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.

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.

PLATES B: Continental Drift

                       Bullard's best (computerized) fit

This is the second of a series of blogs in which I will describe how I dealt with the birth of the plate tectonics paradigm, which now of course now completely dominates geotectonics.  I was in the perfect place to observe: I was at a major university, studying geology, totally absorbed by it, old enough to be skeptical – and completely ignorant!

Well, the paradigm came about in three installments: continental drift, ocean floor spreading, and finally plate tectonics proper.  I will begin with the initial phase.  

In the late 1950s, at Stanford, here is what some of us were mulling over, concerning drift: the “fit” of the Atlantic-bordering continents, geologic “matches” on either side of that ocean, the Glossopteris flora of what came to be known as Gondwana, and evidence of Permian glaciation in those same Gondwana continents but not elsewhere.  The heroes of this stage were Alexander Dutoit, Sir Edward Bullard, and others:  and of course Alfred Wegener, who initiated the whole thing.

Wegener’s evidence basically consisted of the statement that the fit of continental outlines, as well as geologic and fossil similarities, on Atlantic-bordering continents were to exact  to be coincidental.  For instance: the geology of the Caledonian mountains of northern Europe is an excellent match to the geology of the Appalachian mountains of North America.  Moreover, a peculiar and distinctive fossil flora was known to be present, along with glacial deposits, in Africa, India, South America, and Antarctica – but not elsewhere.  The overwhelming precision of the Bullard fit (shown) should alone have proved the case but, of course, it didn’t.

In the meantime, I was tearing through my elementary classes as quickly as possible, going to all our guest lectures, and reading on my own.  By the time the next stage in the revolution rolled around I was pumped and raring to go!

If you want my guess as to why every geologist of that era didn’t welcome continental drift with open arms and a kiss on the cheek, click on: 

https://frivilousessays.blogspot.com/2020/10/s-warren-carey-part-2.html 

THE BIRTH OF PLATE TECTONICS, A

 

This is the first of a number of blog entries concerning my recollections of the early development of what has become the dominant paradigm of modern geotectonics – plate tectonics.  You might call it “viewing the birth of modern tectonic thought, from way back in the bleachers”.  I will explain.

I became seriously involved with geology in 1957, when I already had attained the advanced age of 24.  After graduating from Stanford with what proved to be a well-nigh useless degree in Economics, and then whiling away two years in Germany with the United Stated army, I had returned to Stanford to study geology.  As I had only completed a single course in the subject previously (oddly enough – taught by Bill Dickinson, who later became my principal antagonist) - I was essentially a freshman,  Consequently, I enrolled in Geology 1a, or whatever they called it – general physical geology.  As I recall, it was taught by Art Howard, who was essentially a classical geomorphologist. 

For me, the most exciting topics in that course were big-picture things; the origin of continents, ocean basins and mountain ranges.  (I have always enjoyed big-picture speculation: to hell with the details!).  I do not recall Dr. Howard spending much time on such topics.  He lectured that mountain ranges probably were the result of contraction of the earth due to cooling.  If he mentioned continental drift at all,  I don’t remember it.  The text book did cover drift, in two short paragraphs; the first a half-assed summary of Wegener’s ideas, and the second a scornful rejection.  The main lines of counter argument were that (a) drift was impossible, given the properties of the earth’s interior*, and (b) Wegener’s evidence was inexact and/or could be explained otherwise.

Well, two years later I was an all-in drifter.  I will describe why, next time.  

*see https://frivilousessays.blogspot.com/2020/10/s-warren-carey-part-2.html 

EGYPT 2. SOME READING

         Linda didn't really like camels a whole lot

You would think that, at my age, a person would have learned how not to become voluntarily self-overburdened with “projects”.  You might think that – but you would be wrong.  At present I am juggling my cancer blog (MyrlsBlog), as well as several other topics, “The minimum you need to know in order to get the most out of a trip to Egypt”, and a collection called ‘Terranes”, supplemental to  Nick Zentner’s You Tube geotectonics lectures (“Nick from Home”).  Now has arisen another topic, one which I really want to pursue – my personal recollections and reflections on the birth of what became plate tectonics.  I was an observer, not (until much later) a participant – but my observations may be of some little historical value someday, and with luck will be interesting right now.  I will post them on my “Frivolities” blog, maybe under the title of “The birth of plate tectonics, as virerd from way up in the bleachers”.

But, in the meantime, if you are planning to visit Egypt any time soon, in lieu of personal advice I suggest several books:

Mertz, Barbara, Temples, Tombs and Hieroglyphs.

Perhaps a little dated, but beautifully written

Tyldesley, Joyce, Egypt: How a lost civilization was rediscovered.  History of Egyptology

Clayton, Peter, Chronicle of the Pharaohs.  Probably more than you really wanted tp know about 3000 years of Egyptian history.

