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Unformatted text preview: 817 W hile spacecraft travel ever farther in space, the rocky world deep below us remains mysterious. Yet, the present-day state and dynamics of Earths interior hold the keys to under- standing the early conditions of the solid Earth and its biosphere, hydrosphere, and atmosphere, and how these have evolved to the planet we now know. Earths stable stratification into crust (between 5 and 70- km thick), mantle (from base of crust to ~2890-km depth), and core (2890- to 6371-km depth) has been known for half a century from seismic velocity measure- ments, but characterizing the heterogeneity within and the interaction between these concentric shells is a frontier of modern, cross-disciplinary research. On page 853 in this issue, Trampert et al . ( 1 ) break new ground with compelling evidence for large-scale variations in composition in Earths mantle. Man-made probes into the Earths inte- rior barely reach a depth of ~10 km, and volcanism rarely brings up samples from deeper than ~150 km. These distances are dwarfed by Earths dimensions, and our knowledge of the deeper realms is pieced together from a range of surface observ- ables, meteorite and solar atmosphere analyses, experimental and theoretical mineral physics and rock mechanics, and computer simulations. A major unresolved issue concerns the scale and nature of man- tle convection, the slow (1 to 5 cm/year) stirring that helps cool the planet by trans- porting radiogenic and primordial heat from Earths interior to its surface. The mantle displays a velocity discontinuity at 660 km. Does convection occur within separate layers or over the whole mantle? Is the mantle effectively homogenized or has large-scale compositional heterogene- ity survived long-term mixing? Classic models have focused either on convective layering (with the upper and lower layers having different, but constant, composi- tion), or on isochemical whole-mantle overturn, but neither satisfies all multidis- ciplinary constraints ( 25 ). Over the past decade, several discover- ies have begun to reveal a lower mantle that is far more interestingand enigmatic than the bland shell of near-constant prop- erties considered in the classic models. Seismic tomography demonstrates that the 660-km discontinuity is, at least locally, permeable to convective flow ( 6 , 7 ), imply- ing that any chemical stratification must be deeper. Moreover, slabs of subducted tec- tonic plates that sink into the lower mantle do not appear to all reach the core-mantle boundary ( 8 , 9 ), which may suggest poor vertical mixing of the mantle ( 5 ). Mantle plumes remain a topic of debate ( 10 ), in part because seismological constraints on their nature and size are still ambiguous....
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- Fall '07