. This majestic mountain range formed at a continent-continent convergent boundary between two tectonic plates.

Figure 3. A GPS station in Denali, near Tokosha. The Plate Boundary Observatory (PBO) has installed more than 1,100 Global Positioning System stations in North America, including two in Denali—near Tokosha (above) and on Wickersham Dome.

Figure 4. Capital Reef, Utah. Long after the sedimentary rocks were deposited, the entire region was uplifted thousands of vertical feet, due to large-scale plate tectonic forces. Most of the Colorado Plateau was uplifted relatively evenly, keeping the layers roughly horizontal, creating the ‘layer cake’ appearance common throughout the region (such as at the Grand Canyon). Capitol Reef is a giant exception to this pattern–due to the Waterpocket Fold. The Waterpocket Fold defines Capitol Reef National Park. A nearly 100-mile long warp in the Earth’s crust, the Waterpocket Fold is a classic monocline, a ‘step-up’ in the rock layers. It formed between 50 and 70 million years ago when a major mountain building event in western North America, the Laramide Orogeny, reactivated an ancient buried fault in this region. Movement along the fault caused the west side to shift upwards relative to the east side. The overlying sedimentary layers were draped above the fault and formed a monocline. The rock layers on the west side of the fold have been lifted more than 7,000 feet (2,134 m) higher than the layers on the east.

Figure 5. Dante’s View in Death Valley National Park, California. Time passed and the sea began to slowly recede to the west as land was pushed up. This uplift was due to movement occurring far beneath the Earth’s surface. Scientists have discovered that the Earth’s crust is composed of inter-connected sections, or plates. Death Valley lies near the boundary between two of these plates. As the plates slowly move in relation to each other, compressional forces gradually fold, warp and fracture the brittle crust. This widespread rock deformation and faulting occurred through most of the Mesozoic Era (251 – 65.5 million years ago.) While the Rocky Mountains and the Sierra Nevada formed, active mountain building alternated with times when erosion prevailed, working to breaking down the mountains that had formed. (National Park Service)

Module Objectives

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

1. Describe the development of the theory of plate tectonics, including the concepts of seafloor spreading and continental drift.
2. Name the three types of plate boundaries, describe the type of motion, and describe the features associated with each.
3. Explain the distribution and origin of most volcanoes, earthquakes, young mountain belts and major seafloor features.

---

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 10 (Plate Tectonics)

Discussion 2 – Plate Tectonics

30 points

This week we will be having a conversation about plate tectonics and the ways in which the hazards associated with movement of the plates might personally affect you or your family. Now that you have completed Module 3 (Plate Tectonics), you should have some basic knowledge about the subject and can make educated post(s).

Discussion 2 Instructions

Pay close attention to the Course Schedule for when each of your posts are due. Some are due earlier than others. Failure to post on time will result in lost points.

Module 3 Assignment: Using Google Earth to Visualize Plate Boundaries

10 points

After you complete the reading, you can start working on Module 3Assignment: Using Google Earth to Visualize Plate Boundaries

Module 3 Quiz

10 points

Module 3 Quiz has 10 multiple-choice questions and is based on the content of the Module 3 readings and Assignment 3.

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.