Plate Tectonics - Introductory College Level

View Window Image	Student tasks & Questions
	If starting from scratch: Open ArcView and add the "Country.shp" and "World30.shp" themes (in the ESRIDATA folder) to make them visable in your view.  Make the "Country.shp" theme active and double-click the theme to bring up the "Legend Editor" dialog box.  Change the "legend type" to "single symbol" from "unique value" using the pull-down menu.  Assign a pastel color to the "Country.shp" theme; click "Apply", and then click "OK" to complete your changes.  Your image should look similar to the image to the left.  If you're starting from the project level, all you need to do is start ArcView and click on the "Country.shp" and the "World30.shp" themes to make them visible.
	Click the "shallow_quakes.dbf" theme to display the epicenters of shallow earthquakes that took place between 1/1/1997-7/12/1999.  Notice that these quakes are scattered about the map, but that they tend to define relatively linear paths.  Some of those paths are narrow compared to others, which appear more diffuse on this image.
	Click the "intermediate_quakes.dbf" and the "deep_quakes.dbf" themes to display the epicenters of intermediate- and deep-focus quakes.  What is the relationship of the locus of these quakes to the width of the earthquakes zones you studied previously?
	Click the "volcanoes_all.dbf" theme to superimpose the locations of active and historically active volcanoes on the epicenter data.  What do you notice about the distribution of volcanoes relative to earthquakes?  What do you notice about the distribution of volcanoes relative to the locations of deep-focus earthquakes? How does this compare to areas that experience only shallow-focus earthquakes?
	The distribution of earthquake epicenter data, coupled with the locations of volcanic activity, helped geologists understand the locations of plate boundaries across the Earth.  Where rocks of the Earth's crust move relative to each other, faults and earthquakes form.  Where large pieces of the crust move relative to one another, plate boundaries exist.  Plate boundaries are thus delimited on maps showing earthquake and volcanic activity.  From the "View" menu, choose "New Theme" to create a place to draw in plate boundaries.  A "feature type" dialog box appears; choose "line" from the pull-down menu and click "ok."  Navigate to your home folder and save the new theme (give it an appropriate name first).  Now you are ready to draw plate boundaries.
	Choose the line tool from the tool pull-down button if it is not already highlighted.  Click on the view to begin your line; click once in each place you'd like your plate boundary to turn, and then double click to end your line.  Continue drawing plate boundaries until you've outlined the major plates of the Earth.  Notice that some plates contain only ocean areas, while other plates contain both continental and oceanic areas.  How many plates cover the Earth's surface?
continue with previous image	Now that you've identified the location of plate boundaries, it's time to think about the different types of boundaries that exist.  It's possible to recognize boundaries where one plate (always oceanic) is sinking (subducting) beneath another plate (either oceanic or continental) by studying the earthquake data.  Remember that shallow-focus earthquakes are taking place nearer the Earth's surface, while deep-focus quakes take place well beneath the surface of the Earth.  Study the image and you'll begin to easily pick out the subducting plate boundaries.
	Click the "Select Feature" button from the toolbar, hold down the shift key and begin to select the subduction zone boundaries.  These will be highlighted in yellow in your view (see left; note that this view is only representative and not complete).  After you've selected all your subduction boundaries, you'll edit the attribute table so that you can display these boundaries in a different color on your map.  With all the subduction zones selected, click the "Open Theme Table" button on the tool bar.  Click the "Promote" button and all your selected rows move to the top of your attribute table (selected rows are displayed in yellow).
	Now you're ready to edit the table to name your subduction zone boundaries.  Choose "Start Editing" from the "Table" menu, then choose "Add Field" from the "Edit" menu.  The "Field Definition" dialog box pops up.  Type "Bound_type" in the "Name" box, and choose "string" in the "Type" pull-down menu.  Click "OK" and a new field appears in your table.  You should notice that there several records appear in yellow (selected); these are your subduction boundaries.  Click the "Promote" button to move these records to the top of the table.  Click the "edit" button to get the editing tool, and click in the first cell of the new "Bound_type" field.  Type in "subduction" and press the Enter key to accept your entry.  Type "subduction" in each of your yellow records. Type "other" in the "Bound_type" field for all records that were not selected.  When you've finished, choose "Stop editing" from the "table" menu, save your edits and close the table.  Go back to the View window and double-click on your plate boundary theme to bring up the legend editor window.  Change the "legend type" from "single symbol" to "unique value" via the pull-down menu.  From the "values Field" pull-down, choose "Bound_type", and you'll see two categories for plate boundaries.  Edit the symbols as you like and click "apply" to change the view.  Close the "legend editor" window, and save your project.  CONGRATULATIONS!! You've just outlined plate boundaries of the world!

page created by C. Summa, 7/15/99