Final answer:
Isostasy is the principle that describes the rocks of the crust and upper mantle as being in gravitational balance, floating on the more fluid layers beneath them.
Step-by-step explanation:
The concept that rocks of the crust and upper mantle are floating in gravitational balance is known as Isostasy. This geological principle states that the Earth's crust is in gravitational equilibrium when it is at rest, and it applies as well to the upper mantle. These earth layers are part of the lithosphere, which is broken into tectonic plates that are in constant motion due to the convection in the mantle below. Subduction is the process where one plate is forced beneath another at a subduction zone, and Orogeny refers to the process of mountain building.