Tag: mantle plumes

New views on oceanic-plate volcanism

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Ocean floors, especially those of the Pacific and Indian oceans, are bespattered by tens of thousands of seamounts and volcano-based islands. Some of them define chains roughly aligned with the direction of sea-floor spreading and whose age of activity changes progressively along the chains. A few include bends linked to past changes in plate motions (see: The Great Bend of the Pacific Ocean Floor; May 2009). There are also vast, drowned plateaus, which mark oceanic equivalents of flood-basalt provinces on continents. The July 2026 issue of Nature Geoscience is dominated by four papers that are focused on such magmatism within oceanic plates, together with several commentaries and a striking cover image.  The findings reported by the authors are briefly summarised by the Issue’s editor and two News & Views items (Editor 9 July 2026. Emerging insights on oceanic intraplate volcanism. Nature Geoscience v. 19, p. 739; DOI: 10.1038/s41561-026-02051-9; Ito, G. 2026. The deep link between intraplate volcanism and plate tectonics. Nature Geoscience v. 19, p. 743-744; DOI: 10.1038/s41561-026-02027-9. Whittaker, J. 2026. Linking plates to plumes in the Cretaceous Pacific. Nature Geoscience v. 19, p. 745-746: DOI: 10.1038/s41561-026-02026-w).

Fig 3-D simulation of geodynamics beneath the Pacific and Indian Ocean floors on the cover of the July 2026 Issue of Nature Geoscience. Credit: Hao Dong, Institute of Geology and Geophysics, Chinese Academy of Sciences.

Age-progressive seamount chains have been ascribed since the birth of plate tectonics to the movement of an oceanic plate over ‘hot spots’ or mantle plumes. Yet the vast majority of sea mounts are apparently distributed at random. They have posed a bone of contention for oceanographers. One of the papers proposes a clear explanation (Hao Dong et al. 2026. Deep mantle plume origin of oceanic intraplate volcanism. Nature Geoscience v. 19, p. 828-836; DOI: 10.1038/s41561-026-02006-0. See also: Iyer, D, 2026 New 270‑Million‑Year Simulation Reveals Hidden Heat Zones Behind Thousands of Seamounts. Bioscience/Marine Science10 June 2026). The authors, based at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, have produced a simulation of changing mantle dynamics beneath the Indian and Pacific Oceans over the last 290 Ma. In their model, hot mantle rising from the core-mantle boundary has repeatedly interacted with the oceanic upper mantle and lithosphere. This rise of hot mantle seems not always to manifest as discrete plumes from the depths. Rather it diffusely heats the asthenosphere to produce large-scale hot regions over which clusters of isolated seamounts formed while the thermal anomaly remained. They liken them to “seamount breweries”, and posit fragmentation of the rising mass of hot mantle, sometimes at depth or within the upper mantle. Such mechanisms drift with the overall mantle and lithospheric flow, so that seamount-volcanoes continue to develop, provided the ‘brewing zones’ retain sufficient heat.

But what might have launched such rising deep mantle? Another study notes that seamount volcanism is more voluminous on ocean floors that once passed over large low-shear-velocity provinces (LLSVPs) in the lowermost mantle beneath Africa and the Pacific Ocean (Conrad, C.P. & Domeier, M. Seamount volcanism associated with Earth’s basal mantle structures. Nature Geoscience v. 19, p. 822-827; DOI: 10.1038/s41561-026-02007-z) These massive deep-mantle provinces are hotter than their surroundings so they are less rigid, which is why seismic S-waves travel more slowly through them. They also lie on opposite sides of the Earth and may have done so for several billion years.  Two other, independent studies in the same issue of Nature Geoscience also imply a connection to the deep mantle (Jinchang Zhang et al. 2026. Ontong Java Plateau formed by a thermochemical mantle plume. Nature Geoscience v. 19, p.846-854; DOI: 10.1038/s41561-026-02019-9; Dingshan Deng et al. 2026. Vigorous mantle convection triggered the Cretaceous Pacific large igneous provinces. Nature Geoscience v. 19, p. 837-845; DOI: 10.1038/s41561-026-02016-y).

