Internal Layers of the Earth & Seismic Waves

• Earth is currently the only known planet that sustains life.

• Though it appears as a single rigid sphere, it is a composite of distinct layers that interact dynamically.

• Constant shifting of interior material drives plate tectonics, volcanism, and the geomagnetic field.

Earth’s Structure

The planet is composed of four principal concentric layers:

• Crust

  • Mantle

  • Outer Core

  • Inner Core

• Each layer has unique thicknesses, compositions, physical states, and geophysical roles.

The Four Internal Layers of Earth

1. Crust

• Outermost solid shell; the zone where all terrestrial life resides.

• Sub-divided into:

  • Continental crust (thicker, less dense, granitic).

  • Oceanic crust (thinner, denser, basaltic).

• Dominant chemical elements: Silicon (Si), Oxygen (O), Aluminum (Al), Calcium (Ca), Sodium (Na), Potassium (K).

• Interfaces:

  • Conrad discontinuity: boundary between upper & lower continental crust.

  • Mohorovičić (Moho) discontinuity: base of crust → upper mantle.

2. Mantle

• Occupies roughly $60\%$ of Earth’s volume and $68\%$ of its mass.

• Composition: silicate minerals rich in Mg, Fe; also substantial Si and O.

• Physical state: mostly solid but behaves plastically; molten regions can flow under pressure.

• Sub-layers / discontinuities:

  • Upper Mantle (includes asthenosphere).

  • Repetti discontinuity separates upper & lower mantle.

  • Lower Mantle (more rigid).

3. Outer Core

• Position: beneath the mantle.

• Composition: liquid iron (Fe) and nickel (Ni).

• Temperature reaches up to $2000\,^{\circ}\text{C}$.

• Liquid metal circulation generates Earth’s geomagnetic field via dynamo action.

• Boundary with mantle: Gutenberg discontinuity.

4. Inner Core

• Central, deepest layer.

• Composition: solid iron and nickel alloy.

• Temperature approximates $5000\,^{\circ}\text{C}$.

• Remains solid because immense overlying pressure causes “pressure-freezing.”

• Boundary with outer core: Lehmann discontinuity (transition from liquid → solid).

How Do Scientists Know These Layers Exist?

• Direct drilling reaches only a few kilometres; interior knowledge is indirect.

• Primary method: analysis of seismic waves generated by earthquakes and recorded at global seismic stations.

• Key observations:

  • Velocity changes, reflection, refraction, and diffraction of waves mark boundaries (discontinuities).

  • S-waves do not pass through liquids → absence in certain zones (S-wave shadow) implies liquid outer core.

  • P-waves slow and refract in outer core, creating P-wave shadow zones between $105^{\circ}$ and $140^{\circ}$ from the epicentre.

  • Multiple travel paths (e.g.
    $\text{PKP},\ \text{P},\ \text{K}$ phases) reveal layered structure.

Seismic Waves: General Classification

• Vibrational energy released at an earthquake focus propagates as seismic waves.

• Two overarching categories:

  1. Surface Waves (confined to near-surface layers, arrive last, often most destructive).

  2. Body Waves (propagate through Earth’s interior, higher frequency, essential for probing deep layers).

Surface Waves

• Generated when body waves interact with the free surface.

• Arrive after the main P and S waves.

• Two principal types:

Love Waves

• Named after mathematician Augustus Edward Hough Love (1911).

• Motion: horizontal, side-to-side shear (snake-like).

• Particle displacement is perpendicular to direction of propagation and confined to surface layers.

• Cause intense twisting → often the most damaging to man-made structures.

Rayleigh Waves

• Predicted by Lord John William Strutt, 3rd Baron Rayleigh (1885).

• Motion: retrograde elliptical rolling, similar to water waves.

• Ground moves both vertical (up–down) and horizontal (front–back) components.

• Responsible for most of the prolonged shaking felt during earthquakes.

Body Waves

• Travel through Earth’s interior; recorded at great distances.

• Two main types with distinct propagation mechanics:

Primary (P) Waves

• Also called compressional or longitudinal waves.

• Particle motion: alternating compression & dilation parallel to propagation.

