Exploring Our Fluid Earth

Materials

• Figs. 7.48–7.51
• Table 7.6
• Ruler
• Three-dimensional seafloor model (or other object(s)) in a cardboard box, covered in paper
• Paper
• Colored pencils
• Masking tape
• Wooden skewers
• Pen or fine-tipped marker (optional)

Fig. 7.49. Three-dimensional computer model of the seafloor looking west under the Golden Gate Bridge, San Francisco, California. Red indicates the shallowest areas of the bay; purple indicates the deepest area of the bay.

Image courtesy of United States Geological Survey (USGS)

Image

Fig. 7.50. Color legend for seafloor grid corresponding to depth. Red indicates shallow seafloor and purple indicates deep seafloor.

Image by Byron Inouye

Procedure

A. First survey in 1940.

Imagine that it is 1940 and you have landed your dream job as an oceanographer on a research ship using single-beam sonar technology to explore a never-before mapped section of the ocean floor. You wish to conduct a large survey of your study area, but your granting agency is first requesting general sea floor topography. Your job is to investigate your seafloor effectively, reporting back to the funding agency on prominent sea floor topography, so that you can request additional funds.

1. Obtain a 3-dimensional seafloor model in a box covered with paper from your teacher. The paper represents the water surface. If your teacher has not already done so, tape the paper with a 1 cm grid on top of the paper cover.
    
2. Mark a straight line across the grid paper using a ruler to represent a transect line. The line should extend from one edge of the grid to the opposite edge, but does not have to be perpendicular to the edges. This is the line your boat is traveling over this area of the ocean.
    
3. A funding agency has given you money for 20 depth soundings (wooden skewer probes) along your transect. You do not have to use all 20 soundings, but you should use enough to adequately sample the seafloor.
    
4. Make marks along your transect where you will make your depth soundings. Your depth soundings should be made at regular intervals along your transect. The intervals should be no closer than 0.5 cm apart.
    
5. Measure the interval between each skewer probe (e.g., probes can be 0.5 cm apart, 1 cm apart, etc.).
    
6. Insert the sharp skewer probe. Keep the probe vertical, being careful not to let it slide down a slope on the model.
    
7. For each location, measure the distance in centimeters that the probe is inserted. This is equivalent to the seafloor depth. Use one of the following methods for mesuring the depth of your seafloor (or come up with your own way):
   
   1. Pinch the skewer after it touches the seafloor bottom where the skewer meets the water (paper). Carefully pull out the skewer and measure to your pinched fingers.
   2. Mark the location where the skewer meets water (paper) with pencil or tape. Carefully pull out the skewer and measure to your pencil mark or tape. Remove the tape or erase the pencil mark before continuing.
   3. Make your skewer into a centimeter ruler. Make clear ruler marks on your skewer (starting from the point) with a pen or fine-tipped marker. When your skewer touches the bottom, read the ruler mark where it meets the water (paper).
       
8. Record each depth at its proper location on the grid. You can mark this data directly on the grid covering your seafloor box or have a second grid printed out where you record your data. Note that depth measurements (below the paper, or water line) are negative because they represent elevations below sea level.
    
9. Use the data to make a profile of the seafloor feature(s) along the transect line in Fig. 7.51.
   
   1. Record the interval between each skewer on the x axis.
   2. The top line on this grid represents the water line.
   3. Record the depth from the water line (paper on your seafloor model) to the seafloor on the y axis.
       
10. Examine, identify, and label the seafloor features on the profile. Use the terms in Table 7.6.

B. Second survey in 1943.

Based on your preliminary transect, your funding agency has granted you additional research money to prepare a more complete map of your seafloor area using single-beam sonar. You can take an unlimited number of depth soundings, but have only 10 minutes for further exploration.

1. Use your initial transect to develop a mapping procedure for the remaining area. Will you continue to do transects or will you look more in depth at a particular area of your sea floor? Write down your intended procedure.
    
2. Follow your procedure for 10 minutes.
    
3. If you wrote your measurements directly on the grid taped to the seafloor box, carefully remove the grid from the paper covering the seafloor box without uncovering the seafloor (you should not look into your seafloor box!).
    
4. In the next part of the procedure, you will be color-coding and drawing contour lines on your seafloor grid. Have a discussion with your class to standardize your colors and contour lines so different seafloor grids can be compared.
   
   1. a. Color-coding (refer to Fig. 7.49 and Fig. 7.50 for an example)
      1. Your class’s color scheme should be (from shallowest to deepest): red —orange—yellow—green—blue—purple.
      2. Determine a depth scale to correspond to these colors. For example, zero to -1 cm could be colored red, -1 to -2 cm could be colored orange, etc. Develop a legend for your color-coded depth scale (you can use Fig. 7.43 as a sample).
   2. Contour lines
      
      1. Determine with your class at what regular depth intervals to draw contour lines on your grid (e.g., every 0.5 cm or every 1 cm).
      2. Note this interval on your grid.
          
5. Color-code the known sonar points of your seafloor grid. If there is a section of your seafloor you did not investigate, do not make assumptions about the features (you may leave uncharted areas blank).
    
6. Draw contour lines on your seafloor grid. If there is a section of your seafloor you did not investigate, do not make assumptions about the features (you may leave uncharted areas blank).
    
7. Identify any additional features discovered in your exploration. Use the terms in Table 7.6).

C. Revisiting the survey in 2005.

You have been given the opportunity as a preeminent oceanographer to descend to the ocean floor in a submersible on your 90th birthday. Because you are so famous, you get to choose the location of the submersible dive. You choose to revisit the section of seafloor you originally studied at the start of your career and compare your original single-bean sonar mapping to the actual seafloor.

1. Remove the cover on the box and examine the actual sea floor.
    
2. Describe similarities and differences between your profile map, your subsequent research, and the actual sea floor.