Earth’s interior Earth’s rocky crust encloses a silicate mantle and a metallic core. Seismic waves reveal their boundaries and show that the outer core is liquid while the inner core is solid.
The crust is Earth’s thin, rocky skin. Oceanic crust is usually about 5–10 km thick; continental crust is commonly 30–70 km thick. Below it, the mantle begins at the Mohorovičić discontinuity.
The upper mantle extends to about 660 km depth. Its mostly solid silicate rock deforms slowly over geological time, allowing plates to move. Mineral transformations between about 410 and 660 km form the mantle transition zone.
The lower mantle extends from about 660 to 2,891 km depth. Increasing pressure stabilizes dense silicate minerals. Although solid, this immense rocky layer transports heat by slow convection.
The outer core is a liquid iron-rich alloy between depths of about 2,891 and 5,149.5 km. Motion of this electrically conducting fluid sustains Earth’s magnetic field. Shear waves cannot travel through it.
The inner core is a solid iron-rich sphere with a radius of about 1,221.5 km. Despite temperatures comparable to the Sun’s surface, enormous pressure keeps it solid. Its growth releases heat and light elements into the liquid outer core.
Seismic studies indicate an innermost region of the solid inner core with different directional wave speeds, possibly reflecting a different alignment of iron-alloy crystals. A 2023 study examined a central region of about 650 km radius; its extent and transition remain uncertain.
Earth’s interior Crust Upper mantle Lower mantle Outer core Inner core Innermost inner core (IMIC) Beneath the seafloor NASA · Earth 