Earth's Core

The Earth's core was the first structure in the Earth's interior to differentiate (not crystallize!!) after the Earth's formation → differentiation of lithophilic and siderophilic elements. The process was completed approximately 30–50 myrs after CAI (W-Hf isotope analysis; Rubie et al. 2007).

The percolation model (metal flowed along grain boundaries of a solid mantle towards the center) would be too slow → Therefore, Earth with a deep magma ocean model is more likely, in which metals precipitated as droplets up to diapirs and sank through a liquid mantle towards the center (Rubie et al. 2003, Wade & Wood 2005).

Seismic density measurements revealed that the Earth's core has a composition almost identical to an iron-nickel alloy (Jeffreys 1929). However, the density deviation from an Fe-Ni alloy is approximately 5-15%, indicating the presence of lighter elements in the Earth's core.
Iron meteorites are analogs of the Earth's core, which is why the average composition of the Earth's core could be determined from them (Table 1). Models for the composition of the inner and outer core were calculated using high-pressure experiments and differentiation models.

Table 1: Composition of the Earth's inner and outer core

Magnetization in magnetic inclusions in Jack Hill's zircons indicates a 3.3–4.2 Ga old magnetic field of the Earth (Tarduno et al. 2023). The existence of a magnetic field implies convection (warm material rises > cools > sinks…) of core metals ( geodynamo ), which means: high heat flow from the Earth's core to the mantle → exceeding the adiabatic value, mantle cooling → through plate tectonics or massive volcanism at the surface.

Inner core (4a)

The inner core of the Earth probably only crystallized around 1000 to 1500 Ma (e.g., Biggin et al. 2015), in some models even ~ 500 Ma (Bono et al. 2019) → Detected by an increase in magnetic field strength in rocks (inner core crystallizes (ICN) ⇒ enrichment of light elements (Si, S, O) in the outer core ⇒ faster convection ⇒ stronger magnetic field)

Crystallization through homogeneous nucleation is unlikely: Subcooling is required for spontaneous nucleation ⇒ Heterogeneous nucleation is more likely around impurities. Pressure at ICB: 136 GPa: Melting point of Fe (136 GPa) ~ 6200°C (< 6200 °C: onset of crystallization)

The inner core has been growing steadily at the inner core boundary (ICB) since then [approx. 1 mm/year (Nimmo 2015)].

Composition: Fe-Ni alloy in hexagonal close packing (95 – 100%) + lighter elements such as Si, S, C, O (< 5%)

Seismic measurements show anisotropy in the inner core: sound waves travel faster along the rotational axis than in the equatorial direction → preferred orientation of the iron crystals. Explanation by

1. Asymmetric growth (Cottaar & Buffett 2012)

2. Faster rotation than the mantle, superrotation: differential rotation favored the alignment of the iron crystals (Song & Richards 1996)

3. Stratification in the IC, through convection in the solid inner core (IC transition zone: Song & Helmberger 1998) or through viscous deformation (Vidale et al. 2025)

4. Influence of the magnetic field on IC crystallization (Buffett & Wenk 2001)

Today, IC has a radius of 1220 km (6371 – 5150 km), density: 12.8 – 13.1 g/cm ³ , temperature: 6000 °C, pressure: 330 GPa (Dziewonski & Anderson 1981).

Outer core (4b)

The remaining molten core decreases over time due to crystallization of the inner core. The Lehmann discontinuity lies between the inner and outer cores.

Composition: Fe-Ni alloy + light elements (see Table 1)

The outer core (OC) is characterized by strong convection currents that generate the magnetic field (geodynamo). There are two main sources of OC convection currents:

1. Thermal convection (heat transfer from the Earth's core to the mantle).

2. Compositional convection (release of lighter elements during crystallization of the inner core).

Convection and the Coriolis force (due to the Earth's rotation) lead to a spiral, upward-descending motion (helical motion) of the conducting metal. Movement of free electrons in the iron > induction of an electric voltage > magnetic field ( dynamo effect )

Polar reversals occur approximately every 200 ka to 1 Ma (randomly, without a pattern). The last one was the Brunhes-Matuyama reversal before 780 ka, followed by the Brunhes chron. However, there are exceptions, such as the Laschamp excursion before 41 ± 2 ka, a short-lived reversal that lasted about 440 yr (Laj et al. 2014), or the Mono Lake excursion before 34 ka (duration ~1800 yr). Long periods of the same polarity have also occurred in Earth's history, such as the Cretaceous Normal Superchron (CNS) from 120 to 83 Ma (Ogg, 2020).

Magnetic field reversals are well stored in remanently magnetized minerals (e.g., magnetite) in constantly newly formed crust at the Mid-Ocean Ridge (MOR).

The cause of magnetic field fluctuations, excursions, and reversals is not fully understood. It may simply be due to changes in flow within the ocean, but perhaps triggered by cooling from the mantle (e.g., from hot-spot volcanism or plate tectonics).

The outer core has a thickness of 2260 km (5150 – 2890 km), density: 9.9 – 12.2 g/cm ³ , temperature: 4000 – 5700 °C, pressure: 136 – 330 GPa (Dziewonski & Anderson 1981).

literature

Biggin, AJ, Piispa, EJ, Pesonen, LJ, Holme, R., Paterson, GA, Veikkolainen, T., & Tauxe, L. (2015). Palaeomagnetic field intensity variations suggest Mesoproterozoic inner-core nucleation. Nature , 526 (7572), 245-248.

Bono, RK, Tarduno, JA, Nimmo, F., & Cottrell, RD (2019). Young inner core inferred from Ediacaran ultra-low geomagnetic field intensity. Nature Geoscience , 12 (2), 143-147.

Buffett, BA, & Wenk, HR (2001). Texturing of the Earth's inner core by Maxwell stresses. Nature , 413 (6851), 60-63.

Cottaar, S., & Buffett, B. (2012). Convection in the Earth's inner core. Physics of the Earth and Planetary Interiors , 198 , 67-78.

Dziewonski, AM, & Anderson, D.L. (1981). Preliminary reference Earth model. Physics of the earth and planetary interiors , 25 (4), 297-356.

Laj, C., Guillou, H., & Kissel, C. (2014). Dynamics of the earth magnetic field in the 10–75 kyr period comprising the Laschamp and Mono Lake excursions: New results from the French Chaîne des Puys in a global perspective. Earth and Planetary Science Letters , 387 , 184-197.

Ogg, J.G. (2020). Geomagnetic polarity time scale. In Geologic time scale 2020 (pp. 159-192). Elsevier.

Song, X., & Richards, P.G. (1996). Seismological evidence for differential rotation of the Earth's inner core. Nature , 382 (6588), 221-224.

Vidale, JE, Wang, W., Wang, R., Pang, G., & Koper, K. (2025). Annual-scale variability in both the rotation rate and near surface of Earth's inner core. Nature Geoscience , 1-6.