Hotspots and plumes

One of the pioneers of plate tectonics, W. Jason Morgan, recognised in the 1970s that chains of volcanic islands and seamounts that rise from the ocean floor may have formed as the movement of lithospheric plates passed over sources of magma that lay in the mantle beneath the plates. He suggested that such hotspots were fixed relative to plate movements at the surface and likened the formation of chains such as that to the west of the volcanically active of the Hawaiian ‘Big Island’ to linear scorching of a sheet of paper moved over a candle flame. If true, it should be possible to use hotspots as a framework for the absolute motion of lithospheric plates rather than the velocities of individual plates relative to the others. But Morgan’s hypothesis has been debated ever since he formulated it. A test would be to see whether or not plumes of rising hot material in the deep part of the mantle can be detected. This became one of the first objectives of seismic tomography when it was devised in the last decade of the 20th century: a method that uses global earthquakes records to detect parts of the mantle where seismic waves traveled faster or slower than the norm: effectively patches of hot (probably rising) and cold rock. The first such evidence was equally hotly debated, one view being that the magma sources beneath oceanic islands such as Hawaii and Iceland were actually related to plate tectonics and that the hotspot hypothesis had become a kind of belief system.

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Global distribution of hotspots ( credit: Wikipedia)

The problem was that mantle plumes supposedly linked to magmatic hotspots in the upper mantle would be so thin that they would be difficult to detect even with seismic tomography. Geophysicists have been trying to sharpen up seismic resolution partly by using supercomputers to analyse more and more seismic records and also by improving the theory about how seismic waves interact with 3-D mantle structure. This has culminated in more believable visualisation of mantle structure (French, S.W. & Romanowicz, B. 2015. Broad plumes rooted at the base of the Earth’s mantle beneath major hotspots). The two researchers from the University of California at Berkeley in fact showed something different, but still robust support for Morgan’s 40-year old ideas. Instead of thin plumes, they have been able to show much broader conduits beneath at least 5 and maybe more active ends of hotspot chains. The zones extend upwards from the core-mantle boundary to about 1000 km below the Earth’s surface, where some bend sideways towards hotspots, perhaps as a result of another kind of upper mantle circulation.

Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;
Whole-Earth seismic tomography cross sections beneath a variety of volcanic islands, (Credit French and Romanowicz;

The sources of these hot columns at the core-mantle boundary appear to be zones of very low shear-wave velocities; i.e. almost, but not quite molten blobs. French and Romanowicz suggest that the columns are extremely long-lived and may even have a chemical dimension – as in the hypothesis of mantle heterogeneity. Another interesting feature of their results is that the striking vertical linearity of the columns could indicate that the overall motion of the lower mantle is extremely sluggish and punctured by discrete convection.

Mantle structures beneath the central Pacific

Since it first figured in Earth Pages 13 years ago seismic tomography has advanced steadily as regards the detail that can be shown and the level of confidence in its accuracy: in the early days some geoscientists considered the results to be verging on the imaginary. There were indeed deficiencies, one being that a mantle plume which everyone believed to be present beneath Hawaii didn’t show up on the first tomographic section through the central Pacific. Plumes are one of the forms likely to be taken by mantle heat convection, and many now believe that some of them emerge from great depths in the mantle, perhaps at its interface with the outer core.

The improvements in imaging deep structure stem mainly from increasingly sophisticated software and faster computers, the data being fed in being historic seismograph records from around the globe. The approach seeks out deviations in the speed of seismic waves from the mean at different depths beneath the Earth’s surface. Decreases suggest lower strength and therefore hotter rocks while abnormally high speeds signify strong, cool parts of the mantle. The hotter mantle rock is the lower its density and the more likely it is to be rising, and vice versa.

Using state-of-the-art tomography to probe beneath the central Pacific is a natural strategy as the region contains a greater concentration of hot-spot related volcanic island chains than anywhere else and that is the focus of a US-French group of collaborators (French, S. et al. 2013. Waveform tomography reveals channeled flow at the base of the oceanic lithosphere. Science, v. 342, 227-230;  doi 10.1126/science.1241514). The authors first note the appearance on 2-D global maps for a depth of 250 km of elongate zones of low shear-strength mantle that approximately parallel the known directions of local absolute plate movement. The most clear of these occur beneath the Pacific hemisphere, strongly suggesting some kind of channelling of hot material by convection away from the East Pacific Rise.

