My next step in understanding geophysics came when my father took me along to a lecture. At that point I was somewhat immunized against continental drift by the professor at Harvard, in spite of my unanswered questions. Since the topic of the talk was something to do with continental drift, I was prepared to be quite critical. Remember this was in the early ‘60’s, before the idea of plate tectonics. The notion that sea floor was the youngest, not the oldest, crust on Earth had not even entered anyone’s mind, and the lack of traces of the continents plowing through the sea floor seemed definitive.
That lecture totally changed my attitude.
The lecture was about paleomagnetism, the fact that when lava cools, it retains the signature of the terrestrial magnetic field present at the time. The horizontal part of the field gives the direction to the north pole; the vertical part gives how far the pole is from the site — the latitude. There were complications – sometimes two lava flows close enough in time and space that they should have pointed to the same pole had exactly opposite directions. But if the north and south poles were considered interchangeable it was found that rocks of the same age on the same continent pointed to a consistent pole location at any given time in the past.
(Why the magnetic signature seemed to reverse at times was a mystery at the time and is still not totally understood, though it now known to be a reversal of the magnetic field rather than a reversal of the magnetism of the rocks.)
The next step was to produce what are called apparent polar wander curves: plots of how the pole moved through time as seen from the site of the lava flow. Again, it was found that these curves were quite consistent for a given continent. (There are exceptions, but I’ll get to them in a later post.)
But the curves for different continents were quite different.
In particular, if the curves for north America, Africa and Europe were compared, and the continents were assumed to move in such a way that they “saw” the same pole, those continents must have been snuggled together back in the late Triassic.
I walked into that lecture convinced that the apparent fit of the continents (and the geology) of the continents across the Atlantic was a coincidence, and that Wegener’s continental drift hypothesis was wrong. Certainly his mechanisms were; there was no evidence that continents had ever plowed through seafloor. But I walked out convinced that while Wegener’s hypothesis was wrong in detail, the continents had indeed moved.
Formally, I shifted my studies for the next few years to ice fog. Informally, I kept trying to make sense of solid-earth geophysics. Could there be some sort of underground erosion going on? What about those faults with hundreds of miles of displacement that disappeared when they reached continents?
Luckily, the major journal of my field was the Journal of Geophysical Research, so I was able to follow the steps people were making in the gradual emergence of plate tectonics. More of that later – but in the order I remember finding out, rather than in the order the discoveries were made.