Outer core Outer core The outer core of the Earth is a fluid layer about thick and composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle. Its outer boundary lies beneath Earth's surface. The transition between the inner core and outer core is located approximately beneath the Earth's surface. Unlike the inner core, the outer core is liquid. The inner core is also referred to as the solid core. Seismic inversions of body waves and normal modes constrain the radius of the outer core to be 3483 km with an uncertainty of 5 km,
Age of the Earth Earth is difficult to determine. It is also difficult to determine the exact age of the oldest rocks on Earth, exposed at the surface, as they are aggregates of minerals of possibly different ages. Studies of strata, the layering of rocks and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contained fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer. Nicolas Steno in the 17th century was one of the first naturalists to appreciate the connection between fossil remains and strata.