PX266 Geophysics (2010/11)
Lecture 2 Handout – Earth’s Internal Structure
Dr. Gavin Bell
Radii and average densities
av given in figure; equatorial (Req) and polar (Rpol) radii
given for Earth, approximate average radii for inner (Ri) and outer (Ro) core.
Crust: cool, low density outer layer of the Earth, mainly silicates (~50% is SiO2);
oceanic and continental crust very distinct in composition and thickness.
Mantle: hotter, higher density shell between crust and core; infer composition from
indirect evidence – mostly magnesium and iron silicate minerals, whose crystal
structures change with pressure or temperature (and hence depth). Upper mantle has
layers of olivine (Mg,Fe)2SiO4,spinel then perovskite (Mg,Fe)SiO3.
Core: mostly iron – outer core is liquid, inner core is solid with a transition region
some 500 km thick. Average density of whole core = 1.2104kg m-3.
Lithosphere: cooler upper layer around 60 km thick – includes crust and some upper
mantle. Heat transferred mostly by conduction. Undergoes elastic deformation and
brittle fracture over geological time – mechanically rigid compared to asthenosphere.
Asthenosphere: below lithosphere. Undergoes viscous flow over geological time, but
behaves as an elastic solid (transmitting shear and compressional elastic waves) over
timescales of seconds or minutes. Heat transferred mainly by convection.
(Mesosphere): more rigid, less ductile than asthenosphere, reaching down to the
outer core. Chemically similar to asthenosphere; properties differ due to pand T.
(Core): very distinct both chemically and mechanically; inner / outer core state
difference (solid / liquid) is due to “pressure-freezing” of iron.
Crust
av = 2.9103kg m-3
average thickness:
35 km (continental)
7 km (oceanic)
Outer Core
Inner Core
Ri
Ro
Req = 6378 km (equator)
Rpol = 6357 km (polar)
Ri= 1220 km (inner core)
Ro= 3480 km (outer core)
Mantle
av = 4.5103kg m-3
