Lesson 2: California's Plate Tectonic Setting
This week's lesson and the two that follow introduce background information on plate tectonics, earth materials, and geologic processes. You'll probably find yourself referring back to these chapters throughout the semester as you apply the "big picture" concepts they present to your studies of California's individual geologic provinces.
In chapter 1, Harden introduces plate tectonics. This model, which became widely-accepted in the 1960s, provides an explanation for how interactions between slabs of Earth's outer rigid shell—the lithosphere—and its hotter interior influence the locations and characters of many of Earth's major geologic features. The chapter also briefly describes the modern plate boundaries found in California, the methods used to determine the directions and rates at which plates move, and the types of faults that develop in response to different stresses in the lithosphere. This week's exercise will give you an opportunity to study several plate boundaries around the world and learn how their characteristics are related to the relative motions of the plates they separate.
As you read about plate tectonics and lithospheric deformation (faulting and folding) in our text and online it will be helpful to take detailed notes. Writing concepts out in your own words or making neatly labeled drawings will help you better understand key points as well as 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:
- Contrast the thicknesses, compositions, and physical properties (e.g. rigidities and densities) of the materials that comprise each of Earth's internal layers (core, mantle, crust, lithosphere, and asthenosphere) and relate these properties to the role each layer plays in plate tectonics.
- Distinguish the three types of plate boundaries—divergent, convergent and transform (or shear)—based on the patterns of topography (elevation), seafloor age, volcanism, and seismicity that occur along each, and describe how plates on opposite sides of each type of boundary move relative to one another.
- Locate the following plate boundaries on a map of California: Gorda-Juan de Fuca ridge, Cascadia subduction zone, Gulf of California rift zone, and the San Andreas fault; and indicate which type of boundary (convergent, divergent, or shear) each is.
- Determine the speed and direction a lithospheric plate is moving using map and age data for volcanoes or volcanic islands along a hotspot track.
- Recognize the common types of faults (normal, reverse, left-lateral, and right-lateral) based on how each offsets features shown in a photograph, map or cross-section, and indicate which kind of stress (extensional, compressional, or shear) is typically associated with each type of fault. (For now, don't worry about whether the crust at a jog between two right-lateral or left lateral faults is being extended (transtension) and or compressed (transpression) as shown in Fig. 1-17; we'll discuss these situations a little in the semester.)
Reading and Browsing Assignment
- Read Chapter 1 carefully and pay special attention to the topics covered by the learning objectives outlined above. One strategy for studying each chapter of our text is to print the learning objectives on a piece of paper and leave plenty of space between them. Then, as you read the chapter, use text, a table, or a labeled drawing to address the issue posed by each of the objective. The benefit of taking notes this way is two fold. First, if there are ideas or topics you don't understand you can jot down questions about them in the margins of your page and then post your questions to the discussion board for your classmates or me to weigh in on. Second, when it's time to review for your weekly quiz it will be easier to review your notes than to re-read the entire chapter.
- Browse through the U.S. Geological Survey's online booklet This Dynamic Earth for a slightly different take on plate tectonics. Although the fundamental observations are the same, can you spot some differences in interpretation between this booklet and our text on issues such as plate driving forces (mantle convection versus gravitational sinking of dense plates)?
