Earth science (7)
I believe the answer is:
geology
'earth science' is the definition.
(I know that geology is a type of earth science)
This is the entire clue.
(Other definitions for geology that I've seen before include "Science that deals with the history of rocks" , "The science of the earth's crust" , "The scientific study of the earth's crust" , "Earth science" , "Study of Earth, its rocks" .)