noun
primary rocks
Geological term for rocks that have not undergone significant alteration since their original formation; often contrasted with secondary or metamorphic rocks.
この地域の原成岩石は約2億年前に形成された。
The primary rocks in this area were formed about 200 million years ago.
Compound of 原成 (primary formation) and 岩石 (rock). The exact historical derivation is uncertain; the term is used in geological contexts.