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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 while their densities are similar. Is this correct? Are there other factors than thickness and density which contribute to the mass?

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  • $\begingroup$ Note that your link talks about the crust, not the plates. The two are not the same. $\endgroup$
    – Gimelist
    Mar 20 at 0:25
  • $\begingroup$ @Gimelist, thanks for the comment. I replaced the link with a better one. $\endgroup$
    – Riemann
    Mar 27 at 18:36

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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).

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".

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    $\begingroup$ 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) $\endgroup$ Mar 18 at 12:33
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    $\begingroup$ 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? $\endgroup$
    – Mark
    Mar 19 at 3:25
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    $\begingroup$ @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. $\endgroup$
    – Gimelist
    Mar 19 at 4:49
  • $\begingroup$ I edited the question to make it more clear $\endgroup$
    – Riemann
    Mar 19 at 13:36
  • $\begingroup$ @Gimelist, in the following link I see that the continental plates are more than twice as thick as the oceanic plates: lisbdnet.com/how-thick-are-tectonic-plates/… $\endgroup$
    – Riemann
    Mar 19 at 13:38
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@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 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.

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