GEOLOGY Mercury's geology is marked by extensive cratering, akin to the Moon, due to impacts from asteroids and comets. It possesses vast plains, scarps (cliffs), and ridges caused by contraction as its iron core cooled, causing the surface to shrink. Unique features include volcanic plains formed by ancient volcanic activity, evidenced by smooth, dark basaltic plains called "maria." Mercury's heavily cratered surface indicates billions of years of geological history, revealing a planet shaped by intense bombardment and geological processes, despite its small size.