MARINE GEOLOGY (GEOL 5533)

LABORATORY 2 - OCEAN BASIN TECTONICS

Due 24 September 2001

For this lab, we will investigate some interesting aspects of the tectonics of the world's ocean basins. However, our method for examining ocean basin tectonics will rely again on data we will import from the Internet and World-Wide Web. Using the map on pages 32-33 of your text, locate the following seafloor areas:

Once you have located these sites, go to the World-Wide Web and visit the National Geophysical Data Center (NGDC), Marine Geology & Geophysics Division (MG & G). At the bottom of their home page:

1. click on the "Bathymetry & Global Relief" link;

2. click on "Global Relief (bathymetry/topography)";

3. click on "Search Gridded 5-minute Global Relief Data ".

For this exercise, download data for each of the ocean basin features listed above. Once you have downloaded your data, generate bathymetric profiles for each and answer the questions below.

1. Using the depth-age equation, estimate the age of the seafloor for each point along each profile.

a. Mid-Atlantic Ridge Profiles:

It is possible in Excel to plot data from 2 different worksheets on the same graph. Do this for the Mid-Atlantic Ridge profiles at 26^oN and 41^oN. Note that this is more difficult than it seems because the x-axis (longitude) will have different values for these two profiles. What you want to do is plot them directly over each other so you can compare their bathymetries directly.

You should discover that the bathymetry for each of these profiles varies at every point. Suggest at least 3 hypotheses to account for this variation. How would you test each of these hypotheses? What data sources would be useful to you?

Using the World-Wide Web, visit ftp://ftp.ngdc.noaa.gov/MGG/images/AtlanticAge.jpg

As best as you can, estimate the age of seafloor at various points along your profiles. How well do the depths predict crustal age at 26^oN? How well do the depths predict crustal age at 41^oN? Can you think of any hypotheses to account for your observations?

b. Mendocino Fracture Zone:

Examine your profile of the Mendocino Fracture Zone. The prominent offset at ca. 39.75^oN is the trace of the fracture zone itself. Using the depth-age equation, estimate the age of the seafloor north and south of the fracture zone. What are these values?

Examine Figure 4-11 (p. 125) in your text. This figure shows known magnetic anomalies on the seafloor of the north Pacific basin. Locate the Mendocino Fracture Zone and note the magnetic anomaly number north and south of the fracture zone along your profile (140^oW). What anomaly numbers do you see?

Next, examine Figure 4-9 (p. 122) in your text. This is a version of the geomagnetic polarity time scale which assigns absolute ages to individual magnetic anomalies. Locate your anomaly numbers on this scale and record the age for each. What are the ages of the magnetic anomalies (and, hence, the seafloor) north and south of the Mendocino Fracture Zone?

Given these ages for the seafloor, how well does the depth-age equation predict this age for this portion of seafloor? Conversely, if you use the ages in the equation, how well does it predict the depth of the seafloor in this area?

b. Mid-Pacific Mountains:

On the previous lab, you discovered that simply entering the coordinates for the Mid-Pacific Mountains produced the wrong profile for the Mid-Pacific Mountains. This is because the line of your profile crosses the International Dateline (180^o longitude) and the software which generates the database cannot cope directly with a request that crosses the line. In order to obtain a proper profile of this area, you need to download data in two steps: 1) obtain data from 165^oE to 180^oE; 2) obtain data from 180^oW (-180) to 165^oW (-165). Note that the data point for 180^oE and 180^oW are the same.

From the depth of the base of the rise which forms the foundation of the Mid-Pacific Mountains, estimate the age of this foundation using the depth-age equation. What age do you get? What geologic time period is this? Go back to the GSA Memoir on the Mid-Pacific Mountains. What age are the rocks which were dredged from the top of the seamounts? Can you think of a logical explanation for this phenomenon?