This is the 7354th most frequent Russian word.
геология
The study of Earth's physical structure, substance, history, and processes.
This sentence uses "геология" as the subject, referring to the scientific discipline.
Геология изучает процессы формирования земной коры.
Geology studies the processes of Earth's crust formation.
Here, "геологии" is in the genitive case, describing the type of class.
На уроке геологии мы узнали о вулканах.
In the geology class, we learned about volcanoes.
In this example, "геологии" appears in the prepositional case, indicating the field of his interest.