It is widely assumed that the boundary layer above the core is the source of intraplate volcanoes such as Hawaii, Samoa, and Yellowstone, and that the sub-plate boundary layer at the top of the mantle is thin and entirely subsolidus. In fact, this upper layer is thicker and has higher expansivity, buoyancy, and insulating power than the lower one, and may have higher potential temperatures. The observed seismic structure of the low-velocity zone (LVZ) including attenuation, anisotropy, sharp boundaries, and a reduction of both compressional and shear moduli can be taken as strong evidence for the ubiquitous presence of melt in the upper mantle. If the LVZ contains as little as 1%–2% melt, then it is the most plausible and accessible source for midplate magmas; deeply rooted active upwellings are unnecessary. The upper boundary layer is also the most plausible source of ancient isotopic signatures of these magmas and their inclusions.

It has been known for over 50 years that seismic anisotropy must be included in a realistic analysis of most seismic data. The evidence for this consists of the observed dependency in many contexts (reviewed briefly here) of seismic velocity upon angle of propagation and upon angle of S-wave polarization. Despite this well-established understanding, many current investigations continue to employ less realistic isotropic assumptions. One result is the appearance of artifacts which can be interpreted in terms of details of Earth structure rather than of the restrictive assumptions in the analysis. The reason for this neglect of anisotropy is presumably the greater algebraic complexity and the larger number of free parameters of anisotropic seismics. However, the seismic anisotropy in the Earth is usually weak, and the equations for weak anisotropy are only marginally more complex than for isotropy. Further, the additional parameters are commonly required to describe the data. Moreover, the parameters of weak anisotropy defined below (combinations of the anisotropic elastic moduli) are less subject to compounding of uncertainty and to spatial resolution issues than are the individual anisotropic moduli themselves. Hence inversions should seek to fit data with these parameters, rather than with those individual moduli. We briefly review the theory for weak anisotropy and present new equations for the weakly anisotropic velocities of surface waves. The analysis offers new insights on some well-known results found by previous investigations, for example the “Rayleigh wave–Love wave inconsistency”, including the facts that Raleigh wave velocities depend not only on the horizontal SV velocity but also on the anisotropy, and Love wave velocities depend not only on the horizontal SH velocity but also on the anisotropy.

The role of decoupling in the low-velocity zone is crucial for understanding plate tectonics and mantle convection. Mantle convection models fail to integrate plate kinematics and thermodynamics of the mantle. In a first gross estimate, we computed at >300 km 3 /yr the volume of the plates lost along subduction zones. Mass balance predicts that slabs are compensated by broad passive upwellings beneath oceans and continents, passively emerging at oceanic ridges and backarc basins. These may correspond to the broad low-wavespeed regions found in the upper mantle by tomography. However, west-directed slabs enter the mantle more than three times faster (~232 km 3 /yr) than in the opposite east- or northeast-directed subduction zones (~74 km 3 /yr). This difference is consistent with the westward drift of the outer shell relative to the underlying mantle, which accounts for the steep dip of west-directed slabs, the asymmetry between flanks of oceanic ridges, and the directions of ridge migration. The larger recycling volumes along west-directed subduction zones imply asymmetric cooling of the underlying mantle and that there is an “easterly” directed component of the upwelling replacement mantle. In this model, mantle convection is tuned by polarized decoupling of the advecting and shearing upper boundary layer. Return mantle flow can result from passive volume balance rather than only by thermal buoyancy-driven upwelling.

Seismic anisotropy is an efficient way to investigate the deformation field within the upper mantle. In the framework of rigid tectonic plates, we make use of recent tomographic models of azimuthal anisotropy to derive the best rotation pole of the Pacific plate in the uppermost 200 km of the mantle. It is found to be in good agreement with current plate motion (NUVEL1, HS3, and NNR). However, when dividing the Pacific plate into two subplates separated by what we refer to as the megagash, an east-west low-velocity and low-anisotropy band extending across the Pacific plate from Samoa-Tonga to the Easter–Juan Fernández Islands, the rotation pole of northern Pacific is still in agreement with current plate motion but not the rotation pole of the southern part of the Pacific, far away from the “classical” rotation pole of the Pacific plate. This result suggests a differential motion between the North and South Pacific and an ongoing reorganization of plates in the Pacific Ocean. The megagash might be a future plate boundary between the North and South Pacific plates, associated with the intense volcanism along this band.

