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long cores were placed in the magnetometer, itself a curious contraption of
iron and wires resembling a 6-foot rounded coffin. Eventually three num-
bers emerged: the strength of the magnetic signal carried within each core
and (when the various attitudes of the core and the sedimentary beds were
sorted out by the computer running the magnetometer) the declination and
inclination of the earth's magnetic field at the time the rock that had been
cored solidified. These precious numbers revealed whether the field at that
time was normal (like today's) or reversed. The numbers from Chico Creek
showed both normal
and
reversed polarity. I was jubilant: We had made the
first discovery of polarity reversal in Mesozoic-aged rocks collected from the
west coast of North America. But were these results "real"? Were they indeed
a magnetic signature frozen in stone since the Mesozoic Era, or were they
merely an artifact of later magnetism imprinting itself on these rocks?
This was the first inkling I had had that paleomagnetism was not so
simple a tool to operate as I had anticipated. It was explained to me that the
magnetic signal frozen in the sediments we had sampled—or in any other
rocks, for that matter—maintained its original direction only as long as the
rocks were neither reheated nor subjected to a great deal of ground water
passing through their pores. If the rock had been reheated enough, the tiny
magnetic rods of magnetite that had so faithfully recorded the ancient mag-
netic field present at their consolidation into this particular sedimentary
rock would have reoriented themselves to the direction of earth's magnetic
field at the time of reheating, and they would have done so without leaving
any clue. There would be no sign saying, in effect, "We have been reheated!
We now give spurious results! Paleomagnetist beware!" Huge mistakes can
be made. They have been made and will be made again.
And is that the end of it? No, there is more. Much of the time, the rocks
you sample have been heated, all right, but not quite to that magic tempera-
tute (called the Curie point) where the magnetic direction resets. Instead
they take on a slight, but not complete, overprint of new magnetic field di-
rection. In these cases the investigator must strip off this overprint, either by
progressively heating the cores in an oven and running them again, or by sub-
jecting them to an alternating electric field that (sometimes) accomplishes
