Earth’s deep carbon cycle is linked to plate tectonic activity, which—within our Solar System—is unique to Earth for reasons yet unknown. Although the Deep Carbon Observatory (DCO) continues to explore this dynamic and intricate relationship, the origin and evolution of plate tectonics clearly had a profound impact on the deep carbon cycle throughout the planet’s history. We cannot understand Earth’s deep carbon cycle without understanding Earth’s tectonic history, especially the origins of deep subduction (modern plate tectonics).
The scientific community began discussing the evolution of plate tectonics and continental drift in 1926, with a session on continental drift at an American Association of Petroleum Geologists meeting in New York City. A robust dialogue continued over the ensuing decades and included a Geological Society of America Penrose Conference in Wyoming in 2006 titled When Did Plate Tectonics Begin?
More recently, the Workshop on the Origins and Evolution of Plate Tectonics, held from 18 – 22 July 2016, brought together a diverse group of 62 geoscientists to further explore these topics. The conference took place at the Congressi Stefano Franscini (CSF) of Eidgenössische Technische Hochschule (ETH) Zürich at Monte Verità, near Lago Maggiore in Ascona, Switzerland. A Wednesday field trip to the Alpine suture in southern Switzerland punctuated four days of talks, posters, and discussions. ETH Zürich, the Swiss National Science Foundation, and the DCO co-sponsored the workshop.
Participants disagreed on many points, especially the question of when plate tectonics began and the definition of plate tectonics in this context. Partial consensus on other issues emerged by the workshop’s close, including the following six points:
- To understand the evolution of plate tectonics it is critical to understand modern subduction—especially the forces that favor and oppose it—because plate motions are powered mostly by subducting dense oceanic lithosphere.
- Recycling of surface material began early in Earth’s history. However, the timing and mechanism of this recycling remains unresolved. In particular, it is unclear whether surface recycling requires plate tectonics or if other processes (e.g., meteorite impacts, drips, and delaminations) can drive recycling.
- Oceanic lithosphere must be sufficiently strong to maintain plate-like behavior. Therefore, initiating new subduction zones and plate tectonics entails mechanisms for weakening strong lithosphere.
- Meteorite impacts may have been important for weakening the lithosphere, especially on early Earth.
- Because early Earth’s interior was hotter by a few hundred degrees Celsius, pre-plate tectonic regimes likely involved a soft, deformable lithosphere that was weaker than the oceanic lithosphere of today. Vigorous magmatic activity likely characterized pre-plate tectonic regimes with convective downwellings of various sorts, including drips, delaminations, and short-lived subduction zones.
- Continents may have been important for plate tectonic evolution by providing zones of density contrast, if not lithospheric weakness, at their margins.
Consensus evolved among workshop participants that Earth’s modern plate tectonic regime developed slowly, with mantle plumes playing an important role by causing lithospheric collapse and short-lived subduction zones (there is evidence for such an episode approximately two billion years ago). The participants also agreed that a significant change in Neoproterozoic time increased the abundance of key plate tectonic indicators such as ophiolites, blueschists, and ultra-high pressure terranes. However, ongoing research is critical, and participants plan to reconvene and continue these discussions at a future conference.
Workshop report contributed by Taras Gerya, Robert Stern, and Paul Tackley.