Whether the volcanism of the Columbia River Plateau, eastern Snake River Plain, and Yellowstone (western U.S.) is related to a mantle plume or to plate tectonic processes is a long-standing controversy. There are many geological mismatches with the basic plume model as well as logical flaws, such as citing data postulated to require a deep-mantle origin in support of an “upper-mantle plume” model. USArray has recently yielded abundant new seismological results, but despite this, seismic analyses have still not resolved the disparity of opinion. This suggests that seismology may be unable to resolve the plume question for Yellowstone, and perhaps elsewhere. USArray data have inspired many new models that relate western U.S. volcanism to shallow mantle convection associated with subduction zone processes. Many of these models assume that the principal requirement for surface volcanism is melt in the mantle and that the lithosphere is essentially passive. In this paper we propose a pure plate model in which melt is commonplace in the mantle, and its inherent buoyancy is not what causes surface eruptions. Instead, it is extension of the lithosphere that permits melt to escape to the surface and eruptions to occur—the mere presence of underlying melt is not a sufficient condition. The time-progressive chain of rhyolitic calderas in the eastern Snake River Plain–Yellowstone zone that has formed since basin-range extension began at ca. 17 Ma results from laterally migrating lithospheric extension and thinning that has permitted basaltic magma to rise from the upper mantle and melt the lower crust. We propose that this migration formed part of the systematic eastward migration of the axis of most intense basin-range extension. The bimodal rhyolite-basalt volcanism followed migration of the locus of most rapid extension, not vice versa. This model does not depend on seismology to test it but instead on surface geological observations.

High-temperature geochemistry combined with igneous petrology is an essential tool to infer the conditions of magma generation and evolution in the Earth's interior. During the past thirty years, a large number of geochemical models of the Earth, essentially inferred from the isotopic composition of basaltic rocks, have been proposed. These geochemical models have paid little attention to basic physics concepts, broadband seismology, or geological evidence, with the effect of producing results that are constrained more by assumptions than by data or first principles. This may not be evident to seismologists and geodynamicists. A common view in igneous petrology, seismology, and mantle modeling is that isotope geochemistry (e.g., the Rb-Sr, Sm-Nd, U-Th-Pb, U-Th-He, Re-Os, Lu-Hf, and other less commonly used systems) has the power to identify physical regions in the mantle, their depths, their rheological behavior, and the thermal conditions of magma generation. We demonstrate the fallacy of this approach and the model-dependent conclusions that emerge from unconstrained or poorly constrained geochemical models that do not consider physics, seismology (other than teleseismic travel-time tomography and particularly compelling colored mantle cross sections), and geology. Our view may be compared with computer printers. These can reproduce the entire range of colors using a limited number of basic colors (black, magenta, yellow, and cyan). Similarly, the isotopic composition of oceanic basalts and nearly all their primitive continental counterparts can be expressed in terms of a few mantle end members. The four most important (actually “most extreme”, because some are extraordinarily rare) mantle end members identified by isotope geochemists are DMM or DUM (depleted MORB [mid-ocean-ridge basalt] mantle or depleted upper mantle), HIMU (high mu, where mu = μ = 238 U/ 204 Pb), EMI, and EMII (enriched mantle type I and type II). Other mantle end members, or components, have been proposed in the geochemical literature (e.g., PHeM, FOZO, LVC, PreMa, EMIII, CMR, LOMU, and C), but these can be considered to be less extreme components or mixtures in the geochemical mantle zoo. Assuming the existence of these extreme “colors” in the mantle isotopic printer, the only matter for debate is their location in the Earth's interior. At least three of them need long-term insulation from convection-driven homogenization or mixing processes. In other words, where these extreme isotopic end members are located needs to be defined. In our view, no geochemical, geological, geophysical, or physical arguments require the derivation of any magma from deep mantle sources. Arguments to the contrary are assumption based. The HIMU, EMI, and EMII end members can be entirely located in the shallow non-convecting volume of the mantle, while the fourth, which is by far the more abundant volumetrically (DMM or DUM), can reside in the transition zone. This view is inverted compared with current canonical geochemical views of the Earth's mantle, where the shallowest portions are assumed to be DMM like (ambient mantle) and the EMI-EMII-HIMU end members are assumed to be isolated, located in the deep mantle, and associated with thermal anomalies. We argue that the ancient, depleted signatures of DMM imply long-term isolation from recycling and crustal contamination while the enriched components are not free of contamination by shallow materials and can therefore be shallow.

