New Study Highlights Connection Between Worldwide Glaciations, Sediments, Plate Tectonics
A geological feature visible in the rock strata Grand Canyon known as the Great Unconformity (visible roughly in the center of the photo) is area where layers of sediments were likely scrubbed away by ancient glaciers. New UMD research suggests that such sediments helped lubricate faults in the Earth's crust, kick-starting the modern era of plate tectonics.
With its outer layer of giant plates that grind together, sliding past or dipping beneath one another to cause earthquakes and volcanoes, Earth is the only known world to feature this global system of plate tectonics.
A study published today in the journal Nature suggests a mechanism for how it all began, proposing for the first time a role for sediments in the emergence and evolution of plate tectonics. Michael Brown, a professor of geology at the University of Maryland, co-authored the research paper with Stephan Sobolev, a professor of geodynamics at the GFZ German Research Centre for Geosciences.
Early in Earth’s history, the planet was covered by a single shell dotted with volcanoes—much like the surface of Venus today. As the planet cooled, this shell began to fold and crack, eventually creating Earth’s system of plate tectonics.
According to the new research, the transition started with the help of lubricating sediments, scraped by glaciers from the slopes of Earth’s first continents. As these sediments collected along the planet’s young coastlines, they helped to accelerate the motion of newly formed subduction faults, where a thinner oceanic plate dips beneath a thicker continental plate.The findings suggest that sediment lubrication controls the rate at which Earth’s crust grinds and churns. Sobolev and Brown found that two major periods of worldwide glaciation, which resulted in massive deposits of glacier-scrubbed sediment, each likely caused a subsequent boost in the global rate of plate tectonics.
“Earth hasn’t always had plate tectonics, and it hasn’t always progressed at the same pace,” Brown said. “It’s gone through at least two periods of acceleration. There’s evidence to suggest that tectonics also slowed to a relative crawl for nearly a billion years. In each case, we found a connection with the relative abundance—or scarcity—of glacial sediments.”
Just as a machine needs grease to keep its parts moving freely, plate tectonics operates more efficiently with lubrication. While it may be hard to confuse the gritty consistency of clay, silt, sand and gravel with a slippery grease, the effect is largely the same at the continental scale, in the ocean trenches where tectonic plates meet.
For their study, Sobolev and Brown used a geodynamic model of plate tectonics to simulate the effect of sediment lubrication on the rate of subduction. To verify their hypothesis, they checked for correlations between known periods of widespread glaciation and previously published data that indicate the presence of continental sediment in the oceans and trenches.
According to Sobolev and Brown’s analysis, plate tectonics likely emerged on Earth between 3 billion and 2.5 billion years ago, around the time Earth’s first continents began to form. This time frame also coincides with the planet’s first continental glaciation.
A major boost in plate tectonics then occurred between 2.2 to 1.8 billion years ago, following another global ice age that scrubbed massive amounts of sediments into the fault trenches at the edges of the continents.
The next billion years, from 1.75 billion to 750 million years ago, saw a global reduction in the rate of plate tectonics. This stage of Earth’s history was so sedate, comparatively speaking, that it earned the nickname “the boring billion” among geologists.
Later, following the global “snowball Earth” glaciation that ended roughly 635 million years ago, the largest surface erosion event in Earth’s history may have scrubbed more than a vertical mile of thickness from the surface of the continents. According to Sobolev and Brown, when these sediments reached the oceans, they kick-started the modern phase of active plate tectonics.
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