How the geo strata are identified as time periods

Some volcanic horizons are indeed very useful for radiometricly dating, and for the dating of sediments between volcanic horizons. But basalt, the mafic (high iron and magnesium) end member of the types of igneous rocks is (and someone correct me if I'm wrong) not radiomentrically datable by the common methods, including such as Potasium/Argon, Uranium/Lead, and variations there of. Basalt is very low in Potasium and Uranium.I don't know if there are more recently developed radiometric methods that are useful for dating basalts.Now, if you had instead said rhyolite (the felsic end member, high in feldspars and silica) you would have been on solid ground. Rhyolite very much so can have useful Potasium and Uranium contents. OK, if you substitute something along the lines of rhyolite for basalt. OK, to a degree. If the fossil has been shown to have a short species life time, then it can be used to date a sedimentary rock to within a fairly narrow time span. But if the fossil has a long species life time, then the siedimentary rock can only be dated to be within that long time span.Moose

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Professor, geology, Whatsamatta U
Evolution - Changes in the environment, caused by the interactions of the components of the environment. "Do not meddle in the affairs of cats, for they are subtle and will piss on your computer." - Bruce Graham

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"The modern conservative is engaged in one of man's oldest exercises in moral philosophy; that is, the search for a superior moral justification for selfishness." - John Kenneth Galbraith

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"I know a little about a lot of things, and a lot about a few things, but I'm highly ignorant about everything." - Moose

THIS IS SERIOUSLY GETTING OFF-TOPIC, SO WE NEED A NEW TOPIC IF THIS LINE OF DISCUSSION GOES ANY FURTHER.From message 1 of the new "A Guide to the Bait & Switch tactics of the Biblical Creationists" topic: The above caused me to look into the radiometric dating of basalts a bit more. "Bolds" newly added.Well, I was wrong (or at least, less than totally right), and no one corrected me. At least sometimes, basalts are datable and dated by the Argon/Argon method (and other methods). In Radiometric Dating there are examples of lunar basalts being dated by the 40Ar/39Ar method, and also by other radiometric methods.In particular, see the above cited for an example of a single rock sample that was dated by a variety of methods. See the "Apollo 11 - High-K basalt 10072" table.Well, I'm not going to go any further here. But I certainly wouldn't mind someone else explaining any proper datings of basalt by the K/Ar, Ar/Ar, and/or Rb/Sr methods. Especially the 40Ar/39Ar method. But it needs to be in a new topicMoose