[Rockhounds] Earth's 1st continents arose hundreds of millions of years earlier than thought

[Paul van den Bergen]

Like the article summary says, it's been known that the cratons rose above
the water in places, but the current research shows they rose up widely in
the time frame given.  Older rocks (non-sedimentary rocks especially) do
exist, of course.

as to the island arc thing, that might be a good model for how plates
formed, but this epoch is pre-plate tectonics.


On Wed, 10 Nov 2021 at 14:25, Alan Silverstein <ajs at silgro.com> wrote:

> > https://www.livescience.com/earth-first-continents-cratons-study
>
> Very interesting, thanks!  Although I wonder about this statement:
>
> > Earth's first continents, known as the cratons, emerged from the ocean
> > between 3.3 billion and 3.2 billion years ago...
>
> ...because I have on my coffee table a several-pound chunk of Sacawee
> orthogneiss I collected years ago from remote Barlow Gap, on the Wyoming
> craton, reputedly radio-dated at 3258 Ma.  (Location = 42.64878
> -107.36938.)  First rocks above the waves?  Formed underwater?  It does
> make me wonder.  See this WGS tweet/photo for example:
>
>   https://twitter.com/wygeosurvey/status/978362569772580864?lang=en
>
> Plus if you Google around, you can find evidence of rocks in the Wyoming
> craton supposedly up to 3600, or even 4000, Ma, like this:
>
>
> https://www.researchgate.net/publication/330047933_Oldest_Rocks_of_the_Wyoming_Craton
>   Oldest Rocks of the Wyoming Craton, January 2019
>
> (Which I just found and haven't read yet, it begins:)
>
> > The Wyoming Craton has long been considered one of the oldest cratons
> > largely on the basis of Nd and Pb isotopic compositions of widespread
> > Meso- to Neoarchaean plutonic and metasedimentary rocks, which have
> > been interpreted to reflect interaction with Paleoarchaean (3.2-3.6
> > Ga) and Eoarchaean (3.6-4.0 Ga) sources...
>
> Also, I thought I read years ago that initially the only dry landmasses
> on the planet were volcanic island arc chains, which began to accrete at
> plate boundaries due to tectonics, basically being "scraped off" the
> underlying crust as it subducted, and slammed together with other blobs.
> This research report indicates that somehow larger cratons emerged,
> rather later, as whole units.  Not that plate boundary accretion didn't
> also occur; eg, the Cheyenne Belt in Wyoming where Colorado joined the
> fun no more than 1750 Ma ago?  (Correct me if I have this wrong.)
>
> Anyway, I like to help youngsters marvel at the concept that we mainly
> live on the "floating scum" of mostly silicaceous rocks we call
> continents, on a crust relatively thinner than an eggshell to an egg.
> Is that a fair summary?
>
> Cheers,
> Alan Silverstein
>
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-- 
Dr Paul van den Bergen