PLATES F: Paleomagnetic poles

 

Continuing my recollections of the early days of plate tectonics…..

We are talking here about the state of MY comprehension circa 1960.  Allan Cox and Dick Doell had just published a Review paper in the GSA Bulletin, in which they did a fair job of outlining nuts and bolts, but were (in my later view) too reluctant to embrace some obvious but less orthodox conclusions.  Ted Irving’s magnificent book, which put it all right, was still a few years in the future.  Thus I was left to confront apparent polar wander more or less on my own.

Here, generally, is what I knew (c. 1960) to be true:

1)     1) To a first approximation the earth’s magnetic field, however  generated, has the same configuration as if it were generated by a very short, very powerful dipole magnet located at the center of the earth and oriented along the axis of rotation.  This was called the axial dipole field.  Temporary departures from this configuration were numerous, but small and – importantly – random.  This was known as the secular variation.

2)     2) Many rock types had the ability to acquire a permanent magnetic direction as they formed, then of retaining that direction essentially forever.

3) Note that magnetic directions are directions (vectors)  in three-dimensional space. They are described as follows.  The angle of the vector above or below the horizontal is known as its inclination; the angle of its horizontal projection  makes with due north is the declination.  Declinations are measured clockwise, hence range from zero (due north), through 90 (due east), to 180 (south) on around 360 degrees back to north.  Inclinations range from negative 90 degrees (straight up at the south magnetic pole), through horizontal (at the magnetic equator), to positive 90 degrees (straight down, at the north magnetic pole).  If this confuses you, check Butler's on line textbook.

4) Given D and I for a given rock unit, it becomes simple (given a dipole field)  to calculate where the pole would have to have been to produce that direction of permanent (we called it remanent) magnetization in that particular rock body.  Simply draw a great circle path from the rock’s location on a sphere in the direction indicated by D, go out an angular distance p, given by the formula cot (p) = tan (I)/2, and plop – there’s your paleopole.  If the D,I used represented an average direction representing a decent average of the normal variability of the geomagnetic, the pole thus determined would be regarded as a best estimate of the location of the geographic pole with respect to the continent on which the sampled resided at that particular time.  Such a pole position was called a paleomagnetic pole.  If it was pretty certain that the variability (called the secular variation) had not been properly averaged out, we called the thing a virtual geomagnetic pole. 

All this stuff was being hashed out as I watched from the bleachers, notebook a’tremble.  Obviously, it was fundamental stuff.  If all the paleomagnetic poles, of whatever age or location, consistently clustered near the present spin axis then the fixists had won; nothing interesting, like continental drift, could ever have occurred – and I might as well go home and work in the family lumber yard.  Another possibility was that the pole appeared to have moved – paleomagnetic poles tracing a path away from the present pole as the rocks sampled increased in age – but that the paths from all the continents coincided.  That would indicate a phenomenon known as true polar wander, which still is investigated today.

Naturally there was a third possibility, which I will take up next time.