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 debate1,2. On the basis of geodynamic computations, plumes of purely thermal origin should comprise thin tails, only several hundred kilometres wide3, 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 waveforms4—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 zones5,6,7. 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-lived8, and may have a thermochemical origin9,10,11. 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.
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We thank the IRIS Data Management Center for providing the waveform data used in this study. This study was supported by an NSF Graduate Research Fellowship to S.W.F., NSF grant EAR-1417229, and ERC Advanced Grant WAVETOMO. Computations were performed at the National Energy Research Scientific Computing Center, supported by the US Department of Energy Office of Science (contract DE-AC02-05CH11231).
The authors declare no competing financial interests.
Extended data figures and tables
These figures correspond to the cross-sections in Fig. 1c and d, oriented normal to the direction of Pacific absolute plate motion45. As in Fig. 1, sections are indicated in the inset maps, while white and purple circles indicate position along section and orientation. Shown are relative shear-wave velocity (Vs) anomalies in models SEMUCB-WM1 (this study), S40RTS (ref. 46), PRI-S05 (ref. 47), HMSL-S06 (ref. 48) and GyPSuM (ref. 49), each plotted with respect to its own one-dimensional reference (where the latter notion is well defined: see for example ref. 48; where defined, the one-dimensional reference is often the global average). Panels a–e correspond to the MacDonald-hotspot-centred view of Fig. 1d; panels f–j correspond to the Pitcairn-centred view of Fig. 1c. This comparison shows that the five models are broadly compatible with each other at long wavelengths. However, in the lower mantle, the MacDonald and Pitcairn plumes are much more clearly defined as vertical conduits in SEMUCB-WM1, and stand out as the strongest and most continuous low-velocity features in the lower mantle in these cross-sections (which span almost half of Earth’s circumference).
These comparisons correspond to Fig. 1e, f, presented in a similar manner to those in Extended Data Fig. 1. We again find that models SEMUCB-WM1, S40RTS, PRI-S05, HMSL-S06 and GyPSuM are broadly compatible with each other at long wavelengths. However, in the lower mantle, the plumes beneath both Cape Verde and Canary are more clearly defined as well isolated vertical conduits in SEMUCB-WM1. Furthermore, while we do observe some degree of correspondence between the two plumes imaged in SEMUCB-WM1 and some anomalies also present in PRI-S05 or HMSL (for example, the plume root at the CMB beneath Cape Verde, or the lateral translation of the plume around 1,000 km beneath Canary), the unambiguously columnar nature of the anomalies imaged in SEMUCB-WM1 stands in stark contrast.
These cross-sections are similar to those in Extended Data Fig. 1, but now feature two approximately orthogonal sections through the African LLSVP: a–e, traversing from northwest to southeast; f–j, traversing from southwest to northeast. The African LLSVP is more massive, and therefore better resolved in S40RTS, PRI-S05, HMSL-S06 and GyPSuM than are other plumes. As in Extended Data Fig. 2, we again note some degree of similarity between SEMUCB-WM1 (a), PRI-S05 (c) and HMSL-S06 (d) below the Cape Verde plume.
Extended Data Figure 4 Linear resolution analysis, examining recovery of synthetic whole- and partial-mantle plumes of width 1,000 km beneath Hawaii and Iceland.
Synthetic plume input models, shown in the upper row of panels, have a peak amplitude of −2% and a cosine-cap lateral amplitude profile (thus, the effective width above 1% anomaly strength is only 500 km). In addition to looking at a whole-mantle plume, we also examine recovery of plumes truncated at successively greater depths (1,000 km, 1,500 km and 2,000 km) to assess vertical smearing. Artefacts seen above the truncation depth in the synthetic input models are due to slight aliasing phenomena associated with the radial b-spline basis functions used to parameterize our model. We find that all four input plumes are recovered quite well beneath both Hawaii (centre row), with relatively denser data coverage, and Iceland (bottom row), with comparatively sparser coverage—although there is a slight difference in amplitude recovery beneath the two (maximum amplitude recovered is shown for each panel). Importantly, we see no evidence of lateral (or, in the case of the truncated plumes, radial) smearing, nor do we detect significant gaps in recovery. However, recovered amplitude does vary as a function of depth, with comparatively weaker, although still satisfactory, recovery in the less well sampled mid-mantle (of the order of half of the input anomaly strength). For a more thorough discussion of the caveats implied by linear resolution analysis in the context of our inversion, as well as additional resolution tests, see ref. 4.
