Geoscience Reference
In-Depth Information
magnetism of Pacific submarine volcanoes and atolls using an airborne magneto-
meter. During a group coffee break, a venue that has given birth to many a pro-
ductive idea, Mason asked a colleague whether anyone had thought of towing a
magnetometer behind a ship, as that would allow scientists to record magnetic data
while the vessel conducted its other operations. As Mason remembered,
“What's that?” came a deep voice from behind me. Roger Revelle, director of Scripps, had
overhead the conversation. After the briefest of explanation, Roger, in his characteristically
direct way, asked, “Well do
you
want to do it?” to which I promptly replied, “Yes,” and I be-
came Scripps' magnetometer man.
2
Mason borrowed his first magnetometer from Lamont, then built his own. In
1955 he learned that the U.S. Coast and Geodetic Survey ship
Pioneer
was going
to measure the depth of the ocean over a wide area off the western coast of the Un-
ited States. The influential Revelle secured permission for
Pioneer
to tow the new
Scripps magnetometer behind it, and the measurements began.
The magnetic intensity at any point on the surface of the ocean is a combination
of the Earth's inherent magnetism, which stems from the core, and the magnetiz-
ation of the rocks on the seafloor. When seafloor rocks magnetized in the same
direction as the Earth's magnetic field lie beneath the magnetometer, they add to
the measured strength, producing an above-average reading: a “positive anomaly.”
Those magnetized in some other direction detract and produce a negative anomaly.
Mason and his colleague Arthur Raff obtained their best results from off the coast
of the Pacific Northwest, where scientists had already discovered that the seafloor
is riven by “transcurrent” faults many hundreds of miles long.
At first the magnetic data so baffled Mason that he was on the verge of giving
up. But as he and Raff plotted the positive anomalies in black and the negative an-
omaliesinwhite,insteadoftherandom,salt-and-peppermotifthattheymighthave
expected, there gradually emerged a curious pattern of “zebra stripes,” as elong-
ated, north-to-south-trending bands of greater and lesser magnetism alternated.
3
The Raff-Mason map appeared in print in 1961 and eventually became even more
crucial in corroborating continental drift than the Heezen-Tharp map of the ocean
as scientists could understand the import of those maps at a glance, several years
would pass before anyone figured out the meaning of the “stripes on the seafloor.”
