Deep Origin of Hotspots--the Mantle Plume Model (original) (raw)

Plumes, or plate tectonic processes?

Astronomy and Geophysics, 2002

W e know little about the deep interior of Earth, but because it is the key to understanding surface geology, volcanism and earthquakes, there is much speculation about its composition and the processes that occur within it. Perhaps the most fundamental question is the depth extent of those structures and processes that influence the surface. Opinion is divided regarding whether the mantle, at depths exceeding~1000 km, has little to do with surface processes, or whether it is actively involved, down to the outermost core at~3000 km depth, in the mass transport system associated with plate tectonics. The latter view would imply that material from the deepest mantle can be sampled at volcanic provinces on Earth's surface. The former would imply that it cannot. An important contribution to this debate came hot on the heels of the newly accepted plate tectonic theory. Morgan (1971) suggested that Hotspots 6.19 December 2002 Vol 43 A mantle plume under Iceland is taken for granted as the cause of the volcanism there. But Gill Foulger argues that the evidence does not stand up. 1: Numerical simulation of a deep mantle plume. The red, mushroom-like feature represents a hot upwelling from the core-mantle boundary. Blue, linear features are cold downwellings. (From Kiefer and Kellogg 1998.

Mantle plumes and their interactions

Hotspots are regions of intraplate volcanism or especially strong volcanism along plateboundaries, and many of them are likely caused by underlying mantle plumes – localizedhot upwellings from deep inside the Earth. It is still uncertain, whether all plumes or justsome of them rise from the lowermost mantle, and to what extent and where theyentrain chemically different materials. Also, large uncertainties exist regarding their size.Some plumes (such as Hawaii) create linear hotspot tracks, as the plate moves overthem and can therefore serve as reference frames for plate motions, whereas others(such as Iceland) show a more complicated distribution of volcanic rocks due to variablelithosphere thickness and plume-ridge interaction. Plumes may also weaken plateboundaries and hence influence plate motions. They may influence surface features suchas ice sheets, and therefore climate, but we are just beginning to study and understandprocesses jointly involving solid earth, hydrosphere and ...

Mantle plumes and their role in Earth processes

Nature Reviews Earth & Environment, 2021

The existence of mantle plumes was first proposed in the 1970s to explain intra-plate, hotspot volcanism, yet owing to difficulties in resolving mantle upwellings with geophysical images and discrepancies in interpretations of geochemical and geochronological data, the origin, dynamics and composition of plumes and their links to plate tectonics are still contested. In this Review, we discuss progress in seismic imaging, mantle flow modelling, plate tectonic reconstructions and geochemical analyses that have led to a more detailed understanding of mantle plumes. Observations suggest plumes could be both thermal and chemical in nature, can attain complex and broad shapes, and that more than 18 plumes might be rooted in regions of the lowermost mantle. The case for a deep mantle origin is strengthened by the geochemistry of hotspot volcanoes that provide evidence for entrainment of deeply recycled subducted components, primordial m an tle domains and, potentially, materials from Earth's core. Deep mantle plumes often appear deflected by large-scale mantle flow, resulting in hotspot motions required to resolve past tectonic plate motions. Future research requires improvements in resolution of seismic tomography to better visualize deep mantle plume structures at smaller than 100-km scales. Concerted multi-proxy geochemical and dating efforts are also needed to better resolve spatiotemporal and chemical evolutions of long-lived mantle plumes.

