<p>You're saying...</p>
<blockquote>
<p>My question is not about the density</p>
</blockquote>
<p>But then...</p>
<blockquote>
<p>So if you take for example a cilindrical section with radius 1 meter
of both kinds of plate, which will have a larger mass?</p>
</blockquote>
<p>And since density is mass divided by volume, your question <em>is</em> about density.</p>
<p>To answer your question - the plates &quot;weigh&quot; about the same. They have to be. If one plate is heavier, it will sink and displace the underlying mantle so that it pushes the lighter plate upwards. Note that these things actually happen - the Earth is not in dynamic equilibrium and various tectonic and surface processes cause this disequilibrium and movement.</p>
<p>But there isn't anything inherently heavier about the oceanic plate compared to the continental plate, or vice versa, when taken as a whole. This is the concept of isostasy (<a href="https://en.wikipedia.org/wiki/Isostasy" rel="noreferrer">with a well illustrated Wikipedia page</a>).</p>
<p>And pay attention to what you're talking about the continental and oceanic <em><strong>plate</strong></em> or the continental and oceanic <em><strong>crust</strong></em>. The two are not the same. It is correct that the continental crust is about double (or more!) than the thickness of the oceanic crust, but the continental plate is not double the thickness of the oceanic plate. The plate includes the crust and the upper mantle - together called the &quot;lithosphere&quot;.</p>

CC BY-SA 4.0
2022-03-18T12:33:36.353Z
I was going to say the same thing about density, but it doesn't mention depth in his description, so I think he's saying to take a bigger chunk for the plates with more depth (so you'd have a taller cutout from the plate with more depth, so it's not quite a return to density)
JeopardyTempest
CC BY-SA 4.0
2022-03-19T03:25:19.257Z
Density is part of the answer to the question, but is not the core of the question.  If I drill a core sample from the summit of Mount Everest down to the crust boundary, and drill a core sample from the Dead Sea down to the crust boundary, I'd expect the first to be heavier even though the two have similar densities.  But what if I drill a core sample from the coast of Florida and one from the coast of Hawaii?
Mark
CC BY-SA 4.0
2022-03-19T04:49:03.767Z
@Mark not sure if intended, but I loved the "core" pun! Anyway, what determines isostasy is the entire plate, not just the crust. I think this is what leads to most of the confusion here.
Gimelist
CC BY-SA 4.0
2022-03-19T13:36:54.677Z
I edited the question to make it more clear
Riemann
CC BY-SA 4.0
2022-03-19T13:38:00.333Z
@Gimelist, in the following link I see that the continental plates are more than twice as thick as the oceanic plates: https://lisbdnet.com/how-thick-are-tectonic-plates/#How_Thick_Are_Tectonic_Plates
Riemann
CC BY-SA 4.0
2022-03-19T15:14:45.537Z
@Riemann I think your question was plenty clear before, it just took some thought.  I wouldn't pay too much attention the first few lines of Gimelist's answer, the whole density attention... but from my non-geologist thinking, the rest seems fairly key
JeopardyTempest
CC BY-SA 4.0
2022-03-20T00:24:32.030Z
@Riemann that link confuses the two concepts "plates" and "crust". The heading says plate, but the content says crust. The "lithosphere" page in wikipedia is a good place to start. https://en.wikipedia.org/wiki/Lithosphere
Gimelist
CC BY-SA 4.0
2022-03-22T08:27:16.470Z
@Gimelist is "litosphere" a synonym for "plate"?
Riemann
CC BY-SA 4.0
2022-03-27T18:35:35.650Z
@Gimelist, you say that the link confuses the two concepts "plates" and "crust". This is indeed the case for the second question of the link, which says "What is the thickness of the continental plates?" However, the first question of the link seems to be online about plates. The name of the first question is "How Thick Are Tectonic Plates?", and the content continues to speak about plates. It says that oceanic plates are 50-100 km thick while continental plates are up to 200 km thick.
Riemann
CC BY-SA 4.0
2022-03-29T12:46:48.053Z
@Riemann as I said in the answer, the concept of [isostasy](https://en.wikipedia.org/wiki/Isostasy) is what you need. It has well illustrated examples, equations, and even example densities you can use. Just plug in the numbers and see for yourself how do thickness, mass, and density relate to each other. The Airy model is sufficiently simple to solve with simple arithmetic. Lots of resources on Youtube as well: https://www.youtube.com/results?search_query=airy+isostasy
Gimelist
CC BY-SA 4.0
2022-03-18T10:57:28.327Z
2022-03-18T10:57:28.327Z
Gimelist
<p>@gimelist answered the question in a technical sense, but if you are interested in whether the material that makes up the continents is more or less dense than the material that makes up oceanic plates, then the answer is that the material in oceanic plates is <em>more dense</em> than the material of continental plates.</p>
<p>In a cartoonish way, we tend to think of continents as the &quot;soap scum&quot; that floats on the rest of the earth. It is lighter than the rest of the material, and so whenever a continental and an oceanic plate meet, the oceanic plate generally subducts and the continent overrides.</p>
<p>In concrete terms, the density of the basalt that makes up most of the oceanic plates is 2.9 g/cm^3, whereas the granite that makes up most of the continents has a density of 2.7 g/cm^3.</p>

CC BY-SA 4.0
2022-03-25T22:07:33.500Z
2022-03-25T22:07:33.500Z
Wolfgang Bangerth
<p>If you take a cilindrical section with radius 1 meter of both kinds of plate, which will have a larger mass?
My guess is that continental plates are heavier than oceanic plates, because they are <a href="https://lisbdnet.com/how-thick-are-tectonic-plates/#How_Thick_Are_Tectonic_Plates" rel="nofollow noreferrer">more than twice as thick</a> while their densities are similar. Is this correct? Are there other factors than thickness and density which contribute to the mass?</p>

CC BY-SA 4.0
2022-03-20T00:25:02.737Z
Note that your link talks about the crust, not the plates. The two are not the same.
Gimelist
CC BY-SA 4.0
2022-03-27T18:36:40.840Z
@Gimelist, thanks for the comment. I replaced the link with a better one.
Riemann
<plate-tectonics>
Are oceanic plates or continental plates heavier?
Riemann
# Are oceanic plates or continental plates heavier?

