Geoscience Reference
In-Depth Information
But what if, once the test is satisfied, the revealed magnetic direction places the
ancient pole at a different place than the pole occupies today? Then there are three
possibilities: the continent has moved, the pole has moved, or both have moved.
Something has moved.
In 1949, John Graham (1918-1971) of the Carnegie Institution of Washington
was the first to apply the “fold test.” He found that when he restored folded rocks
to their original position, their magnetism typically did point to the same pole pos-
ition. Graham concluded: “The direction of magnetization in certain selected sedi-
mentary rocks remains unchanged for at least 200 million years.”
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Beyond the question of the stability of rock magnetism was whether the Earth's
magnetic field has always been dipolar: having a single north pole and a single
south pole, as it does today. If not, then scientists could explain any paleomagnetic
result by appealing to a field with more than one north pole, effectively rendering
the method indeterminate.
In 1949 a young Dutchman named Jan Hospers (1925-2004), having survived
Nazi occupation, arrived in Cambridge to earn a Ph.D. Fortune favored Hospers
when a Dutch research team, planning an expedition to study volcanic rocks and
tectonics on Iceland and looking for a student to join them, chose him. One of
the team leaders suggested that Hospers measure the magnetism of Iceland's vol-
canic rocks. He found many with reversed polarity but could detect no difference
between them and the rocks that displayed normal magnetism. He became con-
vinced that the reversals represented actual flips of the Earth's magnetic field. He
also found that when he averaged all the pole positions, they came to the same po-
sition as the present pole, leading him to conclude that the field had been dipolar
at least back to the Tertiary.
By the mid-1950s, the work of Graham, Hospers, and the other pioneer paleo-
magnetists had shown the following: paleomagnetism is usually stable, roughly
half of all rocks show reversed magnetism, the field averages to a dipole over the
long run, and the Earth has had a magnetic field for at least 200 million years.
The reversals remained a puzzle, but since they were by 180 degrees, they did not
prohibit the scientists from locating the past position of the pole. Which end was
which did not matter for that purpose.
In order for past pole positions to be of any use in deciphering earth history,
however, one had to know the age of the rock formation from which the samples
came. Here was a conundrum. The most highly magnetized rocks, like the basaltic
lavas that Hospers had studied, are volcanic. But in the early 1950s, with the
potassium-argon method of age determination still in its infancy, basalts could not
be dated directly. Scientists obviously cannot date lavas using fossils. Sedimentary
