Lesson 3: Plate Tectonics
This week's lesson is the first of three which will introduce you to some key geologic topics—plate tectonics, earth materials and geologic hazards—that provide essential background for the discussions of specific hazards and resources we'll have later in the semester.
First up is plate tectonics, a model for how pieces of Earth's rigid exterior (map at right) interact both with one another and with the planet's interior. This model, which was only proposed about 50 years ago when I was a kid, is widely accepted today because it accounts for so many different types of geologic data. Plate interactions enable us to understand the locations continents and ocean basins, the depths and magnitudes of earthquakes, and the origins of many ore deposits. As Keller points out, the model draws on an understanding of how key parts of the Earth differ in density and strength to in order to explain why many potentially dangerous phenomena—such as earthquakes and volcanic eruptions—occur mostly along plate boundaries rather than in their interiors. In addition to outlining the basics of plate tectonics, Keller also uses chapter 2 to introduce some related concepts such as paleomagnetism, isostasy and mantle plumes. Although these topics are not strictly part of the plate tectonic model, they have provided evidence that supports some of the model's key elements (seafloor spreading, hotspot tracks, etc.) and so are important to understand in that context.
As you read the discussions of plate tectonics in our text and the accompanying online article it will be useful to take detailed notes so that you can keep track of major points and have them at your fingertips when you want to refer back to them. Our exercise this week asks you to make careful observations of how features such as seismicity, volcanism and elevation differ along and across three specific plate boundaries in order to understand the processes that are occurring along each. Finally, 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:
- Compare and contrast the compositions and physical properties of each of Earth's internal layers (core, mantle, crust, lithosphere and asthenosphere) and relate the characteristics of each of these regions to the role that it plays in plate tectonics.
- Distinguish among the different types of plate boundaries based on the patterns of topography (elevation), seafloor age, volcanism, and seismicity along them, and describe the sense of motion and potential hazards typical of each.
- Determine the velocity (speed and direction) of a lithospheric plate using map and age data for a hotspot track.
- Predict how the elevation of the Earth's surface is likely to respond to changes in the density or thickness of the lithosphere by applying the principle of isostasy.
Reading and Browsing Assignment
- Read Chapter 2 of our text and focus on how (1) the major parts of Earth's interior differ from one another; (2) phenomena such as seismic and volcanic activity enable us to locate plate boundaries and infer the processes that occur along them; (3) the "tracks" of volcanoes produced by hotspots enable us to deterimine plate velocities; (4) isostasy predicts whether the lithosphere will rise of fall if the crust is thickened or thinned; and (5) gravity "pushes" and "pulls" on plates to drive plate motion.
- Browse through the U.S. Geological Survey's online booklet This Dynamic Earth for a slightly different look at 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? (Note that our text provides a more current picture of how we interpret plate tectonic processes than the online booklet does.)
- Check out the animations of: (1) a divergent boundary in the Southern Atlantic ocean; (2) the development of magnetic stripes and the movement of transform faults along a segmented mid-ocean ridge; and (3) the formation of a volcanic arc along a continental subduction zone. These animations were created by Tanya Atwater and her students at UC Santa Barbara. Each is a .mov file, so you will need to have Quicktime installed on your computer to view them.
Exercise 3: Plate Boundaries (Due by 9:00 AM on 5-Sep-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 2. 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 sense 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 #3 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 question 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 2, 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 boundary types shown in Figure 2.4a on page 47 of our text. 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 3.
|The maps used for this assignment are from an exercise called Discovering Plate Boundaries developed 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 3: Earth's Structure and Plate Tectonics (Due by 9:00 AM on 5-Sep-2011.)
After you feel you have met the learning outcomes outlined above, please complete Quiz 3 in the "Assignments, Tests and Surveys" tool of the Etudes site. 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 move on to a look at earth materials next week.