HypotHESSes - Four Revolutions in the Earth Sciences - page 127

respond, “Too late—I can tell you the name. We've called it the Jaramillo event”
(347).
Opdyke returned to Lamont and began months of furious activity. He and his
colleagues reported their results in Science on October 21, 1966. 17 In eight separ-
ate South Atlantic cores, they detected each of the major magnetic reversal bound-
aries and each of the short events, including the Jaramillo. The ages of different
sections of the cores as deduced from paleomagnetism exactly matched those de-
termined from microfossils. Everything dovetailed. Lava flows extruded from vol-
canoes on land, basalts erupted onto the seafloor, and sediments that had settled
through miles of ocean water to accumulate on the seafloor all recorded the same
magnetic field reversals.
Confirming Evidence
Several new studies directly corroborated seafloor spreading and continental drift.
One came from the inventive, and former fixist, J. Tuzo Wilson. He showed that if
seafloor spreading is a fact, then the motion along the offsets to the Mid-Atlantic
Ridge shown in the map of the Atlantic Ocean floor in figure 11 would be opposite
to the traditional view of them as “strike-slip faults.” Studies of earthquakes along
the ridge soon showed that Wilson was right. He named the new faults “transform
faults.” 18 His key insight was that the seemingly offset segments of ridge crest
were not offset: they had never been joined in the first place. Instead, the apparent
offsets mark the original crack where two continents had split apart to give birth to
the Atlantic Ocean!
Geologists determine the age of sedimentary strata at different depths in a deep-
sea core using each layer's unique assemblage of micro fossils. When they com-
pared the ages in the Lamont sediment cores using fossils with the ages derived
from the magnetic reversals found in the cores, the two matched up perfectly. 19
And by either method, nowhere was the floor of the Atlantic Ocean older than
about 180 million years. Far from being ancient and permanent, the ocean basins
are young and ephemeral.
Another study spoke volumes to geologists. It compared the ages of rocks
in Africa with those in Brazil. 20 West Africa comprises two major geologic
provinces, one about 2,000 million years old and the other about 550 million, sep-
arated by a sharp boundary. When Africa and South America are laid alongside
each other in the Bullard fit, the extension of the African age boundary enters the
north coast of Brazil at a certain point. In collaboration with Brazilian geologists,
Patrick Hurley of MIT set out to see if he could find the African age boundary in