Using the Computer Program "Geophysics of North America"to Study the Earth's Crust
Introduction
The instructions that follow will guide you through the program(GNA) and direct you to investigate general concepts concerningthe earth's crust.
Learning to use the program
Starting GNA
Choosing and Designing Maps
There are 14 different base maps to start with. Each allowsyou to zoom in, draw profiles, add contoured or point based dataand save images for later use or a presentation. As a resultthe possibilities are nearly infinite. The following steps willteach you how to select and display some of the possibilities.
1. With Image highlighted in the menu bar press <Enter>. When the next screen appears, select the first choice, Topography(N. America). Press <Enter>.
2. Next, a white box will appear with the word Paletteat the top. A palette is a choice of colors. Use the systemdefault 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 UnitedStates 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> tozoom in. Center a square box on the Basin and Range province,and re-size it to include a portion of the ocean floor and justeast of the Rocky Mountains. To reshape the box use the followingcontrols:
Page Up makes it larger
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 edgeto shrink
arrow keys re-positions the box
6. The new image will appear on the screen. If you zoom inon a small area the new image may be smaller than needed. Youcan enlarge it (or shrink it back) using the Page Up/Down keys.
7. You can view a profile of the map and compare topography toother variables, such as magnetics or gravity. To do this pressCtrl P. This will cause a "+" sign toappear. Move the plus sign to the center of the western edgeusing 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.
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8. Try to pick out the physiographic features of the profile. Identify the continental slope, and the ocean floor, the edgeof the continent, the California Sierra Nevada Mountains, theBasin and Range and the Rocky Mountains.
9. Press F1. This provides options for other data setsto be included in the profile. Select 5 to see DNAGgravity. A second line will appear on the same profile.
10. Press Esc to return to the blue menu page and followthe instructions below.
Exploring Ocean Bathymetry
1. Once again bring up the image Topography (N. America). Position a square box across the ocean such that thereis a little of the North American continent in the northwest corner,and a little of Africa in the southeast. If you avoid inflatingthe box it will fill the screen very nicely.
Q-1 What do the colors mean?
2 Press F3 to view a colored scale the corresponds tovalues.
Q-2 What is the value of each color interval in this view?
Q-3 How deep does the ocean get in this region? (By using CtrlP and the arrow keys you can determine this quite precisely)
Q-4 What is the highest elevation of the Mid-Atlantic ridge inthis view?
3. Make a profile across the ocean. Include some land on bothsides.
Q-5 How much relief in meters is there from the bottom of theocean to the top of the ridge in this view?
4. There is enormous vertical exaggeration in this view. Bypressing the PgDn key you can shrink it to a more realisticprofile. How much do you think you should reduce the verticalto make it match reality?
5. Using the Print Screen button and the program L-viewyou can save, modify and print this cross section. Follow thesteps below. Print and save the resultant image.
A. Press Print Screen. Alt-Tab to the ProgramManager and open L-view.
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 background.
E. Add text to your image that:
-provides a title and vertical exaggeration
-labels the continental shelf, slope, rise, ridge and abyssalplane.
-labels the northwest (NW) and southeast (SE) sides of the profile.
6. Return to the Topography (N. America) view. Create,print and save quality profile that illustrates the bathymetricnature of an ocean trench. Be sure to include some of the continentand a volcano and all appropriate labels.