Disentangling the role of mantle convection in shaping Earth’s topography
Plate tectonics, or the movement of Earth’s rigid outer shell in continent-sized pieces, generally controls Earth’s surface elevation (topography). Where tectonic plates collide, mountain ranges are formed, and where they diverge, we see rift basins. It is these processes that cause variations in the height of the Earth’s surface.
However, new research has shown that this obscures a second control on topography; that of mantle convection. Despite its solid nature, the mantle flows on geological timescales, moving up towards the surface of the Earth in some places and down towards the core in others. Whilst it has been known for some time now that this causes the tectonic plates to move, the direct influence of mantle convection on the height of the Earth’s continental surface was unconstrained, until now.
A new study, led by Oxford Earth Sciences’ Simon Stephenson, and published recently in the Journal of Geophysical Research: Solid Earth, compiled the largest database of continental crustal thicknesses to date (totalling 26,725 measurements), alongside a compilation of laboratory analyses of seismic velocity in continental rocks. The authors then used these databases to quantify the proportion of topography controlled by crustal thickness and density, allowing for the influence of mantle processes (known as residual topography) to be isolated.
“Beneath Oxford the crust is about 35 km thick. Beneath northwest Scotland it is about 25 km thick. People will recall that the reason we get mountains is because we thicken the crust. So why is the topography so much higher in Ullapool than in Oxford? Well, our results suggest that hot mantle is pushing up the Earth’s surface in Scotland by around 500 m relative to southern England.”
Simon Stephenson, Lead Author of the Study
The authors found that a substantial amount of topography, even within the interiors of tectonic plates, results from the behaviour of Earth’s mantle, with areas of greater elevation caused by hot mantle flow towards the surface (upwelling) and areas of supressed elevation caused by cold, sinking mantle material. The most prominent of these topographic swells can be found in East Africa, Iceland, Western North America, and in Eastern Europe, where they are up to 2 km in height or depth and stretch for hundreds to thousands of kilometres. It is likely that these topographic features formed over millions of years.
Global residual topography and bathymetry. Red shading indicates areas of elevated topography/bathymetry caused by mantle upwelling, and blue shading indicates areas of depressed topography/bathymetry caused by sinking mantle material. (Figure 11, Stephenson, S. N., Hoggard, M. J., Holdt, M. C., & White, N. (2024). Continental residual topography extracted from global analysis of crustal structure. Journal of Geophysical Research: Solid Earth, 129, e2023JB026735. https://doi.org/10.1029/2023JB026735)
The greatest challenge to overcome in this analysis was the complexity of the interior of our continental crust, which is thick and structurally complicated. Until now, this has been one of the reasons that our understanding of the effects of mantle processes on topography have gone unaccounted for. This work represents a labour of love for Dr Stephenson, who has been considering the problem for a number of years. Reflecting on the study, he said “It has been known for a long time that some proportion of topography on the continents is supported by mantle flow, but it has been really very hard to actually quantify how much and in which places it is important. This work has been carried out in the oceans, where the crust is a bit simpler, but the continents remained under-analysed, leaving a big gap in our understanding. We had to spend quite a long time building a couple of big databases to actually globally quantify this process.”
Knowledge of how the mantle has behaved through time is extremely valuable to geoscientists; this layer of the Earth has long eluded researchers, who have very few observations of about its’ movements. This study provides vital constraints on present-day mantle convection that can drastically improve simulations of these processes in the past. This can be combined with observations at the surface to further our knowledge of where magma rises from the mantle through the crust, particularly far away from plate boundaries.
“The growth of these swells and basins diverts rivers and controls where we see erosion and deposition in the past. The sediments generated and transported by these processes host a whole range of economic resources. Additionally, we increasingly need to understand the crust’s thermal structure because of the growing importance of geothermal processes in energy generation and mineral resource development.”
Simon Stephenson
This work was conducted in collaboration with researchers at the Australian National University, Canberra, and the Bullard Laboratories at the University of Cambridge. Funding for Dr Stephenson was provided by Geoscience Australia.