Question

Correlation with the magnetic time-scale

The magnetic reversal time scale for the past 4.5 Ma, which is primarily derived from careful work carried out on rocks of the continental crust, is shown in Figure A3-8.

1. Correlate the peaks that you selected on the magnetic profiles with the various events described on the magnetic chronology time scale, and record the dates of the features in the fourth row of the table above. Note that points labelled A to E are the tips of the magnetic peaks. They correspond, therefore, with the centre points of the various positive (normal) magnetic events. For example, peak A represents the midpoint of the Jaramillo event (as shown in blue), and the time should be half way between 0.92 and 0.97 Ma, which is 0.945 Ma or 945,000 years. (5 points)
   Chronology of magnetic field reversals for past 4.5 m.y. Gauss (normal) Gilbert (reversed) Bruhnes (normal) Matuyama A (reversed) 1.67 187 / 2.13 2.43 2.92 /3.13 340 3.88 3.98 4.25/449 (date -m.y.) 73 92 .97 3.03 4.12 4.41 2.02
   

Figure A3-8. Magnetic chronology time scale for the past 4.5 Ma

1. Estimate the spreading rates of the Antarctic ridge at 47.7° and 51.6°. Divide the distances (km) by the number of years, and then convert those numbers to cm/year (multiply by 100,000), and put those numbers into rows 5 and 6 of the table above. Calculate the average rates at the two locations and report them. (5 points)

2. Are the calculated spreading rates reasonable? (2 points)

3. Are you satisfied that Vine, Matthews, and Morely were correct in their interpretation of the sea-floor magnetic patterns? Explain. (5 points)

If you are satisfied, then you have confirmed the VMM hypothesis. Although this, does not mean that it has been proven to be correct, if it can pass several such tests, it can be upgraded from a hypothesis to a theory. In fact, the VMM hypothesis regarding the interpretation of sea-floor magnetic anomalies has been part of the comprehensive theory of Plate Tectonics since the late 1960s.

Answer 1

First of all, to calculate the spreading rate I need a distance, which is not mentioned anywhere in the question. Now coming to the seafloor spreading hypothesis. This hypothesis first came after the continental drift where wagenar couldn't explain a few things about the movements of landmasses over oceans. After recognising the symmetrical patterns around the mid-Atlantic ridge, Vine and Matthews explained that new seafloor is being created at the ocean ridges and the continents sit over the oceanic crust as a result of which the continents also migrates. But the volume of Earth is fixed. So if the new oceanic crust is being created at the ridges, then somewhere it has to be consumed. That they couldn't explain. After that, the theory of plate tectonics came which says that the lithosphere of Earth acts like a large conveyor belt that eventually creates new crust at the ridges and consumed them at the trenches, thereby maintaining the volume of planet Earth.