Geoscience Reference
In-Depth Information
Within a short time, work by the USGS team and a group at Australian National
University had shown that the Earth's magnetic field and not the minerals them-
selves had reversed, that the reversals had occurred at irregular intervals, and that
there had been several, perhaps many, reversals. At first it appeared that once the
polarity flipped, it remained in the new state for half a million to a million years
before flipping back. But the more rocks the paleomagnetists studied, the more
complicated the picture became.
By the fall of 1965, the USGS trio had worked out the magnetic polarity times-
cale for the last four million years. One intriguing possibility was a short event
at 0.9 million years ago. By May 1966, they had corroborated the new event and
described it in a paper that, as the author William Glen judged, “was destined to
become a historical marker in the earth sciences.”
1
The chart shows the modern version of the paleomagnetic timescale back to
about 5 million years, with the longer epochs (now called chrons) and the numer-
ous shorter events (subchrons), the renaming done to conform to accepted geo-
logical nomenclature. The distinctive arrangement allows scientists to date rocks
by measuring the sequence of reversals in a vertical section of unknown age and
matching the pattern with the timescale, as though one were sliding two identical
barcodes alongside each other until the patterns coincide.
The use of the magnetic timescale for dating justified all the effort the pioneer
paleomagnetists had put into it, but it would turn out to have an even greater signi-
ficance. The key was to marry two seemingly unrelated lines of research: the mag-
netic reversal timescale as determined from rocks of the continents and the mag-
netism of rocks from the bottom of the sea.
