Basalt US b ə ˈ s ɔː l t ˈ b eɪ s ɒ l t UK ˈ b 230 s ɔː l t ˈ b 230 s əl t is a mafic extrusive igneous rock formed from the rapid cooling of magnesium rich and iron rich lava exposed at or very near the surface of a terrestrial planet or a moon More than 90 of all volcanic rock on Earth is basalt