Geoscience Reference
In-Depth Information
But how could the Earth's North Pole have moved? What does the concept even
mean? Geologists in the early and mid-twentieth century often spoke of “polar
wandering.” They did not mean the magnetic pole's short-term drunken maypole
dance round the geographic pole. Rather, they referred to the possibility that the
Earth's crust and mantle might have rotated, or slipped, relative to the rotational
pole, which after all cannot move. This would not be the same as continental drift
butinsteadmovementonalarger,globalscalethatcouldcarrythecontinentsalong
with it. The ideas of Frank Taylor and Reginald Daly both envisioned this kind of
large-scale slippage of the Earth's crust.
From the perspective of one standing on a continent observing the rotational
pole, slippage of the crust would make it
appear
that the pole had moved, even
though it was the crust that had actually moved. If all one knows is that the pole
appears
to have moved, one does not know whether continental drift or slippage
of the crust, or both, have occurred. Thus when the early paleomagnetists used
the term “polar wandering,” they really meant “apparent” polar wandering, not the
thing itself.
Now back to Ted Irving. In his unpublished Ph.D. thesis, also from 1954, Irving
reported that the location of the ancient pole as recorded by the Deccan Traps
differed from the present pole by more than 70 degrees, indicating that India had
moved north by somewhere between 4,500 and 5,500 kilometers. Moreover, In-
dia had rotated 30 degrees counterclockwise. This still left open whether the crust
and mantle had rotated (polar wandering), India had drifted, or both had occurred.
Irving realized that scientists could decide among the three possibilities “by col-
if rocks of the same age from two different continents point to the pole in a differ-
ent place, then whether or not the pole has “wandered,” the continents have moved
relative to one another and to the pole: they have drifted.
In the same fertile year of 1954, Ken Creer (b. 1925), Runcorn's second Ph.D.
student, had assembled enough data on British rocks to trace the apparent position
of Britain over a time span of several hundred million years. Creer had been a stu-
dent at Cambridge with Irving and went on to design and build the first Cambridge
magnetometer. He plotted all the paleomagnetic data available from British rocks
on a single map, showing the apparent pole positions from the Precambrian to the
present. This was the first “apparent polar wandering curve” in the scientific lit-
erature. Confusingly perhaps, these curves represented the paleomagnetic data as
though the continents had remained fixed while the pole had “wandered,” even
though scientists were not sure that it might not have been the other way around.
This was simply the convention that they adopted. To repeat the import of the map,
