Geophysics

Earth's Liquid Center

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Science  28 Sep 2001:
Vol. 293, Issue 5539, pp. 2351
DOI: 10.1126/science.293.5539.2351c

Originally, Earth's core was entirely liquid, but cooling has allowed iron to crystallize, producing a solid inner core and a molten outer core. Vigorous convection within the still-liquid outer core, which is thought to produce Earth's magnetic field, is driven in part by the latent heat of crystallization of the inner core, and the geometry of the convection is affected by the presence of the inner core.

When did the inner core begin to form. Precambrian paleomagnetic records do not yet provide a clear signal of the onset of Earth's magnetic field. Hence, Labrosse et al. have modeled the thermal evolution of the core in order to predict the age of the inner core from estimates of Earth's heat flow, in much the same way as Lord Kelvin attempted nearly 150 years ago to calculate the age of Earth. This analysis suggests that the inner core formed about 1 billion years ago, with an upper age limit of 2.5 billion years (under the assumption that the core contains some radioactive elements). Thus, the inner core seems not to have been present for most of Earth's 4.5-billion-year history. — BH

Earth Planet. Sci. Lett.190, 111 (2001).

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