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