Lesson 4: Rocks and Minerals
This is the second of three lessons that introduce background information that we'll use as a framework for understanding a wide range of topics throughout the semester. Rocks and minerals are the "building blocks" of our planet and provide crucial information about the processes that shape it. Whether a volcano at Yellowstone is likely to erupt quietly or explosively, whether sedimentary strata in Zion preserve an ancient seabed or sand dune field, and whether metamorphic rocks in the Great Smoky Mountains preserve a record crustal extension or compressionall of these are questions that geologists attempt to answer by studying earth materials.
As you read through the introduction to earth materials in our text and work through the radiometric dating exercise online it will be helpful to take notes so that you can keep track of major points and have the information readily available when these topics arise in discussions later in the semester. Be sure that you are prepared to meet the learning objectives outlined below before you take Quiz 4 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:
- Distinguish between a mineral and a rock and explain his or her rationale for classifying an earth material as one or the other.
- Determine whether the magma that formed a specific igneous rock likely solidified in an intrusive or extrusive environment based on the rocks's grain size, and whether the magma is of mafic (mantle-derived) or felsic (crust-derived) composition based on the rock's color.
- Determine whether a sedimentary rock is of clastic or biological/chemical origin based on its texture and explain what this difference tells us about the conditions under which it formed.
- Determine whether a metamorphic rock is foliated or nonfoliated and explain what this texture indicates about state of stress in the crust during rock's formation.
- Identify the type of parent rock from which a specific metamorphic rock, such as schist or quartzite, is typically formed.
Reading and Browsing Assignment
- Read the section on Earth Materials in Chapter 2 (p. 30-34) and the introduction to rocks and minerals given on the joint U.S. Geological Survey/National Park Service site. Focus on how (1) how minerals differ from rocks and how you might distinguish them; (2) how rocks from each of the three major families (igneous, sedimentary, and metamorphic) are formed and how you might recognize rocks from each family; and (3) what the characteristics of common rocks tell us about the conditions under which they formed (e.g., how grain size distinguishes intrusive from extrusive igneous rocks, or how the alignment of flaky minerals distinguishes foliated metamorphic rocks formed under directed pressure from non-foliated metamorphic rocks formed by localized heating adjacent to magma bodies.)
- Browse Lynn Fichter's sites on igneous rocks, sedimentary rocks, and metamorphic rocks if you want to get more detail about anything the author of our text presents. Although there is far more information on these sites that we need right now, they may serve as useful references later during the semester.
Exercise 4 (Due by 9:00 AM on Monday, 8-Feb-2010)
In last week's lesson you learned a bit about how the decay of radioactive elements in minerals can be used to determine the absolute ages of geologic events in Earth's history (p. 24-25). This week's exercise will give you a chance to explore (in a virtual setting) how one radiometric dating technique actually works.
Point your browser to the Virtual Courseware exercise on Rb-Sr dating. This program uses Java applets, so if you are using a dial-up connection be patient as the intractive graphics load. You don't want to crash the program by trying to use the applets before they are fully loaded. Also, allow enough time to do this entire exercise in one session (about 1 hour) because, unfortunately, this program will not allow you to save your work and come back later. Work through the exercise by carefully reading each part of the text and then answering the accompanying questions. Don't worry if you get an answer wrong at first; the program will tell you what's wrong and give you a chance to go back, think about it and fix it. You should take notes on key points as you work through this exercise so that you will be able to remember what you did later. Focus on understanding the basics of the decay process and the isochron method, but don't worry about keeping track of the dates of individual samples and such. (Hint: On screen 14, remember that one billion years equals 1,000 million years.)
When you get to the end of the exercise, fill in your name and school and press enter to see your "Virtual Geochronologist" certificate. Right-click on the blue part of the certificate and save the certificate on your computer. Be sure to remember where you put it! Log on to the Etudes site, click on the "Assignments, Tasks, and Tests" tool, and open Exercise 4. There is only a single question in this exercise, so just type a brief note in the answer box and then turn in your certificate by clicking the Browse button near the bottom of the page, selecting the name of the file you saved your certificate in, and then clicking the Upload button. Then click finish on the question, and Etudes will store a copy of your certificate in a place where I can review it. After I've done so I'll enter a score of 10 for you in the gradebook. (Don't worry about the 0 score that Etudes initially gives you.) If you have any questions or problems just let me know.
Quiz 4 (Due by 9:00 AM on Monday, 8-Feb-2010)
After you feel you have met the learning outcomes outlined above, please complete Quiz 4 in the Etudes "Assignments, Tasks, and Tests" 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 earth materials pretty well and are ready to move on to a more detailed look at volcanism and seismicity next week.