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Soil eroded by glaciers may have kick-started plate tectonics

Vast amounts of sediment eroded from Earths continents were necessary to lubricate the wheel of plate tectonics, scientists propose. The idea offers a new angle on long-standing riddles about the origin and evolution of the planets global surface recycling system, one that is unique in the solar system.

Earths interior holds a lot of heat, even 4.6 billion years after the planets formation. For the first 1 million to 1.5 million years of Earths history, the planets insides were still too hot for the lithosphere to cool and thicken (SN Online: 9/21/17). Thats one necessary ingredient for modern plate tectonics, the ongoing collisions and separations of large “plates” of lithosphere, the jigsaw puzzle pieces that make up the hard outer shell of the planet.

Eventually the planet cooled enough for the crust to form. And then, around 3 billion years ago, Earths first continents arose. That ultimately added another key ingredient that allowed plate tectonics to get under way, geophysicist Stephan Sobolev and geologist Michael Brown argue in the June 6 Nature. Massive amounts of sediment scraped by glaciers off the continents were essential to kick off the lithospheric dance, the researchers say. That soft sediment was slowly deposited in deep ocean trenches, where it reduced the amount of friction between a sinking, or subducting, plate and the overlying plate, speeding up plate tectonics.

Giant influxes of sediment to the oceans, related to worldwide glaciation events such as an event that lasted from about 750 million to 630 million years ago, could explain why plate tectonics has sometimes kicked into a higher gear, Sobolev and Brown say. And a dearth of such sediments in the rock record could also explain periods of sluggish tectonic movement, including the Boring Billion, a period of lithospheric — and evolutionary — stability between 1.8 billion and 800 million years ago.

Lubricated slab[hhmc]

Three periods of global glaciation leading to intense continental erosion in the last 3 billion years correlate with times of increased plate motion, a study finds. Researchers say that the eroded sediment helped speed up subduction, and thus plate tectonics, by reducing friction at the interface between plates.

    <img src="https://www.sciencenews.org/sites/default/files/2019/06/060419_cg_tectonic_inline1_370.png" title="~~ W. Behr/&lt;em&gt;Nature&lt;/em&gt; 2019" />  

Sobolev, of the University of Potsdam in Germany, and Brown, of the University of Maryland in College Park, combined data on how sediments reduce friction at subduction zones in modern times with geologic records of glaciation and the growth of mountains as well as geochemical data standing in for ancient subduction speeds. From these datasets, the team created a unified picture of when plate tectonics has sped up or slowed down during the last 4 billion years of Earths history.

The cycle sped up three times, the team found, with each period lasting at least several hundred million years. The first began about 2.8 billion years ago, the second about 2.3 billion years ago and the third about 750 million years ago. Each of those time periods corresponds to a timeRead More – Source
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