Similar to that in Extended Data Fig. 4. This analysis again features whole- and partial-mantle plumes of peak strength −2%, but now of width 600 km (meaning the effective width above 1% anomaly strength is only 300 km). We again find that the synthetic input plumes are recovered quite well, with no evidence of lateral or radial smearing, as well as no gaps in recovery. At the same time, we find that recovered amplitude is poorer than for the larger, 1,000-km-width plumes, in some cases recovering amplitudes of the order of one-quarter of the input, and we note that there is again a slight disparity in amplitude recovery between Hawaii and Iceland. Furthermore, we note that tests using synthetic plumes at or below widths of 600 km push the limits of the spherical-spline lateral basis functions used in our model—particularly in the upper mantle, where the inter-spline absolute distance is larger (although the angular distance remains constant).
This analysis is of a 400-km-width plume-like conduit extending from the CMB to 1,000-km depth, with a similar lateral profile (a cosine cap) and maximum amplitude (−2%) as the test structures in Extended Data Figs 4 and 5. The inherent limits of our spherical spline basis prohibit us from representing this narrow conduit with sufficient fidelity for the purposes of this test above 1,000 km. Upper panel, conduit-like input structure; lower panel, output structure resulting from resolution test. We observe that the output structure is at least 800 km in width, but is also significantly weaker than the input, exhibiting a maximum amplitude of −0.6% near its base, while only reaching −0.3 or −0.4% elsewhere in its core. As such, we can infer that the input-structure amplitudes would need to be increased by at least 10× in order to maintain amplitudes near −2.0% throughout the majority of the lower mantle. This latter observation has implications for effective excess temperature (see Methods).
Extended Data Figure 7 Linear resolution analysis for synthetic ‘hanging’ plume input structures in the upper mantle and transition zone.
Like those in Extended Data Figs 4 and 5, these plumes have an overall width of 600 km and a cosine-cap lateral cross-section, as well as −2% maximum amplitude, but are now cut at 410-km (left panels) or 1,000-km (right panels) depth. This experiment is designed to assess the effect of depth-smearing in SEMUCB-WM1. Upper panels, hanging-plume input models. Lower panels, output models when inputs are placed beneath Hawaii. We note that in general the structures retrieved are quite symmetrical and exhibit the appropriate depth extent, with the exception of the plume truncated at 1,000 km, which shows a weak eastward-trending band extending to the CMB. We note that this artefact is very weak, generally less than 0.1% amplitude, as illustrated in the bottom panel, where structure below 0.1% is masked (that is, the band is at least 20× weaker than the −2% input structure). Furthermore, we note that this feature is not at all like the plume we image beneath Hawaii, as it possesses a very different trend and amplitude profile.
Extended Data Figure 8 Linear resolution analysis for synthetic ‘hanging’ plume input structures in the upper mantle and transition zone.
This figure is similar to Extended Data Fig. 7, but now examines recovery beneath Iceland. Upper panels, hanging-plume input models. Lower panels, output models when inputs are placed beneath Iceland. The retrieved structures are again quite symmetrical and exhibit the appropriate depth extent, although amplitude recovery is slightly less impressive than that observed beneath Hawaii (consistent with the results of Extended Data Figs 4 and 5).
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French, S., Romanowicz, B. Broad plumes rooted at the base of the Earth's mantle beneath major hotspots. Nature 525, 95–99 (2015). https://doi.org/10.1038/nature14876
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