• Fastest seismic phase; first to be detected.

• Can pass through solids, liquids, and gases.

• Provide information on both mantle and core (e.g. $\text{P}, \text{PP}, \text{PKP}$ paths).

Secondary (S) Waves

• Also called shear or transverse waves.

• Particle motion: perpendicular to propagation (shaking side-to-side or up-down).

• Slower than P-waves.

• Travel only through solids; cannot propagate through liquids.

• Non-arrival through core produced the S-wave shadow zone, direct evidence that the outer core is liquid.

Seismic Discontinuities (Key Boundaries)

• Conrad discontinuity: upper vs.
  lower continental crust.

• Mohorovičić (Moho) discontinuity: crust–upper-mantle boundary—identified by sudden velocity increase.

• Repetti discontinuity: separates upper & lower mantle.

• Gutenberg discontinuity: mantle–outer-core boundary—marked by disappearance of S waves and slowing of P waves.

• Lehmann discontinuity: outer-core–inner-core boundary—P-wave reflections (PKiKP) reveal solid inner core.

Evidence for Plate Movement (as required by curriculum code S9ES-Ia-j-36.6)

• Although not elaborated in the slides, standard lines of evidence include:

  • Continental fit (e.g. coastlines of South America and Africa).

  • Fossil correlations across oceans (Mesosaurus, Glossopteris, Lystrosaurus).

  • Rock type and structural similarities (mountain belts, cratons).

  • Paleoclimatic indicators (glacial striations, coal beds).

  • Paleomagnetism: symmetrical magnetic stripes at mid-ocean ridges; polar wandering curves.

  • Age distribution of oceanic crust (young at ridges, older toward trenches).

  • Hot-spot volcanic chains showing age progression (e.g. Hawaii–Emperor).

  • GPS and satellite laser ranging measuring present-day plate velocities.

  • Seafloor spreading rates inferred from magnetised basalt.

Ethical, Philosophical & Practical Implications

• Understanding Earth’s layers underpins hazard mitigation (earthquake engineering, tsunami warning).

• Geomagnetic knowledge is vital for navigation and shielding life & technology from solar wind.

• Insights into mantle convection inform questions on planetary habitability and thermal evolution.

• Plate-movement studies contribute to resource exploration (minerals, hydrocarbons) while raising sustainability and environmental stewardship issues.

Numerical / Statistical Highlights (Presented in LaTeX)

• Mantle volume: $\approx 60\%$ of planet; mass: $\approx 68\%$.

• Outer-core temperature: $\approx 2000\,^{\circ}\text{C}$.

• Inner-core temperature: $\approx 5000\,^{\circ}\text{C}$.

• P- & S-wave global shadow zones: $105^{\circ}\rightarrow140^{\circ}$ distance from epicentre.

• Surface-wave arrival occurs after P and S but often dominates damage levels.

Concept Connections / Prior Knowledge

• Builds on earlier lessons covering plate tectonics, rock cycle, and geologic time.

• Reinforces physics principles of wave propagation (reflection, refraction, interference).

• Links to electromagnetism through discussion of the geodynamo (outer core).

Real-World Applications

• Seismic tomography for imaging subduction zones and mantle plumes.

• Earthquake early-warning systems rely on rapid P-wave detection to forecast imminent S-wave and surface-wave shaking.

• Engineering codes incorporate expected surface-wave amplitudes for earthquake-resistant design.

• Geophysical prospecting (oil, gas, mineral) adapts seismic reflection/refraction principles.

Summary Cheat-Sheet

• Earth = $\text{Crust} + \text{Mantle} + \text{Outer Core} + \text{Inner Core}$.

• Seismic waves are our primary probe:

  • $\text{Body}:\ P,\ S\ \ (interior)$.

  • $\text{Surface}:\ \text{Love},\ \text{Rayleigh}$.

• Discontinuities (Moho, Gutenberg, Lehmann) mark layer transitions.

• Observed wave behaviour (speed, direction, absence) → infer composition & phase (solid vs.
  liquid).

• Plate motion evidenced by paleomagnetism, fossils, GPS, and more.