Which factor would be most useful predicting the amount of mantle a certain volume of crust will displace

density of the crust

Explanation:
The displacement of the mantle by the continental or the ocean crust depends on a condition known as isostasy which describes the gravitational equilibrium between both the mantle and the crust of the Earth.
If isostasy is not preserved, this would mean that the equilibrium between the crust and the mantle is not maintained and that the crust would displace a volume of mantle.
The amount of mantle to be displaced usually depends on the density of the crust.

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