The Origin and Evolution of the Marine Environment (MSCI*112)

The Earth's Interior: Layered Structure

• Structure and key terms

  • Lithosphere: rigid, cool, brittle outer shell including the crust and the uppermost part of the mantle; typically about 100 km thick (locally varies 5–200 km). Brittle and can be broken into tectonic plates.
  • Asthenosphere: the rest of the upper mantle beneath the lithosphere; partially molten and behaves plastically (ductile, "jello-like").
  • Mesosphere: the hotter but stronger region of the mantle due to high pressures; lies below the asthenosphere.
  • Crust (outermost shell):
  • Oceanic crust: very thin, about 5–10 km thick; basaltic rocks enriched in Fe and Mg.
  • Continental crust: thicker, typically 30–85 km thick; granitic rocks enriched in Si and Al; lighter than oceanic crust.
  • Mantle: extends from beneath the crust down to about the outer core boundary; composition mainly peridotite; density and temperature increase with depth.
  • Core: divided into a liquid outer core and a solid inner core; composition dominated by Fe and Ni.

• Boundaries and terminology

  • Moho discontinuity: boundary between crust and mantle; identified in 1909 by Andrija Mohorovičić. Oceanic crust thickness ~5–10 km; Continental crust thickness ~30–85 km.
  • Core–Mantle Boundary (CMB): also called the Gutenberg discontinuity; depth to ~2891 km; boundary between the mantle and the liquid outer core; discovery linked to Beno Gutenberg in 1914.
  • Peridotite: the mantle rock type rich in Fe and Mg and low in Al and Si.
  • Mantle convection: movement in the mantle expressed at the surface as tectonic plate motion.

• Layered structure summary (depth ranges and composition)

  • Crust: 0–30/85 km depending on oceanic vs continental; oceanic crust thinner, continental crust thicker.
  • Upper mantle (part of the lithosphere): solid rock; part of the rigid shell with the crust.
  • Lower mantle: extends down to ~2890 km.
  • Outer core: liquid metal; depth ~2891–5150 km.
  • Inner core: solid metal; depth ~5150–6371 km.

• Seismic interpretation of the interior

  • Seismic waves reveal internal structure; convection and boundaries inferred from wave behavior.
  • Tomographic images show hot (high-velocity anomalies) and cool (low-velocity anomalies) regions in the mantle, e.g., superplumes beneath Africa and descending remnants of subducted slabs under North America.

• Lithosphere vs. asthenosphere (physical properties)

  • Lithosphere: cool, rigid, brittle; behaves elastically; ~100 km thickness but varies.
  • Asthenosphere: hot, weak, plastic; partially melted; ductile.
  • Mesosphere: hot but stronger due to increased pressure.

• Earth’s interior temperatures (geotherm)

  • Temperature generally rises with depth; typical gradient: rac{dT}{dz} \,\approx\, 20\text{--}40\ \frac{^ ext{°C}}{\text{km}}
  • Base of the lithosphere: T \approx 1200\text{--}1400\ ^{\circ}\text{C}
  • Core temperatures: T_{\text{core}} \approx 3000\text{--}6000\ ^{\circ}\text{C}

• Radius and scale example from the slides

  • Radius of the Earth: R_{\oplus} \approx 6400\ \text{km}
  • Largest surface height difference: about \Delta h = 20\ \text{km} (9 km up to highest mountain + 11 km down to deepest ocean).

• Take-away from the scale exercise (Question 2 on the slides)

  • If the Earth is shrunk proportionally to a golf-ball size with radius R' = 20\ \text{mm} starting from R = 6400\text{km}, the largest surface height difference becomes:
  • Scale factor: s = \dfrac{R'}{R} = \dfrac{20\ \text{mm}}{6400\ \text{km}} = \dfrac{0.02\ \text{m}}{6.4\times 10^{6}\ \text{m}} = 3.125\times 10^{-9}
  • Scaled height difference: \Delta h' = \Delta h \times s = (2.0\times 10^{4}\ \text{m}) \times (3.125\times 10^{-9}) \approx 6.25\times 10^{-5}\ \text{m} = 0.0625\ \text{mm}

• The Earth’s magnetic field and the geodynamo

  • Core composition drives the geodynamo; field lines in the mantle resemble a dipole at the core–mantle boundary but are entangled due to convective motions in the core.
  • Magnetic field lines plotted in 3D show complexities caused by fluid motions in the outer core.
  • Reversals: the Earth’s magnetic field reverses on timescales of a few 10^4 to 10^6 years (occurs irregularly).

