PLATES C: Ocean floor spreading

                                           Ocean floor spreading

Continuing my tale of confronting plate tectonics….

Here I plan to record what I can remember of my wrestling match with the second of my three stages – ocean-floor spreading.  It wasn’t much of a struggle; I succumbed immediately.

There were lots of heroes who contributed to this stage; I remember particularly Harry Hess, Bob Dietz, and Fred Vine.

I am writing now about the latest 1950s and the very early 60s.  By that time I had ripped through Stanford’s rather stodgy undergraduate curriculum and was taking graduate seminars, reading on my own, and talking to my fellow grad students.  Most of them simply wanted to finish up, get a good job, and get on with life.  However, a few were as speculative and impractical as I turned out to be.  I remember in particular Bob Speed (later professor at Northwestern), and Dave Scholl (later with the USGS).  I also should mention George Thompson who taught a wonderful advanced seminar called Theoretical Structural Geology.  As for the rest of the Stanford faculty, well – they definitely were not on the cutting edge.

So what was going down at the time?  Well, Harry Hess was reporting results from a series of systematic surveys of the depth of the oceans.  With regard to the Atlantic, he found that there was a ridge running down the middle, and that it got deeper as you approached the bordering continents.  He could do that thanks to instrumentation developed during WW2; Hess had been a naval officer.  As a Princeton professor he also noted, and puzzled over, the abundance and distribution of strange suites of rocks in orogenic belts.  These were called, of course, ophiolites.

At the same time Bob Dietz was working for for the Coast and Geodetic Survey.  He also pondered the meaning of the pattern of ocean bathymetry being revealed at that time.  I believe that Dietz coined the term ocean floor spreading, although Hess and quite a few others must have been thinking along those lines.

There is one important wrinkle here that puzzles me: Who was it that determined the age of the ocean crust, and how did they do it?  Somehow it became known at this time that the crust was very young at the mid-ocean ridges, much older near the continents – but, compared to continental rocks, very young overall.  This thickened the plot immensely.  I assume that K-Ar dating was somehow involved.

So, it became obvious by at least 1962 that young oceanic crust (in the form of basaltic magma) somehow appeared at ridges, and then spread out laterally.  The obvious motor for such behavior was some kind of thermal convection in the mantle.  Confronted in the early 1960s by this body of evidence it became an act of heroic stubbornness to deny that Wegner had been right all along – but, amazingly, the majority of geologists still did.  It took the work of Fred Vine and other dabblers in rock and earth magnetism to drive in the final nail in  the fixist coffin.  That’s where, finally, I got involved.  I will write about it next time.