## Lab 2.2 – Bathymetric charts

Fundamental concept: Recognize seafloor slope and texture in bathymetry maps
Preparation for: Lab 2.3 – Bubble charts, Lab 3 – Tectonics, Lab 4 – Volcanoes
Estimated time to complete: 40 minutes
Materials needed (none)

The ocean’s floor contains a vast array of shapes and textures formed over millions of years of geologic processes. These different seafloor formations provide habitat for marine species and sometimes contain valuable resources, such as oil and natural gas. Scientists and industry professionals use detailed bathymetric maps or charts to explore seafloor habitat and resources.

In this activity you will practice interpreting bathymetric maps to investigate seafloor formations in the Gulf of Mexico.

Figure 2.2.1. Map of Gulf of Mexico. Magenta rectangle shows area of salt domes, including the East Flower Garden Bank (EFGB). Black rectangle shows region of the West Florida continental shelf (WFS) that you will examine later in this activity.

The Texas-Louisiana continental shelf has many active oil wells due to the interesting geology of this region.  The oil in the rocks tends to accumulate around circular shaped features called salt domes.  These features form when salt that is buried deeply in the shelf sedimentary rocks rises toward the surface, as a chimney shaped feature.  In rising the salt deforms the surrounding rocks, and sometimes the sea floor, pushing it up into domes.

One such salt dome is the East Flower Garden Bank, within the Flower Garden Banks National Marine Sanctuary (labeled EFGB in Figure 2.2.1). Figure 2.2.2 represents the shape of the Bank with contour lines. Each line on the contour map outlines an area of the seafloor that is a certain depth below the surface of the water. Locate the contour line labeled “60” and trace your mouse or eye around it. Everything on that path has a depth of 60 meters.

Figure 2.2.2. Contour map of East Flower Garden Bank. Labels indicate depth below the sea surface in meters.

### Interpretation Questions:

View the video below for an exercise to help you connect contours on a map to the changing slopes of the seafloor.

1. Follow along with the video above to complete the bathymetric profile exercise. On your own graph paper or printout supplied by your instructor, draw the slope from point A to B and from point B to C.
2. Which side of the East Flower Garden Bank, northern (A-B) or southern (B-C) is the steepest?
3. Estimate the depth of the seafloor at 27° 56’ N, 93° 37’ W. Remember that the depth changes continuously between contours.
4. Hard corals (the ones that build reefs) are found on top of the bank, down to a depth of approximately 50 meters. If an ROV collected video footage of the location in Question 2 (27° 56’ N, 93° 37’ W), would you expect to see living coral reef habitat? Why or why not?

Contour maps often include color to help draw our eyes to differences in depth. View the filled contour map (Figure 2.2.3) below.

Figure 2.2.3. Filled contour map of East Flower Garden Bank. Color scale indicates depth below the sea surface in meters.

1. Describe the relationship between color shading and depth of the seafloor in Figure 2.2.3.
2. What can you look for in the colors on the map to locate a steep area of the seafloor?

We’ll look at one final map of the East Flower Garden Bank. Figure 2.2.4 shows a representation of bathymetry called a shaded relief map. In this case, there are no contour lines and change in depth is represented only by the color scale. Additionally, some parts of the map contain shadows as if light shone onto the Bank from an angle. The shadows help highlight small seafloor features and abrupt changes in depth.

Brine pool at East Flower Garden Bank. The very high salinity of the brine gives it the appearance of an underwater lake. (Photo credit: 2007 Rhode Island Institute for Archaeological Oceanography; Secrets of the Gulf Expedition. NOAA/NOS/NMS/FGBNMS)

Look for the deep trough on the southeastern side of the bank, near 27° 54’ N, 93° 35’ W. We can tell that it is narrow and deeper than the surrounding seafloor because it is darker blue and forms a shadow along the contour of the bank. This feature contains very salty water, called brine, that spills over the side of the bank from the brine pool (visible as an oval shaped depression) at the top of the bank.

Figure 2.2.4. Shaded relief map of East Flower Garden Bank.

1. Locate the two shallowest features at the top of the bank (depth less than 20 meters). Describe the shape and texture of these two locations in as much detail as possible. Are they round or irregular? Smooth or rough? Flat or peaked?
2. Now examine the areas that are deeper than 100 meters. Which side of the bank, east or west, has small bumpy features? Were those features visible in the contour maps?

### Application Questions:

Practice your bathymetric chart reading skills by traveling east from the salt domes in the Gulf of Mexico. The shaded relief map below (Figure 2.2.5) shows a portion of the continental shelf west of Florida. The edge of the continental shelf is a geologic feature called the Florida Escarpment, which is an unusually steep drop-off to the deep sea.

Figure 2.2.5. Continental shelf off of the west coast of Florida.

The dashed line labeled A-B is approximately 285 km long, from the coast of Florida (the gray landmass) to the edge of the continental shelf. The depth of the water increases from 0 to 300 meters between A and B. Based on this information, we can calculate a slope:

slope = (change in depth)/(distance) = 300 m ÷ 285,000 m = 0.001

Multiply the result by 100 to find that it is a 0.1% slope. This is a very gradual change in depth as we move out across the continental shelf.

1. Between points B and C is the continental slope, the transition to the deep open ocean basin. The depth of the seafloor at point C is approximately 3,300 meters, compared to 300 meters at point B. The distance between points B and C is 36,000 meters. Calculate the slope.
2. Federal highways in the United States generally have a maximum slope of 6% and display warning signs for trucks if the slope of the road exceeds that amount. If the continental slope west of Florida (between B and C) was a mountain roadway, would it need a warning sign?

### Reflection question:

1. We have examined three versions of bathymetric charts (labeled contour lines, filled contours with a color scale, and shaded relief). What are the benefits and drawbacks of using a shaded relief chart compared to a contour map?