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By 1964, paleomagnetic research was already shifting direction. No longer would
the emphasis be on determining past pole positions, which had already confirmed
drift to the satisfaction of anyone with an open mind. Now the research swung to
using the magnetic reversals to date rocks. To everyone's surprise, including the
scientists involved, it would turn out to be the reversals, more even than the differ-
ing apparent polar wandering curves, that would ultimately corroborate continent-
al drift. But first the paleomagnetists had to convince themselves that the reversals
reflected actual changes in the Earth's magnetic field rather than being attributable
to self-reversal or some other unknown process. The way to find out was to de-
termine whether rocks of the same age from different locations all had the same
polarity: normal or reversed. If they did, then the magnetic field, rather than the
minerals, had reversed. Basalts are the most strongly magnetized rocks, and by the
early 1960s, geologists had learned how to measure their ages using the decay of
potassium to argon.
Allan Cox (1926-1987) and Richard Doell (1923-2008) had trained at Berkeley
and joined the U.S. Geological Survey to establish a program of paleomagnetic re-
search. In the summer of 1961, they met Brent Dalrymple (b. 1937), a graduate of
Occidental College and by then a second-year graduate student at Berkeley. Cox
and Doell invited Dalrymple to join them to set up a K-Ar dating program at the
USGS so they could work out the paleomagnetic timescale.
Doell had collected basalts from Hawaii but found that all were normally mag-
netized, so the three turned to rocks from California that they knew had both nor-
mal and reversed magnetism. Dalrymple's K-Ar dating showed that the California
rocks ranged in age to just over 3 million years. Combining these results with three
previously reported measurements on Italian volcanic rocks, the trio published the
first magnetic reversal timescale. But they had only nine rock units to base it on,
so their results did not pin down whether rocks of the same age everywhere have
the same polarity, nor whether the polarity epochs were all of the same duration.
The latter point was also crucial. If the epochs were all of the same duration, then
rocks of different ages would show the same pattern of reversals, obviating the use
of paleomagnetism to date rocks.
