How Does Uranium-Lead Dating Explain Age Differences in Rock Layers?

[Impulse2]

I was looking at a program referring to dating the life of the Earth and I`m puzzled on a bit that I`m sure must be obvious to geologists. The dating was based on the ratio of lead that was present in combination with uranium. Lead being the final element after the uranium decay.
It was this varying ratio from locality that enabled the dating of the area. But doesn`t all the uranium on the Earth decay at the same rate and would not all have the same Ur/Pb ratio? Why would the decay, as was stated, of rocks at the bottom of the Grand Canyon be a billion or so years different from those on the surface as determined by the Ur/Pb ratio?

 

[tiny-tim]

Hi Impulse2!

These particular rocks are created with uranium and no lead.

Yes, the uranium in the rock has been decaying "for ever", but the lead produced has just disapppeared until the uranium got into the rock.

Only then is the lead trapped (and so is the uranium, of course) where we can find and measure it.

(for more details, see http://en.wikipedia.org/wiki/Uranium-lead_dating" )

 

[Cleonis]

Why would the decay, as was stated, of rocks at the bottom of the Grand Canyon be a billion or so years different from those on the surface as determined by the Ur/Pb ratio?

I started reading the link to the wikipedia article mentioned by tiny-tim.

When molten material cools very slowly there is a gradual process of crystallization. The process of crystallization has a purifying effect.

In fact, crystallization is how sugar that is harvested form beets or sugar canes is purified. The unpure suger is dissolved in hot water, and then the temperature of the solution is decreased very slowly. The sugar starts to crystallize. A crystal with no inclusions of molecules with a different shape is the lowest energy configuration. So when the crystallization proceeds slow enough non-sugar molecules that do not have a snug fit in the crystal do not find a place, and you end up with purified sugar crystals. (Sugar that is purified in this way is often branded as 'crystal sugar'.)

For Uranium-Lead dating only specific materials are suitable: they must be crystals that formed during very slow cooling of molten material, and they must be materials in which Uranium is readily incorporated in the crystal structure, while Lead is not incorporated.

Such materials do exist, and the wikipedia article mentions that usually socalled Zircon crystals are picked. Zircon is Zirconium Silicate.

The formation of Zircon crystals results in separation of Uranium from Lead, and that way the "clock" is reset.

Cleonis