<p>The answers that have been provided are correct but they're omitting the fundamental issue that explains <strong>why</strong> they are correct:</p>

<p>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.</p>

CC BY-SA 4.0
2022-05-04T05:11:13.117Z
This is incorrect. Zircon crystals, which are primarily used for uranium dating, can record significant thermal events that can alter uranium chemistries on the the rinds of these crystals. Geochemists may grind off these layers to date unaltered cores and obtain formation dates as well as metamorphism dates.
Knob Scratcher
CC BY-SA 3.0
2014-11-18T20:55:12.913Z
2014-11-18T20:55:12.913Z
Loren Pechtel
<p>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.</p>

<p>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. </p>

<blockquote>
  <p>Wouldn't it be more accurate by doing it with its inner materials?</p>
</blockquote>

<p>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.</p>

CC BY-SA 4.0
2019-01-24T12:21:58.557Z
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
jean
CC BY-SA 3.0
2014-11-18T19:28:58.080Z
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.
Gimelist
CC BY-SA 3.0
2014-11-18T18:28:47.417Z
2014-11-18T18:28:47.417Z
user3541786
<p><a href="https://en.wikipedia.org/wiki/Clair_Cameron_Patterson" rel="nofollow noreferrer">Patterson</a> could not age the Earth from Uranium to lead decay root because the Zircon samples were contaminated by <a href="https://en.wikipedia.org/wiki/Clair_Cameron_Patterson#Measurement_of_age_of_Earth" rel="nofollow noreferrer">lead pollution</a> at that time. So he used meteorites under the assumption that they were formed approximately the same period as our planet thus 4.55 billion years ago.</p>

CC BY-SA 4.0
2022-04-26T17:55:32.747Z
2022-04-26T17:55:32.747Z
Markoul11
<p><strong>Some background:</strong></p>

<p>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.
<br><br></p>

<blockquote>
  <p>Why do we use meteorites if they hit the Earth after its formation?</p>
</blockquote>

<p><strong>Short answer</strong>: because the meteorites formed together with the Earth and the rest of the Solar System.</p>

<p><strong>Long answer</strong>: 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.</p>

<p>You say:</p>

<blockquote>
  <p>Why do we use meteorites if they hit the Earth after its formation?</p>
</blockquote>

<p>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.</p>

<blockquote>
  <p>Wouldn't it be more accurate by doing it with its inner materials?</p>
</blockquote>

<p>It would. There are two problems:</p>

<ul>
<li>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 <a href="https://earthscience.stackexchange.com/q/773/725">related question</a>). 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.</li>
<li>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.</li>
</ul>

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

CC BY-SA 4.0
2022-04-30T09:06:41.253Z
Which meteorites are actually used for dating? Is there any publicly available information on this?
mr_js
CC BY-SA 3.0
2014-11-18T12:42:54.433Z
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.
Chirac
CC BY-SA 3.0
2014-11-18T12:59:58.217Z
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.
Gimelist
CC BY-SA 3.0
2014-11-18T16:16:11.200Z
@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.
f.thorpe
CC BY-SA 3.0
2014-11-19T07:09:00.357Z
@MontyHarder not really. If I legally change my name today to "Mike" instead of "Michael", it doesn't mean I am reborn today. My birth date is still that time years ago when Stack Exchange did not exist yet.
Gimelist
CC BY-SA 3.0
2014-11-19T13:48:43.150Z
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 :-)
Steve Jessop
CC BY-SA 3.0
2014-11-19T19:40:46.467Z
Is there any reason to suspect general relativity has aged these meteorites at a rate faster than our passing of time and they could be an inaccurate indicator?
Acey
CC BY-SA 3.0
2014-11-19T19:53:29.620Z
@Acey: If they had any speed that made relativity important, they'd be much faster than the escape velocity of the solar system, so we'd never see them on earth. Also, if they were that fast, they'd age slower, not faster - this is why many more muons from the sun reach us than mandated by their decay rate.
Guntram Blohm
CC BY-SA 3.0
2014-11-19T19:57:01.740Z
Ah yes of course slower not faster, my bad. I guess I'm just think that in a rocks 4.5 billion year history, maybe it was part of a giant massive star that exploded very far away after some time, or I don't know! Just a random thought.
Acey
CC BY-SA 3.0
2014-11-18T12:17:09.193Z
2014-11-19T12:46:56.330Z
2017-04-13T12:50:30.540Z
Gimelist
<p>Reading a course on Precambrian, I read that:</p>

