Chapter 3 - Energetics of the Solid Earth: Implications for the Structure of Mantle Convection

Abstract

This chapter reviews seismic and energetic constraints on the structure of mantle convection. Both downwelling and upwelling-linked structures are now clearly visible in modern seismic images of the lower mantle. A ∼3000 km-broad N-S circumglobal “ring” of higher-than-average seismic wave speeds has been recognized since the first global models of nonradial seismic structure, being found beneath regions with long-term plate subduction. Broad mantle plumes are now also clearly visible in modern seismic images of the lower mantle; these plume structures narrow significantly above ∼1000 km and appear to be continuous pipe-like structures that connect the lowermost D″ region above the core-mantle boundary to the asthenosphere that underlies surface lithospheric plates. Estimates of present-day gravitational energy release from current plate subduction find this heat source is of the same order (∼13 TW) as radioactive heat generation within the mantle. Current ideas about the conductivity of Earth's iron core based on ab initio calculations imply that mantle heat supplied from Earth's core is or order ≥∼15–20 TW. Furthermore, Earth's early core is likely to have formed hotter than Earth's mantle due to preferential viscous heating within descending iron blebs as they differentiated and sank to form the core. Together, these seismic and energetic observations hint that Earth's mantle has two interlocking modes of flow. The much higher viscosity lower mantle is slowly moving in a low-order axisymmetric degree-2 spherical harmonic P20 mode. Lower mantle flow is in response to the long-term addition of cold dense slabs in a generally circumpolar ring from the upper mantle; it tends to “attract” subduction zones into this long-lived lower mantle flow structure. The broad upwelling antipodal regions in the lower mantle are further heated because they are sites where plumes tend to concentrate and heat the slowly ascending (∼2 mm/yr) mantle to their sides. Earth also has a second less-visible mode of low-viscosity upward counterflow to the well-known downwelling associated with plate subduction. This mode consists of strong lateral flow in D″ and asthenospheric boundary layers linked by focused upwelling in mantle plume pipes that pierce the degree-2 lower mantle flow structure. Since radioactive heat production in the mantle has waned over time, the relative importance of core heat loss in supplying heat to the mantle has correspondingly grown.