Initial Publication Date: October 5, 2007

Why is Earth among all terrestrial bodies in the solar system (e.g. Moon Mars Venus and meteorites) the only body to have sialic crust?

submitted by Paul Mueller, University of Florida

Why is this question important?

Because of the variety of issues noted in the importance section above if is clearly beyond the scope of this short essay to comprehensively answer the Key Question. The following discussion therefore highlights only one set of connections that must be evaluated before a comprehensive answer can be composed.

What we know...

Segregation of the Earth's liquid Fe-Ni core requires very high temperatures throughout the solid earth early in its history in order to melt liquid Fe-Ni alloy and to provide sufficiently low viscosity of the silicate mantle to allow pockets of aggregated Fe-Ni liquid to "fall" to the center of the planet.

The higher mean density of the planet is related to the impact of a large (Mars-sized) body with the Earth in which the impactor had already undergone melting had segregated its own metal core and transferred this core to the earth. The volatile-depleted silicate portion of the impactor mixed with excavated parts of the Earth's mantle to form the moon. These accretionary processes were very energetic and very large fractions of the kinetic energy of the lunar impactor and accreting planetesimals was converted to heat and made it difficult for the earth to retain any substantial volatile inventory.

This set of circumstances suggests that the atmosphere-hydrosphere was accreted late in the growth process and the relative size of the Earth allowed it to retain these late-accreting volatiles more quantitatively than the smaller planets. This indirect but logical deduction is strongly supported by the fact that the abundances of highly and moderately siderophile elements in the mantle have relative abundances that are essentially the same as chondritic meteorites but absolute abundances that are much lower than chondritic meteorites. This pattern is also best explained by the accretion of a late veneer of chondritic material that was mixed in at least the upper mantle with material that had been severely depleted in siderophile elements by having been in equilibrium with liquid Fe-Ni metal. Consequently the Earth developed a reduced Fe-Ni core early in its accretionary history and a partially hydrated upper mantle late in the accretionary history.

These observations are critical to the development of plate tectonics and the formation of continental crust because without a hydrated upper mantle the melting of ultramafic mantle would not have occurred (or continue to occur) within the temperature regime of the Earth. The continued melting of the upper mantel is largely fueled by the reductions in liquidus temperatures resulting from hydration. For sialic crust the hydration of the mantel wedge and transfer of lower density material to the crust is the critical chemical and physical transformation involved in the creation of continental crust.

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