DW

Earth's Interior (Lecture 4)

Course Logistics (transcript overview)

  • Lecture slides are posted at least an hour before class on Google Drive (link on Canvas).
  • Develop habits: complete pre-lab assignments before lab, complete reading quizzes before lecture.
  • Learn to efficiently access and answer Chime In questions during class (Week 2 Assignment referenced).
  • Q&A Discussion Board is live on Canvas.
  • Question prompting: "Did you complete the reading quiz due this morning?" to encourage engagement.

Earth's Magnetic Field: variability and reversals

  • The magnetic field is not constant; the magnetic north pole wanders over time.
  • Magnetic reversals occur periodically, i.e., the field flips direction.
  • Paleomagnetic data suggest reversals occur over timescales of 100 \text{ to } 1000\ \text{years} (likely skewed toward the shorter end).
  • In the centuries before a reversal, the field strength drops significantly, by roughly 90\%, though this kind of dramatic weakening is not currently observed today.

What generates Earth's magnetic field?

  • The geodynamo: convection in Earth's liquid iron outer core creates and sustains the magnetic field.
  • Convection and rearrangement of currents in the outer core drive the magnetic north pole wander and occasional magnetic reversals.

Earth’s interior: structure and labels

  • Radial structure (not to scale in slides):
    • Solid Crust: 0 \text{ - } 40\ \text{km}
    • Liquid iron outer core: 2890 \text{ - } 5150\ \text{km}
    • Solid Mantle: 40 \text{ - } 2890\ \text{km}
    • Solid iron inner core: 5150 \text{ - } 6370\ \text{km}
  • These depths show where the crust, mantle, outer core, and inner core reside.

How do we know what's down there? Evidence & methods

  • Direct sampling: rocks are drilled to sample subsurface material in some places.
  • Historic drilling projects:
    • Project Mohole (USA): drilled through the crust toward the mantle; in 1961 drilled to 183 m below the seafloor in water depth of \sim 3600\ \text{m}
    • Kola Superdeep Borehole (Russia): reached 12.2\ \text{km} (about 7.6\ \text{miles}) depth before drilling ended due to geological conditions.
  • Geophysical and indirect methods fill the gap where direct sampling is not feasible.
  • The Cold War Space Race context: USSR launched Sputnik I in 1957; USA landed the Moon in 1969, illustrating competing scientific programs that spurred deep-Earth research efforts.

How do we know what’s down there? Major inference tools

  • Geothermal gradient: temperature rises with depth; in the crust, approximately dT/dz \approx 25 \text{ to } 30\ \mathrm{°C\,km^{-1}}.
  • Earth pressure gradient: pressure increases with depth due to overlying rock weight; crustal gradient approximately dP/dz \approx 25\ \mathrm{MPa\,km^{-1}} (MPa = Megapascal; 1 MPa is about 1,000,000 N/m^2).
  • If pressure increases with depth, density tends to increase with depth (density is mass per unit volume).
  • Mantle xenoliths: fragments of mantle rocks brought to the surface by rising magma, used to study mantle composition:
    • Mantle xenolith of Upper Mantle: Peridotite; Peridot is a birthstone for August.
    • Mantle xenolith of Lower Mantle: Eclogite with Diamond Xenocrysts; xenocrysts indicate deep mantle formation processes.
  • Geochemistry of igneous rocks formed by melting of the Mantle provides clues about mantle composition.
  • Meteorites: Earth-like meteorites contain high concentrations of iron and nickel, informing us about the early Earth’s composition.
  • Geophysical imaging: seismic, gravitational, and other techniques image the interior structure.
  • Geodetic techniques model Earth’s density to match observed gravity and orbital dynamics around the Sun.
  • If the entire core were assumed liquid due to S-wave shadowing, gravity calculations would be inaccurate; thus, S-waves provide information about the solid/ liquid states inside Earth.
  • Note on illustrations: some figures color the Mantle green to emphasize its solid nature (peridotite) rather than the often-red depiction in other diagrams.

Mantle and Core: composition and states

  • Mantle: solid rock; overall composition in silicate minerals with major elements O, Si, Al, Fe, Mg, and smaller amounts of others; mantle is solid but can flow slowly over geological timescales.
  • Outer Core: liquid iron-nickel alloy; major components include iron (Fe) and nickel (Ni), with minor impurities.
  • Inner Core: solid iron-nickel alloy; high-density composition.
  • Outer Core composition (approximate): Iron (Fe) 85–95%, Nickel (Ni) 5–6%, minor impurities of sulfur and oxygen.
  • Inner Core composition (same general category described for both cores): dense Fe–Ni with minor light-element impurities.
  • Mantle and crust composition (by elemental percent, approximate):
    • Oxygen (O): \approx 46\%
    • Silicon (Si): \approx 28\%
    • Aluminum (Al): \approx 8\%
    • Iron (Fe): \approx 6\%
    • Magnesium (Mg): \approx 4\%
    • Others: \approx 6\%
  • The mantle is Green in some diagrams to emphasize peridotite; it is solid yet can flow on long timescales.

