Part 2: Explore Data with a GIS
In any step, click the Show me link to reveal extra information. If you prefer a printout of the full set of instructions for this part, choose Print from the File menu.
Step 1-Use the Identify Tool to Explore Plate and Country Names
- Confirm that the plat_pol.shp theme is active. Open this theme's Table by selecting Theme > Open Table or clicking the Open Theme Table
button. In addition to appearing as a map layer in the map view, every theme in a GIS also has an associated Table in which you can examine the data in a tabular format.
- How many records (plates) are in the table?
Theme > Open Table
Open Theme Table
There are 14 major tectonic plates covering the Earth's surface.
Close the table when finished.
- Select the Identify
tool and click on some of the plates in the map view. The Identify Results dialog box displays the information contained in the table for each plate you click. Close the Identify Results window when finished. - While holding down the shift key, click the Country.shp theme name to make it active. The shift key allows you to make two or more themes active at the same time. Use the Identify
tool and click on some countries. The Identify Results dialog box displays the information contained in the table for both the country and the plate.
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Step 2-Use the Label Tool to Place Plate and Country Names on Map
- Make only the Plat_pol.shp theme active.
- Select the Label
tool and click on the following plates to label them: African, Indo-Australian, Eurasian, Pacific, and the South American. Label more if you want to. Use the Pointer 
tool to select, move, resize or delete any labels. The labels are attached to the theme: turning any theme off will make its label disappear. - Make the Country.shp theme active. Use the zoom in
tool to zoom in to India and label that country. Select the Zoom to Full Extent 
button to zoom back out to the full world view. - When using the zoom in tool it is most effective to drag-and-draw a box around the area of interest.
- With the zoom in tool selected, left-click one corner of your area of interest and drag the box to the opposite corner while holding down the left mouse button.
- Release the button and the view will zoom to the area.
- Select the Zoom to Full Extent
button to zoom back to the full world view.
- Zoom in on the area around Japan and label that country.
Step 3-Turn on More Themes: Earthquakes and Shear Wave Velocities
Earthquakes generate seismic waves that move through Earth. Changes in the speed of some of these wave help scientists learn about Earth's structure. In this step, you'll compare the types of plate boundaries with seismic shear wave velocites near them. You'll observe different types of plate boundaries around the world and the pattern of shear wave velocities near them.
- Select the Zoom to Full Extent
button to ensure that you are at the full world view. Turn off the Plat_poly.shp theme. Make sure only only Plat_lin.shp and Country.shp are turned on.
- Observe the distribution of tectonic plate boundaries around the world. What do the different colored plate boundaries represent?
- Turn on the Depth_100_grid.shp theme. Observe the relationship betweeen color and velocity for this theme. The values are in kilometers per second (km/sec). What color represents the slowest shear wave velocity? What color represents the fastest?
- Turn on quak7093.shp, the earthquake theme. What do you notice about the distribution of earthquakes and the plate boundaries?
The distribution of earthquakes coincides with the boundaries between tectonic plates.
- In the next step, you'll examine the relationship among shear wave velocities at a depth of 100 km, types of plate boundaries, and earthquake locations. You'll examine these data for Japan, India, and the middle of the Atlantic Ocean.
Look at the Table of Contents to the left of the map in ArcVoyager or use the identify tool.
The plate boundaries show where plates are colliding (magenta: convergent boundaries), spreading apart (green: divergent boundaries) or moving horizontally past one another (blue: transform boundaries). The yellow represents an unknown boundary type.
The orange color shows the slowest velocities (4.1-4.2 km/sec) and the blue represents the fastest velocities (4.9-5.0 km/sec).
Step 4-Examine Data near Japan
- Use the zoom in
tool to zoom in on the area around Japan.
- Examine the plate boundary.
- Is this a convergent or divergent plate boundary?
- Use the identify
tool to determine which plates are east and west of the boundary.
- East: Pacific
- West: Eurasian
- Look at the distribution of velocities on each side of the plate boundary. How do the velocities you observe on each side of the boundary compare to one another?
- How are the earthquakes distributed on either side of the plate boundary - evenly or only on one side?
- Can you think of a reason for this pattern of shear wave velocities and earthquakes?
