Andrew Usher wrote:
Every solid save water ice and a few semi-metals is denser than the
liquid of the same composition, and the same temperature and pressure.

This means that a liquid other than water will not exist beneath a
solid of the same composition in a planet. A magma ocean therefore can
only freeze from the bottom up, except for the granitic crust which,
owing to its composition, is lighter than the liquid mantle. The
amount of granitic crust which could exsolve is pretty high, if the
mean composition is that of albite perhaps 20% of the mantle in the
extreme - but this would not be reached until far below the solidus.

Is the crust really all that different material from the mantle? In the
early days, I just thought the crust was just cooled mantle material.
How do they really know what the mantle is made of?

Anyways, what's the liquid (i.e. magma) that exists below the crust made
of? Is it liquefied crust, or liquefied mantle? If it's liquefied crust,
then isn't that another example of a liquid existing below its solid phase?

Yousuf Khan