Geoscience Reference
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Vine and Wilson published in October 1965, one month before the annual meet-
ing of the Geological Society of America. Meanwhile, the USGS team of Cox,
Dalrymple, and Doell had been hard at work dating and measuring the magnetic
polarity of volcanic rocks of different ages. The discovery of the short magnetic
events, so brief as to be easily missed, complicated their task. But one of those
events would clinch the case for seafloor spreading and throw open the door to
plate tectonics.
The Jaramillo Event
By 1964-1965, the USGS trio had analyzed fifteen specimens in the range
between1.0and1.5millionyears,findingthatallhadreversedpolarity.Specimens
from the present back to about 0.7 million years had normal polarity. For the gap
between 0.7 million years and 1.0 million years they had only a single specimen,
from the Bishop Tuff in California, and it had reversed polarity. To fill in the gap,
Doell and Dalrymple went searching for rocks in the critical age range from 0.7 to
1.0 million years ago.
They found them in a suite of volcanic rocks from the Valles Caldera in the
Jemez Mountains of New Mexico north of Santa Fe. Of six specimens from the
volcanic suite, three dated to near 0.7 million years and had reversed polarity;
one dated to 0.88 million years and had an intermediate polarity, as though it had
frozen while the field was in mid-reversal; one dated to 0.89 million years and had
normal polarity; the oldest, at 1.04 million years, had reversed polarity. Thus, near
the end of a long period of reversed polarity, the field had flipped briefly to normal
and then back again. 14 Dalrymple and Doell named the brief excursion the “Jara-
millo normal event,” after a small creek near the site of the critical outcrops. It is
shown near the top of the paleomagnetic timescale in figure 16 .
FredVineremembered Dalrymple telling himoftheJaramillo eventatthemeet-
ing of the Geological Society of America in November 1965. Vine “realized im-
mediately,” he recalled, “that with that new timescale, the Juan de Fuca could be
interpreted in terms of a constant spreading rate.” Vine and Wilson had been trying
to use the paleomagnetic timescale to match the simulated and the actual magnet-
ic patterns, but with the Jaramillo event missing. When they added it, the real and
simulated profiles came into perfect concordance. 15
Here was a correspondence that defied coincidence: the paleomagnetic times-
cale, when combined with the assumption of a constant rate of spreading outward
fromaridge,preciselypredictedthepatternofseafloormagneticanomaliesaround
midocean ridges. One group of scientists had developed the paleomagnetic times-
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