PLATES E: Magnetostratigraphy

                  The black and white bars are the magstrat record

 

Let’s say that (preposterously) it was of the utmost importance for you to determine if a limestone in Kentucky, a lava flow in Iceland, a young pluton in the Cascades, and a shale in Wales were or were not of precisely the same age.  How to go about it?  Well, as you have guessed the old saying “no way, Jose” applies here, in spades.

 You might get lucky and discover that the two sedimentary units both contained fossils known to represent organisms that flourished briefly, spread all over the place, and then suddenly died out.  Such “index fossils” could establish the approximate age equivalence of the two, but of course says nothing at all about the lava and pluton.  The latter units might be of a sort that can be “dated” radiometrically, but of course the standard workhorse methods of absolute age determination don’t work so well – at all, usually – on sedimentary rocks.  What is needed is a time scale that effects the entire earth, is sensitive and precise,  and works on all types of rock.

Well, we’re lucky – such a scale does exist, courtesy of the geomagnetic field.  When the field undergoes polarity transition it (potentially) leaves a precise mark on every rock body forming at the time.  The only problem is that this new (in the 60s) tool has the property of a berserk traffic light: red, or green, for unpredictable lengths of time, separated by very short intervals of orange.  (Orange representing brief transition intervals.)

Thus:  If your two igneous units have the same geomagnetic polarity they might be contemporaneous, but, then again, they might not be.  However, if they have opposite polarities they certainly are not of the same precise age.  Given modern lab gadgets magnetostratigraphy can be applied to most sedimentary rocks, too.  Thus, given fossils, radiometric dating, and rock magnetism you might be able to establish the age equivalence of your four rock units quite precisely - +/- 5%.  As I said earlier; no way, Jose.   

Well, as you probably guessed already, the phenomenon of geomagnetic reversals stirred up a lot of excitement.  In the United States, the U.S.G.S. under Allan Cox and Dick Doell began a well-funded effort to place absolute ages on each of the latest dozen or so transitions.  They did the obvious: sampling young, fresh volcanic exposures wherever they might lie, and determining their polarity.  Then a Survey colleague, Brent Dalrymple, measured their absolute age.  Plot ‘em on a time chart, and a useful stratigraphic tool emerges.  At the same time, Don Tarling (Newcastle, England) was perusing the same goal.  Fortunately, the two trans-Atlantic data sets were easily merged and complementary.  After not much time the absolute date of perhaps the latest several dozen polarity transitions became very well established.

One exciting result of this work was that it suddenly became possible to measure the velocity of seafloor spreading.  Vine and Mathews had shown that the spreading seafloor was a type of magnetic tape recording of the history of geomagnetic polarity transitions.  Now we could determine when those transitions had taken place!  Thus, for instance: if a reverse-to-normal transition was located, say, 100 km from the ridge at which it had originated and was now known to be 5 Ma old, the seafloor had evidentially moved 10⁷ cm in 5X10⁶ years, or an average of 2 cm/year.  Neat stuff, eh!

So, what has all this have to do with my experience of the early days of plate tectonics?  Quite a lot, actually.  From George Thompson’s wonderful class and my own mulling things about I had decided to do an MS project in paleomagnetism.  It just so happened that at the time I was living about 200 yards from the back door of the Cox and Doell lab!  Through the good offices of George I obtained a key to that back door, and most every night – after everyone had gone home – I made paleomagnetic measurements.  How did I know what to do?  Well, Allan was a bachelor with time on his hands; often he would come into the lab late at night just to fool around.  So, I learned technique from Allan.

My first project was a lava flow, known as Table Mountain, Pliocene in age, in the west Sierra foothills.  It turned out to be Reverse in polarity.  I had verified polarity transitions!  I was excited!  I remember shouting at Allan (doing something elsewhere in the lab):  It’s R!  He smiled indulgently.

So next time, polar wander curves.  Again, time for my nap.