Final answer:
Plate Tectonics is the theory that Earth's lithosphere is composed of several rigid plates 'floating' on top of the asthenosphere.
Step-by-step explanation:
The theory that Earth's lithosphere is composed of several rigid plates 'floating' on top of the asthenosphere is called Plate Tectonics. This theory explains various geologic phenomena such as earthquakes, volcanic activity, and the formation of mountain ranges.
Learn more about Plate Tectonics