(Asked by: username_0 on 2022-03-17)

If you take a cilindrical section with radius 1 meter of both kinds of plate, which will have a larger mass?
My guess is that continental plates are heavier than oceanic plates, because they are [more than twice as thick](https://lisbdnet.com/how-thick-are-tectonic-plates/#How_Thick_Are_Tectonic_Plates) while their densities are similar. Is this correct? Are there other factors than thickness and density which contribute to the mass?

**Tags:** plate-tectonics

> **Comments:**
> **username_1**: Note that your link talks about the crust, not the plates. The two are not the same.  
> **username_0**: @username_1, thanks for the comment. I replaced the link with a better one.  

---

## 2 answers

### Answer by username_1 on 2022-03-18. Score: 6

You're saying...

> My question is not about the density

But then...

> So if you take for example a cilindrical section with radius 1 meter
> of both kinds of plate, which will have a larger mass?

And since density is mass divided by volume, your question *is* about density.

To answer your question - the plates "weigh" about the same. They have to be. If one plate is heavier, it will sink and displace the underlying mantle so that it pushes the lighter plate upwards. Note that these things actually happen - the Earth is not in dynamic equilibrium and various tectonic and surface processes cause this disequilibrium and movement.

But there isn't anything inherently heavier about the oceanic plate compared to the continental plate, or vice versa, when taken as a whole. This is the concept of isostasy ([with a well illustrated Wikipedia page](https://en.wikipedia.org/wiki/Isostasy)).

And pay attention to what you're talking about the continental and oceanic ***plate*** or the continental and oceanic ***crust***. The two are not the same. It is correct that the continental crust is about double (or more!) than the thickness of the oceanic crust, but the continental plate is not double the thickness of the oceanic plate. The plate includes the crust and the upper mantle - together called the "lithosphere".

> **Comments:**
> **username_2**: I was going to say the same thing about density, but it doesn't mention depth in his description, so I think he's saying to take a bigger chunk for the plates with more depth (so you'd have a taller cutout from the plate with more depth, so it's not quite a return to density)  
> **username_3**: Density is part of the answer to the question, but is not the core of the question.  If I drill a core sample from the summit of Mount Everest down to the crust boundary, and drill a core sample from the Dead Sea down to the crust boundary, I'd expect the first to be heavier even though the two have similar densities.  But what if I drill a core sample from the coast of Florida and one from the coast of Hawaii?  
> **username_1**: @username_3 not sure if intended, but I loved the "core" pun! Anyway, what determines isostasy is the entire plate, not just the crust. I think this is what leads to most of the confusion here.  
> **username_0**: I edited the question to make it more clear  
> **username_0**: @username_1, in the following link I see that the continental plates are more than twice as thick as the oceanic plates: https://lisbdnet.com/how-thick-are-tectonic-plates/#How_Thick_Are_Tectonic_Plates  
> **username_2**: @username_0 I think your question was plenty clear before, it just took some thought.  I wouldn't pay too much attention the first few lines of username_1's answer, the whole density attention... but from my non-geologist thinking, the rest seems fairly key  
> **username_1**: @username_0 that link confuses the two concepts "plates" and "crust". The heading says plate, but the content says crust. The "lithosphere" page in wikipedia is a good place to start. https://en.wikipedia.org/wiki/Lithosphere  
> **username_0**: @username_1 is "litosphere" a synonym for "plate"?  
> **username_0**: @username_1, you say that the link confuses the two concepts "plates" and "crust". This is indeed the case for the second question of the link, which says "What is the thickness of the continental plates?" However, the first question of the link seems to be online about plates. The name of the first question is "How Thick Are Tectonic Plates?", and the content continues to speak about plates. It says that oceanic plates are 50-100 km thick while continental plates are up to 200 km thick.  
> **username_1**: @username_0 as I said in the answer, the concept of [isostasy](https://en.wikipedia.org/wiki/Isostasy) is what you need. It has well illustrated examples, equations, and even example densities you can use. Just plug in the numbers and see for yourself how do thickness, mass, and density relate to each other. The Airy model is sufficiently simple to solve with simple arithmetic. Lots of resources on Youtube as well: https://www.youtube.com/results?search_query=airy+isostasy  

---

### Answer by username_4 on 2022-03-25. Score: 1

@username_1 answered the question in a technical sense, but if you are interested in whether the material that makes up the continents is more or less dense than the material that makes up oceanic plates, then the answer is that the material in oceanic plates is *more dense* than the material of continental plates.

In a cartoonish way, we tend to think of continents as the "soap scum" that floats on the rest of the earth. It is lighter than the rest of the material, and so whenever a continental and an oceanic plate meet, the oceanic plate generally subducts and the continent overrides.

In concrete terms, the density of the basalt that makes up most of the oceanic plates is 2.9 g/cm^3, whereas the granite that makes up most of the continents has a density of 2.7 g/cm^3.


---

