21 January 2023
Plate Tectonics: a very short introduction, by Peter Molnar
Posted by Callan Bentley
This slim volume (130 pages of ~10 point type) is the 425th in Oxford University Press’s vast series of dense little books about various subjects. Browsing the geology shelves at my college’s library this week, I saw it and thought I might as well check it out. I’ve shifted through the years in what I put weight on when teaching plate tectonics, and I always appreciate reading/hearing/seeing what different professionals choose to emphasize. One thing that I really appreciated about Chapter 2, on seafloor spreading and magnetic anomalies, was short descriptions of key papers early on in the revelation of seafloor spreading (late 1960s) that were, it turned out, scientific stumbles. One of these issues was the “polluting” signal that came from a submarine seamount’s magnetic signature superimposed over the background seafloor’s pattern of magnetic stripes, creating confusion. Another had to do with the fact that some seafloor in the Pacific had originally formed in the southern hemisphere (and thus had an “upward” vector to its ‘normal’ magnetic inclination, but later moved into the northern hemisphere (where we would expect a “downward” vector to its magnetism during the same periods of a ‘normal’ magnetic field. Sorting this mistake out allowed the measured Pacific seafloor magnetic stripe record to make a lot more sense. Another area of emphasis for Molnar is explaining fracture zones, and the debate over whether they represented strike-slip faulting of the transcurrent or transform variety (with opposite kinematic implications). That was great, if a little dense. It was an excellent encapsulation of data acquisition, interpretation, testing, iteration, and reversal, followed by convincing the majority of geologists that their initial instincts were exactly backwards! One additional thing I will mention: Molnar regards plate tectonics as an idea which does a great job specifically of explaining and making predictions about the oceanic lithosphere, but he pulls out continental tectonics as a separate beast. Despite the book ostensibly focusing on plate tectonics, he devotes a chapter to deformation in the continental crust, with the distinction being the rigidity of the plate. Oceanic plates see all the action at the edges (plate boundaries), while continents often see much more diffuse zones of deformation within their interiors. Witness the thousands-of-kilometer wide belts of scrunching and crunching that make up the Himalayas, Tibetan Plateau, and Baikal rift in Asia, or the weirdness of the Basin and Range or Laramide Rockies in North America. According to Molnar, these aren’t due to plate tectonics, and require a separate explanation (he gets into the “jelly sandwich” model of the strength of the continental crust, and invokes flowing Camembert cheese as an analogy elsewhere). In other words, behavior that doesn’t approximate a “plate.” I guess that’s legitimate, but in a way it feels a bit like hair-splitting. An interesting read, and I think I’ll steal some of the book’s figures in my teaching.
Interesting, since I’m presently reading H.W. Menard’s memoir of the development of plate tectonics (“The Ocean of Truth”) in which he addresses some of those same issues, the complications of understanding the magnetic stripes and the nature of fracture zones (which Menard was involved in discovering). It will be nice to compare his view to Molnar’s. Thanks for sharing this.
This feels like deja vu (all over again). When I started teaching about plate tectonics in 1971, Molnar and Menard were two whose papers and/or books I used. It’s hard to believe that was more than 50 years ago! I still have a big stack of Lamont-Doherty reprints from the 60s and 70s. I was sad to hear that Prof. Molnar passed away last year