7.1: Introduction

Module 7

Weathering, Erosion, and Sedimentary Rocks

Figure 1. View of Monument Valley from Hunts Mesa.

Introduction

This module includes two chapters that are very closely related. Weathering, the subject of Chapter 5, is the process that breaks down existing Earth materials and leads to the creation of soil. Most materials can be broken down by weathering, but some materials are more susceptible than others. For example, igneous rocks are fairly resistant to weathering, but over long periods of time they still erode away. In addition, there are a number of different processes that cause weathering, including mechanical and chemical processes. Different processes affect different materials at different rates, which leads to the formation of different types of sedimentary rocks, as you will discover as you read through Chapter 6.

We are particularly interested in the history and events that occur on the surface of the Earth both because it is easier to directly observe and test, and it has direct relevance to our lives and our own history. Sedimentary rocks are the pages in which Earth’s history is written, since they contain powerful environmental indicators, traces of life, and chemical signatures that can inform us about a wealth of subjects from the occurrence of ancient catastrophes to the productivity of life.

The identification of sedimentary rocks is more than applying names, since each name is a loaded term that conveys information regarding its history, where it was formed, potentially when it was formed, and the processes that lead to its formation. Each sedimentary rock is a puzzle and by identifying a set of rocks, how they are layered, the fossils within, and patterns in the rocks a geologist can reconstruct an entire environment and ecosystem. Solving these puzzles is both an academic exercise to better understand the world around us as well as a tool for finding the resources that are important to our lives. In particular, fossil fuels as well as many other natural resources are made of, or are contained within, sedimentary rocks such as coal, natural gas, petroleum, salt, and cement, and more. Therefore, a better understanding of sedimentary rocks and how and where they are formed directly influences your everyday life.

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Figure 2. This is an example of sheet foliation, or exfoliation, of a granite dome, which is caused by the release of pressure within the granite as overlying materials were eroded away over time. This photo was taken in Enchanted Rock State Natural Area, Texas.

Figure 3. This photo shows a example of how freezing and thawing have weathered this rock in Iceland.

Figure 4. Sedimentary rock layers at Zion National Park. Most of the rocks in Zion National Park are sedimentary rocks –made of bits and pieces of older rocks that have been weathered, eroded, and deposited in layers. These rock layers hold stories of ancient environments and inhabitants very different from those found in Zion today. In this distant past, Zion and the Colorado Plateau were near sea level, and were even in a different place on the globe—close to the equator. The rock layers found in Zion today were deposited between approximately 110 –270 million years ago –only in recent geologic time have they been uplifted and eroded to form the scenery of Zion National Park. (National Park Service)

Figure 5. A stratigraphic column showing the history of the layers of rock in Zion National Park, illustrated by Geoscientist-in-the-Park David Tarailo.

Figure 6. Garden of the Gods, Colorado – Ancient sedimentary beds of deep-red, pink and white sandstones, conglomerates and limestone that were deposited horizontally, but have now been tilted vertically and faulted into fins by the immense mountain building forces caused by the uplift of the Rocky Mountains and the Pikes Peak massif.

Figure 7. The red sandstone formations of Cathedral Rock, Sedona, AZ.

Figure 8. ‘The Wave’ Formation of Vermillion Cliffs, at the border of Utah and Arizona, formed by erosion, water and wind.

Figure 9. Bryce Canyon, Utah. Bryce Canyon is not a ‘real’ canyon. It is not carved by flowing water. Water is the active ingredient here, but in the form of ‘frost-wedging’ and chemical weathering. For 200 days a year the temperature goes above and below freezing every day. During the day, melt water seeps into fractures only to freeze at night, expanding by 9%. Now as ice, it exerts a tremendous force (2,000-20,000 pounds per square inch). Over time this ‘frost-wedging’ shatters and pries rock apart. In addition, rain water, which is naturally acidic, slowly dissolves the limestone, rounding off edges and washing away debris. (National Park Service)

Module Objectives

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

1. Explain why rocks formed at depth in the crust are susceptible to weathering at the surface.
2. Describe the main processes of mechanical weathering and its products.
3. Describe the main processes of chemical weathering and its products.
4. Explain the type of weathering processes that are likely to have taken place to produce a particular sediment deposit.
5. Discuss the relationships between weathering and soil formation, the origins of soil horizons, and some of the different types of soil.
6. Explain the geological carbon cycle, and how variations in rates of weathering can lead to climate change.
7. Describe the differences between cobbles, pebbles, sand, silt, and clay and explain how clast size relates to the mode of transport.
8. Describe the various types of clastic and chemical sedimentary rocks, and apply your understanding of their characteristics to infer past depositional environments and climates.
9. Differentiate between terrestrial and marine depositional environments, and explain how the formation of sedimentary basins can be related to plate tectonic processes.
10. Explain the importance of and differences between groups, formations, and members

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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 5 (Weathering)
• Chapter 6 (Sediments and Sedimentary Rocks)

Module 7 Assignment: Sedimentary Rock Lab

10 points

After you complete the reading, you can start working on Module 7 Assignment – Sedimentary Rock Lab.

Module 7 Quiz

10 points

Module 7 Quiz has 10 multiple-choice questions and is based on the content of the Module 7 readings and Assignment 7.

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.