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.