Bathymetric Mapping, Navigation, and Marine Geography
Objectives:
1) Create and interpret bathymetric (contour) maps and cross sections.
2) Practice making measurements with attention to sampling and statistics.
3) Understand basic marine navigation and geography.
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 an aquarium 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 (next page) 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 aquarium bottom, based on your interpretation
of your bathymetric map:
4. Where is the bottom topography 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; angle of repose demonstration
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 the crosssection line (from point 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: Introduction to navigation
Please show all your work when answering the questions in this part of the exercise.
Lecture: Intro to the spherical and geographical coordinate systems; orbit, rotation, and time
Thought experiment: If you woke up one morning on a deserted island with only enough
power in your Iridium global cell phone to make one call, how would you describe your
location on the Earth to your potential rescuers adequately enough for them to find you?
Useful conversion factors:
1° latitude = 60 nautical miles = 69 statute miles = 110 kilometer
15° longitude = 1 hour of time
1.15 statute mile = 1 nautical mile
1 meter = 3.28 feet
1 mile = 0.6 kilometer
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1.
City
Latitude
Longitude
New York, New York
Reno, Nevada
Los Angeles, California
Hana Bay, Hawaii
San Francisco, California
40° 38' N
38° 30' N
33° 42' N
20° 45' N
37° 19' N
073° 50' W
119° 46' W
118° 15' W
155° 59' W
122° 25' W
(a) Which city is farthest north?
(b) Is Reno east or west of Los Angeles?
(c) Based on the latitude difference, what is the time difference (to the nearest minute)
between Hana Bay, in the Hawaiian Islands, and Los Angeles?
(d) How far north is San Francisco from Los Angeles?
__________________ nautical miles.
___________________ kilometers.
2. San Francisco is approaching Los Angeles at a speed (average velocity) of about 30
mm/yr (3x10-5 km/yr) due to movement along the San Andreas Fault. At this rate, in about
how many years will the two metropolitan areas share the same latitude?
_____________ years
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Part III: Your textbook describes the major features of the sea floor (page 80). Use the
maps provided to answer the following questions.
(How could you use the comp lab to teach this all via inquiry?!)
Maps: The Floor of the Ocean, The Relief of the Surface of the Earth, Chart of the World,
and Geographic Map of the Circum-Pacific Region.
4. (a) Look closely at the Atlantic Ocean and describe what features are found in this ocean.
Where is the bathymetry the steepest? Where is it most flat?
What are the greatest depths (in meters and feet) in the Atlantic Ocean and where are they
found?
What is the approximate depth of the ridge crest?
____________meters ___________feet
Look closely at the Pacific Ocean and describe what features are found in this ocean.
What are the greatest depths (in meters and feet) in the Pacific Ocean and where are they
found?
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 I. The Seafloor and Plate Boundaries
This could be a clear precursor to the Plate Tectonics exercise and the following SAF
field trip! Is it redundant with the Plate Tectonics exercise?
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
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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)
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 II. The World's Biggest Mountain
Figure 1. Below is a bathymetric map of the Hawaiian Islands. If measured from the base to
the top, the biggest mountain on the surface of Earth is Hawaii (southeasternmost island in
the chain). Mount Everest in the Himalayas has the highest elevation above sea level, and a
total relief of 8800 meters (29,000 ft.).
1. What is the total relief of Hawaii (height from base to surface)? This map shows only the
part below sea level; you must also add the part above sea level. The elevation of Mauna Loa
(the highest peak on Hawaii ) is 4200 meters (13,800 feet).
4a. relief in meters:
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4b. relief in feet:
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