• Seismic waves: types and behavior

  • Types of seismic waves:
  • P-waves (primary, compressional): propagate in solids, liquids, and gases; particle motion parallel to the direction of travel; faster speeds.
  • S-waves (secondary, shear): propagate only through solids; particle motion perpendicular to the direction of travel; slower than P-waves; cannot travel through the liquid outer core.
  • Rayleigh waves (surface): retrograde elliptical motion; involve both vertical and horizontal components.
  • Love waves (surface): horizontal shear waves; slower propagation than P-waves but can cause significant surface shaking.
  • Wave paths and boundaries
  • P and S waves are reflected and refracted at boundaries such as the Moho and the CMB.
  • Shadow zones exist because S-waves do not travel through the liquid outer core, creating S-wave shadows, and P-waves refract at the mantle–core boundary creating P-wave shadow zones.
  • Practical seismology: seismographs record arrival times of different wave types; relative arrivals help determine depths to interfaces and the properties of interior layers.

• Isostasy: floating crust and compensation of topography

  • Physical idea: the crust floats on a viscous mantle. Archimedes principle governs the buoyant force: Fb = \rho{\text{fluid}} g V_{\text{displaced}}
  • The crust adjusts its height to restore isostatic equilibrium when mass distribution changes (isostatic adjustment).
  • Models of isostasy
  • Airy model: thicker crust corresponds to higher topography, compensated by a deeper, denser crustal root.
  • Pratt model: topography compensated by lateral variations in crustal density rather than thickness.
  • Evidence: gravity measurements (plumb bob analogy) show mass deficiencies beneath mountainous regions, supporting isostasy.
  • Isostatic adjustment scenarios
  • Ice-sheet growth: crust is depressed under the load; after melting, rebound (isostatic uplift) occurs over thousands of years.
  • The Himalayas, large river deltas, and parts of northern North America show ongoing isostatic adjustments.
  • Ice-sheet history example
  • Canada was covered by an ice sheet ~3–5 km thick about 20,000 years ago; melting completed ~7,000 years ago. The crust responds with uplift (isostatic rebound) after unloading; rebound continues long after the ice has melted.
  • Practical timeline (illustrative):
  • TIME 1: Ice load forms and thickens.
  • TIME 2: Crust bends downward to provide buoyant support.
  • TIME 3: Ice melts and warming begins.
  • TIME 4: Crust rebounds toward pre-ice elevations; raised beaches form due to uplift.

• Quick practice questions (from the slides)

  • Question 3: Which layer has the largest thickness within the Earth? Answer: Mantle.
  • Question 4: Which layer has the largest volume within the Earth? Answer: Mantle.
  • Question 5: The speed of seismic waves beneath Hawaii is lower than beneath the continental US. The most plausible cause is
  • Answer: A. Hawaii is a volcanic island underlain by hot molten rocks. (Lower wave speeds indicate hotter rocks.)
  • Question 6: Isostatic response of crust beneath Canada after melting an ~3–5 km thick ice sheet ~20,000 years ago? Answer: A. The crust rises first and sinks subsequently (initial rebound followed by continued adjustments).
  • Question 7: Himalayan crust vs Appalachian crust thickness. Correct statement: B. Crust is thicker underneath the Himalaya.

• Take-home takeaways

  • Earth consists of multiple concentric layers: crust, mantle, outer core, inner core.
  • Oceanic crust is thin (~5–10 km); continental crust is thicker (~30–85 km).
  • The mantle extends from beneath the crust to the outer core; the upper mantle includes the asthenosphere, which is ductile and partially molten.
  • The outer core is liquid; the inner core is solid; the geodynamo arising from core convection generates Earth’s magnetic field.
  • Seismic P- and S-waves reveal interior structure; P-waves can travel through solids and liquids, S-waves only through solids; their behavior explains shadow zones and boundary depths.
  • Isostasy explains the relationship between surface topography and subsurface density structure; crust thickening increases elevation, while thick roots balance massif topography in the Airy model, and density variation balances topography in the Pratt model.
  • Temperature and geotherm control material properties and phase states inside Earth; gradients indicate hotter, weaker regions at depth and solid/liquid state transitions at boundaries.

