Lesson 2: Plate Tectonics
This week's lesson and the three that follow introduce background information on plate tectonics, earth materials, and geologic processes that we'll use as a framework to interpret park landscapes throughout the semester. Before you dive into this week's reading and browsing assignment or work on the accompanying exercise you should review the learning outcomes listed below. Then, as you read about plate tectonics in our text and online it will be helpful to take notes on topics that address the outcomes so that you'll be able to fit this information into an overall framework of knowledge and recall specific ideas when they come up later during the semester. 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:
- Distinguish the major parts of the Earth based on their different compositions (e.g., crust versus mantle or mantle versus core) and different physical properties (e.g., inner core versus outer core or lithosphere versus asthenosphere).
- Differentiate among the three main types of plate boundaries based on the patterns of seismicity, volcanism, elevation, and seafloor age (if applicable) along them, and explain how these patterns are related to the relative motions of the plates along each boundary type.
- Determine the direction of a plate's motion from the ages and locations of volcanoes formed on the plate above a hot spot.
- Use the principle of isostasy to predict how the elevation of the Earth's surface is likely to change depending on whether the crust is thickened or thinned and more or less dense (due, for example, to cooling).
- Contrast parks in the western and eastern parts of the United States in terms of their elevations and reliefs (elevation ranges), and relate these differences to the tectonic settings (active versus passive) of these different parts of North America.
Reading and Browsing Assignment
- Read Chapter 1, and focus on (1) how various phenomena (e.g., seismic and volcanic activity) reflect the processes that are occurring at different types of plate boundaries; (2) how the "tracks" of volcanoes produced by hotspots enable us to measure plate motions; and (3) how isostasy predicts the lithosphere will rise of fall if the crust is thickened or thinned.
- 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 presented in this online pamphlet are the same, can you spot some differences in its interpretation and that of our text? (Although this pamphlet is a "classic" we'll see that some of its explanations may be a bit out of date.)
Exercise 2 (Due by 9:00 AM on Monday, 24-Jan-2010)
This activity is intended to help you learn about the characteristics of the 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, etc. 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 the principle of isostasy, how would you expect the elevation of an oceanic plate to differ between where it's warm (because of rising mantle rock) 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 this 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 any of the small versions 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 for each of 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 of 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 where we can see when eruptions occur.
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 record. 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 for seismicity, etc. you expect from your reading of 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. 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 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 2 (Due by 9:00 AM on Monday, 24-Jan-2010)
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 plate tectonics pretty well and are ready to move on to a more detailed look at geologic dating and crustal deformation next week.