Since the 1960s, scientists have accepted that mantle convection – the lava lamp-like churning of Earth’s interior, where hot mantle material near the core rises and cooler, denser, material near the surface sinks back down – moves the tectonic plates on Earth’s crust. But what if processes on the surface, like erosion and climate, are also powerful factors affecting tectonic activity?
A new paper in Nature  proposes that tectonic plate movements were made possible by giant continental erosion events caused by widespread glaciers in the past. The resulting sediments coming into the oceans lubricated subduction zones, where the edge of one plate sinks beneath another into the mantle. DCO Reservoirs and Fluxes, and Extreme Physics and Chemistry Community member Stephan Sobolev (German Research Center for Geosciences GFZ, Germany), and Michael Brown (University of Maryland, USA), applied what they know about modern plate tectonics to the evolution of tectonic activity in Earth’s past. The researchers identified two major glaciation events that triggered increased tectonic activity in Earth’s history: one that led to the supercontinent Columbia and a second that likely caused the stable plate tectonics that we see on Earth today.
“Although it looks counterintuitive, it could well be that such global and deep-rooted processes like plate tectonics could be affected and largely controlled by near-surface processes and maybe even by climate,” said Sobolev.
Until recently, Sobolev primarily focused on modeling present-day subduction and collision processes. Modeling suggests that sediments play an important role in reducing friction, as one plate scrapes against another in subduction zones. After talking with colleagues, he suspected that this same principle might also apply to the evolution of tectonic activity.
Sobolev reasoned that lubrication would be even more important in the past than today, because when continents first rose out of the ocean about 3 billion years ago, Earth was hotter and the edges of continents likely crumbled rather than subducted. He thought that without sediments, Earth could not maintain stable subduction processes.
In 2016, DCO members invited Sobolev to the Workshop on the Origin and Evolution of Plate Tectonics in Locarno, Switzerland, where he presented and refined these ideas through conversations with other attendees. “That conference changed my life. Now this is my main interest,” he said.
After the workshop, Sobolev and Brown set out to test the idea that Earth needed sediments to grease the wheels of plate tectonics and looked for existing evidence of erosion. They used three geochemical markers that act as proxies to indicate plate tectonic activity, measured from rocks that formed up to 3.6 billion years ago. The proxies point to two times when glaciers descended over Earth, eroding the continents and sending substantial amounts of sediments into the ocean.
The first was the Paleoproterozoic Huronian glaciation, about 2.45 to 2.2 billion years ago, which may have spurred multiple continental collisions and formed the supercontinent Columbia. The second was the Neoproterozoic “Snowball Earth,” when glaciers covered almost the entire planet, about 650 million years ago. The extensive glaciers scraped kilometers of rock from the continents and created the “Great Unconformity” visible in the Grand Canyon and elsewhere, where more recent sediments formed over much older layers.
Snowball Earth occurred at the end of the “Boring Billion,” a period from 1.75 to 0.75 billion years ago with little tectonic activity, and the sediments it created may have set in motion the modern era of plate tectonics. “The Boring Billion was a period when not much was happening on Earth. I don’t believe it was very boring, but it was much more boring than what was before and after,” said Sobolev.
If the hypothesis is correct, then the cooling of early Earth and mantle convection are not the only things that have contributed to plate tectonics. Surface processes such as erosion and climate also may have had an important role in plate movements. Additionally, these surface processes likely influence the entire carbon cycle on Earth, since plate tectonics also affect the input of carbon into the deep mantle through subduction and the output of carbon through volcanic emissions.
Sobolev is working with a team of researchers including his brother, DCO Reservoirs and Fluxes Scientific Steering Committee member Alexander Sobolev (Université Grenoble Alpes, France) to explore the origins of plate tectonics. Their goal is to use the synergy of geochemical and geodynamic approaches to find further evidence to explain the evolution of plate tectonics on Earth, from its beginning to the present.