Seismic tomograhic model of the mantle beneath the central Pacific. Yellow to red colours represent increasing low shear strength. (credit: Global Seismology Group / Berkeley Seismological Laboratory
Seismic tomographic model of the mantle beneath the central Pacific. Yellow to red colours represent increasingly low shear strength. (credit: Global Seismology Group / Berkeley Seismological Laboratory)

Visually it is the three-dimensional models of the Pacific hot-spot ‘swarm’ that grab attention. These show the low velocity zone of the asthenosphere at depths of around 50 to 100 km, as predicted but with odd convolutions. Down to 1000 km is a zone of complexity with limb-like lobes of warm, low-strength mantle concentrated beneath the main island chains. That beneath the Hawaiian hot spot definitely has a plume-like shape but one curiously bent at depth, turning to the NW as it emerges from even deeper mantle then taking a knee-like bend to the east . Those beneath the hot spots of the west Pacific are more irregular but almost vertical. Just what kind of process the peculiarities represent in detail is not known, but it is almost certainly a reflection of complex forms taken by convection in a highly viscous medium.

A plume drive for tectonics?

Himalaya Formation Source US Gove...
India's tectonic travels. Image via Wikipedia

The theory of plate tectonics resolved Alfred Wegener’s search for a driving force for continental drift around half a century after his discovery faced near-universal rejection for not having one that was large enough or plausible. Plate theory recognises many forces, both driving and in opposition to tectonic movement. By far the largest is the gravitational pull exerted by subducting slabs of dense oceanic lithosphere, followed in distant second place by ridge-push, another gravity-driven force that arises from the slope on the ocean floors away from sea-floor spreading centres as the oceanic lithosphere cools and shrinks as it ages. Until very recently, no place was assigned in the theory to forces associated with the apparently non-tectonic plumes that rise through the mantle from well beneath the lithosphere from which plates are made, quite possibly because it seems logical to expect a vertically upwards force, if any, from hot plumes whereas plate tectonics is mainly concerned with horizontal movements. Looking around the present state of sea-floor spreading, the maximum pace at which plates move is just over 100 mm a-1 (100 km Ma-1) in the case of the Pacific Plate. Yet, during the Late Cretaceous and Early Palaeogene Periods after India had been wrenched away from the Gondwana supercontinent to move towards eventual collision with Eurasia the subcontinent experienced an extraordinary episode beginning around 68 Ma when its pace increased to as high as 180 km Ma-1. This accelerated motion continued over some 15 Ma and then equally abruptly slowed to less than 40 km Ma-1 around the start of the Eocene (Cande, S.C. & Stegman, D.R. 2011. Indian and African plate motions driven by the push force of the Réunion plume head. Nature, v. 475, p. 47-52; see also: Müller, R.D. 2011. Plate motion and mantle plumes. . Nature, v. 475, p. 40-41). The acceleration coincided with the start of continental flood-basalt volcanism that blanketed much of western India with the Deccan Traps across the K-P boundary when the subcontinent lay over the site of the Réunion hot spot. Coincidentally, the Réunion plume head formed at that time; i.e. the Indian continental lithosphere did not drift over an active plume, but was hit from below by one that happened to be rising to the surface. Curiously, while the Indian plate was accelerated, nearby Africa was slowed, explained by a push in the same direction of India’s travel towards a subduction zone beneath Asia and one applied to Africa that opposed its motion. Africa too resumed its usual tectonic progress at the start of the Eocene. But how did a mantle plume exert such a force: was it because it caused a local bulge from which the plates slid, or did mantle motion associated with the mushroom-like structure of the horizontally growing plume head exert viscous drag on the overlying plates? Such shifts in motion of major plates inevitably have an effect on the whole plate tectonic carapace, and the authors list a number of contemporary, distant consequences, speculating that the famous bend in the Hawaii-Emperor island and sea-mount chain in the Early Eocene resulted from the final waning of the Réunion plume head’s influence and major readjustment of tectonics.

Himalayan Horizon From Space
The result of India's final collision with Eurasia - the Himalaya. Image via Wikipedia