Question

to correctly classify a rock what would a geologist need to know?

Answer 1

Well an igneous rock is a rock that came out directly from a volcano, and it may have cooled instantly, and it makes the rock all the same colour, or cooled very slowly giving it time to form minerals, like in granite.

A sedimentary rock, as the name says it a rock formed by layers of sediments from other rocks, whenever there's an alteration in the sedimentation process, there's a new layer. This sediments come from the erosion of other rocks by the water, wind,animalss digging caves, etc...

The metamorphic rocks are rocks formed by igneous or sedimentary rocks. In the process of formation of a metamorphic rock, the rock sinks into the earth and as it's sinks the temperature and pressure increase. This will cause the atoms of the minerals to rearange into a more stable formula, and thus changing the rock.