Geoscience Reference
In-Depth Information
same lack of precision remains a limitation. But there is one pos-
sibility: the magnetic reversal time scale that led Walter Alvarez
to Gubbio in the first place. It would be ironic if that same scale
could be used to falsify the Alvarez theory, but that is exactly what
Officer and Drake claimed to have done. Here is the basis for their
approach.
As briefly described in Chapter 1, over the past few decades, geo-
physicists have established that the earth's magnetic field has repeat-
edly reversed its polarity.
3
The north magnetic pole has acted alter-
nately in the way we define a north magnetic pole as acting, then as
a south magnetic pole, then as a north pole again, and so on, over and
over, throughout hundreds of million of years. These reversals have
affected the entire magnetic field, all around the globe. During a
period of reversed magnetism, a compass needle, which seeks a north
magnetic pole, would instead point toward magnetic south. It is
not known why the earth's magnetic field reverses, though super-
computer modeling is beginning to shed light on the mystery. But
remember that we discover facts and invent theories. We have dis-
covered that the earth's magnetic field has reversed itself hundreds
of times, on the average about every 500,000 years; so far we have
not been inventive enough to figure out why.
Magnetized rocks of different ages around the world have been
dated using one of the radioactive parent-daughter pairs, and thus
we know, within the precision of those methods, when each reversal
occurred. The K-T section of the magnetic reversal time scale is
shown in Figure 11. The major intervals are represented by shaded
and light bands, called chrons, numbered and designated R for
reversed and N for normal. Ordinarily, the magnetic time scale suf-
fers from the same lack of precision as the other methods and can-
not be used to show that two rocks have precisely the same age. For
example, if all we know is that two rocks belong to Chron 29R, we
have not pinned their ages down to better than ±750,000 years, the
duration of that chron. But suppose on the other hand that we can
establish that one rock unit belongs to 29R while the other belongs
to 28R. We can then be certain that the two are
not
of the same age
and that their ages must differ by at least 800,000 years, the dura-
tion of intermediate Chron 29N. (The absence of 29N in the region
under study means either that rocks from that age were never de-
posited, or that they were subsequently removed by erosion.) Thus,
paleomagnetism may be able to show that although two rock layers
date to the same general part of the geologic time scale, they do not
have identical ages. This was the opening that Officer and Drake
hoped to exploit.
