Bathymetric Mapping, Marine Geography, and Plate Tectonics
Objectives:
1) Create and interpret bathymetric (contour) maps and cross sections.
2) Understand basic marine navigation and geography.
3) Contemplate the evidence that supports the theory of plate tectonics.
Part I: Create a bathymetric map
Lecture: Introduction to contours
Activity:
The earliest oceanographers were explorers who traveled in sailing vessels. Because of the
dire consequences of running the wooden hull of a ship into a rock, methods of ascertaining
and recording the depth of the sea floor were of great value. The standard method of
measuring depth was the lead line. Depth measurements were recorded and usefully
summarized on bathymetric maps, the marine version of an elevation contour map.
In the first part of this laboratory exercise, we will work as a class to map the unknown
bottom topography of a box filled with dark fluid, using the same tools that were available to
the crew of a typical 17th century sailing ship exploring an unknown coast.
1. As depth measurements are made by lab members over the extent of the aquarium, use
graph paper to record the depth taken at each measurement location.
2. Draw contours (lines of equal depth, or isobaths) at equal depth intervals.
3. Write a brief description of the shape of the bottom, based on your interpretation of your
bathymetric map:
4. Denote on your contour map where the bottom topography is steepest.
5. Label on your map any prominent features (valleys, peaks, ridges, slopes) with creative
names.
Vote: Do contours ever cross, or intersect?
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Part II: Creating cross sections from contour maps
Lecture: Introduction to land slides and their causes
Activity: Use your understanding of contour maps to examine the coastal topography near
Mussel Rock (next page), the area south of campus that we will visit next week on our field
trip. The elevation map is annotated with 2 lines. Generate a cross-section of the elevation
change along each line on the grids provided (on the page following the contour map).
To create your cross section, use a strip of paper to record the distance along a cross-section
line (from points A and C, respectively) at which each contour intersects the line, noting the
elevation of the contour at each intersection. Transfer these distances and elevations to the
respective grid, plotting a point for each pair of values. Finally, connect the points to
complete the cross-section. Compare the 2 cross-sections.
Along which cross-section line is the coastal slope steeper, overall?
If you had to build a home along this coastal cliff, would you prefer to locate it at point B or
point D? Why?
Part III: Overview of plate tectonics
Go to the Earth Systems Laboratory and open the following WWW site:
http://geosci.sfsu.edu/courses/geol103/labs/labs.html and open The Seafloor exercise.
Computer Part 1: The Seafloor and Plate Boundaries
Bathymetric Features on the Seafloor
Figure 1. The map below shows the global relief of Earth, including the land area and
seafloor area. On land low-lying areas are shown in light green and high areas are shown in
read and gray. Below the ocean surface, shallow areas are shown in light blue and deep areas
are shown in progressively darker shades of blue.
1. Identify major features on the seafloor:
a. Continental shelves and slopes
b. Abyssal plains
c. Seamount island chains
d. Mid-ocean ridges (mid Atlantic, East Pacific Rise)
e. Subduction trenches (Alaskan, Peru-Chile, Japan, Marianas)
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Plate Boundaries
Figure 2. The map below shows the plate boundaries (yellow lines) superimposed on the
global relief map.
2. How do the plate boundaries correspond to the features you observed in Figure 1?
Earthquakes
Figure 3. The map below shows earthquake locations (red dots) superimposed on the global
relief map.
3. How do the locations of earthquakes relate to the features you observed in Figure 1 and the
plate boundaries you observed in Figure 2?
Age of the Seafloor
Figure 4. The map below shows the age of the seafloor (the basaltic crust beneath the
sediment cover) in color coding. The red colors are 0-5 million years; the brown colors are 540 million years; yellow colors are 40-80 million years; green colors are 80-110 million
years; light blue colors are 80-150 million years; dark blue colors are 150-180 million years.
4a. How old is the oldest ocean floor (in million of years) and where are the oldest parts of
the ocean floor found?
4b. How young is the youngest ocean floor and where are the youngest parts found?
Computer Part 2: Plate Tectonics Interactive Software
To access the software open the Plate Tectonics folder on the desktop, and then
double click on the icon. After a short introductory sequence, you will see the "home page"
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that contains 19 clickable topics. You may begin with the Introduction if you have extra
time, or you can jump directly to the section on Plate Tectonics and proceed from there. The
program will take you through the topics successively unless you click on the square button
to return to the home page. Review questions are provided after each topic. These are a
good way to test your comprehension of the topic. If a topic is not clear, you may click on
the backward arrows to reverse direction in the program. Otherwise, click the forward button
when you are ready to move to the next "page". Take turns clicking the buttons and be sure
that each person in the group is ready before you move forward.
Questions
Plate Tectonics
(a.) Complete the plate identifying exercise to familiarize yourself with the names and
locations of the tectonic plates. What are the names of the plates that underlie the west coast
of the United States?
Plate Boundaries
(a.) List below the 3 types of plate boundaries. Briefly describe what kind of motion
occurs at each plate boundary type and provide an example of each.
i.)
ii.)
iii.)
Testing the Plate Model
What evidence is given here to support the plate tectonic model?
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Geomagnetic Reversals and Plate Tectonics
(a.) If sea-floor spreading at the East Pacific Ridge is occurring at a rate of 5 cm/yr, how
long does it take to create 100 km (60 miles) of new crust on either side of the ridge?
Plate Tectonics and Earthquakes
(a.) Answer the review questions. One of the largest historical earthquakes in the U.S. was
the great 1906 San Francisco earthquake. This earthquake was the result of movement along
which type of plate boundary?
What two plates are interacting at this plate boundary? What is the name of the boundary?
How deep are the earthquakes that occur along this plate boundary near SFSU?
When you see the topic Hot Spots and Plate Tectonics, return to the home page and click on
topic 19: Plate Tectonics into the Future.
Plate Tectonics into the Future
At the rate of 5 cm/yr (50 km/My), how long will it take for Los Angeles to become a
suburb of San Francisco? (Hint: the cities are now about 700 km or 425 miles apart.)
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