S

Earth Structure & Investigation

Earth’s Layering

  • Formed by differentiation from interstellar material ext{~}4.6\ \text{Ga}

  • Density‐stratified concentric shells:
    • Crust (silica-rich, low \rho)
    • Mantle (ultramafic Fe–Mg silicates)
    • Core (Fe–Ni metal; outer liquid, inner solid)

  • Mechanical divisions: lithosphere (rigid), asthenosphere (plastic), mesosphere, outer core (liquid), inner core (solid)

Crust & Lithosphere

  • Oceanic crust: 5! –! 8\ \text{km} thick, \rho\approx 3\ \text{g cm}^{-3}, seismic v_P\approx 7 \text{ km s}^{-1}

  • Continental crust: <70\ \text{km}, \rho<3\ \text{g cm}^{-3}, v_P\approx 6 \text{ km s}^{-1}

  • Moho = Mohorovičić discontinuity; base of crust

  • Lithosphere = crust + rigid uppermost mantle (down to \sim100\ \text{km})

Investigation Techniques

  • Geology: field mapping, core sampling (deepest drill 12.3\ \text{km})

  • Geodesy: GPS triangulation; tracks plate motions to cm/yr precision

  • Geophysics (seismic reflection): source → reflected/refracted waves → sub-surface images via velocity contrasts

  • Gravimetry: gravimeters detect anomalies \pm0.5\%; reveal density variations, isostasy, groundwater, etc.

  • Heat-flow: average geothermal gradient \sim25\ ^\circ\text{C km}^{-1}; continental crust insulates, oceanic crust vents core heat

  • Geomagnetism: magnetometers map field anomalies tied to magnetic mineral concentration & past plate positions

  • Seismology: P-waves (compressional, travel through solids & liquids, fastest); S-waves (shear, solids only). Core states deduced from P/S shadow zones.

Tectonic Plates

  • Lithosphere split into \approx12 major plates

  • Boundary types:
    • Divergent – plates separate; new lithosphere created
    • Transform – plates slide; lithosphere conserved
    • Convergent – plates collide; subduction or orogeny

  • Relative vs. absolute plate motion (tracked with mantle plumes / hot-spot chains)

Mantle

  • Composition: peridotite (olivine-rich); 84\% volume, 67\% mass of Earth

  • Solid yet ductile on Myr scales; convection transfers internal heat

  • Subduction drags cold lithosphere downward; upwelling creates ridges & intra-plate volcanism

Core

  • \sim33\% of Earth’s mass, 15\% volume

  • Outer core liquid Fe–Ni → seismic S-wave shadow zone 103^\circ–142^\circ

  • Inner core solid; inferred from P-wave refraction patterns

Earth’s Magnetic Field

  • Geodynamo from convection of liquid outer core

  • Remanent magnetization in lavas records field polarity & paleolatitude; key to plate reconstruction