- Zoom back out to the full world view by clicking the Zoom to Full Extent
button.
Colored box for reference only. It will not show up on screen.
Do a screen capture of View1. Save it with the name "Japan."
Convergent boundary.
The velocities on the western side are slower than those on the eastern side. The darker orange color on the western side represent slower wave velocities and the lighter orange on the eastern side are faster.
The majority of earthquakes are on the western side of the boundary.
Think about how the plates here are moving at this convergent boundary. Consider the three dimensional shape of the boundary and what you would find deep below the surface on each side of it. Which side of a subduction zone would you expect to have lower density (and therefore, slower shear wave velocities) overall? On which side of the boundary is a plate being subducted?
The slower shear wave velocities on the western side of the boundary are the result of the Pacific plate plunging below the Eurasian plate (and others). Seafloor sediments and water that are subducted along with the Pacific plate have a lower density than the undisturbed mantle material below the surface on the eastern side of the boundary. Earthquakes on the western side provide further evidence that the Pacific plate is grinding its way under the Eurasian plate. The figure below shows the relationship of those plates from a viewpoint looking south.
You may want to examine shear wave velocities near other locations along this plate boundary to see if the relationship you discovered holds true.
Step 5-Examine Data near India
- Zoom into the area around India.
- Examine the plate boundary.
- Is this a convergent or divergent plate boundary?
- Identify the tectonic plates north and south of the boundary.
- North: Eurasian
- South: Indo-Australian
- In terms of geographic features, how is India different from Japan?
- What major mountain range is located along India's northern border?
- Are the earthquakes evenly distributed on each side of the plate boundary or only on one side?
- Look at the distribution of velocities on each side of the plate boundary. How do the velocities you observe on each side of the boundary compare to one another?
- Can you think of an explanation for the shear wave velocities being faster south of the Himalayas?
- Zoom back out to the full world view by clicking on the Zoom to Full Extent
button.
Colored box for reference only. It will not show up on screen.
Do a screen capture of View1. Save it with the name "India."
Convergent boundary.
India is on a continent rather than an island in the ocean.
The Himalayas, the largest mountain chain in the world.
No, the majority of earthquakes are on the northern side, under the Himalayas.
The velocities on the northern side are slower than those on the southern side. The darker blue color on the southern side represent faster wave velocities and the lighter orange on the northern side are slower.
On which side are you still in the crust and which side are you in the mantle?
Because this is a convergent boundary between two continents, mountain building is occuring. The reason that the earthquakes are to the north of the actual plate boundary is because the subducted slab is contacting the base of the crust north of the plate boundary. This is where the earthquakes are generated. The reason that the velocities are faster in India than below the mountains is because at this level (100 km), under India we are well into the mantle while under the mountains we are just crossing into the mantle.
Step 6-Examine Data in the Mid-Atlantic
- Zoom in on the Mid-Atlantic Ridge.
- Examine the plate boundary.
- What is the primary type of plate boundary here?
- Which plates are on the east and west side of the boundary?
- East: African
- West: South American
- What other type of plate boundary is seen along the Mid-Atlantic Ridge?
- Look at the shear wave velocities on each side of the plate boundary. Are they similar on each side?
- How are the earthquakes distributed in regard to the plate boundary - evenly or or only on one side?
- Can you think of a reason for this pattern of velocities and earthquakes?
- Examine more plate boundaries around the world. See if the patterns you discovered in shear wave velocities at the three plate boundaries above can be generalized for the whole world. If you find locations that exhibit a different pattern, hypothesize about the factors that might be affecting that situation.
- Zoom back out to the full world view by clicking on the Zoom to Full Extent
button.
Colored box for reference only. It will not show up on screen.
Do a screen capture of View1. Save it as a file named "Mid-Atlantic."
Divergent boundary.
Transform boundary.
Yes, the velocities are similar on both side of the boundary.
The earthquakes are evenly distributed on each side of the plate boundary.
Think about how the plates are moving in relation to each other at this divergent boundary. Would you expect to find any differences below the surface of a spreading center?
Because this is a divergent boundary (spreading center), each side of the boundary is similar in material and plate movement. This causes the shear wave velocities and earthquake patterns on both sides of the boundary to be similar.