Extrusion of large igneous provinces (LIPs) on the floor of the Pacific Ocean peaked during the Early Cretaceous. Dingshan Deng and colleagues modelled mantle flow that links subduction, plume activity and mid-ocean ridge activity, which suggests that deep-mantle upwelling peaked around 130 to 125 Ma ago and was driven by increased subduction around the ocean’s margin. This slowed down the rate of spreading at mid-ocean ridges so that heat had to be dissipated by increased melting elsewhere to outpour magma that created LIPs such as the Ontong Java Plateau. The article by Jinchang Zhang et al. focuses on the Ontong Java Plateau, the largest such volcanogenic structure known on Earth. Long regarded as having been produced by a huge buoyant, hot plume, they calculate that such a phenomenon would have had to have uplifted the ocean floor above sea level. That clearly did not happen. To reconcile the sheer volume of magma production with extrusion on deep ocean floor, the authors considered a denser and hotter source mantle, perhaps 135 to 200°C higher than ambient mantle temperature. One possibility is that it had incorporated a lot of older subducted slab materials and was hot enough to thermally erode the plate on which the Ontong Java Plateau was emplaved.

In the 70 years since ideas on plate tectonics began to develop the approach has changed. It began in a ‘compartmentalised’ fashion: spreading at mid-ocean ridges; descent at subduction zones; hot spots and mantle plumes, and so on. Most, if not all Earth scientists involved in this scientific revolution adhered to a reductionist philosophy; i.e. breaking down the hugely complex Earth system into simpler components – the whole is the sum of its parts – an approach begun in the 17th century by by René Descartes. Now, instead of a focus on and separation of cause and effect, it is becoming clearer that the Earth system is fundamentally one of global interconnections, in continuous motion and change. A change in one part inescapably rebounds on all the others, so that the system continually evolves. The tools available to geoscientists have evolved too.

Did Precambrian BIFs ‘fall’ into the mantle to trigger mantle plumes?

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How the Earth has been shaped has depended to a large extent on a very simple variable among rocks: their density. Contrasts in density between vast rock masses are expressed when gravity attempts to maintain a balance of forces. The abrupt difference in elevation of the solid surface at the boundaries of oceans and continents – the Earth’s hypsometry – stems from the contrasted densities of continental and oceanic crust: the one dominated by granitic rocks (~2.8 t m-3) the other by those of basaltic composition (~ 3.0 t m-3). Astronomers have estimated that Earth’s overall density is about 5.5 t m-3 – it is the densest planet in the Solar System. The underlying mantle makes up 68% of Earth’s mass, with a density that increases with depth from 3.3 to 5.4 t m-3 in a stepwise fashion, at a number of discontinuities, because mantle minerals undergo changes induced by pressure. The remaining one third of Earth’s mass resides in the iron-nickel core at densities between 9.5 to 14.5 t m-3. Such density layering is by no means completely stable. Locally increased temperatures in mantle rocks reduce their density sufficiently for masses to rise convectively to be replaced by cooler ones, albeit slowly. By far the most important form of convection affecting the lithosphere involves the resorption of oceanic lithosphere plates at destructive margins, which results in subduction. This is thought to be due to old, cold oceanic basalts undergoing metamorphism as pressure increases during subduction. They are transformed at depth to a mineral assemblage (eclogite) that is denser (3.4 to 3.5 t m-3) than the enveloping upper mantle. That density contrast is sufficient for gravity to pull slabs of oceanic lithosphere downwards. This slab-pull force is transmitted through oceanic lithosphere that remains at the surface to become the dominant driver of modern plate tectonics. As a result, extension of the surface oceanic lithosphere at constructive margins draws mantle upwards to partially melt at reduced pressure, thus adding new basaltic crust at mid-ocean rift systems to maintain a form of mantle convection. Seismic tomography shows that active subducted slabs become ductile about 660 km beneath the surface and below that no earthquakes are detected. Quite possibly, the density of the reconstituted lithospheric slab becomes less than that of the mantle below the 660 km discontinuity. So the subducted slab continues by moving sideways and buckling in response to the ‘push’ from its rigid upper parts above. But it has been suggested that some subducted slabs do finally sink to the core-mantle boundary, but that is somewhat conjectural.

