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Reading a course on Precambrian, I read that:

Earth Age (around 4.5 billion years) is dated thanks to the meteorites hitting Earth during its formation rather than the inner materials composing the Earth.

Wouldn't it be more accurate by doing it with its inner materials? Why do we use meteorites if they hit the Earth after its formation?

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  • $\begingroup$ Where does the text about the Precambian come from? $\endgroup$ – arkaia Nov 18 '14 at 14:01
  • $\begingroup$ It comes from : www2.ggl.ulaval.ca/personnel/bourque/intro.pt/… But if you can't understand French, I doubt you'll be able to read it, I'm sorry. $\endgroup$ – Chirac Nov 18 '14 at 22:56
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Some background:

We are able to determine the age of certain rocks and minerals using measurements of radioactive and radiogenic isotopes of certain elements. The most common are U-Th-Pb, Rb-Sr and Sm-Nd. Simply put, the resulting date is the time that has passed from the crystallisation of that mineral. Obviously there are complexities, but there are not critical for this answer.

Why do we use meteorites if they hit the Earth after its formation?

Short answer: because the meteorites formed together with the Earth and the rest of the Solar System.

Long answer: The Earth formed together with the rest of the Solar System and its meteorites around 4.5 billion years ago. When meteorites fall on Earth and you pick them up, you are able to date the time of their formation.

You say:

Why do we use meteorites if they hit the Earth after its formation?

You have to distinguish the time that the meteorites form and the time that they hit the Earth. If I throw a meteorite at you, and you date it, it still records the formation time and not the time that I threw it at you. Hitting the Earth does not reset the radioactive clock in the meteorite's minerals.

Wouldn't it be more accurate by doing it with its inner materials?

It would. There are two problems:

  • We don't have materials from the time of Earth's formation. The Earth is a dynamic place, and rocks are getting formed and destroyed all the time (also see related question). It may be that such old rocks exist on Earth, but because they are so old there are either metamorphosed and buried deep in the Earth or covered by sedimentary rocks. The oldest exposed rock on Earth available for study is the 4 billion year Acasta Gneiss in Canada. The oldest mineral on Earth is a zircon found in Australia, which is 4.4 billion years old. These are the only two materials that are known to be older than 4 billion years on Earth. There could be more, but we just couldn't find them.
  • The moon forming event occurred some tens of million of years after the formation of the Earth. This event destroyed the Earth's crust and any evidence of the age of the Earth, on Earth itself.

This is why meteorites are excellent for this task - they mostly formed during the formation of the Solar System.

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  • $\begingroup$ If we, I suppose, use uranium datation for rocks, it means fluctuations of the Earth stratums implies modifications that make this datation irrelevant? We could use rock datation, but most of them are unreachable, am I right? Thank you for this Michael. $\endgroup$ – Chirac Nov 18 '14 at 12:42
  • $\begingroup$ Think that you are standing in front of a group of 2000 people and you want to know who is the oldest person in that crowd, but you can only ask the age of the 10 or so people standing right next to you. So they are between 20 and 30 years old. But there could be someone who is 40 years old. You just can't reach him to ask him for his age. $\endgroup$ – Gimelist Nov 18 '14 at 12:59
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    $\begingroup$ @Chirac ... what Michael is eluding to is that you need a large enough sample, because radioactive dating basically uses a ratio of the amount of material that has decayed relative to the amount that has not decayed. So both the uranium and the lead need to still be together in a large sample. Furthermore, there are two decay pathways. $\endgroup$ – farrenthorpe Nov 18 '14 at 16:16
  • $\begingroup$ Actually, everyone who said that a meteorite was formed 4.5 bn years ago is wrong. The meteoroids formed then. A meteoroid becomes a meteor when it enters the atmosphere, then if anything makes it all the way to the planetary surface, that remnant is a meteorite. $\endgroup$ – user1158 Nov 18 '14 at 20:47
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    $\begingroup$ To be pedantic and avoid Monty's objection, you could say that the minerals of which meteorites are composed, were formed back then. "The meteorite" as an object, since it is by definition a remnant from a meteoroid and later a meteor, was formed on impact. Similarly, we say that Michelangelo's David was formed in 1501-1504, not whenever the marble formed :-) $\endgroup$ – Steve Jessop Nov 19 '14 at 13:48
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The answers that have been provided are correct but they're omitting the fundamental issue that explains why they are correct:

When you date a rock you get the point that it solidified, not the point the material came into being. Most rocks on Earth have melted time and again and thus are useless for figuring out how old the Earth is.

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Simple enough, see the meteorites formed alongside the planet, however, since the planet was pretty much a molten soup you can't date it properly, because we can only date it after it cooled down. Meaning that the meteorites hold a much more accurate time-frame since they were not melted down to form our crust! which means their dates (I am using the term loosely) are correct.

In other words meteorites formed with Earth but just decided to chill out there and not be totally melted down for spare parts. While most of our nice rocks formed quite a bit after the planet came to be since they are the cooled down soup that forms our dear crust.

Wouldn't it be more accurate by doing it with its inner materials?

Do you want to go swimming for Uranium? No really, its pretty hard to obtain old, OLD rocks and minerals. Best bet is to analyze meteorites.

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    $\begingroup$ Actually the crust cooled rather quickly after the formation of the Earth. Not that it matters since everything was blown up again in the moon forming event. But then you had continental crust at least 4.4 billion years ago (that Jack Hills zircon), so it's not THAT long after. $\endgroup$ – Gimelist Nov 18 '14 at 19:28
  • $\begingroup$ When "it was a soup" the most dense mats "sanked". Some of it "float up" thanks to crust dynamics but I guess the extreme pressure and temperatures for millions of years affected the decay of the Uranium on some sort and, most important, affectes how Lead and Uranion proportions comes up. In a tiny piece of rock you just can measure how much Uranium and Lead there are $\endgroup$ – jean Jan 24 at 12:21

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