Naked Science Forum
General Science => General Science => Topic started by: thedoc on 22/08/2012 10:30:02
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Liza Le Roux asked the Naked Scientists:
Hi Chris,
I have a question about the methods used to age the Earth. I understand that the age of particular rocks are estimated by measuring the rate of radioactive decay, and that the age of the Earth is based on the age of meteorites measured using the same methods.
So my question is, why would meteorites show the age of the Earth, would the radioactive minerals within them not decay at the same rate in space as the rocks on Earth?
Thanks,
Liza le Roux
Port Elizabeth
South Africa
What do you think?
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I think the meteorites pick up new radioactive elements from the solar wind/cosmic rays, or with interactions with the high energy particles in space.
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The problem with dating the earth, is that you can only date the oldest rocks avaliable at/ near the surface of the the crust - any rock that has melted may have had the radionuclide clock reset - depending on which minerals you are trying to date and how you are trying to date them (some "zoned" zircons can be used to date formation and subsequent metamorphic events).
In most places the crust is a lot younger than the earth due to volcanic action or the oldest "basement" rocks being buried under sediments or have been metamorphosed etc. The oldest rocks that we have found are dated at around 4 billion years old from some very old rocks in Canada.
If our theories about planetary formation are correct, some types of meterorite would have formed at the same time as the earth and spending the last few billion years in space have not been buried or metamorphosed so the radionuclide clock has never been reset. Thus, these can be used to date the formation of the earth.
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There is some evidence that the rate of radioactive decay varies with the Neutrino flux from the sun but it is is controversial and the effect is small.
If we define space as interstellar there would be some change in the decay rate compared to within the Solar system.
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There are at most very minor differences -- less than about 1% -- between these rates.
Several different dating methods based on radioactive decay are used in dating various features of the Earth's ancient history. One of the problems is -- when you ask how old a sample of material is, what are you really counting as its birth? For example, with radiocarbon dating -- only a useful method for the most recent 100,000 years or so -- it is the date when the sample stopped exchanging carbon with the atmosphere, which usually means "when did this carbon stop being part of a living plant?"
One of the methods for dating rock samples that runs back to the origins of the Earth is potassium/argon dating. It regards the birth of a rock at the time it became a solid which trapped any Ar-40 from decay of the K-40 then contained in it. An accurate date depends on the melt it came from being thin enough (non-viscous) to allow argon to escape that had previously been generated by K-40 decay, and in a position where it would not have been trapping Ar-40 generated and escaping from deeper rock while it was solidifying.
(A fairly standard ploy of "young-earth" activists is to go out and grab a bit of scoria, obviously full of gas bubbles, from last year's volcanic eruption, do a conventional K/Ar dating on it and say that you cannot rely on K/Ar dating because this rock that we know solidified only last year has been dated at 2 billion years old!)
I do not know much about dating techniques other than these two, but each of the other methods would have its own particular definition of the "birth" of a sample, and its own particular sampling caveats.
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I think that in dating the age of meteorites, astronomers/geologists (astrologists?) are trying to date the formation of the solar system, not the Earth in particular.