Hotspots and mantle plumes: Some phenomenology

Journal of Geophysical Research, 1990

The available data, mainly topography, geoid, and heat flow, describing hotspots worldwide are examined to constrain the mechanisms for swell uplift and to obtain fluxes and excess temperatures of mantle plumes. Swell uplift is caused mainly by excess temperatures that move with the lithosphere plate and to a lesser extent hot asthenosphere near the hotspot. The volume, heat, and buoyancy fluxes of hotspots are computed from the cross-sectional areas of swells, the shapes of noses of swells, and, for on ridge hotspots, the amount of ascending material needed to supply the length of ridge axis which has abnormally high elevation and thick crust. The buoyancy fluxes range over a factor of 20 with Hawaii, 8.7 Mg s-1, the largest. The buoyancy flux for Iceland is 1.4 Mg s-1 which is similar to the flux of Cape Verde. The excess temperature of both on-ridge and off-ridge hotspots is around the 200øC value inferred from petrology but is not tightly constrained by geophysical considerations. This observation, the similarity of the fluxes of on-ridge and offridge plumes, and the tendency for hotspots to cross the ridge indicate that similar plumes are likely to cause both types of hotspots. The buoyancy fluxes of 37 hotspots are estimated; the global buoyancy flux is 50 Mg s-1, which is equivalent to a globally averaged surface heat flow of 4 mWm-2 from core sources and would cool the core at a rate of 50 ø C b.y.-1. Based on a thermal model and the assumption that the likelihood of subduction is independent of age, most of the heat from hotspots is implaced in the lower lithosphere and later subducted. I.NTRODUCWION ridge plumes using Iceland as an example. The geometry of flow implied by the assumed existence of a low viscosity Linear seamount chains, such as the Hawaiian Islands, are asthenospheric channel is illustrated by this exercise. Then the frequently attributed to mantle plumes which ascend from deep methods for obtaining the flux of plumes on a rapidly moving in the Earth, perhaps the core-mantle boundary. The excessive plate are discussed with Hawaii as an example. These methods volcanism of on-ridge hotspots, such as Iceland, is also often involve determining the flux from the plume from the crossattributed to plumes. If on-ridge and midplate hotspots are sectional area of the swell and taking advantage of the kinematreally manifestations of the same phenomenon, one would ics of the interaction of asthenospheric flow away from the expect that the temperature and flux of the upwelling material plume and asthenospheric flow induced by the drag of the would be similar under both features. In particular, the core-lithospheric plate. The methods for extending this approach to mantle boundary is expected to be nearly isothermal so that the hotspots on slowly moving plates are then discussed which Cape temperature of plumes ascending from the basal boundary layer Verde as an example. An estimate of the global mass and heat should be the same globally provided that cooling by entrain-transfer by plumes is then obtained by applying the methods to ment of nearby material and thermal conduction are minor. 34 additional hotspots. The magnitude of this total estimated Finally, the global heat loss from plumes should imply a reason-flux is compatible with the heat flux expected from cooling the able cooling rate of the core [Davies, 1988a]. core in agreement with Davies [1988a]. Finally, the results are The most important point of this paper is that the properties used to obtain general properties of plumes and inferences on of mantle plumes are constrained by observations, mainly topog-the underlying mantle dynamics. "• raphy, geoid, and heat flow. I follow Davies [1988a], Richards eta!. [1988], and Sleep [1987a, b] in assuming that the ICELANDIC PLUM•FLUX geometry of plumes consists of narrow vertical pipes which supplies hot mantle to a thin low-viscosity asthenosphere beneath The Icelandic hotspot is directly on the Mid-Atlantic Ridge. the moving plate (Figure 1). Such plumes provide a much Methods developed for off-ridge hotspots are thus inappropriate. better explanation for geoid and topographic anomalies than The excess volcanism of the ridge near the hotspot is probably does secondary convection from the cooling of the oceanic plate the best way to constrain the excess temperature and volume of Davies [1988a, b]. This geometry allows simplification of the material supplied by the plume. discussion and the physics because the model plume is much narrower than the hotspot swell and the rapidly flowing layer of Excess Crustal Thickness and Temperature asthenosphere is thin compared to the thickness of the mantle of Icelandic Plume and the width of the swell. The expense is some model dependency of the results primarily because the existence of a thin Higher temperatures beneath hotspot ridges lead to more parlow-viscosity channel has not been established. tial melting and explain the 20-km-thick crust beneath Iceland I begin by developing methods for estimating the flux of on-[McKenzie, 1984]. An excess temperature of the upwelling material between 200 ø C and 250 ø C is needed to generate the .... excess 15 km of crust in the computations of McKenzie [1984]. This average excess temperature is also compatible with the Copyfight 1990 by the American Geophysical Union. depth and volume of extensive melting beneath Hawaii [McKen-Paper number 89IB03587. zie, 1984] and with the maximum excess temperature of 300øC 0148-0227/90/89JB-03587505.00 of Wyllie [1988]. 6715 Presnall and Hoover [1984] state that extensive partial melting It is useful to define a buoyancy flux for later comparison begins at 30 km depth. The former hypothesis implies that with off-ridge hotspots extensive melt remains in the manfie and thus precludes deter-B = p,,, otAT Q/, (2) mining excess temperature from crustal thickness. Forms of the latter hypothesis are thus used by McKenzie [1984]. For-where AT is the average excess temperature, Pm is the density tunarely, the various melting relationships used by McKenzie of the manfie, about 3300 kg m-3, and 0t is the thermal expanß 5o 1 [1984] and a simplified relationship by Sleep and Windley s•on coefficient, about 3 x 10-C-. The buoyancy flux of the [1982] give similar excess temperatures. I thus make no Iceland plume is thus about 1.4 Mg s-1. As shown below this attempt to improve estimates of melting relationships. An esti-flux is similar to the flux of Cape Verde and much less than the mate of 225øC for the average excess temperature of the Ice-flux of Hawaii. land plume is used below. The Jan Mayen hotspot is supposed to exist along the ridge Flux of Icelandic Plume The volume flux of the Icelandic plume may be computed from the kinematics of spreading. The plume needs to supply the oceanic lithosphere at least down to the depth of extensive melting, about 80 km, and probably the underlying asthenosphere as well. I assume that the plume flux balances the flow at a great distance (Figure 2). The flow is assumed to consist of north of Iceland [e.g., Vink, 1984]. However, it has litfie topographic expression along the ridge axis. It is possibly related to southward propagation of the Moins ridge axis [Saemundsson, 1986]. The flux of this plume if it exists is included here in that of the Iceland hotspot. HAWAUAN I•UM• FLUX The Hawaiian hotspot is isolated from other features on the a lithosphere moving at the plate velocity and and asthenosphere Pacific plateß Thus much speculation on manfie plumes and channel where the velocity decreases linearly from the plate hotspots has used it as the primary example. The flux estimates York, 1982.

