Wandering Poles or Drifting Continents? - Four Revolutions in the Earth Sciences - page 98

rocks, on the other hand, are well dated by fossils and by stratigraphy, and they do
contain magnetic minerals like magnetite and ilmenite. Graham had gotten encour-
agingpaleomagneticresultsonthesedimentaryrockshestudiedbutultimatelyhad
lost confidence in his results. It was apparent by the early 1950s that if paleomag-
netism were to lead anywhere, scientists needed to be able to apply the method to
sedimentary rocks.
In 1951, Runcorn hired a young geologist named Edward “Ted” Irving (b. 1927)
as a research assistant. The pair set about collecting sedimentary rocks and meas-
uring their magnetism. Inventing their techniques as they went along, they soon
found that fine-grained “red beds”—iron-rich sandstones and siltstones—gave ex-
cellent paleomagnetic results.
Since the 1500s, it has been known that a compass needle not only swings to
point to the North Pole but if freely suspended “dips” or inclines at an angle that
corresponds to latitude. At the Equator the dip is zero, at the poles it is 90 degrees.
Paleomagnetic measurements not only point to the ancient pole; they also show
the dip, or “magnetic inclination,” thus allowing scientists to determine latitude at
the time the rock formed. Examining the maps that Wegener and his father-in-law
had produced, Irving realized that whereas South America and Africa might have
separated in an east-west direction, maintaining roughly the same latitude, India
had moved several thousand miles, from the Southern to the Northern hemisphere,
changing its latitude by tens of degrees. If the continents had drifted, the paleo-
magnetism of rocks from India would be most likely to reveal it. Irving was not
the first to realize the significance of India to drift, but he was the first in a position
to do something about it.
In 1952, Irving received seven samples of a thick volcanic sequence from India
known as the Deccan Traps. We will meet them again in part 4 , when we take up
the role of meteorite impact in geology. The traps date from uppermost Cretaceous
into the Tertiary, straddling the “K-T boundary,” when the dinosaurs disappeared.
Breakthrough
Meanwhile, the Manchester paleomagnetists under Blackett were focusing on
nearby British rocks. In 1954, his team reported that Upper Triassic fine-grained
red sandstones showed pole positions that deviated markedly from the present
field. Relative to the pole, England appeared to have moved hundreds of miles
north and rotated through 34 degrees. 2 But what had actually moved: England, the
pole, or both?