Earth is the only body in our solar system with plate tectonics, but no one knows why or when the process began. To explore these questions, 62 geoscientists, representing all career stages and a wide range of disciplines, met at the Swiss Federal Institute of Technology’s (ETH) Congressi Stefano Franscini conference center at Monte Verità. The conference center overlooks beautiful Lago Maggiore, which is situated on an ancient plate boundary in southern Switzerland.
This was the fourth conference devoted to understanding the evolution of continental drift and plate tectonics. The series of discussions began with the 1926 American Association of Petroleum Geologists session on continental drift, followed by a 1975 Geological Society of America (GSA) Penrose Conference in Vail, Colo., “Pre-Mesozoic Plate Tectonics: How Far back in Earth History Can the Wilson Cycle Be Extended?” The most recent conference was the 2006 GSA Penrose Conference in Lander, Wyo., “When Did Plate Tectonics Begin?”
Conferees debated when plate tectonics began, how it began, what Earth’s tectonic style was before plate tectonics, and how to define plate tectonics. Participants reached agreement on five key points:
Because the sinking of dense oceanic lithosphere (the conductive outer thermal boundary layer of Earth) in subduction zones powers plate motions, we must understand modern subduction, especially the forces that favor and oppose it, to understand when conditions were ripe for plate tectonics to begin.
- Deep recycling of surface material began early in Earth’s history, although it is uncertain how early and whether or not this required plate tectonics.
- Oceanic lithosphere must be sufficiently strong to remain intact and sustain subduction but locally weak enough to nucleate new subduction zones required for plate tectonics. The fact that half of Earth’s 65,000 kilometers of convergent plate margins formed in the past 65 million years implies that it is reasonably common for lithospheric collapse to make a new subduction zone.
- Because the early Earth’s interior was hotter by 100°C to 200°C than it is now, pre–plate tectonic regimes likely involved weaker and more buoyant oceanic lithosphere than today, impeding the onset of sustained plate tectonics. Vigorous magmatic activity characterized pre–plate tectonic regimes, which were essentially single-plate, mobile “stagnant lid” tectonic regimes. At various stages, heat pipes, mantle plumes, drips, delaminations, and short-lived subduction zones accommodated convective overturn, similar to conditions on modern Venus.
- Continents assisted plate tectonic evolution by providing density contrasts at their margins and by shedding weak sediments to the seafloor, which is required to lubricate subduction zones.
Workshop participants recognized that Earth’s modern plate tectonic regime evolved over time. Lithospheric damage led to the accumulation of weak zones that evolved into plate boundaries. Mantle plumes may have first induced lithospheric collapse and short-lived subduction zones.
Participants agreed that complex rheological interactions and mineralogical processes at the scale of crystal grains play a key role. There was also a sense that important changes occurred in Neoproterozoic time, when key plate tectonic indicators, including ophiolites, blueschists, and ultrahigh-pressure terranes, began to appear in abundance.
The presentation of the outstanding young scientist award to Kiran Chotalia (University College London, United Kingdom) brought the meeting to a close. The next meeting will be held in Sudbury, Ontario, Canada, in 2018.
—Robert J. Stern (email: [email protected]), Geosciences Department, The University of Texas at Dallas; and Taras V. Gerya and Paul J. Tackley, Earth Sciences Department, Swiss Federal Institute of Technology, Zurich, Switzerland