A person who studies rocks is called a geologist. Geologists also study how the Earth is made and how the planet changes in time.
Geologists study rocks to discover metals and minerals and develop ways to remove the metal and minerals from the rocks. They also study rocks to determine the ages of landmarks and other places on Earth.
Geology covers a wide range of areas and most geologists choose a field of study specific to their interests. For example, a geologist may specially study volcanoes. The geologists would study the structure of volcanoes, the eruptions and possibly predict future eruptions.