12.3 Core dynamics and the geomagnetic field

Earth's Core Structure and Composition

Layers and Boundaries

Earth's core has two distinct layers separated by a sharp boundary. The outer core is a liquid shell roughly 2,260 km thick, sitting beneath the core-mantle boundary (CMB) at about 2,890 km depth. Below it, the inner core is a solid sphere with a radius of about 1,220 km. The boundary between them, the inner core boundary (ICB), is where liquid iron freezes onto the growing inner core.

Composition and Physical Properties

Both layers are composed primarily of iron and nickel, but their physical states differ because of the extreme pressure gradient through the core.

• The outer core is liquid and contains lighter alloying elements (sulfur, oxygen, silicon, and possibly hydrogen). These light elements lower the melting point and play a key role in driving convection.
• The inner core is solid despite being hotter, because the pressure at Earth's center (around 360 GPa) is high enough to force iron into a solid phase. It is compositionally purer iron-nickel than the outer core, since lighter elements are expelled as the inner core crystallizes.
• Core temperatures range from roughly 4,000 K at the CMB to about 5,000–6,000 K at the center. The pressure increases from ~136 GPa at the CMB to ~364 GPa at Earth's center.

The density jump at the ICB (from ~12,200 kg/m³ in the outer core to ~13,000 kg/m³ in the inner core) was first identified through seismic observations by Inge Lehmann in 1936.

Geodynamo and Earth's Magnetic Field

Geodynamo Process

Earth's magnetic field originates in the liquid outer core through the geodynamo process. The basic requirement is simple: you need a rotating body containing a large volume of electrically conducting fluid undergoing vigorous convection.

Convection in the outer core is driven by two main energy sources:

• Thermal buoyancy: Heat released at the ICB (both from the latent heat of inner core crystallization and from secular cooling of the core) creates temperature differences that drive fluid motion.
• Compositional buoyancy: As iron crystallizes onto the inner core, lighter elements (O, S, Si) are rejected into the outer core fluid. This lighter fluid is buoyant and rises, providing a powerful and efficient source of convection.

Earth's rotation organizes these convective flows through the Coriolis effect, producing columnar, helical flow structures aligned roughly parallel to the rotation axis. These are sometimes called Taylor columns.

Dynamo Effect and Magnetic Field Generation

The dynamo effect is a positive feedback loop. Here's how it sustains itself:

1. Convective motions move the electrically conducting iron-alloy fluid through a pre-existing (even very weak) magnetic field.
2. This motion induces electric currents in the fluid (by Faraday's law of electromagnetic induction).
3. Those electric currents generate their own magnetic field, reinforcing and reshaping the original field.
4. The reinforced field interacts with continued fluid motion, sustaining the cycle.

The key parameter governing whether a dynamo can operate is the magnetic Reynolds number, $Re_m = \frac{UL}{\eta}$, where $U$ is a typical flow velocity, $L$ is a characteristic length scale, and $\eta$ is the magnetic diffusivity. For self-sustaining dynamo action, $Re_m$ must exceed a critical value (typically $Re_m \gtrsim 10$–$100$). In Earth's outer core, $Re_m$ is estimated to be on the order of several hundred, well above the threshold.

The geodynamo is inherently a chaotic, nonlinear system. Small changes in flow patterns can produce large changes in field geometry over time, which is why the field fluctuates in strength and direction and occasionally reverses polarity entirely.

Evidence for Core Dynamics

Secular Variation

Secular variation refers to changes in Earth's magnetic field on timescales of years to centuries. You can observe it directly: the position of magnetic north drifts measurably over decades, and the field intensity at any given location changes over time.

• The westward drift of magnetic field features (about 0.2° per year on average) suggests that flow patterns in the outer core are not static but evolve continuously.
• Regional changes in field intensity, such as the growth of the South Atlantic Anomaly (a region of unusually weak field strength), provide a window into how convection patterns in the outer core shift over human timescales.

Secular variation is the most direct evidence that the core is dynamically active right now.

Paleomagnetic Records

On longer timescales, the history of Earth's magnetic field is preserved in rocks and sediments. When igneous rocks cool through the Curie temperature of their magnetic minerals (e.g., magnetite at ~580°C), they lock in the direction and intensity of the ambient field. Sediments can also acquire a magnetic signature as magnetic grains align with the field during deposition.

These paleomagnetic records reveal several important features of core dynamics:

• Polarity reversals: The north and south magnetic poles have swapped positions hundreds of times over Earth's history. The most recent reversal, the Brunhes-Matuyama reversal, occurred about 780,000 years ago.
• Irregular reversal frequency: The average interval between reversals is roughly 200,000–300,000 years, but this varies enormously. During the Cretaceous Normal Superchron (~84–124 Ma), the field maintained a single polarity for about 40 million years. At other times, reversals occurred every few tens of thousands of years.
• Apparent polar wander paths: Because paleomagnetic data record the paleolatitude of a rock at the time it formed, tracking the apparent position of the magnetic pole over time for a given continent reveals how that continent has moved. This was one of the early lines of evidence supporting plate tectonics.

The geomagnetic polarity timescale (GPTS), constructed from marine magnetic anomaly patterns on the seafloor and radiometric dating of volcanic rocks, provides a detailed chronology of reversals extending back roughly 170 million years.

Core Dynamics: Implications for Earth's Evolution

Thermal and Chemical Evolution

The core doesn't exist in isolation. Its thermal and chemical evolution is coupled to the rest of the planet.

• Heat flowing out of the core across the CMB helps drive mantle convection, which in turn drives plate tectonics. The estimated heat flux across the CMB is roughly 5–15 TW, a significant fraction of Earth's total heat budget.
• The inner core is growing over time as the core slowly cools. Current estimates suggest the inner core nucleated somewhere between 0.5 and 1.5 billion years ago, though this remains debated. Before inner core nucleation, the geodynamo would have been powered solely by thermal convection, which is less efficient than the compositional convection available today.
• Changes in core cooling rate, influenced by mantle convection patterns and the thermal insulating effect of large low-shear-velocity provinces (LLSVPs) at the base of the mantle, may explain long-term variations in reversal frequency.

Magnetic Field Reversals and Their Consequences

During a polarity reversal, the dipole field doesn't simply flip instantaneously. The transition takes roughly 1,000–10,000 years, during which the field weakens substantially (perhaps to 10–25% of its normal strength) and becomes dominated by complex, non-dipolar components.

This weakening matters because Earth's magnetic field deflects charged particles from the solar wind and cosmic rays. A significantly weakened field could lead to:

• Increased radiation exposure at Earth's surface, potentially raising mutation rates in organisms
• Changes in atmospheric chemistry, particularly increased production of cosmogenic isotopes like $^{10}\text{Be}$ and $^{14}\text{C}$ (which are actually used to detect past field weakening in ice cores and sediments)
• Possible effects on ozone concentrations, though the magnitude of this effect is still debated

Whether reversals have caused measurable biological extinctions remains an open question. The paleontological record does not show a clear correlation between reversals and mass extinctions, but subtler ecological effects are harder to rule out.

For modern civilization, even a partial weakening of the field (not necessarily a full reversal) poses practical risks to satellite electronics, power grid stability, and navigation systems that rely on the geomagnetic field. Understanding the dynamics of the core is therefore not just an academic exercise but relevant to predicting how the field will behave in coming centuries.