Here is the definition of Biology as given by Dictionary:
* the study of living organisms, divided into many specialized fields that cover their morphology, physiology, anatomy, behavior, origin, and distribution.
Tectonic plates are not living, thus it isn't biology. It would be Geology:
* Geology is the study of the Earth, the materials of which it is made, the structure of those materials, and the processes acting upon them
Source: Geology.com