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I know subduction in plate tectonics is mainly density driven.

Probably a hypothetical question, but can weaknesses caused by tidal forces have an influence on the start of a subduction zone? Would this have been more likely in earlier earth history when the Moon's tidal forces where much stronger due to it having been closer?

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My gut feeling would be, "no". Not because the forces are too small, but because they reverse with such a high frequency. People have been looking in databases of earthquakes and volcanic activity for decades for periodicities like this, because it seems reasonable to expect the state of the tide (position of the Moon) to have an effect. TTBOMK, any signals that have been reported are at quite low significance, and are disputed.

There's a philosophical challenge too. Does a subduction zone have a beginning in time that is meaningful at the "month" level? Except during aftershock swarms, it's not normal to get multiple major earthquakes in a month on a modest length of a subduction zone.

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    $\begingroup$ My "gut feeling" is similar, it's only I'd be happier if there was something more tangible. Btw., the question arose when discussing the possibility of biological activity helping the first subduction zones to form by building up massive BIFs, there was a paper recently ... $\endgroup$
    – user29219
    Commented Jul 16, 2023 at 22:49
  • $\begingroup$ Life -> BIFs -> density contrasts at surface -> initiation of plate tectonics. Well, it's an idea. Unfortunately for the idea, the oldest evidence for life that we have (though it's considerably disputed) is from isotope ratios in graphite grains in 3700~3800 Myr apatite crystals in felsic dykes exposed on Akilia Island, Greenland, which is probably in a plate tectonic setting. While the large accumulation of BIFs didn't get started until around 3000 Myr. I think your interlocutor's basic idea is a non-starter. Was there a reference after that ... ? $\endgroup$
    – Rockdoctor
    Commented Jul 17, 2023 at 15:55
  • $\begingroup$ Reference: agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2023GL103553. I am not a follower of the idea, simply because plate tectonics seem to be much older than the paper's setting, as you say. $\endgroup$
    – user29219
    Commented Jul 19, 2023 at 9:30
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Tons of articles on tidal triggering of global earthquakes. Like this one:

"A review of tidal triggering of global earthquakes" August 2022 Geodesy and Geodynamics 14(1) DOI:10.1016/j.geog.2022.06.005

https://www.researchgate.net/publication/362666022_A_review_of_tidal_triggering_of_global_earthquakes

And subduction can't be density driven. The average density of old and new crust is the same cuz their average temperatures are the same. Older crust is not getting colder, it's getting thicker. They both have the same temperatures on their bottoms and tops, thus their average temperatures are the same.

Edit: Actually, on de-lamination, an older slab produces a layer of material of density higher than the average density of the slab. But that happens at the end of subduction and couldn't normally be considered as the driving force for subduction.

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    $\begingroup$ This does not answer the question and anyway needs sources for some claims. It should at least differentiate between types of crust, and then the density-driven and temperature parts are wrong. $\endgroup$
    – user29219
    Commented Aug 21, 2023 at 22:08

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