Planetary differentiation and interior heat

  • Planetary differentiation: Early in Earth's history, when still molten, dense, heavy elements sank toward the center while lighter elements floated toward the surface.
  • Proto-crust formed as heat escaped into space; a conductive lid trapped heat, influencing interior evolution.
  • Total interior heat flow is distributed among layers (approximate shares):
    • Crust: \approx 24\% of total interior heat flow
    • Upper mantle: \approx 22\%
    • Lower mantle: \approx 32\%
    • Core: \approx 22\%
  • Heat transport mechanisms by layer (from Dye 2012 and Arevalo Jr. et al. 2009):
    • Crust: Conduction
    • Upper mantle: Advection
    • Lower mantle: Convection
    • Core: Convection
  • The Earth’s interior engine is driven by trapped heat and its transport through conduction, convection, and advection.

Rheology and internal layering

  • Rheology: how rocks deform under stress; critical for understanding deformation and plate tectonics.
  • Brittle rheology: hard, rigid materials that deform by cracking/breaking (typical of the Lithosphere).
  • Ductile rheology: materials that flow or stretch under stress without cracking (typical of the Asthenosphere).
  • Viscous rheology: liquids that flow under applied forces (relevant to deep mantle flow).
  • Internal subdivision by rheology:
    • Lithosphere: brittle—crust plus uppermost mantle; rigid and can fracture.
    • Asthenosphere: ductile—mantle region capable of slow flow and deformation.
    • Mesosphere (lower mantle): more rigid due to high pressures but still capable of slow flow under long timescales.

The Mantle: solid yet flowing, and convection timelines

  • The mantle is solid (like rock) but flows extremely slowly; convection occurs in the mantle, driven by temperature and density contrasts.
  • Important aside: convection also occurs in the liquid outer core, which drives the magnetic field.
  • Mantle convection is extremely slow:
    • Estimated time for one full mantle convection cycle: \tau \approx 5 \times 10^{7} \text{ years} (about 50 million years).
    • In scale terms: since ancient times (e.g., Egyptian pyramids), the mantle has moved roughly 0.005\ \text{km} \ (= 5\ \text{m}) along a path of about 2\,200\ \text{km} in its convection circuit.
  • The driving question at times: what sustains plate tectonics if mantle convection is so slow and confined to limited regions? The current view emphasizes mantle flow in convection cells and lateral pressure gradients that drive plate motions.

Seafloor spreading and plate tectonics: a synthesis

  • Seafloor spreading hypothesis (Harry Hess, 1960): oceanic crust is created at mid-ocean ridges and recycled at ocean margins.
  • Mechanism: mantle convection currents generate the driving forces that push sea-floor plates apart at ridges and recycle old crust at subduction zones.
  • This mantle convection is a central component of plate tectonic theory, linking interior dynamics to surface geology.

What changes with depth? Rheology, composition, temperature, and pressure

  • Depth-dependent variations alter rheology and behavior of rocks:
    • Changes in composition, temperature, and pressure affect how rocks deform under stress.
    • Rheology defines whether rocks fracture (brittle), flow (ductile), or melt (viscous).
  • Rheological classifications used in teaching:
    • Brittle Rheology: hard, rigid materials that crack
    • Ductile Rheology: solids that flow/stretch slowly without breaking
    • Viscous Rheology: liquids that continually flow under stress

Layered Earth: Lithosphere, Asthenosphere, Mesosphere

  • Lithosphere: crust and uppermost mantle; brittle rheology; participates in plate tectonics through fracture and rigid plate behavior.
  • Asthenosphere: central mantle region with ductile rheology; capable of slow flow, allowing plate motion.
  • Mesosphere: lower mantle; higher pressures make rocks more rigid and less mobile.

Important aside: mantle flow and the question of movement

  • The mantle is solid, but it behaves like play-dough or silly putty: it can flow very slowly over geologic timescales.
  • The mantle’s slow convection is still a powerful long-term driver of tectonics and interior evolution.

Convection in the outer core and the geodynamo

  • An important aside: convection also occurs in Earth’s liquid outer core.
  • The flow of liquid iron-nickel in the outer core generates and sustains Earth’s magnetic field via a geodynamo.
  • Changes in core currents drive the wandering of the magnetic north and occasional magnetic reversals.

