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Geophysics

Latest spin on the core

Nature volume 402, pages 26–27 (1999)Cite this article

Is the Earth's inner core rotating slightly faster than the mantle and crust? This is a question that has perplexed geophysicists since it was first raised five years ago. The latest study comes from Laske and Masters (page 66 of this issue1), who have taken a new approach to the problem and conclude that the higher estimates of relative rotation rate can be ruled out.

The centre of the Earth consists of a nearly spherical ball of crystalline iron, 1,220 kilometres in radius; this solid inner core is surrounded by a fluid outer core of molten iron, whose motions are the source of the geomagnetic field. Numerical models of the geodynamo2,3 suggest that electromagnetic torques might cause the solid inner core to exhibit a slow differential rotation with respect to the silicate mantle and overlying crust, upon which we live. On the other hand, gravitational torques exerted by slight irregularities in the boundary topography and density of the solid inner core and mantle should keep them locked in a state of co-rotation, unless the viscosity of the inner core is low enough to allow it to deform on the requisite timescale4. Small but systematic temporal variations in the travel times of seismic waves passing near the centre of the Earth have been interpreted as evidence that the solid inner core is, in fact, rotating faster than the mantle and crust at a rate between 1° yr−1 (ref. 5) and 3° yr°−1 (ref. 6). Subsequent seismic travel-time studies have, however, found lower differential rotation rates, ranging from 0.2°–0.3° yr−1 (ref. 7) to essentially zero8.

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References

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  1. the Department of Geosciences, Princeton University, Princeton, 08544, New Jersey, USA

    F. A. Dahlen

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  1. F. A. Dahlen
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Correspondence to F. A. Dahlen.

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Dahlen, F. Latest spin on the core. Nature 402, 26–27 (1999). https://doi.org/10.1038/46918

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