▁common ▁vol can ic ▁rock ▁type ▁on ▁Earth , ▁being ▁a ▁key ▁component ▁of ▁ocean ic ▁cr ust ▁as ▁well ▁as ▁the ▁principal ▁vol can ic ▁rock ▁in ▁many ▁mid - o cean ic ▁islands , ▁including ▁I cel and , ▁the ▁Far oe ▁Islands , ▁Ré union ▁and ▁the ▁islands ▁of ▁Hawai ʻ i . ▁ ▁Bas alt ▁commonly ▁features ▁a ▁very ▁fine - gra ined ▁or ▁glass y ▁matrix ▁inter sp ers ed ▁with ▁visible ▁min eral ▁gra ins . ▁The ▁average ▁density ▁is ▁ 3 . 0 ▁g / cm 3 . ▁ ▁Bas alt ▁is ▁defined ▁by ▁its ▁min eral ▁content ▁and ▁texture , ▁and ▁physical ▁descri ptions ▁without ▁min eral og ical ▁context ▁may ▁be ▁un re li able ▁in ▁some ▁circumstances . ▁Bas alt ▁is ▁usually ▁grey ▁to ▁black ▁in ▁colour , ▁but ▁rapidly ▁we athers ▁to ▁brown ▁or ▁rust - red ▁due ▁to ▁ox id ation ▁of ▁its ▁ma fic ▁( ir on - rich ) ▁min er als ▁into ▁h emat ite ▁and ▁other ▁iron ▁ox ides ▁and ▁hydro x ides . ▁Although ▁usually ▁character ized ▁as ▁" dark ", ▁bas alt ic ▁rocks ▁exhib it ▁a ▁wide ▁range ▁of ▁sh ading ▁due ▁to ▁regional ▁ge och em ical ▁processes . ▁Due ▁to ▁weather ing ▁or ▁high ▁concentr ations ▁of ▁pl agi oc l ase , ▁some ▁bas al ts ▁can ▁be ▁quite ▁light - col oured , ▁super f icial ly ▁res emb ling ▁and es ite ▁to ▁un tra ined ▁eyes . ▁Bas