The Way of Science

Now examine the two types of paleomagnetic evidence that were important in the geologic revolution of the 1960s. The first kind - evidence for apparent polar wandering - convinced a few more geologists to accept Wegener's model, but not a majority. The second - Sea-floor "stripes" - was the final nail in the coffin for the "shrinking apple" model.

1. Wandering magnetic poles
   Using the techniques and logic described above, it appeared that over the past few hundred million years the north magnetic pole had wandered over vast distances. The first path to be plotted was from North American rocks, and the great distances of wandering were very disturbing to many geologists. Worse yet, when the same techniques were applied to European geology, the two paths (although roughly similar) were displaced from each other by 30 degrees. That displacement was very difficult to explain, and that still wasn't the end of the story. Data from the southern hemisphere, combined with the first two sets, showed conflicting positions and paths! Were there multiple sets of poles, moving independently? Absurd! Figure 6 shows a very diagrammatic representation of these north polar paths, and it might strike you (as it struck some geologists) that the simplest way to reconcile these divergent patterns was to fuse South America, Africa and Australia. That is, support continental wandering. Why did these data not convince all the geologists? Among other reasons (see the next section), part of the problem was technical: It's not all that easy to get reliable and consistent output from such sophisticated new technology, and it was relatively easy for some of these data to be regarded as errors and anomalies.

   Figure 6a. If "Shrinking Apple" model were correct (no continental lateral movement), then there were two south magnetic poles in the past. Rocks from South America indicate the path of one pole (#1); African rocks show a second pole, which followed a different path (#2) hundreds of miles from #1.

   Figure 6b. If contents moved laterally, then the two paths are easily reconciled. There was only one south magnetic pole; the two contents were once fused; the fused pair moved over the fixed, single pole tegether.

   Figure 6c. The path shown here is a more extensive version (longer time period) of the apparent south polar wandering, based on African rock paleomagnetism. Note that the pole apparently moved from north of the equator to its present position, a trip equal to more than one-third of the earth's circumference. (South American data, not shown here, indicted a second pole that moved as far, but by a separate path.)

   Figure 6d. If Africa moved laterally, (rather than polar movement), then it must have moved to its present location from the other side of the globe (where, as shown here it was fused with South America.)

   
   
   
2. Sea-floor spreading
   The final blow to the "shrinking apple" model, the magnetic "stripes" that led to the concept of sea-floor spreading, requires one more morsel of background . At irregular intervals, the Earth's polarity reverses itself 180 degrees, and the north pole becomes the south, and vice-versa. Determining this fact was relatively straightforward. Successive layers (oldest to recent) of hardened basaltic lava were found to have reversed magnetism. When the same dated sequences were found at widely separated locations, these reversals were accepted as real events, and not just artifacts. We now know that there have been over 300 such reversals over the past 200 million years. There is an interesting aside to be made here. Reversal was actually first discovered in 1906, in France. Why was this striking fact put aside and ignored for 50 years, when it was rediscovered, and accepted? Think about that when you investigate "prematurity" in the last section.

   In any case, reversals became a fact of Earth history. You should be aware that these shifts in the field entail a gradual weakening of the lines of force. These decreases of field strength may go to completion (no field at all; no auroras!), and then reverse and strengthen, or they may not reverse, or they may reverse before reaching zero. We will keep it simple, and look at the clear reversals that occurred at irregular but datable intervals.

   With the enormous interest in paleomagnetism of the 1950s and 1960s, it should be no surprise that these techniques were applied to the sea floor. Technically, it's not quite as daunting as it might sound, in that one need not lower fancy equipment to great ocean depths. Sensitive magnetometers can be towed behind a ship, or even an aircraft, and they will pick up NRM from the basalt deep below the surface of the ocean. As data began to accumulate, a very distinctive pattern of magnetic "stripes" appeared. A highly diagrammatic representation of this pattern may be found in Figure 7; take a look at Hurley's article on Library Reserve for a more realistic view. In any case, the following observations (facts) were taking center stage:

   On either side of the mid-Atlantic ridge, there is a symmetrical pattern of magnetic reversals. The ridge itself, remember, was not discovered until after Wegener's death. What a shame that he was unaware of this massive structure!

   Figure 7. Polarity reverses at the mid-Atlantic ridge (highly diagranmatic). At different portions of the ridge, crust may move at different times/rates, creating transform faults (where portions of the crust move laterally).

   The "stripes" nearest to the ridge show today's polarity.
   The next set (one on either coastal side of the previous set) show reversed polarity.
   The next set, again, shows reversal, and so on to produce mirror-image symmetry around the mid-Atlantic ridge.

   As you may know from the section on Plate Tectonics, the "stripes" are broken and irregular from the formation of transform faults. Ask your instructor about how they form if you wish; they are not significant for our present discussion, except as another illustration of plate material moving sideways.
   
   

   1. With the information available (a mirror-image pattern of magnetic reversals) what conclusion might you reach? Remember, as yet you have no information on the ages of any of these stripes, so you cannot talk about younger vs. older sea floor. Think! It's important to understand what you can and cannot conclude from these data so far.
      
      
   2. Now suppose we add new information. First, it gradually became apparent that the mid-Atlantic ridge was not only an enormously long and continuous range, but it was also a rift system: That is, there is a major "crack" in the ridge. Second, the more difficult task of dating the "stripes" began to show different ages, with the youngest basalt flanking the ridge, and progressively older dates occurring as one moves toward the two continents

This final accumulation of data constituted extraordinary evidence for sea-floor spreading, and hence lateral movement of the continents perched, passively, on the moving plates. The revolution had truly won; the heretic was right after all. Yet one major question remains: why did it take so long?