Rising through Earth's mantle

Ritter, Joachim

Volcanism and the breaking of the continents continually remind us of the hot and dynamic interior of our planet. Driven by temperature and density gradients, convective currents carry vast amounts of partially molten rock through Earth's mantle (see the figure). Subducting oceanic plates have been traced during their downward flow from the top to the bottom of the mantle (l), but the nature of the upward flow is still a matter of debate. In this issue, Goes et al on page 1928 (2) and Ritsema et al. on page 1925 (3) present fascinating seismic images that indicate large-scale upwellings of hot material in the lower mantle that extend all the way to the surface. The results are interpreted as direct evidence for the deep driving forces that cause rifting and intraplate volcanism in Central Europe and East Africa. In recent years, advances in seismological research and geodynamical modeling have led to important refinements to the traditional layered Earth model (see the figure). Seismic tomographya geophysical analog to medical computer tomographyhas been particularly instrumental by providing detailed three-dimensional images of the spatial distribution of seismic velocities in the mantle. The seismic images can be interpreted as spatial temperature contrasts, with low velocities corresponding to high temperatures and high velocities corresponding to low temperatures. Today's images are sufficiently detailed to show the continuation of cold subduction zones across the 660-km boundary (see the figure) deep into the lower mantle and possibly even down to the core mantle boundary (CMB) (1). Geodynamical models can thus be tested and refined by seismic tomography. These advances are helping to resolve a question that has been hotly debated for decades, namely whether material flows across the boundary between the upper and lower mantle at 660-km depth or whether the convection patterns above and below the boundary are isolated from each other. Mantle processes may drive the Cenozoic rift systems cutting through the Central European and East African continental plates that developed during the past 40 million years and the intraplate volcanic fields that developed at the same time. The seismic low-velocity anomaly underneath Europe discovered by Goes et al.[see fig. 3 in (2)] is most visible in the lower mantle at 600 to 2000 km depth. The Central European and African plumes are obviously more complicated then simple straight tubes. With a lateral extent of more than 500 km, the observed plumes are much wider than expected, even when smearing effects inherent to global seismic tomography are accounted for. Upwelling is likely to be broad in the high-viscosity lower mantle, followed by narrowing in the low-viscosity upper mantle (10). In the upper mantle, the plume may split into several tall plumes (7, 8), as seen in local tomography studies (7). To resolve such details, high-resolution regional imaging of the interaction of plumes with mantle flow, subducting slabs, and lithospheric plates should be performed. Carefully designed seismic field projects on the continents and in the oceans are required to achieve this challenging task. The resulting estimates of lateral and vertical mass fluxes in the mantle will provide a firmer basis for geodynamic computer models and should lead to a consistent three-dimensional Earth model.

Rising through Earth's mantle

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