Migrating and incipient ridges and triple junctions sample the heterogeneous mantle created by plate tectonics and crustal stoping. The result is a yo-yo vertical convection mode that fertilizes, cools, and removes heat from the mantle. This mode of mantle convection is similar to the operation of a Galileo thermometer (GT). 1 The GT mode of small-scale convection, as applied to the mantle, differs from the Rayleigh-Taylor (RT) instability of a homogeneous fluid in a thermal boundary layer. It involves stoping of over-thickened continental crust and the differences in density and melting behavior of eclogites and peridotites in the mantle. The fates of subducted and delaminated crust, underplated basalt, and peridotite differ because of differences in scale, age, temperature, melting temperature, chemistry, thermal properties, and density. Cold subducted oceanic crust—as eclogite—although denser than ambient mantle at shallow depths, may become less dense or neutrally buoyant somewhere in the upper mantle and transition zone, and may be gravitationally trapped to form mafic eclogite-rich blobs or layers. Detached lower continental crust starts out warmer; it thermally and gravitationally equilibrates at shallower depths than do slabs of cold mature lithosphere. The density jumps at the depths of 400 and 650 km act as barriers. Trapped eclogite is heated by conduction from the surrounding mantle and its own radioactivity. It is displaced, entrained, and melted as it warms up to ambient mantle temperature. Both the foundering and the re-emergence of mafic and ultramafic blobs create midplate magmatism and uplift. Mantle upwellings and partially molten blobs need not be hotter than ambient mantle or from a deep thermal boundary layer. The fertile blobs drift slowly in the opposite direction to plate motions—the counterflow model—thereby maintaining age progressions and small relative motions between hotspots. Large-scale midplate volcanism is due to mantle fertility anomalies, such as large chunks of delaminated crust or subducted seamount chains, or to the release of accumulated underplate when the plate experiences flexure or pre-breakup extension. Eclogite can have lower shear velocities than volatile-free peridotite and will show up in seismic tomograms as low-velocity, or red, regions, even when cold and dense. This model removes the paradoxes associated with deep thermal RT instabilities, propagating cracks and small-scale thermal convection. It explains such observations as relative fixity of melting spots, even though the fertile blobs are shallow.

In this article we examine whether it is viable to form an age-progressive ridge-crossing seamount chain using a nonplume mechanism. Nonthermal melt sources considered include fertile mantle blobs and subsolidus mantle while lithospheric stresses generated at the ridge and at ridge-transform intersections (RTIs) are tapped to bring the mantle to the surface. Finite element models, analog models, and an analysis of the Tristan de Cunha chain all show that ridge-crossing seamount chains may be created using these mechanisms. Essentially, as a ridge migrates or reorganizes, excess magmatism may appear to switch sides of the ridge as areas of extensional stress at the RTI migrate with the ridge.

