Explore the vast range of titles available.

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.

Ultralow-velocity zones are localized regions of extreme material properties detected seismologically at the base of Earth’s mantle. Their nature and role in mantle dynamics are poorly understood. We used shear waves diffracted at the core-mantle boundary to illuminate the root of the Iceland plume from different directions. Through waveform modeling, we detected a large ultralow-velocity zone and constrained its shape to be axisymmetric to a very good first order. We thus attribute it to partial melting of a locally thickened, denser- and hotter-than-average layer, reflecting dynamics and elevated temperatures within the plume root. Such structures are few and far apart, and they may be characteristic of the roots of some of the broad mantle plumes tomographically imaged within the large low-shear-velocity provinces in the lower mantle.

How cratons—extremely stable continental areas of the Earth’s crust—formed and remained largely unchanged for more than 2,500 million years is much debated. Recent studies of seismic-wave receiver function data have detected a structural boundary under continental cratons at depths too shallow to be consistent with the lithosphere–asthenosphere boundary, as inferred from seismic tomography and other geophysical studies. Here we show that changes in the direction of azimuthal anisotropy with depth reveal the presence of two distinct lithospheric layers throughout the stable part of the North American continent. The top layer is thick (∼150 km) under the Archaean core and tapers out on the surrounding Palaeozoic borders. Its thickness variations follow those of a highly depleted layer inferred from thermo-barometric analysis of xenoliths. The lithosphere–asthenosphere boundary is relatively flat (ranging from 180 to 240 km in depth), in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. Our findings tie together seismological, geochemical and geodynamical studies of the cratonic lithosphere in North America. They also suggest that the horizon detected in receiver function studies probably corresponds to the sharp mid-lithospheric boundary rather than to the more gradual lithosphere–asthenosphere boundary. Lithospheric layering in a craton Huaiyu Yuan and Barbara Romanowicz show that changes in the direction of seismic anisotropy with depth across the stable part of the North American continent reveal the presence of two distinct lithospheric layers. The top layer, which they infer to be chemically depleted, is approximately 150 kilometres thick under the ancient core of the continent and tapers out along its younger borders. They find that the underlying bottom of the lithosphere is relatively flat, varying from 180 km to 240 km in depth, in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer. These authors show that changes in seismic anisotropy with depth across the stable part of North America reveal the presence of two lithospheric layers. The top layer, which is chemically depleted, is ∼150 km thick under the ancient core of the continent and tapers out along its younger borders. The bottom of the lithosphere is relatively flat, in agreement with the presence of a thermal conductive root that subsequently formed around the depleted chemical layer.

Understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists. Using full-waveform seismic tomography, we imaged a pattern of horizontally elongated bands of low shear velocity, most prominent between 200 and 350 kilometers depth, which extends below the well-developed low-velocity zone. These quasi-periodic fingerlike structures of wavelength ~2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers. Below 400 kilometers depth, velocity structure is organized into fewer, undulating but vertically coherent, low-velocity plumelike features, which appear rooted in the lower mantle. This suggests the presence of a dynamic interplay between plate-driven flow in the low-velocity zone and active influx of low-rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer.

Seismic studies continue to refine the elusive boundary that defines the depth at which the lithosphere ends. A fundamental premise of plate tectonics on Earth is that rigid lithospheric plates, formed at mid-ocean ridges, float above a more deformable substratum, the asthenosphere ( 1 ). The precise nature of the asthenosphere is still debated. Mechanical models predict a well-defined, sharp lithosphere-asthenosphere boundary (LAB), but evidence for such a boundary from conventional seismic measurements is ambiguous. On pages 499 and 495 of this issue, Kawakatsu et al. ( 2 ) and Rychert and Shearer ( 3 ) present analyses of more sophisticated seismic studies that help refine the LAB and hence the thickness of the lithosphere and tectonic plates, although challenges still remain in picking out this boundary versus other structures within the lithosphere.

