Core geophysics

The deep interior of Earth is inaccessible directly, yet determining our planet’s structure and composition is critical for understanding the origin and evolution of our planet. In recent years, important advances have been made using indirect methods to infer the nature of the Earth’s deep interior. These advances have been made by applying an interdisciplinary approach, using seismic, geochemical, and geomagnetic methods.

On the basis of seismic wave propagation through the earth we divide the planet into the crust (10–70 km thick), the mantle (which extends down to 2,900 km), the liquid outer core (from 2,900 km to 5,080 km), and a solid inner core (5,080 km to 6,371 km). The composition of the Earth is estimated by assuming that the planets and the Sun formed from a parent nebula of gas and dust. Therefore the elemental abundances in the Sun together with those in chondritic meteorites (which are considered remnants of the early material that accreted to form the planets) can be used to infer the bulk composition of the earth. The result is that the outer core must be composed of an iron and nickel alloy (80 wt% Fe, 5 wt% Ni) along with a smaller percentage of a less dense element (up to 15%) necessary to meet the density required by seismic wave velocities through the core. Likely candidates for the lighter element are oxygen, silicon, carbon, and to a lesser extent hydrogen or nitrogen.

The core began to grow after the formation of the Earth as the temperature increased to the point where dense, liquid iron began to sink to the center of the planet. The time of the core formation can be estimated by using isotopic signatures of radiogenic element pairs as natural chronometers. For example, hafnium-182/tungsten-182 ratios of various terrestrial and extraterrestrial rocks (Hf has the tendency to concentrate in the Earth’s mantle, whereas W will partition into the Earth’s core) suggest that the last core formation events occurred at least 4.7 × 10⁷ years after the formation of the Earth (4.5 × 10⁹ years ago).

The geomagnetic field is a fundamental feature of our planet, and yet the origin of the field remains as one of the major outstanding problems in the earth sciences. The field is thought to be generated by some sort of dynamo process acting in the outer core. Liquid iron and nickel flowing in a convecting core provide a moving conductor capable of generating magnetic fields. But just how this happens is unknown. Solving the nonlinear equations for a dynamo with earth-like conditions has proven extremely difficult.

Dramatic progress has been made recently by including the effects of the inner core in dynamo models. The inner core is solid iron and has a finite electrical conductivity. As a result of its electrical conductivity, it is difficult for the magnetic field generated in the outer core to diffuse through the inner core. The inner core therefore dampens out fluctuations in the magnetic field that are shorter than the diffusive time scale of the inner core.

The first three-dimensional dynamo model using earth-like conditions was solved recently (1). This model included an inner core with a finite electrical conductivity. This dynamo model successfully predicted that the inner core should rotate faster than the earth’s mantle. Seismic studies showed that the inner core is rotating about 1.1°/year faster than the mantle (2). This dynamo model also caught the magnetic field in the act of reversing its polarity—one of the most dramatic properties of geomagnetic field behavior.

Polarity reversals remain one of the most enigmatic properties of the field. Under certain conditions magnetic minerals lock in the direction and intensity of Earth’s magnetic field at the time the rock forms. The resulting magnetizations can be treated as fossil compasses and used to map out the changes in the geomagnetic field. By obtaining high-resolution paleomagnetic records of sediments or lava flows that formed as the field was reversing, it is possible to document how the reversal occurs.

A ubiquitous feature of polarity transition records is a decrease in the intensity of the field to roughly 10% of its full polarity strength. While the intensity is low, the directions change by 180° and then the field grows in the new direction. The directional change takes 1,000 to 4,000 years to occur, and the intensity change takes longer. Changes in the field may occur much more rapidly, though. In what is arguably the best documented record, the Steen’s Mountain record, the field changes by as fast as 6° per day (3). The drop in the field intensity and fast changes in the local field direction have implications for the environment at Earth’s surface, but there exist no known correlations between polarity reversals and biological extinctions.

A number of recent polarity transition records exhibit recurring field behavior observed at widely separated sites. This suggests that something provides a memory for the rapidly changing dynamo. The two possible candidates are the solid inner core and the core mantle boundary.

Seismic tomography shows that the core–mantle boundary is laterally heterogeneous. Observed as differences in velocity of seismic waves, these variations are interpreted as lateral variations in temperature or composition. Either of these interpretations could influence the geodynamo by entraining thermal convection in the outer core, or lateral variations in the electrical conductivity could focus the magnetic field lines emerging from the core.

The primary energy source that maintains the dynamo action is convection driven by the latent heat of crystallization of the solid inner core. If the inner core provides a stabilizing influence on the magnetic field, then the growth of the inner core likely affected the frequency of polarity reversals. By extending the polarity reversal chronology back into the Precambrian, paleomagnetism may provide constraints on the timing of the origin and growth of Earth’s inner core.

Suggested Reading

Fuller, M., Laj, C., & Herrera-Bervera, E. (1996). Am. Sci. 84, 552–561.