The standard model of mantle dynamics and chemistry involves complex interactions between rigid plates and hot plumes, and exchanges of material between a homogeneous upper mantle and a “primitive” lower mantle. This model requires many assumptions and produces many paradoxes. The problems and complexities can be traced to a series of unnecessary and unfruitful assumptions. Dropping these assumptions, or assuming the opposite, removes many of the paradoxes. A theory of plate tectonics can be developed that is free from assumptions about absolute plate rigidity, hotspot fixity, mantle homogeneity, and steady-state conditions. Here, a simpler and more general hypothesis is described that is based on convective systems that are cooled and organized from the top. Plate tectonics causes thermal and fertility variations in the mantle and stress variations in the plates, thus obviating the need for extraneous assumptions about the deep mantle. The general theory of plate tectonics is more powerful than the current restricted forms that exclude incipient plate-boundary (also known as volcanic chains and hotspot tracks) and athermal (e.g., melting point, fertility, and focusing) explanations of melting anomalies. Plate tectonics, geology, mantle dynamics, magmatism, and recycling are upper-mantle processes, largely independent of the deep mantle. These ideas came about by examining the paradoxes and assumptions in current models of mantle structure, evolution, and chemistry. By identifying the assumptions that generate the anomalies, one can have a zero-paradox hypothesis. Eventually, new paradoxes will be identified, and a new paradigm will be introduced. This is the way science progresses. “…. In science, conventional wisdom is difficult to overturn. After more than 20 years some implications of plate tectonics have yet to be fully appreciated by isotope geochemists… and by geologists and geophysicists who have followed their lead. “ A myth is an invented tale, often to explain some natural phenomenon… which sometimes acquires the status of dogma… without a sound logical foundation. It is a dogma that has distorted thinking about the Earth for decades. In science this is an old story, likely to be repeated again, as the defenders of conventional wisdom are seldom treated with the same scepticism as the challengers of the status quo… the dogma has been defended with false assertions, defective data, misconceptions and misunderstandings, and with strawman arguments… The justification … boils down to a statement of belief, an opinion, rather than a deduction from observations. “… geochemists are reluctant to abandon cherished concepts they grew up with and have vigorously defended during their education and research careers.” —Richard L. Armstrong, 2002, The Persistent Myth of Continental Growth: Australian Journal of Earth Science, v. 38, p. 613–630.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

This article briefly describes the derivation of S20RTS, a model of shear wave velocity variations in the mantle. In particular, I illustrate how interpretation of tomographic models is complicated by heterogeneous resolution, taking as an example the Icelandic upper mantle. GSA Data Repository item 2005055, Preliminary Reference Earth Model (PREM), is available online at www.geosociety.org/pubs/ft2005.htm , or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.

This chapter provides a catalog and maps of those volcanic, tectonic, and geochemical features that have become known as “hotspots,” including those that may have a shallow plate tectonic or asthenospheric origin. Many proposed hotspots, including isolated structures and the active portions, or inferred ends, of seamount chains, do not have significant swells, substantial magmatic output, or tomographic anomalies. A hotspot catalog, as opposed to a volcano catalog, is therefore subjective. Recent lists of those purported to be underlain by deep mantle plumes disagree strongly. A melting anomaly, or hotspot, may result from localized high absolute mantle temperature or from a localized fertile or fusible patch of the asthenosphere. Some have been called “wetspots,” and some have been called “hotlines.” The localization may be due to lithospheric stress or architecture. The common characteristics of features designated as hotspots suggest an underlying common cause. This chapter provides references and brief evaluations of individual features and mechanisms that can be used to evaluate the origins of hotspots. Discussions of individual hotspots, volcanic chains, and tomographic results are given in three appendixes.

