Geoscience Reference
In-Depth Information
the Danian, the first stage of the Tertiary, which started then and
lasted until about 60 million years ago.
Walter had not gone to Gubbio to study dinosaur extinction.
He and a group of American and Italian geologists were there to
measure the magnetism frozen in the Cretaceous and Tertiary sed-
imentary rocks handsomely exposed in a deep gorge nearby. They
hoped to be able to locate sections where the rocks had recorded
reversals of the earth's magnetic field—times at which the north
pole of the earth had acted as a south pole, and vice versa. (A mag-
netized rod or needle develops two poles that act oppositely.
Because one end of the rod points toward the current north mag-
netic pole of the earth, we say that it is the north-seeking end. This
property is the basis for the common compass.)
While his father, back at Berkeley, had begun to worry that
physics had started to leave him behind and that his career had
stalled, Walter and his co-workers were in Italy, engaged in research
that no one could have expected would aid in jump-starting Luis's
career. The geologists were attempting to determine the precise
patterns of magnetic reversals in rocks of known age, which would
then allow those same unique patterns to be used to date rocks of
unknown age. Thus Walter Alvarez and his colleagues were aiming
to fill in a gap in geologic knowledge, a vastly more common en-
deavor than launching a paradigm shift.
Geologists had discovered that, for reasons unknown, magnetic
reversals were frequent (on their time scale), occurring on the aver-
age about every 500,000 years. Because all rocks of a certain age,
wherever found, show the same magnetism—either normal (de-
fined as the situation today) or reversed—we know that the rever-
sals affected the entire earth at once. In the 1960s, analysis of the
magnetic reversal patterns in rocks from the seafloor showed that
sections of the floor on one side of, and parallel to, a mid-oceanic,
deep-sea volcanic ridge, could be matched exactly with the pattern
on the other side. Some clever scientists deduced that lavas were
being extruded at these ridges and, as they cooled, took on normal
or reversed magnetism, whichever was prevalent at the time. Later,
the frozen lavas were dragged out to either side as the seafloor
spread away from the ridge, to be replaced by a new batch of lava
that, if the earth's magnetic field had meanwhile flipped, would be
magnetized in the opposite direction. This proved that the seafloors
diverged from ridges, and it was only a small leap to conjecture that
continents, made of light, buoyant rock, would ride on top of the
spreading seafloors. Thus emerged the theory of plate tectonics, the
modern version of the theory of continental drift.