Mantle plumes from top to bottom

Earth-Science Reviews, 2006

Hotspots include midplate features like Hawaii and on-axis features like Iceland. Mantle plumes are a well-posed hypothesis for their formation. Starting plume heads provide an explanation of brief episodes of flood basalts, mafic intrusions, and radial dike swarms. Yet the essence of the hypothesis hides deep in the mantle. Tests independent of surface geology and geochemistry to date have been at best tantalizing. It is productive to bare the current ignorance, rather than to dump the plume hypothesis. One finds potentially fruitful lines of inquiry using simple dynamics and observations. Ancient lithospheric xenoliths may reveal heating by plumes and subsequent thermal equilibration in the past. The effect at the base of the chemical layer is modest 50-100 K for transient heating by plume heads. Thinning of nonbuoyant platform lithosphere is readily observed but not directly attributable to plumes. The plume history in Antarctica is ill constrained because of poor geological exposure. This locality provides a worst case on what is known about surface evidence of hotspots. Direct detection of plume tail conduits in the mid-mantle is now at the edge of seismic resolution. Seismology does not provide adequate resolution of the deep mantle. We do not know the extent of a chemically dense dregs layer or whether superplume regions are cooler or hotter than an adiabat in equilibrium with the asthenosphere. Overall, mid-mantle seismology is most likely to give definitive results as plume conduits are the guts of the dynamic hypothesis. Finding them would bring unresolved deep and shallow processes into place.

Mantle plumes: Dynamic models and seismic images

Geochemistry, Geophysics, Geosystems, 2007

1] Different theories on the origin of hot spots have been debated for a long time by many authors from different fields, and global-scale seismic tomography is probably the most effective tool at our disposal to substantiate, modify, or abandon the mantle-plume hypothesis. We attempt to identify coherent, approximately vertical slow/hot anomalies in recently published maps of P and S velocity heterogeneity throughout the mantle, combining the following independent quantitative approaches: (1) development and application of a ''plume-detection'' algorithm, which allows us to identify a variety of vertically coherent features, with similar properties, in all considered tomographic models, and (2) quantification of the similarity between patterns of various tomographic versus dynamic plume-conduit models. Experiment 2 is complicated by the inherent dependence of plume conduit tilt on mantle flow and by the dependence of the latter on the lateral structure of the Earth's mantle, which can only be extrapolated from seismic tomography itself: it is inherently difficult to disentangle the role of upwellings in ''attracting'' plumes versus plumes being defined as relatively slow, and thus located in regions of upwellings. Our results favor the idea that only a small subset of known hot spots have a lower-mantle origin. Most of those that do can be associated geographically with a few well-defined slow/hot regions of very large scale in the lowermost mantle. We find evidence for both secondary plumes originating from the mentioned slow/hot regions and deep plumes whose conduits remain narrow all the way to the lowermost mantle. To best agree with tomographic results, modeled plume conduits must take into account the effects of advection and the associated displacement of plume sources at the base of the mantle.