<h1>It's in the rocks!</h1>
<p>The atmosphere is a tiny fraction of Earth's mass; there's no reasonable way of talking about &quot;the Earth&quot; that ends up with gaseous dioxygen making up even a millionth of its mass.</p>
<p>But the crust and mantle are almost entirely silicates - various oxides of silicon and magnesium which together are about half oxygen by mass. The mantle is two-thirds of the Earth's mass; most of the rest is the nickel-iron core (which also contains some oxygen, but less). All in all, oxygen <a href="https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Earth" rel="nofollow noreferrer">makes up 30.1% of the Earth's mass</a>, second only to iron at 32.1%. The &quot;50% oxygen by weight&quot; in the referenced comment is a little high, but I suppose that counts as &quot;almost&quot;.</p>

CC BY-SA 4.0
2024-08-14T09:53:53.033Z
Doesn’t the weigh less relative to it’s mass as it close to the centre of the earth
Harrychink
CC BY-SA 4.0
2024-08-14T12:24:16.533Z
The simplest interpretation is that "by weight" is a slightly careless/sloppy attempt to say "by mass".
Peter Erwin
CC BY-SA 4.0
2024-08-14T12:53:23.160Z
Yeah the commenter was almost certainly looking at a crustal elemental composition, where it looks to be about 46%.
notovny
CC BY-SA 4.0
2024-08-14T17:22:08.393Z
Yea, it was my mistake, I grabbed the wrong reference, the crust is "almost" and the entire planet, well, I appreciate kind phrasing. Was too late to edit the comment, but not worth deleting it now.
David S
CC BY-SA 4.0
2024-08-14T09:34:05.523Z
2024-08-15T00:27:44.187Z
2024-08-15T00:27:44.187Z
Toph
<p>In <a href="https://astronomy.stackexchange.com/questions/58333/how-much-air-escapes-into-space-every-day-and-how-long-before-it-makes-earth-ai#comment134287_58333">this comment</a>, Astronomy SE community member David S. writes:</p>
<blockquote>
<p>Just an FYI, Earth is almost 50% oxygen by weight.</p>
</blockquote>
<p>How, specifically, is &quot;Earth&quot; defined to make this statement correct?</p>
<p>(Or, is the comment mistaken?)</p>

<earth><planetary-formation><planetary-science>
How to define "Earth" so that it is almost 50% oxygen by weight?
Amazon Dies In Darkness
# How to define "Earth" so that it is almost 50% oxygen by weight?

(Asked by: username_0 on 2024-08-14)

In [this comment](https://astronomy.stackexchange.com/questions/58333/how-much-air-escapes-into-space-every-day-and-how-long-before-it-makes-earth-ai#comment134287_58333), Astronomy SE community member username_5. writes:

> Just an FYI, Earth is almost 50% oxygen by weight.

How, specifically, is "Earth" defined to make this statement correct?

(Or, is the comment mistaken?)

**Tags:** earth, planetary-formation, planetary-science


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## 1 answers

### Answer by username_1 on 2024-08-14. Score: 8

# It's in the rocks!

The atmosphere is a tiny fraction of Earth's mass; there's no reasonable way of talking about "the Earth" that ends up with gaseous dioxygen making up even a millionth of its mass.

But the crust and mantle are almost entirely silicates - various oxides of silicon and magnesium which together are about half oxygen by mass. The mantle is two-thirds of the Earth's mass; most of the rest is the nickel-iron core (which also contains some oxygen, but less). All in all, oxygen [makes up 30.1% of the Earth's mass](https://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements#Earth), second only to iron at 32.1%. The "50% oxygen by weight" in the referenced comment is a little high, but I suppose that counts as "almost".

> **Comments:**
> **username_2**: Doesn’t the weigh less relative to it’s mass as it close to the centre of the earth  
> **username_3**: The simplest interpretation is that "by weight" is a slightly careless/sloppy attempt to say "by mass".  
> **username_4**: Yeah the commenter was almost certainly looking at a crustal elemental composition, where it looks to be about 46%.  
> **username_5**: Yea, it was my mistake, I grabbed the wrong reference, the crust is "almost" and the entire planet, well, I appreciate kind phrasing. Was too late to edit the comment, but not worth deleting it now.  

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