3 ms·

Can anyone explain something that's always puzzled me: when dating the ages of rocks, geologists look at the proportions of the isotopes that are in the rocks.

Can anyone explain something that's always puzzled me: when dating the ages of rocks, geologists look at the proportions of the isotopes that are in the rocks. Because isotopes decay at a well known rate (they have a half life), then by knowing the proportion of the isotopes that the rock would have had when it formed and comparing to the observed ratio now, they can tell how long ago the rock formed. http://en.m.wikipedia.org/wiki/Isotopic_dating http://en.m.wikipedia.org/wiki/Isotopic_dating By "formed" I guess this means "cooled down" so it's not magma or lava anymore. But how do they know the initial isotope proportion? When an "old" rock with some ratio of isotopes melts and becomes magma or lava again, why does this "reset" the proportion of its isotopes? To put it another way: the isotope will have also been decaying when it was part of the magma wouldn't it? Why don't new rocks have the same isotopic ratios as old rocks?

deciplex 12y ago
You can get some idea of how this is done from the link you provided. It differs depending on the dating method, like for Carbon-14 the amount of it in the environment is pretty much constant (Carbon-14 is generated in the upper atmosphere). So you can date an organism by when it stopped taking in new carbon, i.e. when it died. For other, longer-lived isotopes, keep in mind you are not just measuring the amount of radioactive material left, but are also measuring the amount of the daughter product mixed in some proportion with it. And the proportions in the material being dated might not be the same as what they are in the surrounding environment (in fact, can not be, if the dating method is going to work). As a simple example, you could date rocks with uranium and lead, since you would expect there to be more lead in old rocks because some of the uranium decayed to lead. However, if that rock melted and mixed with younger rocks, the proportion of lead and uranium would cease to be of any interest.
jofer 12y ago
Great question! In a nutshell, it's because we choose minerals whose crystal structure can only bond with the original element and not with it's "daughter" that's produced after radioactive decay. For example, Uranium/Lead dating of zircons is one of the most common methods (particularly for very old rocks). There's a reason we use zircons for uranium/lead dating, and not just any mineral that contains trace amounts uranium. In zircon (ZrSiO4), uranium can (and does, in small amounts) substitute for zirconium in the crystal structure and bond with the silica tetrahedra. However, lead cannot bond with the silica tetrahedra in zirconium's place. Therefore, the zircon crystal structure "rejects" lead, and any lead in the crystal lattice at the present day must be the result of radioactive decay. If we know the rate of radioactive decay of the uranium isotopes in question (there are two uranium isotopes that decay to two different lead isotopes), all we need to date the rock is the present-day ratio of a particular uranium isotope to the corresponding lead isotope. In practice, you also exploit the fact that there are two uranium isotopes that independently decay to two different lead isotopes. This gives two independent dates, and disagreements between them can be use to test for contamination, etc. (There are also a few other checks I won't get into... I'm not a geochronologist, and each different isotopic system and instrument used to measure it has slightly different protocols. Reality is messy, but this gives the basic picture.)
- Patient0 12y ago
  Thanks so much! Such a great answer! This is why I love Hacker News :-)

deciplex 12y ago

You can get some idea of how this is done from the link you provided. It differs depending on the dating method, like for Carbon-14 the amount of it in the environment is pretty much constant (Carbon-14 is generated in the upper atmosphere). So you can date an organism by when it stopped taking in new carbon, i.e. when it died. For other, longer-lived isotopes, keep in mind you are not just measuring the amount of radioactive material left, but are also measuring the amount of the daughter product mixed in some proportion with it. And the proportions in the material being dated might not be the same as what they are in the surrounding environment (in fact, can not be, if the dating method is going to work). As a simple example, you could date rocks with uranium and lead, since you would expect there to be more lead in old rocks because some of the uranium decayed to lead. However, if that rock melted and mixed with younger rocks, the proportion of lead and uranium would cease to be of any interest.

jofer 12y ago

Great question! In a nutshell, it's because we choose minerals whose crystal structure can only bond with the original element and not with it's "daughter" that's produced after radioactive decay. For example, Uranium/Lead dating of zircons is one of the most common methods (particularly for very old rocks). There's a reason we use zircons for uranium/lead dating, and not just any mineral that contains trace amounts uranium. In zircon (ZrSiO4), uranium can (and does, in small amounts) substitute for zirconium in the crystal structure and bond with the silica tetrahedra. However, lead cannot bond with the silica tetrahedra in zirconium's place. Therefore, the zircon crystal structure "rejects" lead, and any lead in the crystal lattice at the present day must be the result of radioactive decay. If we know the rate of radioactive decay of the uranium isotopes in question (there are two uranium isotopes that decay to two different lead isotopes), all we need to date the rock is the present-day ratio of a particular uranium isotope to the corresponding lead isotope. In practice, you also exploit the fact that there are two uranium isotopes that independently decay to two different lead isotopes. This gives two independent dates, and disagreements between them can be use to test for contamination, etc. (There are also a few other checks I won't get into... I'm not a geochronologist, and each different isotopic system and instrument used to measure it has slightly different protocols. Reality is messy, but this gives the basic picture.)

Patient0 12y ago
Thanks so much! Such a great answer! This is why I love Hacker News :-)