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Geodynamic estimates of the viscosity of the Earth's inner core

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Abstract

Recent seismological studies1,2 have suggested that the inner core is rotating relative to the bulk of the Earth, a situation which (according to numerical simulations3) may be sustained by convective flow in the liquid outer core. On the other hand, large gravitational forces due to the heterogeneous distribution of mass in the Earth's mantle should be sufficient to keep the inner core aligned with the mantle4. Here I show that the differential rotation of the inner core can be reconciled with these strong gravitational forces by allowing the shape of the inner core to adjust as it rotates, so permitting an estimate of the effective viscosity of this innermost region of the Earth. The inferred rotation rate constrains the viscosity of the inner core to be less than 1016 Pa s or greater than 1020 Pa s, as two different dynamical regimes are possible. The viscosity estimates for these two regimes have very different implications for the origin of seismic anisotropy in the inner core.

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Figure 1: Velocity field induced by a misalignment of the inner core from its equilibrium position.
Figure 2: Adjustment of the inner core when the rotation rate φ̇ is variable.

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Acknowledgements

I thank J. Bloxham, B. Hager, P. Richards and D. Stevenson for comments and suggestions. J. Bloxham and D. Stevenson independently suggested the possibility of the high-viscosity solution. This work was supported by NSERC.

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  1. Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, V6T 1Z4, BC, Canada

    Bruce A. Buffett

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  1. Bruce A. Buffett
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Correspondence to Bruce A. Buffett.

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Buffett, B. Geodynamic estimates of the viscosity of the Earth's inner core. Nature 388, 571–573 (1997). https://doi.org/10.1038/41534

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