• Key numerical references and boundaries (for quick recall)

  • Oceanic crust thickness: 5$-$10\ \text{km}
  • Continental crust thickness: 30$-$85\ \text{km}
  • Moho depth (crust–mantle boundary): oceanic ~5$-$10\ \text{km}; continental ~30$-$85\ \text{km}
  • Core–Mantle Boundary (Gutenberg discontinuity) depth: \approx 2891\ \text{km}
  • Outer core: 2891\ \text{km} \le r \le 5150\ \text{km} (liquid)
  • Inner core: 5150\ \text{km} \le r \le 6371\ \text{km} (solid)
  • Geotherm gradient: \dfrac{dT}{dz} \approx 20\text{--}40\ \frac{^{\circ}\text{C}}{\text{km}}
  • Base of lithosphere temperature: T \approx 1200\text{--}1400\ ^{\circ}\text{C}
  • Core temperatures: T \approx 3000\text{--}6000\ ^{\circ}\text{C}
  • Scale example for Earth shrinking to golf-ball size: scale factor s = \dfrac{R'}{R} = \dfrac{20\ \text{mm}}{6400\ \text{km}} = 3.125\times 10^{-9} and scaled surface relief \Delta h' = \Delta h \times s = (2.0\times 10^{4}\ \text{m}) \times (3.125\times 10^{-9}) \approx 6.25\times 10^{-5}\ \text{m} = 0.0625\ \text{mm}

• Connections to broader concepts

  • Plate tectonics and mantle convection explain continental drift, mountain building, ocean basin formation, and seismic activity.
  • The geodynamo links deep Earth processes to the observable magnetic field, affecting navigation and shielding from cosmic radiation.
  • Isostasy links surface processes (glaciation, sedimentation) to subsurface density and crustal architecture, explaining long-term topographic evolution.

• Real-world relevance and implications

  • Seismic studies inform earthquake hazard assessment and the depths of faults.
  • Understanding the geotherm helps model magma generation, volcanic activity, and tectonic movement.
  • Knowledge of crust thickness and mantle properties underpins resource exploration (minerals, hydrocarbons) and geothermal energy.

• References to lecture content

  • Moho (1909) Mohorovičić; Gutenberg (1914) core–mantle boundary; Lehmann (1936) inner core discovery.
  • Mantle tomography and geophysical cross-sections show heterogeneity in the mantle with hot plumes and subducted slabs.
  • Isostasy evidenced by gravity measurements and the need for a buoyant crust–mantle system to balance topography.

• Glossary of terms to remember

  • Lithosphere, Asthenosphere, Mesosphere, Moho, Gutenberg discontinuity, Peridotite, Geodynamo, P-waves, S-waves, Rayleigh waves, Love waves, Isostasy, Airy model, Pratt model, Tomography

Take-Home Messages (concise)

• Earth’s interior is layered: crust, mantle, outer core (liquid), inner core (solid).
• Oceanic crust is thinner and basaltic; continental crust is thicker and granitic.
• The lithosphere is rigid; the asthenosphere is ductile; mantle convection drives plate tectonics.
• The outer core is liquid and generates Earth’s magnetic field via the geodynamo; the inner core is solid.
• Seismic waves (P and S) reveal interior structure; S-waves do not traverse the liquid outer core, leading to shadow zones.
• Isostasy explains topography through buoyancy: Airy (thickness-based) and Pratt (density-based) models; crust adjusts height after loading/unloading (e.g., ice sheets).
• Global temperature increases with depth (geotherm): typical gradient ~dT/dz \approx 20\text{--}40\ \dfrac{\circ C}{\text{km}}; base of lithosphere ~1200\text{--}1400\ \circ C; core ~3000\text{--}6000\ \circ C.

Practice and quick review questions

• Which layer has the largest thickness and largest volume within the Earth? Mantle.
• Why do Hawaii and the continental US have different seismic velocities beneath them? Hotter mantle beneath Hawaii due to volcanic activity lowers wave speeds.
• What is the fate of the crust after melting an ice sheet? Initial rebound (uplift) followed by continued isostatic adjustment.
• Where is crust thicker, Himalaya or Appalachian region? Himalaya (thicker crust).

Quick references to the figures and boundaries mentioned in the slides

• Moho discontinuity: crust–mantle boundary; oceanic crust ~5–10 km, continental ~30–85 km.
• Core–Mantle Boundary (Gutenberg): boundary at ~2891 km depth; outer core is liquid; inner core is solid.
• Mantle structure: upper mantle + lithosphere vs. asthenosphere; tomographic images show hot plumes and cool slabs.
• Seismic wave behavior: P-waves and S-waves; shadow zones due to core properties; seismic data collected by seismographs.