Christmas Letter, 2020

                    Damn, how I miss these people!

                         Well, not Santa Claus, necessarily 

Well, not much good one can say about the year 2020, other than that it’s almost over,  My adventures for the year have consisted mainly of trips to the doctor; my health remains pretty good for a man of my advanced years (currently 87 7/12), but numerous consultations with medical science seem to be required.  Like a good boy I am staying home to avoid giving/receiving our friendly local virus.  The only people I ever see, other than my two daughters and my cleaning lady, are happy, mainly be-masked dog owners (through the window), transporting their pets to and from the dog-pooping grounds a few blocks away.  Makes me appreciate cats, especially on rainy days.

But, of course, I have good family news to report.  I have two new great grandchildren to make me smile and give me hope for the future of the human species:  Thomas Robert Moreno, only a few months old, and Hugo Wilde Weise, who is a half-year or so older.  Both cute as buttons, of course.  Add them to Evelyn and Seamus, both seven and in school, for goodness sake, and Finnegan, not far behind – and you have proof that I have done my part to replenish humanity.  Now if this damned virus would only let me go see them…..

For the last few months I have been thinking about geology, a little bit, anyway.  This is mostly owing to the fact that there seems to be renewed interest in some ideas I wrote about 40-50 years ago.  In essence, I apparently am regarded as a kind of interesting archeological specimen!  Anyway, to see how it all started, go to Google and ask for Nick Zentner You Tube * 13, and don’t be dismayed at how old I look.

So, anyway, I hope that your holiday season is as nice as can reasonably be expected, and that 2021 proves to be much better than our current unhappy year.  Hang in there.

 

SUGGESTED READING

 MAGNETIC LINES OF FORCE OF A GEOCENTRIC AXIAL DIPOLE

I have a suggestion for all of you who follow Nick Zentner’s lectures, as well as any other people out there with an active, if perhaps comparatively non-professional, interest in geology and, especially, geotectonics.  Much of current discussion surrounding Cordilleran tectonics revolves around data from the field of paleomagnetism.  Thus it would be very useful to understand how rocks acquire a permanent magnetic direction, as well as what that direction can tell us.  Bob Butler has written an excellent book on those and related subjects which is available free on line.  However, I suspect that Bob’s treatment of rock and earth magnetism may be a bit beyond the average non-professional reader.  For that reason, I suggest that you go to your local library and bully them into obtaining a copy of Paleomagnetism and its application to geological and geophysical problems, written by E. Irving and published by John Wiley and Sons  in 1964.  Section 2 explains rock magnetism in a way that is easy to comprehend, and the rest of the book – while outdated in some areas, of course – is also worth studying.

You might even think about buying a copy.  Although currently out of print, there is a used one for sale listed on the web site www.abebooks.com,  But I warn you – it’s not cheap!

ANCIENT REFLECTIONS

 

As read by Nick last night

  For what they may be worth, here are some of my reflections on the state of geotectonic thinking about the Western Cordillera during the 1970s and 80s.  Note that they are MY reflections and recollections: other players such as Ted Irving and Davey Jones may have had somewhat different views; too bad they’re not around to comment.

Well, first and foremost, I didn’t regard the bulk of displaced crustal blocks as in any way “exotic”.  Without much thought, I casually assumed that they had originated as part of North America itself, and had been transported relatively north by the north-oblique nature of subduction of the Farallon plate.  In fact, I spent inordinate amounts of time and energy pondering the circumstances under which this would occur.  It turns out that, theoretically, what is needed are:  a shallow subduction angle, a high angle of obliquity – and a mysterious factor measuring  the “stickiness” between the interacting plates, which I never figured out how to measure.  I wrote several papers on the subject which you can find very easily; the most cited one appeared in the 1991 volume of Physics of the Earth and Planetary  Interiors. 

Another player in the northward-transport game was, of course, the Kula plate – but we didn’t know how far south it had ever extended.  Clearly, detach a chunk of crust and attach it to the Kula – and northward displacement would be very fast.  But we didn’t know how, when, or even if this (might have) occurred.

Also, in those days I thought of northward displacement as occurring piecemeal, along a series of moderate-sized strike-slip faults and shear zones.  Then Ted introduced the concept of Baja BC, which seemed to imply existence of a San Andreas-type super through-going break.  That really got the geologists stirred up!

I said earlier that I thought of these allochthonous terranes as originally part of North America:  not “exotic” by today’s terminology.  The one universally acknowledged, indubitably exotic terrane was of course Wrangellia, which contained rocks that must have originated in the ocean.  Davey Jones, being a paleontologist, may have thought more along “exotic” lines, but I don’t recall him talking about it.

The next exotic terrane to be discussed in those early days was, I think, Stikinia, but by that time I had departed for the relative simplicity (and excellent cuisine) of the Central Andes.