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  #1  
Old 12th March 2013, 03:55 PM
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Default How do we know the age of the Earth?

What are the best references which demonstrate our current understanding?
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  #2  
Old 12th March 2013, 07:14 PM
Gorinnosho Gorinnosho is offline
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Default Re: How do we know the age of the Earth?

I am going to go with http://en.wikipedia.org/wiki/Age_of_the_Earth
Plenty of references and links.
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Old 28th January 2015, 11:49 PM
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Default Re: How do we know the age of the Earth?

Hi djarm67,
Once again, this is a very late reply, but as I just completed a geology/biology double major I can provide the information in case any other forum members have the same question. Absolute age of the earth is determined by zircons in volcanic ash beds. Zircons (zirconium silicate) are isomorphic minerals which are clusters of silicate tetrahedra with a gap in the middle large enough to hold a zirconium cation. They can also substitute a uranium cation, but not a lead cation, as lead is too big (so any lead in the zircon is the result of radioactive decay of the uranium). When this cools it reaches closure temperature. The uranium, being radioactive, then starts to decay to lead, so by measuring the ratio of lead to uranium, you get the age of the closure temperature of the zircon. This can be cross-checked as there are 2 separate decay chains U238-Pb206, and U235-Pb207. One of these has a half-life of 4.5 billion years, and the other 700 million years.
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Old 29th January 2015, 09:31 PM
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Default Re: How do we know the age of the Earth?

Fascinating, now I know.
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Old 29th January 2015, 10:49 PM
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Default Re: How do we know the age of the Earth?

Thank you STOKER. Most people don't think beyond a lifetime, or just the year. Not so geologists.
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Old 11th March 2015, 12:38 AM
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Default Re: How do we know the age of the Earth?

Very nice section about this in Bill Bryson's 'A Short History of Nearly Everything'. Worth the read.
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Old 11th March 2015, 03:26 PM
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Cirrusly Cirrusly is offline
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Default Re: How do we know the age of the Earth?

Quote:
STOKER said View Post
Hi djarm67,
Once again, this is a very late reply, but as I just completed a geology/biology double major I can provide the information in case any other forum members have the same question. Absolute age of the earth is determined by zircons in volcanic ash beds. Zircons (zirconium silicate) are isomorphic minerals which are clusters of silicate tetrahedra with a gap in the middle large enough to hold a zirconium cation. They can also substitute a uranium cation, but not a lead cation, as lead is too big (so any lead in the zircon is the result of radioactive decay of the uranium). When this cools it reaches closure temperature. The uranium, being radioactive, then starts to decay to lead, so by measuring the ratio of lead to uranium, you get the age of the closure temperature of the zircon. This can be cross-checked as there are 2 separate decay chains U238-Pb206, and U235-Pb207. One of these has a half-life of 4.5 billion years, and the other 700 million years.
Hi Stoker. Never read this kind of explanation before. Very succinct. May I have your permission to quote it to an acquaintance on my FB timeline please?
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Old 11th March 2015, 05:31 PM
STOKER STOKER is offline
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Default Re: How do we know the age of the Earth?

Quote:
Cirrusly said View Post
Hi Stoker. Never read this kind of explanation before. Very succinct. May I have your permission to quote it to an acquaintance on my FB timeline please?
By all means Cirrusly, it's all yours.
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Old 11th March 2015, 05:50 PM
STOKER STOKER is offline
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Default Re: How do we know the age of the Earth?

Hi Cirrusly,
I should add, that was a very simplified explanation. I didn't want to bore readers with too much detail. But seeing you showed an interest I can go into a bit more detail. I used the example of volcanic ash beds simply because they cover a larger geographical area than lava flows, so are more useful for stratigraphic correlation. Also zircons are only in lava that has chemistry ranging from andesitic through to rhyolitic, NOT basalt. Their is simply not enough silica in mafic (basaltic) lavas to form zircons. The oxygen atoms, instead of binding to silica, to form silicate tetrahedra, bind to zirconium to form baddelyite (which is rare). So do date basalt flows you need to use Argon-Argon in plagioclase or whole rock. Theirs lots of other amazing details of course, volcanic ash beds in different locations can be shown to be from the exact same eruption by their geochemistry etc, etc.
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Old 11th March 2015, 07:51 PM
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Default Re: How do we know the age of the Earth?

Stoker - Cirrusly was not the only person in this forum who read your responsemore than a month ago now with interest
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