[Rockhounds] WAS EARTH ONCE A WATER WORLD?

[pmodreski]

All sounds about right to me Alan.Sent from my Verizon, Samsung Galaxy smartphone
-------- Original message --------From: "(Alan Silverstein)" <ajs at silgro.com> Date: 3/26/21  1:15 PM  (GMT-06:00) To: rockhounds at rockhounds.drizzle.com Subject: Re: [Rockhounds] WAS EARTH ONCE A WATER WORLD? It seems to me the news here is, Earth once had up to 2x as much surfacewater as now; not just that it used to be covered by water.  I readyears ago that the planet started out with no continents, justoccasional volcanic island arc chains, otherwise mostly water-covered.(Of course not entirely resolved yet how the water got here from outsidethe "snow line" in the early solar system.)As plate tectonics ramped up (never mind the debate about exactly when),and lighter "crud" rock (granitics) separated from heavier mafic(basaltic, ocean-bottom) rocks, these arcs were "scraped off at themargins" (accreted) by the plate-based conveyor belt to start formingthe continents.  Is that a fair summary?Consider the Cheyenne Belt in Wyoming, the suture between the ancientcraton rocks to the NW (up to say 3600 MY) and the much younger rocks(max 1750 MY) to the SE.  I read that except for about 40 acres in farNW CO, no CO rocks are over 1750.  On a table at home I have, and enjoyfondling, a big chunk of innocent-looking Sacagawea gneiss from remoteBarlow Gap, WY, age ~3200 MY.I've also read how fortunate and unlikely it is for us to have "just theright amount" of surface water.  Too much, and it's a water-world withno dry land for more complex lifeforms to emerge.  Too little, and it'sa desert planet instead.  If you consider how thin our shell of water isrelative to the crust and the rest, it's really remarkably Goldilocks-ish.Cheers,Alan Silverstein_______________________________________________Rockhounds mailing listSubscription Services:  http://rockhounds.drizzle.com/mailman/listinfo/rockhounds_rockhounds.drizzle.comList Usage Policy: http://Tomaszewski.net/Kreigh/Rockhounds/Rockhounds.shtml