Lesson 4: Earth Materials
This is the second of three lessons that introduce key geologic topics and provide essential background for the discussions of specific hazards and resources we'll have later in the semester. This week we'll learn about earth materials, the rocks and minerals (like quartz, at right) that are the "buiding blocks" of our planet. In addition to providing many of the critical raw materials on which humanity depends, earth materials also provide important insights into specific geologic processes and hazards. Whether a volcano is likely to erupt effusively or explosively; a town is susceptible to damage from sinkholes; or a roadway can be built safely on a hillside underlain by metamorphic rocksall of these questions are ones that geologists attempt to answer by studying earth materials.
Developing a working understanding of rocks and minerals in only a week is a tall order, and it's made more difficult in an online class because reading about these materials and looking at pictures of them are not substitutes for actually being able to examine them yourself. Earl Bennett, my boss at the Idaho Geological Survey, used to say, "The best geologist is the one who has seen the most rocks," and there's a lot of truth to that. So, if you live in or near Weed and would like to see some of the common rocks and minerals described in this week's lesson just send me a private message. I'll put together a suite of samples and we'll arrange a time when you can come in and study them. If you live elsewhere you might want to contact a geology instructor at a nearby college or science teacher at a nearby school and ask if you could come in and take a look at the samples he or she might have.
In chapter 3 Keller briefly describes what minerals are and how their properties depend both on their elemental compositions and atomic structures. He goes on to explain that rocks are aggregates of one or more minerals and describes what the characteristics of rocks in each family—igneous, sedimentary and metamorphic—tell us about the conditions under which they formed. For example, coarse-grained igneous rocks crystallized from magmas that cooled slowly deep underground, whereas fine-grained and glassy igneous rocks solidified rapidly from magmas that were erupted onto Earth's surface. Finally, Keller outlines three "rock laws" that enable us to establish the relative ages of adjoining rock units, and describes some common structures—folds, faults and unconformities—that develop where rocks are either deformed within the Earth or deposited at very different times.
As you read through the introduction to earth materials in our text and work through the rock and mineral identification and relative dating exercises online it will be helpful to take careful notes. A lot of information that bears on this week's learning objectives is presented in the chapter, and writing out key facts in your own words or making neatly labeled drawings will help you better understand the significance of what you've read and spot any gaps in your knowledge. Having complete notes will also make it easier for you to review for this week's quiz and and access what you've learned when you want to refer back to it for future assignments. Be sure that you are prepared to meet the learning objectives outlined below before you move on to the quiz at the bottom of the page.
Weekly Learning Objectives
Upon successful completion of this week's lesson, a student is expected to be able to:
- Identify common, geologically significant minerals and rocks by observing their properties and comparing your observations to information in a table (e.g., Table 3.3, p. 95) or key (e.g., Appendix A, Table A3).
- Estimate the general composition of a crystalline igneous rock (mafic = rich in dark ferromagnesian minrals or felsic = rich in light colored quartz and feldspars; see Fig. 3.15, p. 88) from its color, and use its grain size to infer whether it solidified in an intrusive or extrusive environment.
- Determine whether a sedimentary rock is of detrital or chemical origin based on its texture, and whether it is likely to pose a hazard due to weakness (e.g., shale) or solubility (e.g., limestone or rock salt.)
- Determine whether a metamorphic rock is foliated or nonfoliated and explain what this texture implies about state of stress in the crust during its formation.
- Infer the sequence of geologic events that have taken place in a region by applying the three rock laws (relative dating principles) and an understanding of unconformities.
Reading and Browsing Assignment
- Read Chapter 3 and skim through Appendix A. Don't worry about learning the details of folding and faulting for now; we will discuss these structures, especially faults, in a couple weeks when we study earthquakes.
- In order to get a sense of how physical properties (color, luster, etc.) can be used to identify common minerals go to Mineral Physical Properties and Identification on Richard Harwood's Physical Geology course website. Using the information Harwood provides and the descriptions of common rock-forming minerals given in Appendix A of our text, identify minerals 6, 12, and 19 in the exercise. Note that clicking a gray button lets you perform a test or make an observation. When you press a number for the hardness test you will be told if your sample can be scratched by a reference mineral of that hardness; the hardness of your sample will be one less than that of the softest reference mineral that scratches it. Record your observations by clicking the little round "radio buttons" in the table. Be sure everything is answered before you ask the program to check your identification.
- Based on your reading of our text be sure that you understand (1) what makes rocks different from minerals; (2) how each of the three major families of rocks is formed; and (3) what the characteristics of common rocks tell us about their sources (e.g., light-colored felsic igneous rocks like rhyolite and granite crystallize from magmas formed by melting of the crust whereas dark-colored mafic igneous rocks like basalt and gabbro crystallize from magmas formed by melting of the mantle) and the conditions under which they formed (e.g., how grain size distinguishes intrusive from extrusive igneous rocks, or the alignment of flaky minerals distinguishes foliated metamorphic rocks that formed under directed pressure from non-foliated rocks that may not have.) In order to try your hand at rock identification, go to Igneous Rock Identification, Sedimentary Rock Identification, or Metamorphic Rock Identification on Richard Harwood's Physical Geology class website. Identify igneous rocks 1 and 6, sedimentary rocks 4 and 10, and metamorphic rocks 2 and 6 based on the information given in our text and on Harwood's site. Note that Harwood uses the term phaneritic to mean coarse-grained and aphanitic to mean fine-grained when referring to igneous rock textures.
- Finally, visit the site on interpreting geologic cross-sections and apply the relative dating principles ("rock laws" and unconformity information) discussed on pages 88 and 103-106 and practice what you've studied by "working out" the sequences of events that have taken place in one or two of the sample cross-sections.
Exercise 4: Rocks, Minerals and Relative Dating (Due by 9:00 AM on 12-Sep-2011)
Exercise 4 will give you an opportunity to demonstrate what you've learned about the identification of rocks and minerals and the sequencing of geologic events using the "rock laws". When you have completed the "Reading and Browsing Assignment" above, point your browser to the class' Etudes site, go to the "Assignments, Tests, and Surveys" tool, and complete Exercise 4.
Quiz 4: Earth Materials (Due by 9:00 AM on 12-Sep-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 4 in the Etudes "Assignments, Tests, and Surveys" tool. There are ten questions, each worth one point. If you can answer all of them correctly it means that you know your way around the basics of Earth materials and relative dating pretty well and are ready to move on to a more detailed look at natural hazards next week.