igneous or sedimentary, which are heated and compressed underground.Igneous rocks are formed when molten rock from volcanic activity cools down either underground or on the surface.This slow but ...

These rocks, laid down during the Archaean Eon, are metamorphosed mafic volcanic and sedimentary rocks ... s carbon-rich layers suggested biological activity had occurred there.

They form in sedimentary rocks like limestone ... Their formation is closely tied to volcanic activity and the crystallization of minerals within the gas bubbles of rhyolitic ash.