Broad plumes rooted at the base of the Earth's mantle beneath major hotspots

• Published in: Nature (London) Vol. 525; no. 7567; pp. 95 - 99
• Main Authors: French, Scott W., Romanowicz, Barbara
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 03.09.2015; Nature Publishing Group
• Subjects: 704/2151; 704/2151/210; 704/2151/508; Anisotropy; Conduits; Earth; Earth mantle; Heated water; Humanities and Social Sciences; letter; Magnesium composites; Mantle; multidisciplinary; Natural history; Plumes; Science; Thermal plumes; Tomography; Upper mantle; Upwelling; Velocity; Volcanic hotspots; Volcanic plumes; Volcanoes; Hawaii; United States > US
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/nature14876

A whole-mantle seismic imaging technique, combining accurate wavefield computations with information contained in whole seismic waveforms, is used to reveal the presence of broad conduits beneath many of Earth’s surface hotspots, supporting the idea that these conduits are the source of hotspot volcanoes. Plume-like conduits beneath surface hotspots Scott French and Barbara Romanowicz use a whole-mantle seismic imaging technique, combining accurate wavefield computations with information contained in whole seismic waveforms, to reveal the presence of wide, quasi-vertical conduits beneath many of the Earth's surface hotspots. The conduits they image extend from the core–mantle boundary, where they are rooted in patches of strongly reduced shear velocity, and correspond to known locations of large ultralow-velocity zones beneath Hawaii, Iceland and Samoa, in support of the idea that they may be the source of hotspot volcanoes. As the conduits are broader than classical thermal plume tails, the authors suggest that they are long lived and may have a thermochemical origin. Plumes of hot upwelling rock rooted in the deep mantle have been proposed as a possible origin of hotspot volcanoes, but this idea is the subject of vigorous debate 1 , 2 . On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide 3 , and be difficult to detect using standard seismic tomography techniques. Here we describe the use of a whole-mantle seismic imaging technique—combining accurate wavefield computations with information contained in whole seismic waveforms 4 —that reveals the presence of broad (not thin), quasi-vertical conduits beneath many prominent hotspots. These conduits extend from the core–mantle boundary to about 1,000 kilometres below Earth’s surface, where some are deflected horizontally, as though entrained into more vigorous upper-mantle circulation. At the base of the mantle, these conduits are rooted in patches of greatly reduced shear velocity that, in the case of Hawaii, Iceland and Samoa, correspond to the locations of known large ultralow-velocity zones 5 , 6 , 7 . This correspondence clearly establishes a continuous connection between such zones and mantle plumes. We also show that the imaged conduits are robustly broader than classical thermal plume tails, suggesting that they are long-lived 8 , and may have a thermochemical origin 9 , 10 , 11 . Their vertical orientation suggests very sluggish background circulation below depths of 1,000 kilometres. Our results should provide constraints on studies of viscosity layering of Earth’s mantle and guide further research into thermochemical convection.