Exploring Our Fluid Earth

Materials

• Figs. 7.41–7.44
• World map, globe, or atlas
• Strip of paper
• Colored pencils
• Centimeter ruler

Procedure

A. Develop a contour map.

1. Fig. 7.41 is a nautical chart of the Maug Islands in the western Pacific ocean basin.
   1. Using the latitude and longitude information given on the chart, locate this island group on a map of the world. Write down the location of the Maug Islands.
   2. The Maug Islands are three volcanic islands. Describe each island, including its size and elevation above sea level.
       
2. On Fig. 7.41, locate the deepest sounding on the chart. Draw a box around the number.
    
3. Using a light color, circle all depths 30 m or less. Choose a new color and circle all depths between 31 and 60 meters. Repeat this for 30-meter intervals.
    
4. On Fig. 7.41, draw the contour lines for each interval of 30 meters (see Fig. 7.42).
   
   1. The 30 m contour line will encircle all of the depths less than 30 m.
   2. Remember that contour lines follow a single depth; lines should only go directly through depth readings that are the same as the labeled line.
   3. Soundings that are not the same as the labeled line will be intermediary between the 30-meter contour lines.
   4. Your contour lines should never intersect one another; they should continue completely around the island group (chart all of the islands as one group).

Image

Image caption

Fig. 7.42. Draw the 30-m contour line.

Image copyright and source

Image by Byron Inouye

5. Lightly shade the intervals between the contour levels using the shades you selected for each depth. Make a depth color key on the side of the chart.
    
6. Suppose you were on a sailing ship that draws (extends below the waterline) 4 m. You want to sail your ship safely into the protected waters in the center of the Maug Island group and drop anchor.
   
   1. Study the chart to learn about each channel that leads from the open ocean into the protected central water. Describe each channel in terms of depth and hazards to navigation.
   2. Decide how you would sail from open ocean into the center of the island group. With the ruler, mark a straight line on the map showing the course you would choose.
       

B. Develop a depth profile.

Your contour map shows a “bird’s eye view” of the Maug Islands. To get a “fish’s eye view,” a depth profile will be needed.

7. Extend the line you drew in Procedure 6 to mark your course into the center of the island group so that it becomes a transect line cutting across the entire chart, including a portion of any of the islands.
    
8. Lay the edge of a strip of paper alongside the line you drew to mark the course you took to the center of the island group (Fig. 7.43 A). This line is your transect, a line that you follow to record data in specific locations.
    
9. Mark the strip of paper at each point where the edge of the paper strip touches a contour line and record the depth or elevation alongside the mark (Fig. 7.43 A).

Image

Image caption

Fig. 7.43. Constructing a profile drawing from a transect line drawn on a contour map

Image copyright and source

Image by Byron Inouye

10. Make your own grid like that in Fig. 7.44. Label each axis.
    1. The horizontal axis represents the distance across the transect line.
    2. Note that the chart in Fig. 7.44 gives a scale for distance. The vertical axis represents the depths below sea level and the elevations above sea level in meters.
    3. Do not forget that sea level is 0, so some of your points will be above and some will be below 0.

Image

Image caption

Fig. 7.44. Grid for plotting profile data

Image copyright and source

Image by Byron Inouye

11. Place the paper “transect” along the bottom of the grid as shown in in Fig. 7.43 B, and transcribe the depth information along the base of the graph.
    1. Plot each depth and elevation marked on the paper strip.
    2. Hold the strip along the horizontal axis of the grid and make a dot corresponding to each contour line marked.
    3. Read the depth or elevation marked on the strip, beginning with the first mark on the left.
    4. Move your pencil straight up from the mark on the bottom to the appropriate level on the grid and make another mark at the corresponding depth or elevation.
    5. Use a ruler to help you stay directly above the mark on the horizontal axis.
        
12. Draw the profile by connecting all the depth and elevation marks. This will show you the side view of the islands.