Lecture 23

1. The framing question

• Seattle's compass points 15.5° east of true north in 2026 — it pointed 22.1° east in 1955.
• KSEA's main runway was renamed 16L/34R → 16R/34L in 2019 to keep its number honest.
• A planetary-scale physical quantity changes fast enough to alter aviation infrastructure.
• What is the field, where does it come from, and how do rocks record it?

Read more → Lecture 23 §1

Learning objectives

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

1. Decompose the field into (D, I, F) and (X, Y, Z).
2. Locate the three sources of the surface field — core, lithosphere, ionosphere — in their physical context and on the power spectrum.
3. Distinguish induced from remanent magnetisation, use the Königsberger ratio , and recognise the five categories of magnetic ordering.
4. Describe the three remanence-acquisition mechanisms — TRM, DRM, CRM — and the Curie temperatures of PNW magnetic minerals.
5. Apply as a forward and an inverse problem with uncertainty.

2. The dipole field and the tangent-cylinder geodynamo

• Surface field ≈ 90% dipolar, axis tilted ~11° from rotation axis.
• The outer-core geodynamo is cylindrical: convection is organised into columns coaxial with the rotation axis (the tangent cylinder).
• At a station, three numbers describe the field: declination D, inclination I, total intensity F.

Read more → Lecture 23 §2

2. (D, I, F) (X, Y, Z) at a station

→ (X, Y, Z) = (18 300, 5 070, 49 360) nT

The vertical component is ~2.6× larger than the horizontal at Seattle's latitude — the compass needle only sees .

Read more → Lecture 23 §2

3. Three sources of the surface field

A surface magnetometer adds three contributions from three depths:

1. Core (geodynamo) — 2 900–5 150 km depth, km
2. Lithosphere — upper ~30 km, – km
3. Ionosphere / magnetosphere — 80–500 km altitude, time-varying

Magnetic surveying = separating these three.

Read more → Lecture 23 §3

3d. Spectral fingerprint — same field, three scales

Spatial profile (top) decomposes into three colour-coded components; the Mauersberger–Lowes spectrum (bottom) maps each one to its own degree range — core dominates , crust dominates , external is a floor.

Read more → Lecture 23 §3.4

4. The field drifts — secular variation at Seattle

• D: +22.1° → +15.5° over 71 yr
• I: 71.0° → 68.9°
• F: ≈ 3 000 nT drop

Drift is outer-core fluid flow; corrected in surveys via the IGRF reference epoch.

Read more → Lecture 23 §4

5. The mineral scale — a rock is an ensemble

Bulk magnetic response = volume-weighted average of the mineral assemblage. A "magnetite-bearing" basalt is mostly plagioclase + pyroxene; a granite carries only trace magnetite; a red bed is dominated by hematite cement.

Read more → Lecture 23 §5.1

5b. Induced vs. remanent — the central distinction

• Induced: — vanishes when (dia-, para-).
• Remanent: hysteresis loop, at (ferri-, ferro-).
• Königsberger ratio tells you which regime dominates a real rock in Earth's field.

: anomaly points along today's field. : along the ancient field.

Read more → Lecture 23 §5.2

5c. Five categories of magnetic ordering

• Dia- (quartz, halite): — induced only.
• Para- (olivine, biotite): — induced only.
• Ferro- (Fe metal, rare in nature): — remanent.
• Antiferro- (hematite, with spin canting): weak parasitic remanence.
• Ferri- (magnetite, titanomagnetite): unequal antiparallel — the workhorse.

Read more → Lecture 23 §5.3

6. Curie temperatures of PNW magnetic minerals

Above → paramagnetic (no ordering). Below → ordering returns; spins lock in as the grain cools through a narrow blocking interval.

Read more → Lecture 23 §6.1

6a. TRM — cooling through the Curie temperature

A grain cooled through in field locks in a TRM parallel to in the blocking interval just below .

Igneous rocks → TRM is the dominant remanence carrier. Oceanic basalts of the Juan de Fuca plate record the field at their eruption (→ Lecture 24).

Read more → Lecture 23 §6.1

6b. DRM and CRM — sediments and chemistry

• DRM (detrital remanent magnetisation) — magnetite grains settling through the water column align with the ambient field, then are locked at the sediment-water interface. The remanence carrier of marine and lacustrine sediments; Cascadia accretionary wedge cores record Pleistocene–Holocene field.
• CRM (chemical remanent magnetisation) — a new magnetic mineral grows in place in the ambient field at . Dominant in red beds and oxidised basalt tops.
• Bonus: IRM (lightning strikes) and VRM (slow long-time tail) are the routine overprints removed in the lab.

Read more → Lecture 23 §6.2–6.4

7. The forward problem — GAD inclination from latitude

For a Geocentric Axial Dipole:

The factor of 2 comes from at the surface for a dipole.

Read more → Lecture 23 §7

7b. The inverse problem — paleo-latitude from inclination

• Propagate → .
• Largest uncertainty near the equator (steep slope of forward curve).
• Reliable at high paleo-latitudes; soft at low.

Read more → Lecture 23 §7

8. Research Horizon — Swarm and the geodynamo

• ESA Swarm constellation (3 satellites, 2013–present) maps the vector field at 460 km altitude to degree .
• Tracks westward drift (~0.2°/yr) → azimuthal core flow.
• Detects geomagnetic jerks — abrupt year-scale changes in .
• Resolves lithospheric features: oceanic fabrics, impact structures, continental margins.

The core is not in steady state, and we now have the data to watch it.

Read more → Lecture 23 §8

9. AI Literacy — derivation as a verification task

Reasoning Partner activity:

1. Ask an LLM to derive from the dipole field expressions in polar coordinates.
2. Verify, do not trust: check the inclination definition, the algebra, and the latitude-vs-colatitude convention.
3. Disagree productively: if it errs, name the specific step in your correction prompt — never ask "is this right?"

LLMs accelerate derivations; they do not replace the student's responsibility to check.

Read more → Lecture 23 §9

10. Concept checks

1. D, I, F at Seattle in 1955. Compute (X, Y, Z) from D = +22.1°, I = +71.0°, F = 55 980 nT. Which component has changed most since 2026 in relative terms? In absolute terms?

2. Induced or remanent? A +800 nT anomaly above a granite () and a +800 nT anomaly above a basalt (). Demagnetise both in zero field — which signal survives?

3. Paleo-latitude error budget. With , compare for vs . Why are they different?

Read more → Lecture 23 §10

11. Connections — what's next

Lecture 24 takes today's framework and asks the next question:

If a magnetised body sits in the crust, how does its small () perturbation to F appear on the surface?

• Forward problem: anomaly shape depends on magnetic latitude.
• Half-width depth rule with measurement-noise propagation.
• Inverse problem with m–z trade-off — the ridge.
• Application: Juan de Fuca magnetic stripes and plate tectonics.

Continue → Lecture 24 — Magnetism and Plate Tectonics