Goals: This week's homework involves the theory of plate tectonics, and how geomagnetic reversals and sea floor spreading have contributed to the understanding of continental drift and plate tectonics.  We will learn about some of the evidence, which scientists have used to bolster plate tectonic theory, and we will reconstruct the mathematical evidence, which shows South America and Africa were once joined together.
 

 Introduction
 
Background Information
 

The Assignment:

(NOTE:  Click on the image of any of these figures to see a larger clearer image that you can print and use for answering the Questions)
 

 1. Figure 2 shows the geomagnetic stripes along the mid-Atlantic ridge near Iceland.  The ridge axis is shown on the drawing, and the stripes are more or less symmetric about the ridge.  Figure 3 shows the chronological information about the epochs of geomagnetic polarity reversals.  Use this chronological information to assign ages to the boundaries of each of the major stripes along the mid-Atlantic ridge in Figure 2.  On the south-east side of the ridge, what is the age range of the oldest (major) black stripe?
 

 2.  Using a ruler, find the average distance (in nautical miles) from the ridge to the oldest edge of the oldest (major) black stripe.  Use the latitude scale on the map as a nautical mile scale [1 nautical mile = 1 minute of latitude (i.e. 1/60 of 1 degree of lat.)].  Explain how you arrived at your answer.
 
 

 3.  Convert your answer in number 2 above from nautical miles to kilometers.  (there are 1.852 kilometers in every nautical mile, use unit cancellation and show your work)
 
 

 4.  Convert your answer in number 3 above from kilometers to centimeters.  Then, use the number of centimeters traveled, and the time it took to get that far (from question number 1 above) to determine the spreading rate of the ridge (show your work, using the formula described in the Background section).
 
 

 5.  From Figure 1 above, determine the distance between the coastlines of Africa and South America along the Tropic of Capricorn (denoted by a dashed line on the map).  In order to do this, you must first determine the scale for the map using latitude as you did in Question 2.  Then, using the spreading rate you calculated in Question 4, determine the approximate time in geologic history when these two points first drifted apart.  Show your work.
 

6.  Using the skills you learned in the first five questions.  Determine the rate of sea floor spreading for each color on the map between South America and Africa.  To do this, measure the distance from the ridge to the edge of the red region on the east side of the ridge.  Convert this distance to cm, as before, and determine the spreading rate for the time period covered by red (see scale at bottom of map).

Next, determine the distance from one edge of the orange area to the other edge of the orange area (remember, since seafloor spreading is relatively symmetric you need only measure distances on one side of the ridge.  For this exercise, make all measurements on the east side of the ridge) along the Tropic of Capricorn.  Again determine spreading rate in cm/yr, based on your measurement of distance and the time span encompassed by the orange area.

Continue to determine spreading rates for each color zone until you hit the coast of Africa.  Show all work for each calculation.
 
 

7.  According to the map in Figure 1, when did Africa and South America drift apart?
 
 

8.  Does the date in Question 7 agree with the number you calculated in Question 5?  List at least five errors that may have occurred to cause these two numbers to be different.
 
 
 
 

 
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