Which statement best describes amphibolite?

• A. Regionally metamorphosed, nonfoliated rock consisting principally of hornblende and plagioclase
• B. Formed by contact metamorphism with biotite and andalusite
• C. Contains schistose foliation
• D. Is composed mainly of quartz and mica

Answer: (A)

Amphibolite is a medium- to high-grade metamorphic rock formed during regional metamorphism, typically from basaltic or gabbroic precursors. Its defining mineral assemblage is abundant hornblende (an amphibole) and plagioclase feldspar, with little quartz and few other minerals. This combination—regional metamorphism plus hornblende-rich, plagioclase-dominated composition—best captures what amphibolite is. Texture can be foliated or schistose due to deformation, but the key identifying features are the regional metamorphic setting and the hornblende–plagioclase mineralogy. The other statements describe features not characteristic of amphibolite: formation by localized contact metamorphism, or rocks rich in quartz and mica, or textures that aren’t defining for amphibolite.