Computer models show how hot material that rises from Earth's interior is affected by plate tectonics, producing unexpected irregularities in Earth's topography and assisting in the break-up of continental plates. See Letter p.85
High mountains and deep valleys are eye-catching features of our continents. Such differences in surface elevation are the result of the converging and diverging motions of the tectonic plates that form Earth's strong outer layer — the lithosphere. Superimposed on this landscape is low topography over wide regions that is generated by slow movement in Earth's interior. How do these two processes interact to shape the ground beneath our feet? On page 85 of this issue, Burov and Gerya1 demonstrate how plate tectonics turns symmetric deep mantle flows into irregular surface topography, thereby touching on the debate over what drives the break-up of continental plates.
Earth's mantle, which underlies the tectonic plates, moves slowly, rather like a liquid being warmed up. Earth's heat comes from the heat left over from processes that formed the planet and from the radioactive decay of elements such as uranium. Rising upward through this slowly circulating matter are columns of warm, buoyant material — mantle plumes — that effectively transport heat from great depths to the surface2,3. These plumes are probably the source of large lava outpourings, such as those found in Siberia and India, that may have caused mass species extinctions in the past4. But unambiguous detection of mantle plumes has so far been difficult5. This is especially so for the detection of their deeper parts, which can be imaged only indirectly.
Furthermore, the effect of mantle plumes on Earth's surface is far from simple. Plumes are thought to have a symmetric shape that would generate a circular surface uplift once they impinge on the lithospheric plate. The Hawaiian plume, however, shows that the situation may be more complicated: horizontal plate motion, a possibly tilted plume conduit, small-scale convection in the upper mantle, and heterogeneities in the overlying plate all combine to destroy the expected surface symmetry6,7.
The topography generated by mantle flow occurs over wide areas, but with low magnitudes — up to hundreds of metres of surface elevation over distances of hundreds to thousands of kilometres. Locally, this signal is easily overwhelmed by the larger surface displacements generated by tectonic processes such as mountain building. Topography caused by mantle flow is therefore usually determined by filtering Earth's topography from the effects of density variations caused by plate tectonics. But Burov and Gerya present three-dimensional (3D) computer models showing that Earth's topography cannot always be separated into deep and shallow contributors when plate–mantle interactions come into play. This is particularly so for continental plates, which are thicker than oceanic plates and characterized by a horizontal layering of alternating strong and weak materials. Earlier 2D computer experiments8 showed that the continental lithosphere above a plume head can develop drip-like instabilities. Deformation along the weak layers inside the lithosphere may lead to alternating small-scale uplift and subsidence patterns, instead of a single, dome-shaped surface uplift. Using high-resolution models of plume–lithosphere interaction, Burov and Gerya demonstrate that this also holds for 3D models.
These models combining mantle flow with deformation of tectonic plates imply that the contributions from plate tectonics or mantle flow no longer need to be considered individually. Instead, combined 3D models predict a surface topography that can be directly compared with observations. Nonetheless, such comparisons may be difficult, because Burov and Gerya's results show that continental topography above mantle plumes not only varies in space and time, but also can be asymmetric. It therefore becomes a challenge to identify surface uplift related to mantle plumes unambiguously, or even to use such surface observations to constrain properties of Earth's interior, such as viscosity.
Plume–lithosphere interaction models such as those of Burov and Gerya may contribute to the debate over a chicken-and-egg question in Earth science: can mantle plumes cause so much uplift and deformation in a continent that it breaks and creates a new ocean? Or do continents rift apart under stresses that are generated by tectonic-plate motions, and does the break-up process cause the upwelling of warm mantle from which the ocean crust is formed? As is often the case, the answer is probably a bit of both.
Continental break-up and the eruption of large amounts of magma are often closely related9. For example, the opening of the central Atlantic Ocean occurred shortly after the formation of the Central Atlantic Magmatic Province (about 200 million years ago), and magmatism and rifting in the Afar region of northeastern Africa are closely related in space and time (Fig. 1). In other places, however, continental break-up occurred without much magma, for example in the break-up between Iberia and Newfoundland. The fact that the processes occur at the same time does not favour one scenario over the other. It is here that Burov and Gerya's models offer new insight. The authors show that slow continental extension might not lead to break-up when continents do not contain heterogeneities inherited from earlier deformations, such as faults or changes in rock strength. But if a plume impinges on a continent that is already undergoing slow extension, it may localize deformation and help the plate to break up. The mantle plume alone does not cause continental break-up, but it could be the deciding factor.
Such a picture of plume-assisted rifting might apply to the separation of Norway and Greenland about 54 million years ago. Here, rifting events over a few hundred million years led to break-up only shortly after the North Atlantic Igneous Province formed. It would be interesting to see if a plume that is offset from a developing rift could lead to “upside-down drainage” flows10, in which plume material moves upward along the base of the plate towards the rift area. This would considerably increase the likelihood of plume–rift interactions.
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