Three thermometers for Earth’s upper mantle

The density and viscosity of Earth’s mantle govern its convection and therefore plate tectonics, and the density and viscosity are each functions of temperature and composition. At the mid-ocean ridges—an interconnected global network of volcanoes—new ocean crust is formed from the solidification of molten rock derived from the mantle. Thus, mid-ocean ridges hold important clues about the inaccessible mantle, in particular about its local temperature and composition. So Earth scientists Colleen Dalton (Brown University), Charles Langmuir (Harvard), and Allison Gale (University of Wisconsin–River Falls) set out to examine the link. The data they used include velocities of seismic shear waves 300 km beneath the ocean floor, taken from 242 different vantage points along six different ridges; how far those ridge tops lay beneath the ocean’s surface; and the chemical composition of recently erupted ocean crust. Although each of those three data sets has its limitations, the authors found correlations among them that require a common cause. Further scrutiny showed that mantle temperature was by far the main driver of global variations in ridge height, seismic speed in the mantle, and crustal chemistry, with mantle composition causing scatter about the thermal trend. Turning their analysis around, the trio then used ridge height, seismic speed, and chemistry to map the mantle’s temperature beneath the ridges, as shown in the figure. The black squares represent known mantle hot spots, some of which are labeled. (C. A. Dalton, C. H. Langmuir, A. Gale, Science 344, 80, 2014 .)