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Isotope Dating

  1. Feb 28, 2009 #1
    Hello my smart fellows, I was wondering why scientists say that when an ancient civilization constructs a structure, they can tell the date that the structure was built with radioactive dating methods? Surely the materials used to make the structure already HAD an age, so how would they be able to discern between the age of the raw mud/sand and the finished structure's age? Thanks.
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  3. Feb 28, 2009 #2


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    Carbon 14 has a half life of about 5730 years. Carbon 14 is produced continuously by radiation hitting the upper atmosphere and so the percentage of carbon which is carbon-14 is relatively fixed in plants as they continuously produce sugar using sunlight.
    Similarly animals which eat plants and the animals which eat those animals etc will also have this fixed ratio until they die.

    Once tree is used for a fire, or to build a spear or an animal hide is used to make clothing then the carbon-14 decay causes the amount to decrease. If you find in a sample only half the initial carbon 14 then you know about 5700 years have passed since the organic object came from a living organism.

    BTW one of the tricks that has been used to forge ancient documents is to get ancient blank parchment and use charcoal also from that period to make your ink. That way radiocarbon dating will show the material of the document is as old as the claim even though the document has been written recently.

    Similarly one must be careful in dating a sample to prevent contamination by living bacteria or other organic matter so that the level of C14 reflects the true age. The shroud of Turin has been carbon dated by 3 independent labs and they agree that it is much too young to be authentic according to radiocarbon dating. But the true believers can dismiss this evidence by assuming the shroud has been contaminated by contemporary organic matter a la bacteria...faith can let you believe mountains have moved.
  4. Feb 28, 2009 #3
    Actually, I am inquiring about everything except organic matter. Most buildings (pyramids, colloseums, ect) are constructed of rock, or inorganic matter..... why would the carbon clock start just by using the material to build a building?
  5. Mar 1, 2009 #4


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    Well if you're just looking at stone then you can't use carbon date it directly. Or if it is say something like limestone you can only date the age at which the sea-life formed the carbonates which later became the rock (generally much longer than the scale of carbon dating can measure anyway). So you are right the carbon clock doesn't start just because it was reformed into a building.

    But most structures will also have organic components. Straw for bricks wooden doors and flooring and roofing. One may assume a door isn't built centuries before the doorway. There's also carbon pigment in adornments. Soot from torches and lamps and fires. One may assume a building is older than the charcoal you find in the fireplaces.

    There are other isotope ratios and decay product ratios which can be used to date inorganic material but only if, as you point out, a baseline can be established. Some baselines can be established by knowing the chemistry of the rock formation. I leave it to you to research the details.
  6. Mar 1, 2009 #5
    I have a related question -- I read that rocks are often dated by measuring the abundance of Pb 206 ... knowing the half life of U 238 turning into Pb 206 ... that most "natural" Pb is Pb 208, so almost all Pb 206 must have started out as U 238. But how does this indicate when the rock was formed? Why doesn't it just indicate how much time has passed since the supernova created the atoms? What does the formation of the rock have to do with it?
  7. Mar 1, 2009 #6


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    This I know less about but it has to do with the chemistry of formation. Uranium is quite reactive as compared to Lead. So I assume that when the rock forms from molten ingredients this fixes a particular percentage of U vs Pb.

    One could also figure that during formation certain daughter products of U decay will not be present e.g. radon gas being inert would not be in the molten precursor to the rock while radon produced from U decay will be trapped in solid rock. Thus one would compare percentage of radon with U and Pb and other products to try and fit them to the decay model which starts with effectively 0% Radon larger amount of U and less Pb.

    Ultimately there must be some chemical or mechanical cause for the baseline from which one then determines the starting point for the decay process used in radiometric dating.

    [Edit]P.S. Here is a good article on the topic I found in a quick google search: http://facstaff.gpc.edu/~pgore/geology/geo102/radio.htm"
    Last edited by a moderator: Apr 24, 2017
  8. Mar 1, 2009 #7


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    The archeologists will be looking at many more than one sample from a site. Not infrequently they have found that wood in buildings has been recycled from older ones.
  9. Mar 1, 2009 #8


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    See e.g. http://en.wikipedia.org/wiki/Isochron_dating" [Broken]
    Last edited by a moderator: May 4, 2017
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