How low-velocity anomalies in the lower mantle influence convective flow, and their broader role in mantle dynamics, remain a topic of ongoing debate. One such anomaly, located roughly beneath the Russian city of Perm, is exceptionally well sampled by core-traversing seismic phases. We investigate seismic anisotropy, propagation- and polarization direction-dependent seismic wave speeds caused by deformation, within and around the Perm Anomaly at a sharp lateral resolution. Here we show a quasi-symmetric pattern of strong seismic anisotropy delineating the boundary of the Perm Anomaly, and linear streaks of strong anisotropy pointing towards the boundary. Geodynamic modeling experiments suggest such patterns of anisotropy are signatures of convergent upwelling mantle flow; ‘frozen-in’ fossilized deformation of ancient origin is a plausible explanation supported by geodynamic modeling. Furthermore, we detect seismic anisotropy within the anomaly indicative of internal deformation, though it is substantially weaker than that observed near the edges. By analysing deep-Earth seismic waves, Wolf et al. mapped the Perm Anomaly in the lower mantle. Unique deformation patterns at its edges suggest the structure is a fossilized remnant of ancient, upward-flowing rock.

Applying full-waveform methods to image small-scale structures of geophysical interest buried within the Earth requires the computation of the seismic wavefield over large distances compared to the target wavelengths. This represents a considerable computational cost when using state-of-the-art numerical integration of the equations of motion in three-dimensional earth models. “Box Tomography” is a hybrid method that breaks up the wavefield computation into three parts, only one of which needs to be iterated for each model update, significantly saving computational time. To deploy this method in remote regions containing a fluid-solid boundary, one needs to construct artificial sources that confine the seismic wavefield within a small region that straddles this boundary. The difficulty arises from the need to combine the solid-fluid coupling with a hybrid numerical simulation in this region. Here, we report a reconciliation of different displacement potential expressions used for solving the acoustic wave equation and propose a unified framework for hybrid simulations. This represents a significant step towards applying ’Box Tomography’ in arbitrary regions inside the Earth, achieving a thousand-fold computational cost reduction compared to standard approaches without compromising accuracy. We also present examples of benchmarks of the hybrid simulations in the case of target regions at the ocean floor and the core-mantle boundary. This study introduces a framework for hybrid seismic simulations in regions with solid-fluid boundaries, enabling imaging of fine-scale structures anywhere within the Earth.

Forty years after the discovery of seafloor spreading and the acceptance of the theory of plate tectonics, important gaps remain in the understanding of the pattern of convection that drives the motions of the plates, leading to earthquakes, tsunamis and volcanic eruptions. Now a new study of seismic waves generated in Earth's interior provides images that help scientists to better understand the pattern of mantle convection that drives plate motions.

The existence of a thin, weak asthenospheric layer beneath Earth’s lithospheric plates is consistent with existing geological and geophysical constraints, including Pleistocene glacio-isostatic adjustment, modeling of gravity anomalies, studies of seismic anisotropy, and post-seismic rebound. Mantle convection models suggest that a pronounced weak zone beneath the upper thermal boundary layer (lithosphere) may be essential to the plate tectonic style of convection found on Earth. The asthenosphere is likely related to partial melting and the presence of water in the sub-lithospheric mantle, further implying that the long-term evolution of the Earth may be controlled by thermal regulation and volatile recycling that maintain a geotherm that approaches the wet mantle solidus at asthenospheric depths.

We review the present status of global mantle tomography and discuss two main classes of models that have been developed in the past 10 years: P velocity models based on large datasets of travel times from the International Seismological Centre bulletins, often referred to as \"high resolution\" models, and S velocity models based on a combination of surface wave and hand picked body wave travel times, or waveforms, referred to as \"long wavelength\" models. We discuss their respective strengths and weaknesses, as well as progress in the resolution of other physical parameters, such as anisotropy, anelasticity, density, and bulk sound velocity using tomographic approaches. We present the view that future improvements in global seismic tomography require the utilization of the rich information contained in complete broadband seismic waveforms. This is presently within our reach owing to theoretical progress as well as the increase in computational power in recent years.