Final answer:
The concept whereby Earth's lithosphere is floating in gravitational balance upon the asthenosphere is known as plate tectonics.
Step-by-step explanation:
The concept whereby Earth's lithosphere is floating in gravitational balance upon the asthenosphere is known as plate tectonics. Plate tectonics is the theory that explains how the slow motions within the Earth's mantle cause large segments of the crust to move, resulting in continental drift and the formation of mountains and other geological features.