Re: Age of the Earth

[EMAIL PROTECTED] (George Prehmus) writes:

> Joseph Lazio <[EMAIL PROTECTED]> wrote in message news:<[EMAIL PROTECTED]>...
>>>>>>> "GP" == George Prehmus <[EMAIL PROTECTED]> writes:
>> 
>> GP> I understand that geologial dating is done by comparing long
>> GP> halflife radioactive elements in rocks.  The data so gotten shows
>> GP> the time since the rock last was molten.  The formation of the
>> GP> Earth is supposedly determined by this method as 4.6 BYA.  Yet I
>> GP> undestand that the giant impact which caused to formation of the
>> GP> Moon occurred at about 4.3 BYA.  When that happened, I understand
>> GP> that the entire surface of the Earth was heated to the melting
>> GP> point, resetting the Earth's atomic clock.
>> 
>> I don't think that's quite right.  The age of the solar system is
>> dated by meteorites, e.g.,
>> <URL:http://pubs.usgs.gov/gip/geotime/age.html>.  The ages of
>> meteorites are all around 4.5 billion years old.  Presuming that the
>> Earth and meteorites did not form at radically different times, this
>> must then be the age of the Earth.
>> 
>> Last summer I was at a conference in which a couple of researchers
>> claimed to have dated zircons (mineral inclusions in rocks) to ages
>> around 4.2 billion years old.  I think they were claiming that this
>> would make them the oldest known objects from Earth.
> 
> Thanks. That was a help, but OK, so where did I go wrong? Was the heat
> of that giant impact not enough to reset the clocks?

Given the uncertainties involved in the both the radiochemistry and the
chain of inferences as to when the "giant impact" took place, the few
percent difference between 4.2 Gyr and 4.3 Gyr represents a remarkably 
good agreement, IMO !!!


-- Gordon D. Pusch   

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