Okay, here I
am again, at Stanford, eagerly absorbing all the new stuff that eventually
brought us to Plate Tectonics. Possibly
the most conclusive bit that came along at that time was what was (and still
is) known as apparent polar
wandering. I started to explain
it last time, but didn’t get very far.
By now you
are comfortable with the notion that the magnetic direction locked in some
types of rock formations can tell us where the pole was at that time, with
respect to that rock unit. Now, for
reasons best left to Physics 101, it appears extremely unlikely that the pole
itself moves much; consider the dynamics of a spinning top – or a
gyroscope. Thus, if paleomagnetic poles
for a given continent or tectonic terrane appears to move, it is the rock mass –
not the pole – that must be in motion. Ergo, curves of apparent polar wander. The rock mass is moving, not the pole. We abbreviate this as APW.
As I tried
to convey last time, if there had never been any change in the configuration of
the earth’s surface there would be no apw – every proper paleomagnetic study
would return a paleomagnetic pole very near the spin axis – the geographic
pole. Polarity reversals were not
precluded – but there would have been no apw.
However, it
might happen that the whole earth moved, as a rigid mass, with respect to the
spin axis. This can be illustrated by
setting a slightly asymmetric basketball to spinning on a table top - it will
orient itself so as to position any excess mass on the equator. Try it yourself. This whole-earth repositioning, which might actually
involve only what we now call the lithosphere, apparently does occur, but seems
to be minor. It would reveal itself as identical
apw on every continent.
Well, of
course, that’s not at all what was found.
Each continent as studied showed evidence of apw, and none of the
resulting apw paths came at all close to coinciding. The evidence was clear and, to my mind at
least, completely irrefutable – the continents had moved with respect to one another, as
well as with respect to the spin axis. Moreover,
if one graphically replaced the continents to what already was known as the Pangaea
fit, these disparate apw paths tended to merge – to coincide. That was it, end of story. Wegener was right!
I should
mention that much – most – of this work was done by people trained at the university of Newcastle
upon Tyne, by a brilliant, enigmatic man named S.K. Runcorn. Ted Irving (Australia, later North America), Ken Creer (South America, later Scotland), Ernie Hailwood (Africa, later
England), Don Tarling (mainly Europe), and a bunch of others I can’t remember all
were trained by Runcorn. In the
meantime, the USA was lagging far behind.
But in the next phase, we caught up.
On to (mathematical) plate tectonics!
thank you
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