PLATES D Magnetic polarity

 

Continuing my personal reflections on the birth of plate tectonics…..

During my graduate years at Stanford (1957-60) continental drift/seafloor spreading were much discussed elsewhere, but not – as I recall – by my own faculty.  Bill Dickinson was there and must have been mulling mobilist concepts – but my only contact with Bill was field camp which, for me, was a near-total bust.  Emphasis at Stanford at the time was almost entirely “classical”: how to use the petrographic microscope, make a map, find mineral deposits, etc.  Fortunately there was one “big picture” guy for me to lean on – George Thompson.  As I believe I mentioned earlier, George taught a wonderfully broad graduate course called Theoretical Structural Geology.  George’s course it was that entrapped me into what became the rest of my career – magnetism.

So, in about 1960 here is what definitely was known about rock and earth magnetism:

(1)             The earth had an intrinsic magnetic field, as had been demonstrated for thousands of years by the usefulness of the magnetic compass.

(2)             Some rocks were magnetic, as could be shown by merely waving a large stone (preferably basalt) near a reasonably sensitive compass.

(3)             Some rocks were magnetized in an anti-parallel direction – that is, north-seeking needle pointing south.  This had been shown by a guy named Brunhes early in the century.

(4)             These “reversely” magnetized rocks could not all be explained by mineralogical peculiarities, although some – a tiny minority. It turned out – could be.

(5)             Ergo, whatever caused the earth to be magnetic had the property of spontaneous reversal – north and south magnetic poles changing places.  Moreover, the exchange of polarity was accomplished swiftly – no more than ~ 2 ky, and often much less.

So, knowing all this (say, early 1960s), all hell was justified in breaking loose.  Take seafloor spreading, for instance.  It had been determined previously that there was a young, seismically active, occasionally volcanic ridge in the middle of the Atlantic Ocean (and similar ridges were known in other oceans).  It also was known that the age of the ocean floor increased monotonically away from the ridge, but that no ocean floor older than Jurassic existed anywhere.

SO, enter Fred Vine, Drummond Mathews, Alan Cox, Dick Doell, Don Tarling – and a huge mob of others.

First, Vine and Mathews drove the final, irrefutable nail into the coffin of fixist dogma by showing that heretofore mysterious linear magnetic anomalies that had been found running parallel to the mid-Atlantic ridge were symmetrical – that is, if you found a broad one so many kilometers east of the ridge, you would find an identical one that many kilometers west of the ridge.  And so forth.  Vine and Mathews drew the obvious, but paradigm-shattering conclusion: hot magma was continuously being extruded at oceanic ridges, acquiring a magnetic direction when cool enough – thereby recording the polarity of the geomagnetic field – then being passively carted away to either side by seafloor spreading.  Moreover, simultaneous studies were underway to determine an absolute age for each polarity transition – I will describe them next time, it’s time for my nap!

But, one last thing…  The oldest ocean floor rock was only Jurassic in age.  Did that mean that seafloor spreading only began ~ 200 Ma ago?  Seems unlikely, no?  Thus began the study of what came to be known as subduction zones.  Your homework:  Google Hugo Benioff and read about Benioff Zones.