Questions: Core Crystallization Dynamics and Magnetic Field Reversals

Question 1

A planet's iron core has cooled until the liquid outer core has completely solidified. What is the most direct consequence for the planet's magnetic field?

• A. The magnetic field strengthens because solid iron is more electrically conductive than liquid iron
• B. The magnetic field collapses because compositional convection in the outer core ceases when there is no longer a liquid layer
• C. Magnetic reversals become more frequent because the solid core has higher magnetic susceptibility
• D. The mantle takes over as the primary driver of the dynamo through thermal convection alone

Answer: (B)

Earth's magnetic field is generated by convective motions in the liquid outer core — the dynamo. When the outer core solidifies, both thermal and compositional convection cease, eliminating the mechanical energy source that drives the dynamo. The solid inner core itself is not ferromagnetic at core temperatures (far above the Curie point) and contributes nothing to field generation. Mars is the canonical example: its smaller core likely cooled and solidified, terminating its dynamo and leaving it without a global magnetic field.

Question 2

What is the primary driver of compositional convection in Earth's outer core?

• A. Temperature differences generated by radioactive decay in the surrounding mantle
• B. Buoyant, light-element-enriched liquid rejected from the crystallizing inner core boundary
• C. Direct electromagnetic induction from the Sun's magnetic field interacting with the core
• D. Coriolis-forced rotation of iron dendrites growing at the inner core surface

Answer: (B)

When liquid iron freezes at the inner core boundary, the crystal lattice selectively incorporates iron while rejecting lighter elements (sulfur, silicon, oxygen). This element-depleted solid leaves behind a liquid enriched in light elements, which is less dense than the surrounding outer core liquid. The resulting buoyancy drives vigorous upward flow — compositional convection — that powers the dynamo independently of (and in addition to) thermal convection from latent heat release.

Question 3

Earth's solid inner core is the primary source of Earth's magnetic field because solid iron is ferromagnetic.

• T. True
• F. False

Answer: False

This is a fundamental misconception. Iron loses its ferromagnetic properties above the Curie temperature (~770°C), and the inner core is at roughly 5,000°C — far above this threshold. The solid inner core plays no direct role in generating the magnetic field. The field is generated by convective motions in the liquid outer core — the dynamo. The inner core's significance is indirect: its slow growth provides the latent heat and light-element rejection that power the convection sustaining the dynamo.

Question 4

Magnetic field reversals leave a permanent paleomagnetic record in volcanic rocks that cooled and solidified during or after the reversal.

• T. True
• F. False

Answer: True

When molten rock cools through the Curie temperature, magnetic minerals (like magnetite) become locked into alignment with the ambient magnetic field direction at the time of cooling. This 'fossil' magnetism is preserved indefinitely, recording the field's polarity. Ocean floor basalts formed at mid-ocean ridges provide a continuous record of polarity reversals as the seafloor spreads, giving us the primary evidence for the irregular timing of reversals throughout Earth's history.

Question 5

Why is the crystallization of Earth's inner core an energy source for the outer core dynamo, rather than simply removing magnetic material from the system?

Model answer: Crystallization releases latent heat (warming the surrounding liquid) and rejects light elements (sulfur, silicon, oxygen) from the iron crystal lattice. These rejected elements concentrate in the remaining liquid, making it less dense than the overlying outer core fluid. This buoyant, compositionally distinct liquid rises, driving compositional convection — vigorous mechanical stirring that supplies the energy sustaining the dynamo. The magnetic field is generated by this convective motion, not by the magnetic properties of the iron itself.

The key insight is that the energy for the dynamo comes from phase change (latent heat) and chemical segregation (compositional buoyancy), not from the magnetic properties of the material. This is why planets with actively growing inner cores tend to have strong magnetic fields, and why the cessation of inner core growth — or its completion — would eliminate the energy supply and collapse the dynamo.