Typical banded iron formation

There are sedimentary rocks whose density at the surface exceeds that of the upper mantle: banded iron formations (BIFs) that contain up to 60% iron oxides (mainly Fe2O3) and have an average density at the surface of around 3.5 t m-3. BIFs formed mainly in the late Archaean and early Proterozoic Eons  (3.2 to 1.0 Ga) and none are known from the last 400 Ma. They formed when soluble iron-2 (Fe2+) – being added to ocean water by submarine hydrothermal activity –was precipitated as Fe3+ in the form of iron oxide (Fe2O3) where oxygen was present in ocean water. With little doubt this happened only in shallow marine basins where cyanobacteria that appeared about 3.5 Ga ago had sufficient sunlight to photosynthesise. Until about 2.4 Ga the atmosphere and thus the bulk of ocean water contained very little oxygen so the oceans were pervaded by soluble iron so that BIFs were able to form wherever such biological activity was going on. Conceivably (but not proven), that BIF-forming biochemical reaction may even have operated far from land in ocean surface water, slowly to deposit Fe2O3 on the deep ocean floor. After 2.4 Ga oxygen began to build in the atmosphere after the Great Oxidation Event had begon. That time was also when the greatest production of BIFs took place. Strangely, the amount of BIF in the geological record fell during the next 600 Ma to rise again to a very high peak at 1.8 Ga. Since there must have been sufficient soluble iron and an increasing amount of available oxygen for BIFs to form throughout that ‘lean’ period the drop in BIF formation is paradoxical. After 1.0 Ga BIFs more or less disappear. By then so much oxygen was present in the atmosphere and from top to bottom in ocean water that soluble iron was mostly precipitated at its hydrothermal source on the ocean floor. Incidentally, modern ocean surface water far from land contains so little dissolved iron that little microbiological activity goes on there: iron is an essential nutrient so the surface waters of remote oceans are effectively ‘wet deserts’.

Plots of probability of LIPs and BIFs forming at the Earth’s surface during Precambrian times, based on actual occurrences (Credit: Keller, et al., modified Fig 1A)

Spurred by the fact that if a sea-floor slab dominated by BIFs was subducted it wouldn’t need eclogite formation to sink into the mantle, Duncan Keller of Rice University in Texas and other US and Canadian colleagues have published a ‘thought experiment’ using time-series data on LIPs and BIFs compiled by other geoscientists (Keller, D.S. et al. 2023. Links between large igneous province volcanism and subducted iron formations. Nature Geoscience, v. 16, article; DOI: 10.1038/s41561-023-01188-1.). Their approach involves comparing the occurrences of 54 BIFs through time with signs of activity in the mantle during the Palaeo- and Mesoproterozoic Eras, as marked by large igneous provinces (LIPs) during that time span. To do this they calculated the degree of correlation in time between BIFs and LIPs. The authors chose a minimum area for LIPs of 400 thousand km2 – giving a total of 66 well-dated examples. Because the bulk of Precambrian flood-basalt provinces, such as occurred during the Phanerozoic, have been eroded away, most of their examples are huge, well-dated dyke swarms that almost certainly fed such plateau basalts. Rather than a direct time-correlation, what emerged was a match-up that covered 74% of the LIPs with BIFs that had formed about 241 Ma earlier. They also found a less precise correlation between LIPs associated with 241 Ma older BIFs and protracted periods of stable geomagnetic field, known as ‘superchrons’. These are thought by geophysicists to be influenced by heat flow through the core-mantle boundary (CMB).

The high bulk density of BIFs at the surface would be likely to remain about 15 % greater than that of peridotite as pressure increased with depth in the mantle. Such slabs could therefore penetrate the 660 mantle discontinuity. Their subduction would probably result in their eventually ‘piling up’ in the vicinity of the CMB. The high iron content of BIFs may also have changed the way that the core loses heat, thereby triggering mantle plumes. Certainly, there is a complex zone of ultra-low seismic velocities (ULVZ) that signifies hot, ductile material extending above the CMB. Because BIFs’ high iron-content makes them thermally highly conductive compared with basalts and other sediments, they may be responsible. Clearly, Keller et al’s hypothesis is likely to be controversial and they hope that other geoscientists will test it with new or re-analysed geophysical data. But the possibility of BIFs falling to the base of the mantle spectacularly extends the influence of surface biological processes to the entire planet. And, indeed, it may have shaped the later part of its tectonic history having changed the composition of the deep mantle. The interconnectedness of the Earth system also demands that the consequences – plumes and large igneous provinces – would have fed back to the Precambrian biosphere. See also: Iron-rich rocks unlock new insights into Earth’s planetary history, Science Daily, 2 June 2023