## Lab 2.3 – Bathymetric charts

Fundamental concept: Variability in the data, finding trends
Estimated time to complete: 30 minutes
Materials needed (none)

The shape of the seafloor often contains information about the processes acting in the ocean and thus are useful for scientists. Submarine volcanic activity can be located by measuring water depth and finding volcanoes. Submarine landslides leave identifiable scars on the continental slope. And tectonic boundaries often have distinctive shapes. The deepest places in the ocean, trenches, occur at subduction zones where two plates collide.

Maps of seafloor depth are called bathymetric charts. The chart below is a part of such a chart that is used for navigation. Note that the depth of the water is shown in three ways; by soundings (the numbers indicating depth in feet), contour lines and color coding. This is because depth is very important to mariners. They must constantly keep track of chart depth and height of tide in coastal waters to make sure they do not run aground!

Figure 2.3.1 NOAA Chart 12318 Absecon Inlet, New Jersey

1. The depths on NOAA charts are given in feet. Locate the deepest point on the chart. Each story of a building is typically 10 feet high. How many stories would a building have to be to project above the water if it sat on the seafloor at the deepest point on the chart?
2. Scientists typically work in the metric system. Take the deepest point on the chart and convert the depth to meters (1 ft = 0.3048 m).
3. Identify the latitude and longitude of deepest point (in degrees, minutes and seconds).

Scientists do not usually need the detail given in charts like the ones used by mariners. Individual soundings would clutter a scientific chart. Contour lines or color coded contouring proves more effective in showing the shape of the sea floor. The figure below shows the same location as the chart above, using color to indicate the depth of the seafloor in meters below the surface. Note the color bar inset which shows how depth is represented on this chart or map. The axes along the edge of the map show the latitude (right side) and longitude (bottom) in degrees, minutes and seconds.

Figure 2.3.2 Absecon Inlet, New Jersey

1. The NOAA chart contains information specifically used in navigation, like buoys that are placed and maintained to aid vessels in safe passages. Notice on the chart there is a symbol labeled G “3” Fl G 2.5s. This is a buoy used to navigate into the harbor. This particular one has a green light that flashes every 2.5 seconds. Locate this buoy and record its latitude and longitude.
2. Why is this buoy not located further to the east? Use the color-coded bathymetry map below the NOAA chart to support your answer.

1. Below is a color coded bathymetric map of part of the coast off of California. It is a nice example of how useful this type of map can be in identifying submarine features. Can you identify valleys in the sea floor from the color coding? Locate the largest valley, it’s landward end is located in Monterey Bay. Note that a river empties into the ocean near this valley.
• How might this submarine valley have formed?
• Follow the submarine valley seaward. How deep is it at the deepest end?
• By what process might this valley have been carved in such deep water?
• Check your answers to the questions by looking at the information here.

Figure 2.3.3 Bathymetric color map of Monterey Bay