Exercise 2: Characteristics of Plate Boundaries (Due by 9:00 AM on 24-Jan-2011)
This activity is intended to help you learn about the characteristics of the different types of plate boundaries you've been reading about in Chapter 1. Before you begin, be sure that you have read through the section on the natures of the different types of plate boundaries in your text and thought about the question, "In light of the different processes that occur along divergent, convergent, and transform boundaries, how would I expect patterns of volcanism, seismicity (earthquake activity), and so on to differ along these different types of boundaries?" For example, pulling apart thin lithosphere might be expected to produce shallow earthquakes along a divergent boundary, but what about pulling dense ocean lithosphere down into the Earth along a subduction zone (convergent boundary)? Or, based on its density, how would you expect the elevation of an oceanic plate to differ between where it's warm (because of rising asthenosphere beneath it) versus cold (because of a long history of cooling at the surface)? Once you feel that you have a good understanding of how different types of plate boundaries differ in terms of their basic geologic characteristics (and, ideally, some good notes), start this exercise by following the steps below:
- Open a large version of the first map ("Plate Boundaries") by clicking on the small version below (this works for any of the maps). On the large version of the first map identify the locations and numbers of the three plate boundaries that I would like you to study (#1, 2, and 3; the boundaries are shown in black and are surrounded by colored circles). There is also a sample boundary marked, and observations for it are recorded in the form below. You may want to print the first large map and keep it handy for reference when you look at each of the other maps because the boundaries aren't shown directly on those maps.
- Next, carefully study the four remaining large maps. Remember, clicking on the small version of any of them will bring up the corresponding large version. The first two show surface elevation and seafloor age, and the second two show earthquake and volcano locations. Take a moment to review the legends on the seafloor age and earthquake maps to find out what the different colors mean. Gray and white areas on all of the maps (except the surface elevation map) simply mean "no data available". As you work through this exercise you are going to be observing data along and adjacent to each of the three boundaries (#1, 2, and 3) marked on the first map on each of other four maps (surface elevation, seafloor age, earthquakes, volcanoes).
- To guide your observations, answer the following questions for boundaries #1, 2, and 3.
On the surface elevation and seafloor age maps note if the elevations and seafloor ages along the boundaries are:
- uniform* or variable* along the length of each boundary;
- if they are uniform, are elevations along the boundary higher or lower, and are the seafloor ages younger or older, than those of the crust on either side;
- are elevations or seafloor ages symmetrical* (about the same on both sides) or asymmetrical* (different on opposite sides) across each boundary.
* To see examples of uniform, variable, symmetrical, or asymmetrical properties along a boundary check out the accompanying images. In each panel the location of the boundary is marked by a black line.
On the earthquake and volcano maps note if earthquake epicenters and volcanoes along or near the boundaries are:
- closely or widely spaced along the boundary;
- located right on the boundary or off to one side of it;
- and, if earthquake epicenters are located off to one side, do their depths differ with distance from the boundary (e.g., are their depths greater or less farther from the boundary).
Note: On the volcano map you may assume that the density of volcanoes along boundary #2 is similar to that seen in Iceland. Much of this boundary is underwater which makes eruptions difficult to detect, but Iceland is an exposed part of the boundary and so we can see the eruptions that occur there.
When you have finished studying the maps, print this form and use it record your observations. It includes a set of observations for the sample boundary so that you can see what level to detail I expect you to be looking for. Also, note that a "negative" answer ("There are no volcanoes along this boundary.") is perfectly valid if that's what you observe. If you have questions about working with the maps, making observations along specific boundaries, or predicting what pattern you might expect data to have along a given type of boundary please post a query to the discussion board in Etudes. One or more of your classmates may have insights to share, and I will weigh in from time to time to keep everyone moving forward.
After you have completed your observations compare them to the patterns of seismicity, volcanism and so on that you expect from reading chapter 1, and use this comparison to decide which type of boundary (divergent, convergent, or transform) each of the three you studied is most likely to be. To check your predictions, refer to the symbols shown on the accompanying plate boundary map. Finally, go to the Etudes "Assignments, Tests, and Surveys" tool and use the observations and boundary identifications you've made to answer the questions in Exercise 2.
|The maps used for this assignment are from an exercise called Discovering Plate Boundaries created by Dale Sawyer at Rice University. Elevations on the surface elevation map are in meters, and colors on the earthquake map indicate the depths—not the magnitudes—of the quakes.|
Quiz 2: Plate Tectonics (Due by 9:00 AM on 24-Jan-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 2 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's structure and plate tectonics pretty well and are ready to start learning about the types of rocks and minerals found 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.