1. The framing question

"If the spreading floor hypothesis is correct, then normally magnetised blocks alternate with reversely magnetised blocks…"
— Vine & Matthews, Nature (1963)

• Ship-towed magnetometer profiles across the Juan de Fuca Ridge show alternating stripes of high and low magnetic intensity, symmetric about the ridge axis.
• A pattern this clean requires that newly-formed basalt records the field at the moment it cools and keeps that record for millions of years.
• What atomic-scale physics turns a rock into a magnetic tape recorder?

Read more → Lecture 24 §1

Synthetic Juan de Fuca stripes

• Half-rate 30 mm yr⁻¹ converts polarity timescale → distance.
• Smoothed ΔF profile reaches ±350 nT with FWHM ≈ 6 km.
• Crustal cross-section: blue blocks = normal-polarity TRM, red = reversed.

Learning objectives

By the end of today, students will be able to:

1. Classify minerals into the five magnetic-ordering categories and predict and .
2. Distinguish induced from remanent magnetisation and compute the Königsberger ratio .
3. Identify the three principal acquisition mechanisms — TRM, DRM, CRM — and the rocks that carry them.
4. Read a geomagnetic polarity timescale (GPTS) ribbon and convert chron ages to seafloor distance.
5. Use the Siletzia post-Eocene rotation as a worked example of inverting paleomagnetic vectors for tectonic motion.

2. Magnetic ordering at the mineral scale

• The constitutive law hides a factor of 10⁵ between minerals.
• Five categories of electron-spin alignment:
  • Diamagnetic (calcite, quartz):
  • Paramagnetic (olivine, pyroxene): , small
  • Ferromagnetic (rare in nature: native Fe)
  • Ferrimagnetic (magnetite, pyrrhotite): large
  • Antiferromagnetic / canted (hematite): small but stable

Read more → Lecture 24 §2

Curie temperatures of common carriers

3. Induced vs. remanent magnetisation

• Induced: , disappears when .
• Remanent: persists with no external field — locked in at formation.
• Königsberger ratio :
  • — magnetisation tracks today's field
  • — magnetisation records the paleofield
• for fresh MORB ≈ 5–50; for granite ≈ 0.1–1.

Read more → Lecture 24 §3

4. A rock is an ensemble of grains

• Bulk = vector sum over millions of grains.
• Single-domain grains (≲ 0.1 μm) carry the most stable remanence.
• Multi-domain grains lose memory through domain-wall motion.
• Stability ↑ as grain volume ↑ and temperature ↓ — captured by Néel relaxation time:

Read more → Lecture 24 §4

5a. TRM — thermoremanent magnetisation

• Lava erupts at ~1100 °C — well above . Spins are randomly oriented.
• As cooling crosses (e.g. 580 °C for magnetite), spins align with the ambient field at that instant.
• Below the blocking temperature , that alignment is locked for yr.
• TRM is what records the polarity timescale on the seafloor.

Read more → Lecture 24 §5a

5b. DRM — depositional remanence

• Detrital magnetic grains settle through a water column with the ambient field acting as a weak torque.
• Hydrodynamic and biological forces leave a biased alignment preserved at the sediment–water interface.
• DRM is typically 10×–100× weaker than TRM but covers the long sedimentary record (deep-sea cores, lakes).

Read more → Lecture 24 §5b

5c. CRM — chemical remanence

• New magnetic minerals grow during weathering, diagenesis, or hydrothermal alteration.
• As grain volume exceeds the superparamagnetic threshold, the field at that moment is locked in.
• CRM records secondary events, not the original cooling age — a source of paleomagnetic noise unless cleaned by thermal/AF demagnetisation.

Read more → Lecture 24 §5c

6. The geomagnetic polarity timescale

• The field reverses on irregular timescales (10⁴ – 10⁶ yr).
• The last reversal (Matuyama → Brunhes) was 781 ka.
• The GPTS, built from ocean stripes + radiometric dating, is the master clock for Cenozoic tectonics.

Read more → Lecture 24 §6

7. Forward model — Vine-Matthews-Morley

• New crust at the ridge cools through → records the current polarity.
• Spreading at half-rate carries that crust laterally.
• Distance from ridge polarity-stripe age.
• For JdF half-rate mm yr⁻¹: the 781 ka Brunhes/Matuyama boundary sits at km from the ridge — exactly where the field measurement shows the first reversal.

Read more → Lecture 24 §7

8. Cascadia worked example — Siletzia rotation

• The Eocene Crescent Formation (Olympic Peninsula, Willapa Hills) carries a TRM locked in ~50 Ma.
• Measured paleomagnetic inclination → paleolatitude agrees with today's.
• Measured declination is rotated ~50° clockwise from north.
• Inverse: Siletzia (the accreted oceanic plateau under western WA / OR) has rotated clockwise since accretion — consistent with GPS-tracked block rotations today.

Read more → Lecture 24 §8

9. Research Horizon

• Single-crystal paleointensity (e.g. on IODP cores) — recovering field strength, not just direction, back to the Cretaceous.
• Magnetic stratigraphy of Cascadia subduction-zone turbidites — tying recurrence intervals to the GPTS.
• Anisotropy of magnetic susceptibility (AMS) as a non-invasive fabric indicator for fault-zone deformation.

Read more → Lecture 24 §9

10. AI Literacy

• LLMs reliably describe TRM but routinely confuse Curie temperature with blocking temperature (they differ by 50–150 °C and matter for paleointensity).
• Ask: "What is the Néel relaxation time and how does it depend on grain volume?" — verify the exponential dependence, not linear.
• Always check that any generated GPTS dates are referenced to a published timescale (Cande & Kent 1995, Ogg 2020) — chron numbering has been revised.

Read more → Lecture 24 §10

11. Concept check

1. A basalt sample has A m⁻¹ and . With A m⁻¹, what is ?
2. If JdF spreading half-rate were half today's value, where would the Brunhes/Matuyama boundary sit?
3. Why does hematite preserve a CRM with high stability despite being antiferromagnetic?

Read more → Lecture 24 §11

12. Looking ahead

• Lecture 25 — Magnetic anomalies: forward dipole modelling, half-width depth rule, three-scale reading culminating in the Seattle Fault Zone aeromagnetic survey.
• The polarity record we built today becomes the target the next lecture inverts for.

Read more → Lecture 25 — Magnetic Anomalies & Surveys