Earth’s Inner Core Rotation Deceleration

The Earth’s Inner Core Rotation Deceleration refers to the recently observed phenomenon in which the solid inner core of the planet appears to be slowing down relative to the Earth’s mantle and crust. For decades, geoscientists believed that the inner core rotated slightly faster than the outer layers of the planet—a behaviour termed super-rotation. However, modern seismic evidence indicates that this rotation has begun to decelerate, marking a significant development in the understanding of the Earth’s internal dynamics. This discovery sheds new light on the interactions between the solid inner core, the liquid outer core, and the mantle, as well as on the processes governing the generation of the Earth’s magnetic field.

Structure and Composition of the Inner Core

The Earth’s inner core is the deepest and most central layer of the planet. It is a solid metallic sphere composed mainly of iron and nickel, measuring about 2,440 kilometres in diameter. Surrounding it is the outer core, a fluid layer of molten iron and nickel approximately 2,200 kilometres thick. Above the outer core lies the mantle, which is semi-solid, and beyond that is the solid crust, the outermost layer on which life exists.
The inner core exists under immense pressure—over three million times the atmospheric pressure at the surface—and temperatures approaching those of the Sun’s surface (around 5,400°C). Despite these extreme conditions, the inner core remains solid because of the enormous pressure that prevents the metal from melting. The boundary between the solid inner core and the fluid outer core is known as the inner-core boundary (ICB), and this interface is critical in controlling how the inner core rotates.

Discovery of the Inner Core’s Rotation

The concept that the inner core might rotate at a rate different from the rest of the Earth emerged in the 1990s. Seismologists noticed that seismic waves produced by earthquakes travelled through the Earth’s interior at slightly different speeds when compared over time. These variations were interpreted as evidence that the inner core was not stationary but rotating independently within the fluid outer core.
Initially, estimates suggested that the inner core was rotating about 0.1 to 0.5 degrees per year faster than the mantle—a phenomenon known as super-rotation. This discovery was groundbreaking because it implied that the inner core and the mantle were mechanically decoupled, with their relative motion influenced by the fluid dynamics of the outer core.

Evidence for Deceleration

Over the past two decades, increasingly sophisticated analyses of seismic data have revealed that the inner core’s rotation is not constant. Scientists studying repeating earthquake pairs—known as earthquake doublets—observed subtle changes in the travel times of seismic waves passing through the core. These time shifts provide a means of tracking how the inner core’s orientation changes relative to the mantle.
Data from the early 2000s showed clear evidence of super-rotation, but from around 2008 onwards, these travel-time differences began to diminish. By the mid-2010s, observations suggested that the inner core had slowed down to the point where it was rotating nearly in synchrony with the Earth’s mantle. More recent analyses indicate that it may now be rotating slightly more slowly—a state referred to as sub-rotation.
Although this change amounts to only a fraction of a degree per year, it represents a major shift in the behaviour of the deep Earth. The results suggest that the inner core’s motion may be part of a multi-decadal cycle of acceleration and deceleration rather than a constant rotation pattern.

Causes and Mechanisms of the Deceleration

The deceleration of the inner core is attributed to the complex interplay of electromagnetic, gravitational, and viscous forces acting between the planet’s internal layers.

1. Electromagnetic Coupling: The molten outer core is a conductor of electricity and is in constant motion due to convection. This movement generates the Earth’s magnetic field through the geodynamo process. The magnetic field, in turn, exerts torque on the solid inner core, influencing its rotation. Variations in the flow of molten iron or changes in magnetic field strength can either accelerate or decelerate the inner core’s rotation over time.
2. Gravitational Coupling: The Earth’s mantle contains regions of varying density, such as the African and Pacific large low-shear-velocity provinces (LLSVPs). These dense regions exert gravitational forces on the inner core, which can modify its rotational speed. When the distribution of mass in the mantle changes, it can act as a gravitational brake or accelerator for the core.
3. Viscous Friction and Fluid Dynamics: The interface between the solid inner core and the fluid outer core experiences viscous drag. As the flow pattern of the outer core evolves, this friction can resist the inner core’s motion, gradually reducing its relative rotation.
4. Thermal and Compositional Effects: The inner core is slowly growing as the Earth cools, with iron crystallising from the outer core and adding to its mass. This gradual growth alters the dynamics of the inner-core boundary, potentially affecting the forces that control rotation. Variations in heat flow and compositional layering can therefore contribute to the observed slowdown.

Cyclic Behaviour and Timescales

Some geophysical models propose that the inner core’s rotation follows a cyclic pattern with a period of about 60 to 70 years. During each cycle, the inner core alternately rotates faster and slower than the mantle. This pattern may be linked to oscillations in the Earth’s magnetic field and to small fluctuations in the planet’s rotation rate—phenomena recorded in historical observations of day length.
The connection between these cycles suggests a deep coupling between the inner core’s dynamics, the outer core’s convection, and the overall rotation of the planet. If this periodicity is confirmed, the current slowdown could represent a natural phase in a repeating sequence of acceleration and deceleration rather than a one-time event.

Scientific and Geophysical Implications

The deceleration of the inner core has profound implications for understanding Earth’s internal processes and long-term stability.

• Magnetic Field Behaviour: The motion of the inner core influences the convective flows in the outer core that generate the magnetic field. A slowdown could correspond to changes in magnetic field strength, polarity drift, or regional anomalies in geomagnetic intensity.
• Thermal Evolution: The rate of heat transfer between the inner and outer core affects the geodynamo and the cooling rate of the planet. Variations in inner-core rotation may provide indirect evidence of changing thermal conditions deep within the Earth.
• Rotational Dynamics of the Planet: The exchange of angular momentum between the inner core and the mantle may cause minor variations in the Earth’s overall rotation speed. These changes could, in turn, produce measurable fluctuations in the length of the day by fractions of milliseconds.
• Seismic Anisotropy and Structure: The inner core is known to be seismically anisotropic, meaning that seismic waves travel faster along its rotation axis than across it. Changes in rotational behaviour might relate to shifts in crystal alignment or deformation within the core, offering insights into its physical state and evolution.

Challenges and Uncertainties

Despite recent advances, many aspects of the inner core’s deceleration remain uncertain. Direct observations are impossible; all data are inferred from seismic signals that pass through multiple layers of the Earth. Differences in seismic models, assumptions about anisotropy, and uneven data coverage across the globe can lead to varying interpretations.
It also remains unclear whether the current slowdown is part of a regular cycle or represents a longer-term trend. Understanding this behaviour requires continuous seismic monitoring and the integration of geomagnetic and geodynamic data.

Broader Perspective

The discovery that the Earth’s inner core is decelerating redefines our understanding of the planet’s deep interior. Far from being a static, unchanging sphere, the inner core is an active and dynamic component of the Earth system. Its rotation interacts with the magnetic field, influences global rotation, and participates in complex feedbacks that extend from the core to the surface.