<blockquote>
  <p>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.</p>
</blockquote>

<p>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?</p>

CC BY-SA 3.0
2014-11-18T14:01:28.487Z
Where does the text about the Precambian come from?
arkaia
CC BY-SA 3.0
2014-11-18T22:56:29.070Z
It comes from : 
http://www2.ggl.ulaval.ca/personnel/bourque/intro.pt/planete_terre.html
But if you can't understand French, I doubt you'll be able to read it, I'm sorry.
Chirac
<geology><geochemistry><earth-history><meteorite><precambrian>
Why is Earth's age given by dating meteorites rather than its own rocks?
Chirac
# Why is Earth's age given by dating meteorites rather than its own rocks?

(Asked by: username_0 on 2014-11-18)

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?

**Tags:** geology, geochemistry, earth-history, meteorite, precambrian

> **Comments:**
> **username_1**: Where does the text about the Precambian come from?  
> **username_0**: It comes from :  http://www2.ggl.ulaval.ca/personnel/bourque/intro.pt/planete_terre.html But if you can't understand French, I doubt you'll be able to read it, I'm sorry.  

---

## 4 answers

### Answer by username_6 on 2014-11-18. Score: 39 | Accepted answer

**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](https://earthscience.stackexchange.com/q/773/725)). 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.

> **Comments:**
> **username_0**: 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.  
> **username_6**: 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.  
> **username_9**: @username_0 ... 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.  
> **username_6**: @MontyHarder not really. If I legally change my name today to "Mike" instead of "Michael", it doesn't mean I am reborn today. My birth date is still that time years ago when Stack Exchange did not exist yet.  
> **username_10**: 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 :-)  
> **username_11**: Is there any reason to suspect general relativity has aged these meteorites at a rate faster than our passing of time and they could be an inaccurate indicator?  
> **username_12**: @username_11: If they had any speed that made relativity important, they'd be much faster than the escape velocity of the solar system, so we'd never see them on earth. Also, if they were that fast, they'd age slower, not faster - this is why many more muons from the sun reach us than mandated by their decay rate.  
> **username_11**: Ah yes of course slower not faster, my bad. I guess I'm just think that in a rocks 4.5 billion year history, maybe it was part of a giant massive star that exploded very far away after some time, or I don't know! Just a random thought.  
> **username_8**: Which meteorites are actually used for dating? Is there any publicly available information on this?  

---

### Answer by username_2 on 2014-11-18. Score: 19

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.

> **Comments:**
> **username_3**: This is incorrect. Zircon crystals, which are primarily used for uranium dating, can record significant thermal events that can alter uranium chemistries on the the rinds of these crystals. Geochemists may grind off these layers to date unaltered cores and obtain formation dates as well as metamorphism dates.  

---

### Answer by username_4 on 2014-11-18. Score: 6

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.

> **Comments:**
> **username_6**: 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.  
> **username_5**: 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  

---

### Answer by username_7 on 2022-04-26. Score: 1

[Patterson](https://en.wikipedia.org/wiki/Clair_Cameron_Patterson) could not age the Earth from Uranium to lead decay root because the Zircon samples were contaminated by [lead pollution](https://en.wikipedia.org/wiki/Clair_Cameron_Patterson#Measurement_of_age_of_Earth) at that time. So he used meteorites under the assumption that they were formed approximately the same period as our planet thus 4.55 billion years ago.


---

