Lesson 4: Dating and Depicting California's Geology
This is the last of our three lessons that introduce the basic geologic concepts we'll be applying throughout the rest of the semester. Our reading this week covers two chapters—3 and 4—that explore how we establish the ages of geologic features and depict their locations, sizes, and shapes on geologic maps and cross-sections. As you can see from the accompanying geologic map of California, many different types of rocks (marked by different colors) and hundreds of faults (dark lines) are exposed across the state. Even at this scale, however, you'll notice that rocks of similar compositions and ages are found together into several distinct areas. You can see that the Central Valley is underlain by young sedimentary rocks (yellow), for example, whereas the High Cascades and Modoc Plateau in the state's northeastern corner are underlain by volcanic rocks (orange). These areas with similar rock types and geologic structures are California's geomorphic provinces (p. 62) and will provide a framework for our study of the state's geology in the weeks ahead.
In chapter 3 Harden explains how geologists use field observations to determine the relative ages of rock units, and measurements of parent and daughter isotopes to determine their radiometric ages. She also describes how changes in life on Earth have enabled us to establish the geologic time scale, and introduces some of the organisms whose fossils tell us how California's enviroment has changed during the past several hundred million years. In chapter 4 she explains how geologic maps, cross-sections, and block diagrams are used to depict the ages, locations, and orientations of the rock formations that define state's geology. With basic understandings of these topics, as well as plate tectonics and earth materials, you'll be ready to start learning about California's individual geomorphic provinces next week—beginning with young volcanoes here in the High Cascades.
As you read about geologic dating and map interpretation in our text and online it will be important to take detailed notes. A lot of important concepts are covered in these chapters, and writing them out in your own words or making neatly labeled drawings will help you better understand key points and also recognize any gaps in your knowledge. Having good notes will also make it easier for you to review for this week's quiz and and access what you've learned when you need 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:
- Apply relative dating principles such as original horizontality, superposition, cross-cutting relations, and inclusion relations to order the sequence of geologic events that have affected an area shown on a geologic map or cross-section.
- Calculate how the ratio of parent to daughter isotope will change over time in a sample undergoing radioactive decay and use this ratio and the parent isotope's half-life to determine the sample's radiometric age.
- Contrast the types of geologic samples you might be able to date using radiocarbon (14C) versus potassium-argon (K-Ar) dating.
- Infer which of the major eras of Phanerozoic time (Paleozoic, Mesozoic, or Cenozoic) a sample is likely to come from based on the types of fossils it contains. (Do not try to remember the details of all of the fossils Harden describes, but be aware of when major groups such as invertebrates, mammals, and dinosaurs appeared in the fossil record (see Fig. 3-3).)
- Briefly outline what fossil assemblages tell us about how California's environment has changed from the Paleozoic to the Cenozoic Era.
- Recognize rock units, contacts, faults, and folds (anticlines and synclines) from their symbols on geologic maps or from their depictions in geologic cross-sections and block diagrams.
Reading and Browsing Assignment
- Read Chapters 3 and 4, focusing on the topics outlined in the learning objectives above.
- Apply the relative dating principles you've learned by interpreting the geologic histories of one or two of the hypothetical cross-sections on Athro's Interpreting Geologic Sections site. (These are fun; just choose the events in the correct sequence from the drop down menus and then run the animation!)
- To learn a little more about how geologists use fossils to date geologic events browse through the U.S. Geological Survey's Fossils, Rocks, and Time site. Especially note how different groups of plants and animals appeared at different times in Earth's history on the page entitled "Fossil Succession".
- To learn the basics of reading geologic maps, browse through the joint U.S. Geological Survey/National Park Service website on geologic maps. It gives a clear overview of the meanings of the symbols commonly found on these maps; maps that we'll be using—in conjunction with geologic cross-sections—to depict California's geology throughout the semester. Just two notes about this presentation. First, the base maps on which most geologic maps are drawn show the shape of the land surface using the topographic contours you learned about in last week's map reading exercise. Be careful not to confuse contours with geologic contacts or faults when both are shown on the same map. Second, the map symbols for fold axes are different for anticlines and synclines. Paired arrows on either side of a fold axis line point inward for a syncline because beds dip in towards the center of such a fold (see location 6 on the website). Similarly, the paired arrows point outward for an anticline because the beds dip away from its center.
- Finally, to look in detail at the geology of any part of California check out this interactive, online version of the California geologic map. You can pan around, zoom in or out, and see exactly which geologic units crop out where you live. Be careful, though. I've put this last because it can be addictive!
Exercise 4: Rb-Sr Isochron Dating (Due by 9:00 AM on 7-Feb-2011)
In Chapter 3 you learned a bit about how the decay of radioactive isotopes in minerals or organic materials can be used to determine absolute (numerical) ages of geologic events (p. 44-45). This week's exercise will help you learn more about how radiometric dating works. This is a very important technique because it enables us to reconstruct California's geologic history and estimate the rates of geologic processes operating here.
- Point your browser to the Virtual Courseware exercise on Rb-Sr dating. This program uses Java applets, so be patient as you load the intractive graphics if you are using a dial-up connection. 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 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 don't worry too much about the details of the isochron method. (Hint: On page 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, choose Save Image, and save it to your computer in a standard graphics format (.jpg, .gif, etc.) In order to be sure that you actually captured the image of the certificate correctly, you may want view it in program like Windows Picture and Fax Viewer before you close the web page you copied it from.
- Finally, when you have the image of the certificate safely stored on your computer, log on to the Etudes site, click on the "Assignments, Tests, and Surveys" tool, and then on the link tor Exercise 4. Attach your certificate by clicking the "attach file" radio button about halfway down the page and then selecting the name of your image file. (Please do not try to "paste"your certificate into the text box; this commonly produces unpredictable results on my end.) Finally, press the submit button near the bottom of the page. This exercise is worth 10 points, and everyone who completes it and submits the certificate will earn full credit.
Quiz 4: Geologic Dating, Maps, and Cross-sections (Due by 9:00 AM on 7-Feb-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 about geologic dating, maps, and cross-sections, each worth one point. If you can answer all of them correctly it means that you know your way around the basics of the techniques we use to determine the ages and locations of rock units in California pretty well and are ready to start learning about volcanism in California next week. Like all of our weekly quizzes, this one is timed (you'll have 30 minutes) and must be completed in one "sitting". (That is, you will only be granted access once.) So, be sure you're ready to complete your quiz when you start it—and be sure you're using Firefox. Good luck.