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The fourth revolution

Plumes, plates, and climate. Trond Torsvik is working to decipher the Earth system as a whole and, ultimately, what makes Earth so uniquely habitable in our Solar System

14 June 2024

“The solid Earth’s dynamic surface is the interface between its interior and the ocean and atmosphere, and palaeogeography is therefore a critical component of our Earth System Model,” explains Trond Torsvik

“A fourth revolution is underway,” explains Trond Torsvik, Professor of Geodynamics at the University of Oslo, Norway.

“Over the last century our description of the movement and deformation of Earth’s outer layer evolved from the hypothesis of Continental Drift (1915) into Seafloor Spreading (1962) and then to the theory of Plate Tectonics (1967) – a theory as fundamentally unifying to the Earth Sciences as Darwin’s Evolution Theory is to Life Science. Now Plate Tectonics is being subsumed into a new framework of Mantle Dynamics, linking surface volcanism and mantle plumes, and perhaps explaining plate motions both quantitatively and dynamically.” Trond is currently focused on this fourth revolution, attempting to gain a bigger-picture understanding of the links between deep and shallow Earth processes, but his passion for Earth science was initially sparked by palaeomagnetism.

Palaeomagnetism

Originally primarily interested in mathematics, Trond trained in military marine navigation before discovering a fascination for geophysics and geology while studying for his undergraduate degree at the University of Bergen in 1978.

“I became passionately interested in geomagnetism and notably how we could use Earth’s magnetic field (palaeomagnetism) to reconstruct continents in the deep past. My training in marine navigation required a fundamental knowledge of differential geometry (Euler’s theorem), so in a sense I went from steering boats to reconstructing continents.”

Trond’s earliest work focused on regional plate reconstructions and palaeogeography. However, by pairing palaeontological time constraints with palaeomagnetic data, and using plate reconstruction software that Trond himself developed, the work evolved into global palaeogeographic reconstructions covering the past half a billion years of our planet – all of which are synthesised in the remarkable, ambitious, and beautifully illustrated book Earth History and Palaeogeography, which Trond wrote together with Robin Cocks (Torsvik & Cocks 2016, Cambridge University Press).

An incomplete theory

Trond’s research is motivated by fundamental gaps in our understanding of how surface and deep mantle process are linked: “Plate tectonics and the Wilson cycle were recognised as key elements of geodynamics in the 1960s. Plate Tectonic Theory was extremely successful in providing a framework for understanding deformation and volcanism at plate boundaries, and allowed us to understand how continent motions through time are a natural result of heat escaping from Earth’s deep interior. Plate tectonics was, however, an incomplete theory: For instance, we lacked a generally accepted mechanism that explains plate tectonics in the framework of mantle convection, and the origin of intra-plate volcanism such as hotspots and large igneous provinces (LIPs) was controversial.”

Drawing links between surface and lithospheric processes and the mantle is extremely challenging. However, Trond and his colleagues made this feasible by deriving the longitude-calibrated maps (that is, absolute reconstructions) of ancient continents from before the Cretaceous (achieved, in part, because of an increased understanding of the dynamics of true polar wander), and combining these with more detailed images of Earth’s deep mantle. Seismic tomography reveals the presence of two so-called large low shear-wave velocity provinces, which extend laterally for thousands of kilometres and sit above the core-mantle boundary in diametrically opposite positions (one beneath Africa, one beneath the Pacific). The provinces are interpreted by some as piles of thermally and/or chemically distinct material, possibly relics of Theia, the planetary body that is thought to have collided with Earth to form our Moon, or a graveyard for slabs of cold, dense subducted oceanic lithosphere. As Trond explains, the seismic tomographic images provide compelling evidence that the edges of these provinces or piles are key sites for mantle-plume generation.

“These thermo-chemical provinces on the core-mantle boundary have been semi-stable for 300 million years, possibly for 540 million years and longer, and their edges are the dominant sources of the plumes that generate LIPs, hotspots and kimberlites. LIPs provide a direct link between plume-generating processes in the deepest mantle and the atmosphere and biosphere, enabling us to develop an Earth model, not only integrating plate tectonics and mantle dynamics, but also the ancient environmental and climatic evolution.”

Whole Earth system

To gain a more holistic picture of Earth evolution, Trond is now taking his research one step further and using carbon-cycle modelling to probe the e  ects of plate tectonics and palaeogeography on climate changes over the past 540 million years.

“The solid Earth’s dynamic surface is the interface between its interior and the ocean and atmosphere, and palaeogeography is therefore a critical component of our Earth System Model. Full-plate palaeogeographic models developed over the past decade enable direct estimation of surface-mantle fl uxes to inform time-dependent models for water cycling between Earth’s surface and interior, and to estimate plate tectonic degassing (a source of CO2). Our palaeogeographic models have now also been extended to include the regions that defi ned exposed and fl ooded land through time, and thus allowing us to parameterise silicate weathering (a sink for CO2) for 3D climate and carbon-cycle simulations.”

The constellation of environmental conditions that allowed life to arise from inanimate matter via abiogenesis is still among the greatest unknowns in science

Trond is currently Director of a new Centre for Planetary Habitability (PHAB) at the University of Oslo, the prime endeavour of which is to recognise and characterise the key conditions that make a planet habitable.

“The constellation of environmental conditions that allowed life to arise from inanimate matter via abiogenesis is still among the greatest unknowns in science. Earth is the only planet on which life is known to have originated and appears unique in many ways, including the presence of abundant surface water (life’s medium), a large moon, a long-lived magnetic fi eld, and plate tectonics. Yet, which of these and other characteristics are essential for its long-term habitability? Equally, how have Earth’s physical and chemical attributes, and thus our planet’s proclivity for life, evolved? How can we recognise distant worlds around other stars that have been or could be habitable? These questions, and a new understanding of planetary habitability unfolding from them, are especially important as we now embark on an unprecedented era of exploration and discovery of extrasolar planetary systems.”

Trond Torsvik is Professor of Geodynamics and Director of the Centre for Planetary Habitability at the University of Oslo, Norway, and the 2024 recipient of the Geological Society’s Wollaston Medal.

Interview by Amy Whitchurch 

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