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Re: JUST a question....

Buckaroobwana asked:

Fossils are not carbon dated, but the sediments they're found in are.

And then Jerry Hodges replied:

Carbon dating is only done on organic material,not sediments. Volcanic ash, other sediments, and clean crystal faces are sometimes done with potassium/argon testing. There are variables and inconsistencies in all these tests to some degree, thus the use of age ranges. If a sample is "clean", most tests can be relied upon.

I would only like to add a little more info here (and folks who are more geologically inclined than I, please correct me should I make some errors here). The basic idea behind dating rocks using radioactive elements is comparing the amount of parent product to daughter product in the rock sample and then determining how long the rock has been around based on what is known about the half-lives of the radioactive materials.

Generally, we only date volcanic rocks. When rock is molten inside the earth, radioactive elements pass into and out of it because it is essentially like a liquid. When the molten rock comes to surface as lava and then cools and crystalizes, the now solid rock traps radioactive elements within it and prevents new radioactive elements from entering it. It is now a closed system. Therefore, because nothing can leave or get in, the radioactive elements inside begin to decay and build up their daughter products.

The rate of radioactive decay is constant, and cannot be distrubed by pressure, heat, etc., unless the rock is reheated to the point of re-melting, in which case new radioactive contaminants can enter the specimen. This is why metamorphic and sedimentary rocks are not used to determine the age of a particular stratum or formation: metamorphic rocks have been re-melted in some cases, and sedimentary rocks are formed from the remnants of many types of different rocks. In either case, we would get misleading dates.

Furthermore, many rock samples are taken from a location to be dated. Having a large sample size greatly decreases the possibility of making an error in regard to the age of the rocks, and sample average is then used as the rock "date." Remember, usually only volcanic rocks are dated, and the date refers to when the rocks cooled.

If you had a layer of dino bones that was capped above and below by volcanic rock, you could potentially take the radioactive dates from both volcanic rock layers and get an estimate as to when the dino bone bed you were interested in was laid down. Or, if the dinosaur bones were buried in a volcanic ash sediment, perhaps the ash could be dated as well.

Again, while I have a background in geology, I am somewhat rusty on the radioactive decay front. To the best of my knowledge, this is the general idea, but please add on or correct me if necessary.

Adios for now,

Matt Bonnan
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