The origin of midplate and along-ridge melting anomalies is controversial. Hypotheses involve, at one extreme, concentrated hot mantle upwellings from the deepest mantle and, at the other extreme, shallow processes dominated by stress, plate tectonics, and fertility variations, along with an asthenosphere that is near the melting point. An updated hotspot list is presented and is tested against criteria relevant to both the deep thermal plume and the shallow (plate and asthenosphere) hypotheses. The unique polling approach of Courtillot et al. (2003) is applied to the plume hypothesis and to other hypotheses for melting anomalies. Although some “primary” (i.e., potentially deep-seated) hotspots (Iceland, Hawaii, Easter, Louisville, Afar, Ré-union, and Tristan) score well using the chosen subjective plume criteria, they score poorly using criteria more appropriate to deep or thermal processes, such as magma temperature, heatflow, transition zone thickness, and high-resolution upper and lower mantle seismic tomographic results. In particular, Iceland, Easter, Afar, Tristan, and Yellowstone have not been confirmed by tomography. They are shallow features with well-defined plate tectonic explanations. For most melting anomalies (aka “hotspots”) the plume hypothesis scores poorly against competing hypotheses such as stress- and crack-controlled magmatism, mechanisms that are associated with plate tectonics. Based on the results, most “hotspots,” including proposed “primary” or plume-candidate hotspots, are unlikely to be caused by thermal plumes from deep thermal boundary layers. Melting anomalies, on- or off-ridge, appear to be a natural result of nonrigid plate tectonics, including recycling, and do not require an extraordinary explanation, such as narrow thermal instabilities that traverse the whole mantle. Thus plate tectonics, plate boundaries, global plate reorganization, normal magmatism, melting anomalies, volcanic chains, and mantle geochemistry can be unified into a single theory. We see that many assumptions used in previous hypotheses can be discarded as unnecessary … there is no need to locate the source of plumes in the lower mantle. —Richter and Parsons (1975)

Hotspot activity has been invoked to explain a number of geological observations and phenomena. The genetic relationship between hotspots and continental flood basalts, as well as “tracks” in many oceans, appears to be commonly accepted by Earth scientists. One critical test that must be applied to such connections or hypotheses is that the pertinent radiometric data must be robust. Herein I critically reexamine some sets of data in this regard. The pertinent 40 Ar/ 39 Ar step-heating data must (1) satisfy rigorous statistical tests for validity and (2) be based on material that can be shown to be fresh or minimally altered. I show that most age data published recently for the Isle of Mull (British Tertiary Igneous Province) are invalid as proper estimates of the crystallization age. I apply a similar mode of examination to rocks thought to represent the tracks of (1) the Yellowstone hotspot, Pacific Northwest, USA, (2) the Tristan da Cunha and Great Meteor hotspots, Atlantic Ocean, and (3) the Kerguelen and Ré-union hotspots, Indian Ocean. Few, if any, valid crystallization ages were recovered. These hotspot tracks cannot be temporally defined. Conclusions based on the data rejected herein, in particular those pertaining to the extrapolated paths of such tracks and the calculation of plate velocities, should be subject to critical scrutiny.

Hotspot theory was first proposed on the basis of the observation of linear volcanic chains on the Pacific plate and assumed age progression within these chains. Knowledge of the ages of islands and seamounts is therefore of primary importance to analyzing intraplate volcanism and deciphering the history of hotspot tracks. In this paper we review published radiometric ages of islands and seamounts on the Pacific plate to help further reconstruction. We present a compilation of 1645 radiometric ages sorted by chain and further by island or seamount, along with a brief overview of each chain. Paleomagnetic ages obtained from seamount magnetism have not been considered, except for some oceanic plateaus (e.g., Shatsky rise). We do not consider foraminifer ages, which only give minimum ages of seamounts. Reliability problems intrinsic to the samples and to the radiometric dating methods must be considered. Dating of whole rocks must generally be disregarded unless they have been subject to special treatment, Ar/Ar incremental heating dating should be preferred over other methods, and data that do not pass the reliability criteria discussed by Baksi (this volume) should be disregarded. Thus use of the ages compiled in our database must be done in the light of filtering, and we encourage the user to check critically the initial papers in which the dates were published.

For too long philosophers have debated how scientists should judge hypotheses in glaring ignorance of how scientists in fact judge hypotheses…. Maybe it is now being done better than one thinks. Indeed, attempting to follow the [philosopher's] advice might be detrimental to scientific progress. — R.N. Giere (1988)