Basic Skills
Ocean Bathymetry Lesson
Magnetics Lesson
The instructions that follow will guide you through the program (GNA) and direct you to investigate general concepts concerning the earths crust.
Learning to use the program
Starting GNA
2. Next, a white box will appear with the word Palette at the top. A palette is a choice of colors. Use the system default palette by pressing <Enter>.
3. The next screen should have the word Area highlighted. This box allows us to tell the program which part of the United States we want to look at, so go ahead, press <Enter> again. On the next screen, New will be highlighted. Press <Enter>. Window Image will be highlighted. Press <Enter>.
4. Notice that you just selected the default option for the last 6 options.
5. You can reshape the box and press <Enter> to zoom in. Center a square box on the Basin and Range province, and re-size it to include a portion of the ocean floor and just east of the Rocky Mountains. To reshape the box use the following controls:
Page Down makes it smaller
T, B, L, R moves the top, bottom, left edge and right edge to enlarge
t, b, l, r, moves the top, bottom, left edge and right edge to shrink
arrow keys re-positions the box
7. You can view a profile of the map and compare topography to other variables, such as magnetics or gravity. To do this press Ctrl P. This will cause a "+" sign to appear. Move the plus sign to the center of the western edge using the arrow keys. Press <Enter> to fix its position. Now move the plus sign to the center of the eastern edge. Press <Enter> again to strike a line. A profile will appear.
8. Try to pick out the physiographic features of the profile. Identify the continental slope, and the ocean floor, the edge of the continent, the California Sierra Nevada Mountains, the Basin and Range and the Rocky Mountains.
9. Press F1. This provides options for other data sets to be included in the profile. Select 5 to see DNAG gravity. A second line will appear on the same profile.
10. Press Esc to return to the blue menu page and
follow the instructions below.
1. Once again bring up the image Topography (N. America). Position a square box across the ocean such that there is a little of the North American continent in the northwest corner, and a little of Africa in the southeast. If you avoid inflating the box it will fill the screen very nicely.
Q-3 How deep does the ocean get in this region? (By using Ctrl P and the arrow keys you can determine this quite precisely)
Q-4 What is the highest elevation of the Mid-Atlantic ridge in this view?
3. Make a profile across the ocean. Include some land on both sides.
5. Using the Print Screen button and the program L-view you can save, modify and print this cross section. Follow the steps below. Print and save the resultant image.
B. Select Paste under Edit.
C. Crop the image to a more reasonable size.
D. Invert the colors so you have a black line and white back ground.
E. Add text to your image that:
-labels the continental shelf, slope, rise, ridge and abyssal plane.
-labels the northwest (NW) and southeast (SE) sides of the profile.
Part 1: Map view-color contoured for magnetics
1. Select DNAG magnetics under Image.
2. Select all the default options as you did before.
3. Notice the colored patches and lines. These are the areas where we have data. The black areas are regions with out available data. Therefore, to see the relationship of magnetism to the ocean ridges you must zoom in either on the Juan de Fuca ridge or the section of the Mid Atlantic ridge just south of Iceland. Position the box just off shore of the Washington centered on the Juan de Fuca ridge. Press <Enter> and wait for the image.
Note: You can add geographic boundaries to the map for clarity. Press <Enter> to return to the menu. Select Boundaries. Toggle through the options with the <Enter> key. Type a "y" for yes for Coastline. Choose Black for color and Detail. Press <Enter> several times to skip the other options and return to the menu, Select Go.
Q-2 Can you recognize the symmetry of the magnetic stripes about the ridge?
Q-3 Use Ctrl P to explore the values of magnetics in nT (nanotesla). What colors are positive, what are negative?
Q-4 What is the difference between positive and negative
magnetism of the ocean crust, in regards to the earth’s magnetic field?
Q-5 Explain why the recognition of the patterns of magnetic
stripes was such a revelation for Vine, Mathews, Hess and Cox.
1. Bring up the image Topography (N. America). Position a square box the same region west of Washington and Oregon.
2. Display contoured gravity by selecting Contours from the blue menu. Select DNAG Magnetics at a contour interval of 600 nT.
Q-2 Notice that one set of stripes has a kink in its orientation.
What does this suggest about the history of plate motions in this area?
B. The "help panel" at the top of the screen will instruct you as to how to re-assign colors to topographic intervals. Expect to make several attempts before succeeding at this task.
C. When the image looks good press Print Screen, open I-View and Paste in the image.
D. Give the image an appropriate title. Save and print it. Leave it on the screen for the next set of instructions.
1. Using Ctrl-P, make an east-west profile across the region that includes part of the continent.
2. To add a profile of magnetics press F1 and select Mag 2.5’.