--- "R. Joel Duff" <rjduff@uakron.edu>
> wrote:
Hi,
I just glanced over this story this morning and was wondering if anyone had any comments on the implications of this theory on the polonium (?) halos so often talked about. Story can be found at: http://www.newsandevents.utoronto.ca/bin1/001206a.asp
Earth's continental land masses created in short, fast bursts
Findings challenge multi-million year continental crust theory
Joel
Brent Foster writes:
This has been discussed on the ASA and ACG lists so I'm x-posting this.
I don't think the findings the article cites indicate that Earth's continental land masses were created in short, fast bursts, nor do they challenge multi-million year continental crust theory. As Jonathon mentioned, the article adequately sumarizes "some of the innovations in understanding the genesis of granite that has occurred over the last 20 years" which indicate that emplacement of some plutons may have been quite rapid. But the formation of the continental land masses, and continental crust itself, involved the emplacement of countless intrusions. Stratigraphic, structural, and cross-cutting relationships all indicate that these were emplaced over considerable time intervals. Of course radiometric dates indicate this also, but that's not very convincing to a YEC who rejects the validity of radiometric dates. I'm reading an interesting article on the evolution of the Wyoming Craton ( ftp://204.144.241.6/pub/bulletin/b112564.pdf ). The Wyoming Craton is one of several !
Archean cores in North America. Since the Archean, the continent has grown by accretion of material to the margins of the craton. Where is the western margin of North America? The answer depends on what time is specified. In the Mesozoic the continental margin was east of the Klamath Mountains and Sierra Nevada, and migrated westward with the accretion of successive island arcs and seamounts. But even the Archean cratons, in many cases have long histories of accretionary tectonics.
The rapid formation of large amounts of granitic magma poses petrogenetic problems perhaps as daunting as rapid crystallization. By all accounts derivation of granitic melts directly from mantle rocks requires an unreasonably small degree of partiall melting and/or an unreasonably large degree of fractionation. Some crustal recycling is generally thought to be required for the formation of large amounts of granitic magma.
Brent
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