Practical limits and technological takeaways

  • Takeaway: We currently do not have the technology to drill beyond Earth’s upper crust.
  • Drillings and observations have demonstrated the limits of direct access to deeper layers (Mohole, Kola Borehole, and CHIKYU efforts).
  • Cultural and educational reflections: personal and professional frustration in mantle geology, with the sense of “being blocked by the rock wall” when trying to access deeper layers.
  • The CHIKYU deep-sea drilling vessel (Japan, 2011) illustrates attempts to drill deeper into the crust; the mission showcased the scale and difficulty of deep-ocean drilling (a 106-day endeavor referenced in the slides).
  • The running takeaway statement: continuous limits hinder direct access to deeper mantle and core materials; indirect methods remain essential.

Global knowledge synthesis: multiple lines of evidence bind together

  • Mantle xenoliths and mantle geochemistry provide key mantle constraints when samples reach the surface.
  • Meteorites and early differentiation offer clues about Earth’s early formation and core–mantle differentiation.
  • Geophysical imaging and gravity/orbit modeling help infer density distributions and interior structure.
  • Seismic waves (P and S waves) reveal state (solid vs liquid) and layering, including the S-wave shadow zone indicating a liquid layer within the Earth (outer core).

S-waves, p-waves, and internal structure evidence

  • Seismic body waves show two main types:
    • P waves (compressional waves) can travel through liquids.
    • S waves (shear waves) cannot travel through liquids.
  • Seismometers worldwide monitor earthquakes to map interior structure.
  • The presence of an S-wave shadow zone is evidence for a liquid layer (outer core).
  • If the entire core were liquid (as suggested purely by S-wave considerations), gravity calculations would be inaccurate; thus, a solid inner core and/or a partially solid component must exist.

Additional notes on interior depiction and terminology

  • Some diagrams color the Mantle green to emphasize peridotite (a mantle rock) and to remind that the Mantle is solid, despite its flow.
  • The inner and outer core are composed of dense Fe and Ni with minor impurities (S, O).
  • The lithosphere-asthenosphere distinction reflects changes in mechanical behavior with depth, not just composition.

Key numerical anchors and formulas to remember

  • Magnetic reversal timescale: t_{rev} \in [100, \ 1000] \ \text{years}
  • Pre-reversal field strength drop: about 90\%\
  • Geothermal gradient: \frac{dT}{dz} \approx 25 \text{ to } 30\ \mathrm{°C\,km^{-1}}
  • Pressure gradient: \frac{dP}{dz} \approx 25\ \mathrm{MPa\,km^{-1}}
  • Mantle convection timescale: \tau \approx 5 \times 10^{7} \text{ years}
  • Mantle convection distance/time reference: about 0.005\ \text{km} \ (= 5\ \text{m}) displacement over a ~2.2\times 10^{3} \ \text{km} mantle path during a single circuit
  • Core composition (outer core): approximately \text{Fe} \sim 85-95\%\, ,\ \text{Ni} \sim 5-6\% with minor impurities
  • Core composition (inner core): similar dense Fe–Ni composition
  • Mantle composition (by weight percent): O ≈ 46%, Si ≈ 28%, Al ≈ 8%, Fe ≈ 6%, Mg ≈ 4%, Other ≈ 6%
  • Layer depth references (approximate):
    • Crust: 0 \text{ - } 40\ \text{km}
    • Mantle: 40 \text{ - } 2890\ \text{km}
    • Outer Core: 2890 \text{ - } 5150\ \text{km}
    • Inner Core: 5150 \text{ - } 6370\ \text{km}
  • Total interior heat-flow shares (approximate): Crust 24%, Upper Mantle 22%, Lower Mantle 32%, Core 22%
  • Heat-transport mechanisms by region: Conduction (Crust), Advection (Upper Mantle), Convection (Lower Mantle and Core)

Reading and course reminders

  • Reading assignment for Friday: read Chapter 2.2 and complete the Reading Quiz before class.
  • Lab reminder (in-person): purchase the lab manual and complete the pre-lab before the lab meets this week.
  • Late-policy reminder: 10% per weekday late policy will be enforced.

Quick conceptual recap

  • The Earth’s interior is a complex, layered system where solid mantles can flow slowly, outer core is liquid and drives the magnetic field, and inner core remains solid.
  • Magnetic reversals are linked to chaotic reorganizations of outer-core convection; these reversals are rare on human timescales but can occur over centuries.
  • Our understanding relies on indirect methods (geophysics, geochemistry, meteorites, xenoliths) and on rare direct samples (drill cores, mantle rocks brought to surface).
  • Plate tectonics emerges from mantle convection combined with lithospheric rheology, supported by seafloor spreading data and mid-ocean ridge activity.
  • There are significant limitations to drilling deeper, underscoring the need for indirect techniques to study Earth’s deep interior.