Lecture 3: Earth's Interior

Differentiation of the early earth

earth’s interior

  • Humans have never physically sampled the deep interior of the earth

  • we can estimate the overall density of the earth based on its volume and gravitational pull

  • The overall density of the Earth is greater than the density of rocks at the surface – so there must be denser material in the middle

  • we can also infer where boundaries between layers are using earthquake waves

  • meteorites and magma can also give us and idea of the earths overall composition

  • Earthquakes generate seismic waves

    • primary waves (p-waves)

      • longitudinal waves

      • parallel to direction of travel

      • travel through solids and liquids

    • secondary waves (s-waves)

      • transverse waves

      • perpendicular to direction of travel

      • travel through solids but not liquids

  • Seismic waves reveal the deep earth (s-waves)

    • seismic waves are energy moving through the earth

    • these two waves cause materials to oscillate in different directions

    • when there is an earthquake the s-waves (transverse) are not observed on the far side of the earth

      • s-wave shadow zone


  • Seismic waves reveal the deep earth (p-waves)

    • when there is an earthquake, the p-waves (longitudinal) are observed on the far side of the earth, but not everywhere

      • p-wave shadow zone

    • waves refracted (bend) at solid/liquid boundaries


  • these seismic waves give us other clues about the structure and density of the inner earth

  • generally, seismic waves travel faster through denser materials

  • we can think about earth’s interior in two ways:

    • Composition

    • mechanical properties

    • pressure and temp increases as we approach the center

  • The core

    • mostly iron and nickle

    • the outer core is liquid due to the higher temperatures

    • the inner core is solid despite being hotter than the outer core, due to pressure from the surrounding layers

    • convective flow of the outer core generates a magnetic field

  • Mohorovic Discontinuity (Moho)

    • it was observed in 1909 that seismic waves travelled faster once they got below 5 to 70 km depending on where you are on earth

    • this seismic boundary represents the boundary between the earth’s crust and its mantle

  • Crust-mantle boundary

    • average: 5-7 km below the seafloor

      • remember deep ocean is 3-4 km below water surface

    • Average 35-40 km below the continents

      • even deeper under mountian belts

    • this means the thickness of oceanic crust differs from that of continental crust

  • density of oceanic vs continental crust

    • Oceanic Crust: 3.0 g/cm3

      • Mostly basalt

      • Mafic minerals

      • Rich in Fe and Mg

    • Continental Crust: 2.7 g/cm3

      • Lots of different types of rocks

      • More Felsic minerals

      • Rich in Al, K, and Na

  • The crust

    • the very thin outside layer of the earth - where life exists

      • earth’s diameter: 6,731 km

      • crust: 5-70 km thick

    • major elements are oxygen, silicon, aluminum

  • The mantle

    • denser than the crust, less dense than the mantle

    • the mantle is mostly made of the mineral peridotite and is solid rock

    • lower mantle is plastic and can flow (slowly)

      chemical comp of the earth
chemical comp of the crust

  • The Lithosphere and Asthenosphere

    • the crust and upper mantle collectively form the Lithosphere, the rigid outer portion of earth (it’s brittle)

    • below the Lithosphere is the Asthenosphere the portion of the mantle that while viscous, can flow and deform (it’s ductile)

  • Archimedes

    • any object that floats displaces its own weight of fluid

  • Temperature in earth’s interior

    • 3 sources of heat

    • originally accretion

    • planetary differentiation

    • radioactive decay